Amplifier Burn In

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dineshvarma2k

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Hello Everybody,
I have just joined this forum yesterday. Great to be here. Come with a question. Do Semiconductor Amplifier need 'Break In'.
Let me have your replies.
Thanks in advance
 

doors666

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Hello Everybody,
I have just joined this forum yesterday. Great to be here. Come with a question. Do Semiconductor Amplifier need 'Break In'.
Let me have your replies.
Thanks in advance

Well, I am on my second avr, both yams, i dont think solid state electronics need burn in.
 

venkatcr

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Amplifier burn-in is as controversial a topic as burn-in of any other audio component.

In tubes, a 30 minute warm up is needed for the tubes to be ready to play. But I have not read anywhere about a burn-in for tube amps.

In solid state, the transistors, resistors and other components use low voltage DC current. If a stable power is delivered, they should sound the same as always. Nelson Pass, a legend in amplifier design, says that, during production, his amps are adjusted for bias after about an hour of being on for the first time. This is done over a few days to ensure that the amp sounds the same always. Once it reaches the user, it is ready to be used from the first minute.

Excepting for a warm up requirement, I don't think amps need any burn-in.

Cheers
 
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gobble

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My amp sounded real grainy for the first 10 hours of play. Then in a weeks time it improved. Thats +1 for break-in :)

Then i once left my amp on for 15hours ( I fell asleep and the lights didn't go away :)) ... and my!! Did it sound sweetly delicious? It was heaven! There was a drastic improvement in SQ . +1 for warm-up :)

So I am inclined to believe SQ is affected ... so much for theory.

Regards
 

reju

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Then i once left my amp on for 15hours ( I fell asleep and the lights didn't go away :)) ... and my!! Did it sound sweetly delicious? It was heaven! There was a drastic improvement in SQ

Regards

Since you fell asleep then, are you sure you did not hear those "sweetly delicious" sounds in your dreams? :eek:hyeah:

Just kidding...;)
 

dineshvarma2k

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Sincerely thank you guys for your replies.
By the way has anybody done any tests which shows the difference betweeen the units before and after burn in.
I guess this it is more of getting 'familiar and used to' effect.
If there are any such test reports would be glad if someone can share it here with us.
Thanks
 

marsilians

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Burn in of tubes and SS amps is mainly due to the way the amplification is driven. To me, there are 2 things that need to be at the optimal state : some of these are instantanous and some take a few hours - These are tubes (can feel the sound differences when you start from cold state) and capacitors (cannot feel the difference) even though they go through reactions in order to run at full capacity.
 

suprateep

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Burn in of tubes and SS amps is mainly due to the way the amplification is driven. To me, there are 2 things that need to be at the optimal state : some of these are instantanous and some take a few hours - These are tubes (can feel the sound differences when you start from cold state) and capacitors (cannot feel the difference) even though they go through reactions in order to run at full capacity.

+1 to that.
 

venkatcr

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Your ears also need time to get used to tonality of your system as opposed to what you were hearing earlier, and that is also part of the break-in/burn-in process.

Ah ha !!! I caught this extremely loaded statement only after I ready Cranky's post a few times.

More than anything else, I firmly believe that it is the human ears that adjust to the sound coming in, more than a system breaking-in. A system's sound stage could change, but I doubt if it for the better. Even if it does change for the better, it will be a very marginal amount within the first few hours.

Whatever warm up adjustments are needed should be complete during the testing phase at the factory itself. I am sure some of the better brands run their systems for a couple of hours at least, and that should be enough.

Someone compared the burn-in of electronics to that of a car. In the days of Ambassador and Fiats, the machining capabilities of the manufacturers were not so good, so the tolerances in the pistons, the crankshaft, and other parts were rough. In a mechanical device, when two parts rub against each other there is wear and tear. In the initial stages, since the fitting and tolerances are low, this actually leads to an improvement with the reduction of friction. But, as the usage increases, the wear and tear actually leads to decay and and a gradually worsening of performance. It is because of this that those manufacturers would recommend gentle usage in the initial stage so that parts fit into each other well and friction get reduced.

Modern cars are a different matter altogether. Using robotised machining, the tolerances are measured in micro millimetres and. a brand new car can be driven and abused without any worry immediately after you drive it out of the showroom. But, with usage, you will only see a worsening of the performance - more noise, lowering of pickup, slight reduction in mileage, etc. You will not see any improvement whatsoever.

Modern solid state electronic circuitry is similar. If designed and manufactured well and tuned properly, using high quality parts, a modern amplifier, barring a few minutes to reach stable state after power on, should perform as well from day one. Whatever improvement we 'hear' could possibly be more psychological and in our heads rather than something that can be measured as coming from the system.

Consider me a skeptic if you want, but all matter decays in time and with usage.

