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Subwoofers - A primer

Castle Knight 2 Speakers

murali

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Considering several questions and postings on choosing and integrating subwoofers to main speakers, I think a primer on certain basic facts regarding subwoofers may be helpful to some... (Emphasis here is on subwoofers for music, not just for LFE output in home theaters)
The best definition I have across for a subwoofer is that it is an "add-on" product that extends the bass response of a system so that it is flat or very close to flat all the way down to about 20 Hz. The other major point is that by integrating a subwoofer to main speakers, you don't stop the mains at, say 100 Hz, and start the sub at, say 99.999 Hz. The main speakers and subwoofer must overlap for at least one full octave. For example, if the crossover point is 80 Hz, the main speakers have to be flat to 40 Hz and the subwoofer should perform well up to 160 Hz.
The subwoofers which usually accompany small main speakers have normally little bass response below about 60 Hz and are never true subwoofers, we can call them add-on woofers with 8" or 10" drivers. The real and true subwoofer is capable of extending the bass to close to 20 Hz and usually comes with 10" or 12" single or multiple drivers. Then there are the infra subwoofers with huge 16" or 18" drivers extnding the bass to below 20 Hz but can never operate satisfactorily above 40 Hz or so.
The best way to get good deep bass in a stereo system is to have a pair of full-range main speakers and add a true subwoofer to extend the bass, or if possible, a pair in stereo (many believe all bass frequencies are not directional but this is not true). The best connection for musical subwoofers is at the output terminals of the main amplifier. Relative to the main speakers' 4-8 ohms impedance, the input impedance of the subwoofer amplifier will be several thousands of ohms and virtually no power will come from the main amplifier to subwoofer. Practically all the amplifier power goes to the low impedance main speakers. The subwoofer amplifier just "looks" at the main amplifier and boosts the lowest frequencies. The best type of subwoofer crossover is the one that rolls off bass into the main amplifier and relieves the main speakers and amplifier from having to produce as much bass. This gives the best performance from the main amplifier and speakers for mid-range and higher. To my knowledge, only Vandersteen makes such speaker systems (2WQ subwoofer) where a high pass filter is inserted before the main amplifier and the Vandy subs have a feed forward circuit which compensate the roll-off.
Finally, never believe that LFE subwoofers make good music subwoofers, whatever others might say. The best music subwoofer will be linear in response all the way down to about 20 Hz.
I had already mentioned in one of my postings that companies like Bag End, Rel, M&K, Vandersteen etc make excellent subwoofers for music, mostly with sealed enclosures and low-slope crossovers. May be there are others too I am not aware of.
So before spending your hard-earned money on subwoofers, please consider these facts and read and discuss more to decide. You can always buy a subwoofer and whether it is money wisely spent will depend...
Trust this helps someone.
Thanks for your patient reading and happy listening.

> murali
 

Shuvc

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Thanks ! Can you please elaborate on these two points ..
The main speakers and subwoofer must overlap for at least one full octave. For example, if the crossover point is 80 Hz, the main speakers have to be flat to 40 Hz and the subwoofer should perform well up to 160 Hz.
If the crossover is set at 80hz from the amp, how does one ensure that
A) there is an overlap, if the signal gets split to the main speakers and sub based on that cutoff? Some amps probably allow separate cutoffs for main speakers and sub.
B) the sub goes upto 160?

If the both the front speakers and sub gets full range signals from the amp, and the LPF is set to 80hz at the sub, then
A) does the overlap happen, since the main speakers will then go down to their rated LF
B) how does the sub still go up to 160hz?


The best connection for musical subwoofers is at the output terminals of the main amplifier.
I'm not sure I understood which terminal you implied by 'output terminals'. Is it a full range pre-out?
 

