Can I drive a power amp without a pre?

[asliarun]

Hi All,

I am in the process of slowly building a music system, and am experimenting with using a power amp without a pre. My idea is to use a Squeezebox Classic to control the volume of my power amp. Since I only have one input for now, I can live with this setup until I strongly feel the need for a preamp.

Any thoughts if this is a viable idea? Has anyone tried skipping a pre-amp? My power amp is a Parasound 2125. It also has an option to control the gain on each input channel so I can try tweaking this setting as well.

Thanks,
Arun

 

[ranjeetrain]

Why not. Entirely possible.

A pre does two things. One, bring the voltage (and impedance) of the input source to a level where the power amp can deal with it easily. And two, control the volume of all input source. And possibly a third thing, modify the sound signature of the chain.

It's entirely doable. I am doing the same. I just hook my DAC to the power amp which has a built-in pre. In past I did it with a CDP with variable out control.

In your case, since your amp also has gain control, it's perfectly fine.

 

[jaudere]

Yes. It is very much possible . You need to check input sensitivity of poweramp and output voltage of your sqeezebox and you need to have gain control at source wwhcih your sqeezbox will provide)

 

[doors666]

see the input sensitivity of the power amp. the source output voltage level should be more than that.
Check the output impedance of the source and the input impedance of the amp. the second should be atleast ten times that of the first.

 

[asliarun]

Thanks, Ranjeetrain, Jaudere, and Doors666!

I just hooked up my Squeezebox to my amp (which just arrived today) - it is working beautifully! I experimented with gain control a bit, but it looks like my amp is able to amplify the input signal without too much tweaking. I guess I got lucky.

 

[nowwhat?]

Arun, this is entirely possible from newer digital sources but only as a temporary measure.

Right now I am running a power amplifier without gain control off an iPod using the volume control of the iPod. I will be moving to a preamp soon as any digital volume control loses 1 bit and decreases the signal to noise ratio for every 6db of attenuation.

 

[ranjeetrain]

That's why analog volume controls are preferred. However, when on a budget, such limitations can be lived with. You get what you pay for! Besides, it's better to have a cheap but good digital volume control than a cheap and noisy analog volume control. If digital volume controls suffer from losses, so do analog ones. Cheap analog volume controls can introduce a lot of noise, thus decreasing the SNR. And preamps with high quality analog controls are not exactly cheap.

 

[Thad E Ginathom]

That is a very well-balanced real-world answer, ranjeetrain. There are many things that we might do because they have some theoretical advantage, or avoid because they have some theoretical disadvantage. When they are easy or affordable, then why not? The problem is that many of these theoretical [dis]advantages then become dogma, and many of them do not merit it.

Digital volume control is one of these things. We have an idea that our music should be bit-perfect, that everything in our FLAC file should reach our DAC unaltered, and thus, hopefully, reach our ears unchanged. Thus, most of us would rather reach for an analogue volume control than touch that slider on the screen. I can say me-too to this one.

However there are a couple of items of reality that we should take on board. One is that it is possible to knock quite a few bits off before we hear the difference (especially if it is 24-bit) , and the other is that the SNR of the equipment that we use is way more than we are able to perceive --- not just because of our ears, but because of the normal background noise with which we live.

Try it. For those music players that allow add-ins, there are effects processors that allow you to cut down the number of bits and see where the quantisation errors actually start to be audible. It's interesting to try. I use something called Decimator, a LADSPA add-in, with Aqualung.

To find the limits of SNR perception for your ears in your room, try the tests such as dynamic range on audiocheck.net. Warning: do not go to that site unless you have several hours to spare! :lol:

 

[ranjeetrain]

I agree. There are tons of stereotypes in the hifi industry. Many times they become a bottleneck in the decision making process even as they do not merit it.

 

[corElement]

I have a question, what if you have a tube amp and you have the tube amp without a volume control hooked up to say something like a soundcard.

Also if one uses a soundcard as a pre, will lowering the pre on a soundcard also lower the power consumption of the tubes / lower it's temperature?

I don't know anything about tubes that's why asking.

 

[GeorgeO]

I dont think it will.My tube amps get heated up when left on( forgot to switch off) with no music..

Ranjeetrain: Have to agree with some of the earlier posts-I use my SB connected to valve and ss amps bypassing a pre and the volume control in the digital domain works very well.( I couldnt reply to this post earlier since I was travelling without access to this forum)

 

[ranjeetrain]

I have a question, what if you have a tube amp and you have the tube amp without a volume control hooked up to say something like a soundcard.

Also if one uses a soundcard as a pre, will lowering the pre on a soundcard also lower the power consumption of the tubes / lower it's temperature?

I don't know anything about tubes that's why asking.

Nope, that won't be the case. Tubes must run on a certain voltage. That's what I know. But certainly I don't know about all of them.

 

[ranjeetrain]

Yes George, that's right. People fall for the stereotype that "anything digital is bad".