Cheers
 
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square_wave

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I think the amplifier break-in issue is very amplifier specific. It has mostly to do with the components used in the power supply section. The final amplifier sound has much to do with the power supply. Especially black gate caps and such. These kind of amplifiers take a while to break in. Most average amps just sound the same always. Like cranky said, terrible :D

It is not in the mind because we have checked out burnt in and non-burnt in amps of the same brand side by side to understand this phenomenon.
 

alpha1

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Ah ha !!! I caught this extremely loaded statement only after I ready Cranky's post a few times.

More than anything else, I firmly believe that it is the human ears that adjust to the sound coming in, more than a system breaking-in. A system's sound stage could change, but I doubt if it for the better. Even if it does change for the better, it will be a very marginal amount within the first few hours.

Whatever warm up adjustments are needed should be complete during the testing phase at the factory itself. I am sure some of the better brands run their systems for a couple of hours at least, and that should be enough.

Someone compared the burn-in of electronics to that of a car. In the days of Ambassador and Fiats, the machining capabilities of the manufacturers were not so good, so the tolerances in the pistons, the crankshaft, and other parts were rough. In a mechanical device, when two parts rub against each other there is wear and tear. In the initial stages, since the fitting and tolerances are low, this actually leads to an improvement with the reduction of friction. But, as the usage increases, the wear and tear actually leads to decay and and a gradually worsening of performance. It is because of this that those manufacturers would recommend gentle usage in the initial stage so that parts fit into each other well and friction get reduced.

Modern cars are a different matter altogether. Using robotised machining, the tolerances are measured in micro millimetres and. a brand new car can be driven and abused without any worry immediately after you drive it out of the showroom. But, with usage, you will only see a worsening of the performance - more noise, lowering of pickup, slight reduction in mileage, etc. You will not see any improvement whatsoever.

Modern solid state electronic circuitry is similar. If designed and manufactured well and tuned properly, using high quality parts, a modern amplifier, barring a few minutes to reach stable state after power on, should perform as well from day one. Whatever improvement we 'hear' could possibly be more psychological and in our heads rather than something that can be measured as coming from the system.

Consider me a sceptic if you want, but all matter decays in time and with usage.

Cheers

Count me in your skeptic's anonymous group.
Particularly liked the analogy to cars.

In fact I believe that there is no speaker-break-in as recommended by speaker manufacturers. The manufacturer has got all the time during the quality check / testing to actually break in.

Its our ears and mind that really requires breaking in to find something acceptable, and its a tactic by manufacturers to buy enough time to make their products acceptable to customers who might be in split minds while auditioning.
 
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ajinkya

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Modern cars are a different matter altogether. Using robotised machining, the tolerances are measured in micro millimetres and. a brand new car can be driven and abused without any worry immediately after you drive it out of the showroom.

Venkat,

I am completely with you on your observations on system burn-in, However, your statement on new cars is one I cannot agree with.

I know many car manufacturers (Honda being one of them) who do recommend easing the accelerator and break during the first 500 miles or so, till the car is "driven in" (their term, not mine). Honda also states that mileage will pick up after this. We can drive from showroom in a new car but there is still a break-in period during which the company recommends we go a little more gently. This is not for gears meshing together in older cars but for the piston rings to conform to cylinder walls and friction between all moving parts to reduce. Even though the process is greatly reduced and the requirements less stringent, there is a break-in period. All this is well documented in Honda and Toyota user manuals.

That being said, in audio, I believe solid state requires no burn-in. Speakers I may accept since there are moving parts which can soften over time. Tubes, I don't really know since I am not familiar with tube design methods.

For the most part, it's all in our heads (and hearts) rather than in our equipment specification. (Funnily enough, I believe the same thing is true of love and s** as well :lol:)

-Ajinkya.
 
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venkatcr

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I know many car manufacturers (Honda being one of them) who do recommend easing the accelerator and break during the first 500 miles or so, till the car is "driven in" (their term, not mine).

Quite possible. Strangely I have never read the manual of any car that I have purchased including a Honda City. Old cars used to have a block on the accelerator that would stop you from speeding beyond some 40KMPH.

Cheers
 

ajinkya

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I don't know about Indian Honda manuals but I know that UK and US manuals are very detailed and worth a read. There is a lot of practical information in them. Some even have an index and a glossary :)
Yes, I remember my father telling me about the "regulator" on his ambassador that needed to be taken out after first few months of use. How times have changed! Reliable electronics, reliable mechanics, reliable performance...only constant is unreliable politicians. Well,...actually they are reliable in being unreliable...

-Ajinkya.
 
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dinyaar

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Quite possible. Strangely I have never read the manual of any car that I have purchased including a Honda City. Old cars used to have a block on the accelerator that would stop you from speeding beyond some 40KMPH.