murali

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First let me reiterate that the best connection to a subwoofer for music (not LFE home theater) is from the SPEAKER BINDING POSTS OF THE MAIN AMPLIFIER, EITHER POWER AMP OR INTEGRATED AMP, and NOT from any pre-out. Whatever stereo power amps and intgrated amps I have seen, unlike in a home theater receiver or surround processor, these amplifiers have no bass management features. To read more about these, you can visit Polk Audio website and others.
The bass management, ie. essentially integrating the sub to main speakers, is usually done by adjusting the cut-off frequency (low-pass filter) usually found in decent subwoofers. Remember, the usual input impedance of main speakers will be 4-8 ohms whereas the same of the subwoofer amplifier will run into tens of thousands of ohms. This implies two points: (1)since current is inversely proportional to impedance, practically all the current flow from power or integrated amplifier will be to main speakers with hardly any current flowing to subwoofer amplifier. (2)both main speakers and subwoofer will see the full range signal from the power or integrated amplifier which is very important for sound integration as you do not want to see frequencies divided between main speakers and subwoofer by a wall. However, a good subwoofer, by adjusting the cut-off frequency will just look at the full-range signal input and try to boost up the lower frequencies.
Thus, for example, let us assume a cut-off frequency of 80 Hz adjusted in the subwoofer. The subwoofer will boost up the 40-80 Hz octave and below whereas 80-160 Hz will be rolled off depending on the slope of crossover of filter built in the sub. The mains will continue to reproduce everything that it receives. Ideally the overlaps are the 40-80 and 80-160 Hz octaves. This is where the choice of the subwoofer is very important as IT IS THE SUBWOOFER that has to integrate to the mains and not vice-versa. The mains should be capable of reproducing at least that one octave below the cut-off frequency for the entire system to blend harmoniously. Just to repeat what I said in my first posting, no system works by the main speakers stopping at a brick-wall exactly at the cut-off frequency and the sub starting exactly there. This is the same principle you find in multiple drivers and crossover filters inside the main speakers. Each driver inside a speaker handles a range of frequencies with overlaps and if it is a good speaker, you should not hear any non-uniformity in the blending.
The last point. A musical subwoofer is not expected to handle higher frequencies and typically should reproduce with maximum efficiency one or two of the lowest octaves, say 20-40 and 40-80 Hz. Similarly a main speaker should be capable of going down to at least around 40 Hz. Please note that just adding a subwoofer to a pair of main speakers (again for music, I mean) does not guarantee anything. You have to choose a good subwoofer and then experiment with crossover setting and placement to arrive at the right balance. Needs a lot of patience and back-breaking work in moving the sub to several positions in the room before you find the right blend.
There are several websites, books and articles which you should read about this subject.

> murali
 

flanker.r

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Murali,

Do not get me wrong but I find the information posted has several wrong assumptions, distorted (pun not intended) facts and grossly erroneous electrical concepts

To begin with
Remember, the usual input impedance of main speakers will be 4-8 ohms whereas the same of the subwoofer amplifier will run into tens of thousands of ohms. Why should this be.? Remember- impedance is a function of frequency. As frequency drops, impedance should drop. So your point 1. should be rephrased as At a particular frequency, current is inversely proportional to impedance The moment you connect another speaker in parallel to the amp it sees the combined impedance which would be lesser than the single speaker. If you connect a 4 ohm sub in parallel with a 4 ohm speaker your amp essentially would see a 2 ohm impedance in the x-overs overlap area. Risky thing to do. Also remember impedance is an imaginative value!, ok not imaginative but you would agree if I call it a virtual value

When connected as suggested by you, the sub would rob all the power from the amp as LF needs more energy than a HF driver. The main speakers will be denied of power. How are we going to prevent this?

Also what happens if the sensitivity of the sub is different from that of the main speaker. The one with higher sensitivity would play louder negating the flat frequency response which we set out to achieve in the first place

When you say there has to be an overlap between the frequency range of the sub and main, how will you control this? What if the mains speaker has a 2nd or third order x-over and the sub has a different order. How can one correct for this? Also what about the phase differences your suggested method of correction would lead to?

How is your suggested method of connection going to avoid inter-modulation distortion? You will have sub and mains reproducing some same frequencies.



Please note that just adding a subwoofer to a pair of main speakers (again for music, I mean) does not guarantee anything what do you mean? Then what is the point of this write up?

I think a brick wall filter is the best means to achieve ideal multi driver based speakers. We cant do it (or do not attempt doing it) simply because of phase abnormalities to be addressed, more components, complex designs, cost, etc

Active bi-amping or triamping takes care of all this. So that the way to go. Bi-amp ot tri-amp and get rid of the passive x-overs that rob the precious power

Do not take my comments personally (and I never intended them to be that way). I am open to healthy discussions and arguments of aspects related to enhancing music listening experience and would go to any extend to unravel the obstacles that prevent us from doing it. After all, isnt that the reason we all have subscribed to hifivision and spend precious time writing posts like these?
 

murali

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No comments to add from my side.
Ignorance is bliss, sometimes not always.
If you find time before writing again, please go to some of the science and speaker websites or Physics textbooks and try to understand the basic principles behind sound and electricity.
There is also a FAQ section in Polk Audio website about ALL you need to know about bass management setting up your digital home theater system for the best performance by Paul DiComo. Or, read Max dB with Doug Blackburn in Soundstage.com. After reading the same and understanding it, if you still have points to discuss, we will continue.
By the way, if you know of any 4-ohm input impedance sub (I mean active subwoofer with amplifier), or for that matter even 4000 ohms, please let me know as I am very much interested.
Finally, FYI, I run 2 Nos, Rel subs (1 each to each main speaker) connected as I have explained in my postings, and I am sure many people have similar arrangements, and if you ever happen to come to this part of the country, you can see for yourself.
Good luck.