Whereas a digital volume control will almost always be better (more predictable, more precise) compared to a cheap analog control.

 

[doors666]

digital volume control throws away bits. search google for enob (effective number of bits), effectively reducing your 16 bit recordings to lesser number of bits. I always keep all my digital volumes to 100% due to this.

analog volume control introduces a resistor in the path (which is of course variable in nature). The pots could be bad while changing the volume, but once set, I wonder how bad can it be, its just a resistor then.

My b1 preamp has ldr volume control.

pick your poison.

 

[captrajesh]

I think that the previous posts have by and large covered most of it but let me summarise.

As per my understanding, a preamplifier does the following tasks: -

1. Provide impedance matching between the source and the power amp.

Different types of music would load the amp differently. Some like Western Classical and Hard rock put more load while some like Jazz are easy on the amp.

To be able to play all kinds of music without putting undue strain on either the source or the amp, the thumb rule is, 1:10 i.e. the output impedance of the source must be 1/10th of the input impedance of the Amp or lower.

2. Provide volume control

Traditionally this is done through a potentiometer which is nothing but a carbon based variable resistor. Due to the moving parts involved, this is bound to induce noise into the system the quantum of which is inversely proportional to its cost.

Newer (at least for me) ways of volume control have now come up using 'Light Dependent Resistors' 'Transformers' and the digital ones.

3. Source Selection

Since majority of people use several sources like CDs, TTs, Media players, DVD/BD players, STBs, it provides a convenience of switching between several sources without one having to take the trouble of swapping I/Cs

Can the preamp be eliminated? Yes it can be but subject to above.

 

[nowwhat?]

2. Provide volume control

Traditionally this is done through a potentiometer which is nothing but a carbon based variable resistor. Due to the moving parts involved, this is bound to induce noise into the system the quantum of which is inversely proportional to its cost.

Captain, good summary!

Just to add on this based on my recently acquired knowledge. There are basically 2 types of pots: 1) variable track or 2) stepped attenuator.

The former employs a carbon resistive or conductive plastic track. Here one has to get a log scaled one for audio applications. Alps Blue is conductive plastic and very popular. For the carbon resistive type, I have heard of a mil spec one from Canada (forgot the name but available on Digikey) used in jet planes that is good.

The latter employs a switching system and exactly 2 resistors for each step at any given time. Price depends on the mechanism (Seiden, Elna cost a lot), the resistors (some resistors like Charcroft cost around GBP 20 each!) and the solder (more for silver solder). Cost varies quite a bit and some handmade ones cost more than active preamps! As a result, a product like the DACT appears to cost way less.

There is also a third type which employs relays to select the circuit. The advantage here is that one can use a remote control to change the volume unlike the previous two. Of course one can use a remote with Alps motorized or stepper motor for DACT but it is not as elegant as the relay one.

 

[Thad E Ginathom]

digital volume control throws away bits. search google for enob (effective number of bits), effectively reducing your 16 bit recordings to lesser number of bits. I always keep all my digital volumes to 100% due to this.

Indeed it does, and that is why the theoretical reluctance to use it (and I have this too, as mentioned in my previous post) --- but whilst I wouldn't choose to base my volume control on digital, I understand that the reality is not really bad.

If one does choose to digitally control volume, then it might be a good idea to convert to 24-bit before the control: it gives one more bits to throw away, and they will probably be empty bits too. Throwing away leading zeros from a number leaves the number unchanged!

For serious decision making on this, serious research is needed, and it is best to take the explanations from the engineers rather than from the unqualified.

 

[ranjeetrain]

I can give a very good example why digital volume control isn't as bad as it is made out to be.

Take a FiiO USB DAC and a resolving headphone. Sennheiser 5xx/6xx, AKG 70x, Ultrasone, Grado etc and drive it.

FiiO has a digital volume control, and these headphones are resolving enough. Theoritically, this setup playing 16bit media should sound utterly rubbish when attenuated by 12+ db. Here as I type this, I am playing 16-bit 44.1k FLAC through it into Senn HD 650 attnuated by about 20db and it still sounds as good as at any other volume.

Which theory will explain it?

 

[doors666]

I can give a very good example why digital volume control isn't as bad as it is made out to be.

Take a FiiO USB DAC and a resolving headphone. Sennheiser 5xx/6xx, AKG 70x, Ultrasone, Grado etc and drive it.

FiiO has a digital volume control, and these headphones are resolving enough. Theoritically, this setup playing 16bit media should sound utterly rubbish when attenuated by 12+ db. Here as I type this, I am playing 16-bit 44.1k FLAC through it into Senn HD 650 attnuated by about 20db and it still sounds as good as at any other volume.

Which theory will explain it?

i can say the same thing about a pot based analog volume control.

 

[corElement]

When I set preamp volume to 100% on my stx the sound becomes horrid. If I set it to 50% it becomes horrid. Sweet spot is between 70-80%. So I have to disagree regarding setting digital volume to 100% at all times on digital.