Cheers

Dear Venkat,
It is very much there in all the Honda vehicle manuals i have. It also goes to say the first few thousand Kms are crucial and will determine the performance of the vehicle in the long run.
What u guys are talking about on old cars was called a GOVERNOR. That is hardly ever used anymore except on rebuilt engines of old/vintage cars.
Rgds
 

Asit

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Whatever warm up adjustments are needed should be complete during the testing phase at the factory itself. I am sure some of the better brands run their systems for a couple of hours at least, and that should be enough.

I am really a novice and am curious to know of any basis for the statement above, especially the reference to "a couple of hours".

I would like to know

1) Why is a time interval of that order (2 hours) relevant here? Why not 20 hours, or 200 hours? Is there a scientific reason that sets the scale here?

2) Why isn't a period of 2 hours considered a break-in?

There is a possibility that I have not made myself at all clear here. I am introducing a notion called "orders of magnitude". For example, an ant has a length of the order of a cm, a human being has a length (usually called height) of the order of a meter, a football ground has a length of the order of 100 meters; up to very large distances like astronomical distance scales of megaparsec (1 parsec is 31 trillion km). Now the point is that all the above are lengths, however small or large. Any of them can be relevant depending on the scale of our observation. There exists this notion of order of magnitude with all physical quantities.

With the above explanation of the notion of order of magnitude of a given quantity, now my concern is two fold, as above: 1) if you say, there exists a 2 hour break-in, that is to be considered also a break-in, though a relatively short one and 2) what sets the scale here? Once we agree that there is a break-in, the question naturally arises as to what in an amp determines whether the break-in will be achieved in 2 hours or 10 hours or 20 hours?

Since the time I have owned stereo separates, I have owned 4 amps: Technics (model forgotten), HK 6300, Nad c325bee and Leben CS300. All these amps have had a break-in period. This is of course a subjective statement, but I can say only the stuff that I myself have noticed, and I would not like to theorize or speculate here, just stating the facts I have perceived them to be.

However, I must say in all these cases, the break-in period was not large, between 20 and 30 hours. In the case of the Leben, the statement is not accurate, it's more of a guess (also based on inputs from Sridhar and Leben users), because I also had a brand new IC that was connecting the CDP to this new amp.

I also would like to share with you another relatively recent fact. Three years ago my 20 year old HK amp developed some problem with one channel. There is a very experienced person (Sri Rajesh Tanna) in Kolkata who in his spare time repairs amps, speakers etc and he has a special interest in old HK amps and also holds them with very high regard. He repaired and serviced my amp and claimed that it needed a few cap changes. His explanation was also consistent with my guess, because I suspected that after so many years of running the dielectrics of the caps had to give in and they better get replaced. But when I got the amp home, although it was fully operational, I did not like the sound at all, it sounded very harsh to me, and sounded very much unlike the amp I had owned for such a long time. At that time I already bought my Nad amp, and hence I was listening mostly to the Nad amp in my setup. However, the sound of the HK amp quickly improved and in about 20 hours at the max, it sounded beautiful again, and perhaps with more dynamics than what was there before the amp started giving problems.

The capacitors need a time of the order of milliseconds to get them fully charged. So contrary to many forum discussions (in forums across the globe), I concluded that the improvement could not be due to charging of the new caps, rather it must have had something to do with stabilization of the electric response properties dielectric material of the caps, assuming of course that the new caps were of course of the correct specification. Experienced and knowledgeable people, please explain my observations and confirm or reject my conclusions above.

Please note that I have been specific to break-ins of amps only, no sources, speakers or cables.

In addition, I like to point out that I did not find anything in Cranky's post that says that the amps do not have break ins. He obviously has also added to the subject the new issue of the listeners' ears and subjective adjustments.

Regarding cars (excuse me moderator, since this has been brought up), I have personally bought 11 cars so far in my life (7 of them new) and still do own 2 of them (the rest were sold as and when they got 'old') from early 80's models to last years models, from entry level cars to mid level and I have driven quite a number of relatively high end cars. As I have some interest in the matter, I have gone through the owner's manuals of most of those (even rented cars in the US). All those owner's manuals (irrespective of brands) say very very clearly that for the first X kms one should be careful, shouldn't do this and that and so on. My personal experience (after driving of the order of 10 lakh kms so far in my life, without being a truck driver) has been something like this: 2500 kms is a good average after which the car becomes very smooth in handling (especially pick up). But I must say I do not find any similarities between a car break-in and an amp break in.
 

ajinkya

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All passive components require settling time, specially capacitors (arguably, resistors too, drift with time and temperature). .. in the formation of the oxide layers inside the cap to permit the capacitor to create its capacitance. And that takes voltage, which needs to be applied for a while (say 2 hours) before the cap settles in.
From what I know, the oxide layer coating the anode is formed during the manufacturing process itself. So where does the question of break-in for capacitors play any role? I am assuming you're talking about electrolytic caps.
Resistors drift with temp. but that does not mean that burn-in is required to get them to the 'right state' since temp. drift will occur throughout the lifetime of the resistor. It is not a process which goes to some value and stops. So again I cannot see any reason for burn-in here. Similarly for an inductor (taking into account its non-ideal resistance). I could be missing the point of what exactly you're explaining.