>murali
 

arj

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The moment you connect another speaker in parallel to the amp it sees the combined impedance which would be lesser than the single speaker. If you connect a 4 ohm sub in parallel with a 4 ohm speaker your amp essentially would see a 2 ohm impedance in the x-overs overlap area. Risky thing to do. Also remember impedance is an imaginative value!, ok not imaginative but you would agree if I call it a virtual value

When connected as suggested by you, the sub would rob all the power from the amp as LF needs more energy than a HF driver. The main speakers will be denied of power. How are we going to prevent this?

Uh.. I have a Sealed Box Sub connected to my Amp terminals in the same configuration.

My amp is a 30W tube amp

I run a 87db/8 ohm Speaker via the amp off its 8 ohm tap..i do no see any of these problems. are you sure you got that theory right....it does not seem to work that way in practice !

the sub draws so little current that the amp will just not feel it and thats the way its designed


I am not too much of a techie but i believe the sub lives in the votage domain unlike the speaker which is in the curent domain and this is done by means of a resistive potential divider to bring down the current requirement.


regarding
Remember- impedance is a function of frequency. As frequency drops, impedance should drop.

As far as i remember impedance is a function of 1. Resistance (not impacted by frequency) 2. Capacitance (inversely proportional) and 3. Inductance (directly proportional). hence depending on which factor contributes more, you could have the impedance go in may complex ways.

so are you saying an amplifier has only inductance ?? ;)
 

murali

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Thanks, arj, for your inputs.
To go back to some of the questions posed by flanker and probably appreciated by some others, I think the following illustration may help to make them understand this better:
My Ayre CD player has an output impedance of 55 ohms (unbalanced) and 110 ohms (balanced), the VTL line-stage preamp's input impedance is 100,000 ohms and output 200 ohms, and finally the Vandersteen main speakers' input impedance is 4-8 ohms whereas the Rel subwoofer (amplifier input impedance)is 100,000 ohms (high-level or speaker level connection as described in my postings and not the LFE connection (which has 10,000 ohms).
What do the above imply? The source (CD player) generates a voltage at its output (4 volts balanced or 2 volts unbalanced) and because of the very high input impedance of the next-in-line preamp, very little current flows from source to preamp. The same is the case between preamp and power amp. The voltage at the power amp output will be high (after all the amplifications or gains) enough to generate sufficient current to drive the low-impedance main speakers. If the readers are modest enough to apply fundamental principles of electricity, they can see that the 20 volt sends practically all the current to 8-ohm main speakers and hardly anything to the 100,000 ohm impedance. Probably they believe that this same voltage/current drives the subwoofer driver whereas in reality, the signal at the main power amplifier output is the INPUT TO SUBWOOFER AMPLIFIER and it is the latter that drives the subwoofer driver. That is why even a modest 100W or 150W subwoofer is good enough to produce excellent bass in a normal listening room.
Now, not all subwoofers have high-level or speaker level input connections, ie. being able to get connected from main power amplifier output. There are many subwoofers with only line level connections where one has no choice but to connect from a pre-out. That is why I have been suggesting to look for subwoofers which have speaker level connections for music reproduction whereas for a home theater movie effects, just a LFE or line level input is more than enough.
I had often come across such misconceptions among listeners without fully understanding basic principles in sound reproduction systems, especially as the inmate pointed out that we all spend time and effort in forums like these. I, for that matter, expect at least an understanding of such basics from those who wish to exhibit their knowledge, with all due respect. For example, I was surprised when one pointed out that an amplifier made the same input voltage bigger!!! How do you make a voltage bigger? You never do. All an amplifier does is to take a large amount of electricity from the power line (hence the importance of good plugs, sockets and cables) and control it with the input signal. Electricity from the wall socket is dumped into a loudspeaker and the audio signal it receives from the previous component only tells it what to do. If you are lucky and have good components, the two will resemble each other closely (minimum distortion).
I am sorry, but I think I have made too much postings on this subject and may be it is time to withdraw for some days or weeks (or for ever???).
Thanks to arj and others who read these.
Happy listening.
> murali
 

ajinkya

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Murali,

I think your observations all apply to powered subs. An unpowered sub will not have a high input impedance, being just a crossover and a driver stage.
Having said that, I find all points raised by flanker and arj to be valid. I would be very interested in finding out the answers to their questions, so that I can better understand the interplay between the various sub configurations.