Cars have not changed at all since the days they were first manufactured, they just have gotten better at isolating the occupants from the pollution they are creating. Amplifier technology has taken huge strides forward, in my mind, and a lot of it in the last thirty years when we are able to improve distortion by thousands of degrees of magnitude. If we were to progress at the same rate with cars, we'd have landed one on the moon ten years ago. In an hour.

That is not true. The first Ford car, the Model T had to be hand-started (cranked up). The mechanical engine used was vastly different from the computer-controlled fuel injection engines used today. There are about 50-100 embedded controllers in a modern car, monitoring every aspect of it's drive along with cruise control, anti-lock braking, stability control, active suspension damping, along with mechanical things like crumple zones, airbags...all these things make the modern car a distributed computer system on wheels, something which is orders of magnitude more sophisticated than old cars.

Yes, we still use petrol/diesel as primary fuel but now electric hybrids are changing that scene as well. I think as far as cars and airplanes are concerned, there have been monumental leaps forward in the last 50 years, especially in passenger safety and ride comfort. Cars will soon drive themselves, as witnessed by the recent Urban Challenge held in the US by DARPA.

Comparing this to reducing distortion levels in amplifiers is, in my opinion, comparing apples to cherries. Yes, amplifiers have gained a lot from strides in reducing tolerances on all components, aside from minituarization and VLSI design. But the stride is not comparable to placing a car on the moon. If that was the basis of comparison, amplifiers today should be able to adjust all their individual component characteristics to match the current listener's tastes and mood, all in real-time, apart from being able to adapt tonally and electrically to whichever speakers and source they were connected to, at half the cost of today's budget amps. And all these components (CDP,Amp, Speakers) should be connected wirelessly, without signal interference over a room length. That would be an amplifier equivalent to a "car on the moon". :D

-Ajinkya.
 

suri

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Quite possible. Strangely I have never read the manual of any car that I have purchased including a Honda City. Old cars used to have a block on the accelerator that would stop you from speeding beyond some 40KMPH.

Cheers

that was a device called a "GOVERNER" - lol:lol:

yow!!!! sorry:p jumped the gun!!:eek:
 
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venkatcr

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At the outset I want to make it very clear that I dont want to get into any extended argument for or against Amplifier Burn-in. However, I certainly want to offer a set of reasons why I am so skeptical about amplifier burn-in. When I think through it logically, it does not make any sense to me. Mind you, when an amplifier is used for a period of time, it certainly will have some impact on its performance. What we are discussing here, though, is the statement made by a number of people that, after 100 odd hours of use, an amplifier will actually belt out better sound due top some inexplicable reason.

Asit, Cranky and others have made some comments that got me thinking seriously. Frankly, in all my life, I have heard a different sound from the same system only once. This was in an exhibition where I was hungrily listening to a pair of B&W DM303. When I came back the next day, my favourite singer - OS Arun - sounded more life like. However, I have my doubts whether it was due to burn-in of any component or because the exhibitor had changed some units in the system, specially the speaker cables.

Last few weeks I have been listening to a Music Hall A25.2 amplifier, which I got unopened. I have used that amp for over 6 weeks, and the music sounded great to me from day one until I had to return the amp. I had purposely used the same CDs a number of times through the 6 weeks, carefully listening for any change in tonality or any other characteristic of the music and, as I said, they sounded the same. I did not change any component including cables, interconnects, CD player, or, speakers. I can say this with some confidence as I listened mainly to singers such as Katie Melua, Harry Belafonte and, The Dave Brubeck Quartet. With these artists, the sheer simplicity of the songs helps you in immediately catching any changes in the tone or other characteristics of the song,

Long ago, I had assembled my own amplifier using kits from a now defunct British company called Bi-Amp Kits. This was a fully populated board and all I needed to do was to fit it in a cabinet and fix knobs to the pots. Excepting for an initial pop during power on, the system sounded divine. I had this connected to a Dual 606 with Ortofons Concorde cartridge, and a pair of humongous speakers assembled by me and my friend using 10 inch woofers, mid range and, Philips tweeters. The cross over, again, was a fully assembled kit imported from Britain. I had a set of LPs that I used again and again including the then just released The Wall by Pink Floyd. I remember putting my ears very close to the speakers to hear every bell and every helicopter sound from that album. Again, I remember the system sounding the same throughout the two odd years that I used it.

So when someone says that an amplifier sounds better after it is used, what am I missing? Have I gone deaf? Have I become incapable of recognizing subtle differences in music? What was I doing when I auditioned the Audire system, and more recently, the Lyritas? Was I dreaming when I clearly remember why a friend of mine (an audiophile himself) called the Hsu Research subwoofer liquidy, as it had a very fast decay and we could easily identify multiple low frequency sounds individually without any overhang or rollover? Was I dreaming when I could hear the laboured breathing of a dog in The Gladiator with just the change of a pair of speakers? Mind you, I never listen to music or watch a movie at anything greater than 50 to 60 dB at between 8 to 10 feet from the speakers.

There are numerous arguments on the burn-in effect on cables, interconnects, and even players. But, an amplifier? If nearly all the items of a system are so undependable that we have to burn them in and pray to God that they will sound nice, why are we paying kings ransom for all these high fidelity equipment? Would a designer agree that the specifications that he used to design an amplifier circuit would last only for a few hours of use? More important, how would he agree to the statement that his circuit would somehow sound better after 100 hours of use? What control does he have on the behaviour of his design during those 100 hours or even after?

If a manufacturer or a dealer tells me that I must use the system for 100 hours before it sounds nice, I will have just one answer. Run it yourself for 100 hours and give it me if it sounds better. I can at least save 100 hours of warranty.

As I said, this got me thinking, and I decided to understand how an amplifier circuit works and what could possibly change in the circuit. As Julie Andrews sings in the Sound of Music, let us start at the very beginning.

An audio amplifier has two major parts.

The first part is a power unit that has a power transformer, a rectifier, filter, capacitors, and a voltage regulator. The power unit supplies the requisite DC voltages to the next unit in the amplifier chain the amplifier circuit. The regulated power supply is usually bypassed with high quality capacitors. These are placed next to the supplied circuit and electrically between the power supply rail, and the ground. The capacitor provides a path to ground the noises from the power supply unit. There have been some discussions on the capacitors used before and after the power unit. To not complicate matters for now, we will assume that we are able to supply stable and noise free voltages to the amplifier circuit. However, do keep in mind that we will come back to the power unit later.

There are multiple ways of building an amplifier circuit tubes, op-amps, MOSFETS, JFET, ICs, and others. What we will be discussing, for easy understanding, is a simple discrete transistor based amplifier.

The most important part of the amplifier is the transistor. The transistor is a solid-state semi conductor that has three elements: the base, emitter, and the collector. The transistor converts a small signal at the base to a large signal at the collector. Think of it as a hosepipe carrying water. The water source is the emitter, and the base is a tap in the hose that opens and closes to allow more or less water to flow. Remember, I said more or less, not on or off. In other words, a transistor always has some current flowing through it. You will understand the importance of this further down.



Before we move forward let us understand what we need from an amplifier. An analogue music signal is a pure sine wave as shown in the figure below. At a particular frequency, a sine wave will sound smooth and pleasant to our ears. If at the same frequency, the sound carries an additional wave, this will become noise to our ears. This additional wave is called harmonics.

classaoutputwavform.th.gif


Essentially, we need an identical duplicate of the original sine wave (input signal) to be given to us as an output signal. The amplifier must faithfully reproduce the input signal with perfect bias, never reaching its cut-off or saturation points. This will give us a signal that is perfectly centered between the amplifiers upper and lower limits as shown above. We will be discussing bias a little below.

Let us look at a single transistor amplifier to understand how the signal is amplified.



In the figure above, the object in the central grey circle is an NPN transistor. Other supporting devices such as resistors (R1, R2) and capacitors (C1, C2) surround the transistor.

The DC power supply is connected to the collector at top of the transistor as shown above (+Vcc). The emitter is at ground potential, meaning it is negative with respect to the collector. As a voltage exists between the collector and the emitter, the current will want to flow through the transistor. The base can either allow or deny the current from flowing depending upon the base voltage. Thus, the base acts as a control valve. When the transistor is turned on by a tiny amount of base current, a large current flows from the emitter to the collector. When it is turned off by a lower base current, no current flows between the emitter and the collector.

If we insert an audio signal on the base, it will control the current flow from the emitter to the collector. When the amplitude of audio signal is more, the transistor will be turned on more, allowing more current to flow from the emitter to the collector. A low amplitude audio signal will allow less current to flow through the transistor. This is how an audio signal is amplified. This is also why the volume control is always with the pre-amplifer, as that is the unit that sends the audio signal to the base of the amplifiers transistor.

The quantum of amplification provided by the transistor is called gain. This is expressed either as a ratio between the input and output voltages or in decibels (dB). A gain of 10 means that the input signal has been amplified 10 times. A 1Khz sine wave with a base voltage of 10mV will, when amplified with a gain of 10, produces a 1KHz sine wave of 100mV.

Real life is always more difficult and never follows theory to the T. A transistor can never be turned off completely. A small base current is always present. This is called the bias current and is essential to make the transistor work. Bias makes the transistor conduct quiescent current through the collector when there is no input signal at all. Quiescent means being at rest or being dormant. The amount of bias current is very carefully selected so that the transistor always operates within its linear operating region.



Bias allows the amplifier to amplify both positive and negative phases of the audio signal. As the supply voltage is positive and the emitter is connected to the ground, the transistor can never produce a negative voltage at the collector.

Bias sets the zero crossing point at some voltage between the ground and the supply voltage the designer thinks is optimal for the amplifier to work with minimal distortion. This is determined by the values of the emitter and collector resistors. The quiescent Base voltage (Vb) is determined by the potential divider network formed by the resistors R1 and R2, and the power supply voltage Vcc, and, this is given as Vb=Vcc*R2/R1+R2. This base voltage should not be affected by any external factor, as that will immediately introduce distortion. A pair of what are called coupling capacitors - C1 and C2 - take care of this. These will pass only AC signals and block any DC component that could alter the bias.

Some designers do not like to use capacitors in the audio path. This kind of circuit is called direct coupled. DC can yet be prevented from reaching the output with a device called DC Servo.

The large signal at the output is simply a DC power supply converted to AC audio signal by the transistor. What does this mean? This means that the power supply is effectively always in the path of the audio signal. Both the audio signal and the power supply are connected to the collector. Thus, anything that happens to the power supply noise, high impedance, fluctuations all could affect the audio signal. This also has bearing in what my conclusions are at the bottom.

DISTORTION


One of the major problems with all amplifiers is that, because of the very nature of their operation, they can never reproduce a duplicate of the original signal. They add some signals of their own, and these are called distortion. The most difficult part of an amplifier design is to reduce distortion as much as possible. There are multiple types of distortion and we will understand them briefly now.

In Harmonic Distortion, an overtone is added to the base frequency of the input signal. If an input signal is a 100Hz pure sinewave, the output will contain the 100Hz sinewave, plus a 100Hz harmonics, a 200Hz harmonics, a 300Hz harmonics and so on. These are called the first, second, third harmonics, and so on. In the picture below, the figure on the left shows the input frequencies, while the right shows the distorted signals output by the amp.



Harmonics distortion is expressed as a percentage of the input. The term Total Harmonic Distortion (THD) is something we are all familiar with. If an amplifier outputs 10V and the harmonic distortion additions are 10mV, the amplifier has a THD of 0.1%. A high second order or third order distortion is more acceptable to the human ear than even a low sixth or seventh order distortion.

Another distortion is called Inter Modular Distortion or IMD. If you feed two frequencies as input at the same time, the output signal will have these two frequencies, plus the sum and differences of these frequencies. For example, if you feed a 100Hz and a 10KHz signal, the output will contain these two plus a 9.9KHz signal (the difference) as well as a 10.1KHz signal (the sum). IMD is, in layman terms, referred to as muddiness or lack of definition. Severe IMD could lead to audible artifacts sounds that were never there in the source in the first place.



In the picture above, the amplifier has been fed with a 100Hz and a 3000Hz signal and asked to amplify to a peak level of 1 watt. The tall peak is the 3000Hz input signal. As you can see, it is straddled by a whole set of other signals created by IMD.

Next, if a transistors bias, or quiescent Base voltage changes, the amplifier will stop being linear and result in Amplitude Distortion.

If the bias is low, output waveform will have its negative part of the output waveform chopped off. If the bias is high, the positive part gets chopped. Why does this happen? Simple, actually. When the bias voltage is small, during the negative part of the cycle, the transistor does not conduct the voltage fully so the output is set by the supply voltage. When the bias is large, the positive part of the cycle saturates the transistor and the output drops almost to zero. Look at the figure below.



There is another distortion that is kind of extension to amplitude distortion. Even if the biasing voltage is set correctly, a large input signal could get distorted when amplified. The output waveform gets chopped off on both positive and negative ends and no longer look like a sine wave. This is called Clipping and happens when the input circuit is stretched or over driven. I have been warning people of this in many threads, particularly of connecting an iPod or similar device that has a tiny amplifier of its own. This takes the input signal of the amplifier beyond its specified limits and immediately introduces distortion. Remember, the amplitude control is with the preamplifier, so what you feed into it is very important.



We now come to another distortion that is very important for our discussion. This is called slew-rate, phase, or delay distortion. If the steepness of the input signals attack is faster than the ability of the amplifier to amplify the signal, then slew-rate limiting will occur. An amplifiers slew rate is measured in volts per microsecond (V/s). This describes how much voltage an amplifier can swing in a specified time period. If the input signals transients are faster than this, the amplifier will distort the musics structure, This is also sometimes referred to as the amplifiers speed.

To understand this better look at the following figure:



For an amplifier to work without any slew rate, it must be able to deliver a specified amplitude at the steepest slope of the input wave as shown by the dark line above. Though the input sine wave has negative and positive signals as it passes through zero, the magnitude of change is always nearly identical in both cases.

An amplifiers slew rate is determined by the storage and discharge capacities of the capacitors used as these can only handle a finite amount of current. For an amplifier to pass a sine wave with no distortion, its slew rate must meet or exceed the highest rate of change of the input wave represented by the steepest slope. Why not make an amplifier with very high slew rate? This will require a speaker cone to stop instantly and move in the other direction. So, even if you can construct such as an amp, there is no speaker on earth that can deliver that kind of slew rate.

What we have discussed till now is the configuration of a Class A amplifier. The other classes are B, A/B, C, and D.

A Class D amplifier uses integrated circuits. Such amplifiers are usually very well protected internally because of the construction methodology and have error corrections modes. The chances of such amplifier changing their behaviour pattern are very remote.

Most of the amplifiers used for audio are either Class A or Class B. A Class A amplifier is very inefficient and is power hungry. As against a single transistor in Class A, Class B amps use two complementary transistors in what is called a push-pull configuration.



A phase splitter divides the input signal into two halves. The positive half is fed to a transistor that operates in a NPN configuration. The negative half is fed to the complementary transistor that operates in the PNP configuration. The two output amplified halves are assembled to deliver a full signal. As against the Class A where there is always a quiescent Base voltage, in Class B, when one transistor is operating, the other transistor is switched off. This is more efficient as there is less power consumption as well as less heat generated. Of course there is a catch, and that is called Crossover Distortion. If the two halves of the signal are not biased perfectly, crossover distortion happens. When one transistor does not switch on exactly when the other is switching off, the two halves of the output curve may have differences in amplitude as well as differences in terms of the point at which they meet on the X-Axis.



Most amplifiers operate as Class A up to a particular amplification level, then switch to Class B beyond that. Such amplifiers are called Class A/B. The point at which the switch happens is determined by three factors:

1. The amount of bias current the Class A transistor can handle
2. The amount of heat the transistor generates
3. The amount of current the power supply can handle.

REMOVING DISTORTION

Can distortion or errors in the amplifier circuit be removed completely? No, that is not possible. However, they certainly can be reduced to a very large extent. One method to do this is called feedback. In this, a portion of the output is taken from the collector and fed back to the emitter. Amplifiers may also work in multiple stages of amplification. When output from one or more of the intermediate stages is fed back to the input, this is called local feedback. When output from the final stage is fed back, this is called global feedback.

CONCLUSIONS

What have we learnt till now? An amplifier is, by nature, an unstable device. In addition, amplifiers work on very small margins as far as input and their internal circuitry go. The input, for example, is usually in the 1 to 3 volt range, and increments for amplifications are in small fractions.

Many designers do not believe in feedback and work carefully on every part of the amplifier so as to not create errors in the first place. The amplifier works in a narrow bandwidth of tolerances or margins. If these margins are crossed in either direction, the immediate action of the amplifier is to start distorting the input signal. The higher the output power, the more are the chances for the amplifier to start distorting.

Design goals are generally high slew rate, high bandwidth, low output impedance, and high current capabilities. This means large power units, very carefully chosen parts, and parts of the highest quality possible at the price point. Most of the factors we discussed above are specified by standard mathematical formulas. The designer will use these formulas to calculate the specifications of all the supporting devices for a particular transistor that he uses.

One of the biggest issues with transistors (and associated devices) is heat. As large amounts of current pass through them constantly, all units in the amplifier and, the transistors in particular, will heat up. This is called warm up and takes roughly 30 to 120 minutes.

What happens when a transistor heats up? A number of things:

1. The current between the collector and base increases. Its flow through the biasing resistors drives the base more positive, increasing forward bias on the base-emitter diode. This means immediate amplitude distortion.

2. The emitter voltage required for a given collector voltage will decrease. This means unwarranted increase in the gain of the transistor.

3. If the current flowing through the system is kept constant, the resistance of the devices will increase with temperature. This will lead to a loss of power and further increase in temperature. If this is not controlled, the temperatures at the junctions will rise till the devices are destroyed.

The designer has to take these into consideration. He will measure the performance of the amplifier after the warm up, and relook at his specifications. A prototype will be constructed and used extensively for performance measurements across time and usage. The measurements will be given back to the designer to bring on as much stability as possible to the amplifier.

Specifications of supporting devices such as resistors and capacitors cannot be allowed to change as this will junk the very design of the amplifier. In addition, the manufacturer has to work with the specifications supplied by the manufacturer of these devices. As a designer, I will not even touch, say, a resistor if the manufacturer tells me its specification are valid only for the first 100 hours of usage. The manufacturers of such devices will specify something like a mean time between failures (MTBF). This is the time in the products life cycle when it is expected to meet its manufactured specifications without any change. Since all these devices will generate heat, the manufacturer will also specify the temperature limits within which the device will deliver according to its specifications.

Thus, as long as the specified temperature limits are maintained and a stable power is supplied, 100, 200 or 300 hours of use should not make nor develop any difference to the specification of the amplifier devices. The minute any of the specifications change, distortion will set in. Of course, this could happen when inferior parts are used, but this then becomes a design and construction flaw.

A reputed manufacturer will burn-in a prototype system for such time as he thinks is enough to alter the system specification adversely. He will then measure the specifications of the devices and, if some of them have changed, replace them with parts that are more reliable or with parts that have different specifications.

Since the amplifier devices work within extremely narrow margins and with high current flow, any minute change in the specifications can only affect the amplifier adversely. If there were any possibility of positive improvement in performance, this would already have been considered by the designer. Remember, a designer will want his design to be tested to near destruction point so that he can get the best operating performance possible. It is very rare that a random and uncontrolled factor such as 100 hours of usage could affect the amplifier in a positive way.

So what does happen that make people say that their amplifiers sound better after usage? There is one possibility and this relates more to the power unit.

There is a lot of copper used in the power unit for flow of current. Asit had once explained that the conductivity of copper increases after current flows through it for some time. Essentially the molecules inside the copper align themselves in the direction of flow. This reduces internal resistance within the copper and aids in the smoother flow of current. The power unit will, thus, after some usage, be able to deliver marginally higher current, and more important, the higher current faster. This will affect the amplifier circuit in two ways. One, the amplifiers slew rate will improve enabling the amplifier to match the input signals speed better. Two, the power unit will be able to replenish the capacitors faster, thus enabling the amplifier to attack the next rush requirement with equal gusto. The net effect will be that, to our human ears, the music will sound smoother.

A well designed amplifier, in my opinion, will, with usage, not have any positive change in tonality, headroom, dynamics, or other similar and important characteristics of the sound signal. The only possible positive change is the increase in the slew rate and the resultant attack speed of the amplifier.

Let me try to clarify a point raised by Asit. He wanted me to scientifically explain why a couple of hours of usage would make a difference and not 100 hours or 200 hours. This is the difference between warm-up and burn-in that I have explained above. Because of current flow, an amplifier will reach very near it max operating temperature quite quickly. This, as I explained above, does have an effect on its performance. In addition, the designer is working with specifications given to him about the operating temperature of the various devices he is using. He will thus be able to take this into consideration during design stage.

But burn-in is different. What we are all saying is that something unknown happens to the state or the very matter of the devices used that make them perform better. Unlike friction in mechanical devices, electronic devices, as I mentioned above, are designed to deliver consistent results for a specified time called MTBF. A resistor, for example, will always allow a certain voltage (V) to flow for a given resistance (R) and current (I) as per the equation V=IR. As long as you supply the correct current, you can expect the resistor to provide a consistent resistance and supply the voltage you expect it to for something like 50,000 hours (MTBF). Yes, they do have some more parameters such as tolerance, temperature coefficient, noise, and inductance. But, all these are factors known to the designer of the amplifier when he chooses a resistor. These are the specifications supplied by the manufacturer. What we are expecting is that these specifications change somehow and that is why the amplifier starts behaving differently. My point is that these specifications cannot be allowed to change, and if they dont, the amplifiers behavior also cannot change. If these specification do change, the part has failed to operate according to its specification, and that change can never have a positive impact on the whole unit. It can only be negative in the form of distortion given the small tolerances within which we operate. If the change is small enough to be within the specified tolerances, the result will be so minute that it will have no effect on our listening pattern.

Cheers

Acknowledgements:

1. The Complete Guide To High End Audio by Robert Harley
2. Audio Technology Fundamentals by Alan A. Cohen
3. Most images from Introduction to Amplifiers
 

Asit

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Hi Venkat,

That's a huge report. It will take some time to read it carefully, and that may not happen before Monday because of some deadlines.

However, I must laud you for your seriousness to pin down an issue and work on it.

On my part, let me just say I have shared my direct experiences in my previous posts on this topic, not hearsay, others' opinion and not from any reading.

For once, I have to read your report, and see if I can understand (because my technical knowledge of electronics is very limited, almost zero, as I have said many many times in this forum). I sure will have questions.

Thanks for the post in advance.
 
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