Thanks,
Ajinkya.
 

flanker.r

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Well Murali, I told you it was nothing personal and I repeat again

Just because some guys at 'Polk Audio' forums do or suggest such things does not mean its the 'Audiophile's bible' or its an IEEE certified white paper.

So, were you suggesting in your initial post that you connect the 'Active' Sub to the Speaker output of an integrated amp ie, run an amplified low impedance signal into the high impedance input of an active sub

This is called cascade amplification and is commonly used in PA systems when amplification is provided by means of voltage amplifiers. We do not use voltage amplification in home hifi. All amplification is done only in current domain. Some manufacturers may use a transformer to accept this type of connection so that impedance mismatch is corrected. However, this is not the standard way of doing things

Taking an amplified signal of low impedance and feeding it into the high Z input of another amplifier for further amplification will only cause trouble and loads of distortion. Amplifier inputs accept high Z line level of .689 V. Amplifier output to speakers in extremely low impedance. When this is given to the preamp stage of another amp the level is way toooo high

I urge that you do not resort to personal allegations and judgment when that is clearly not the intention of the person who posts a reply. It is suffice to say if a person counters your point of view, he knows what he is talking about and has adequate experience in the domain
 

murali

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Thank you very much for educating me. No further comments and just one suggestion, if you don't mind. Being in Chennai, please try to visit The Audio People who distribute Rel subwoofers and see how they connect them. Seeing is more believing than writing.
I am sure Sri Muthusami thanks you too as always.
Happy listening.

> murali
 

flanker.r

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Thanks for letting me know of sites where I can enhance my knowledge of science, speakers, physics, electricity and sound (and get to know about people whom you seem to hold in such high esteem and knowledge). I would have definitely pursued them. But as you have clearly mentioned that they are home theater sites, I am reluctant to pursue.

I am an ardent stereophonic and intend to remain this way. ya home theater seems to be changing all aspects of electricity and physics with a micro mini transformer powering first 5,then 6 now 7 speakers plus a sub woofer but I would rather remain faithful to music
 

flanker.r

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I have installed REL subwoofers myself and that's what I was talking about manufacturers taking provisions to include circuitry to match impedance . REL is one of them. Maybe another couple of them might be doing this. This is definitely not the industry standard

To quote from a review of REL

'Like all REL systems, line-level and high-level filter stages are fully regulated to ensure total isolation from the power amp stages, enabling high/low level and LFE signals to be connected to and controlled by the Storm 5 simultaneously. The LFE input bypasses all filtering to enable a clean LFE channel to be reproduced.'

Regulated means impedance matched

The link is here
REL Storm 5 - AVReview News and comment
 

murali

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I don't follow your line of argument, please. Forgive my ignorance.
My contention was that good active subwoofers used for music and to be integrated with main speakers should be connected using high level input (ie. power amplifier output to main speakers). I don't know whether you agree or disagree but Rel and Vandersteen subwoofers (with some others) are considered as some of the best subwoofers for music (not home theater I repeat) and here are extracts from their manuals:
Rel (Strata): "It is helpful to know that you will always connect the Rel to the input on the rear panel labelled Hi-Level which is the first and best choice. This connection is made using the supplied cable, the bare leads of which connect to the speaker output terminals of the power amplifier. By connecting to the Rel's high level input from the amplifier, you bring forward the sonic signature of your main system including the tonal balance and timing cues. In this way the Rel shares the signal that is fed to the main speakers. This connection can be made without affecting the performance of the main amplifier because the Rel amplifier input impedance is 10,000 ohms."
Vandersteen (2WQ): "Conventional powered subwoofers receive their input signal directly from their crossover before the main amplifier but then the sonic signature you hear from full range speakers will be missing from the subwoofer. This causes blending and integration problems as the sonic characteristics of the system are different above and below the subwoofer crossover point. This deterioration is why conventional subwoofers are never totally accepted as part of high performance audio system. The 2WQ uses a unique connection method by inserting a high pass filter in the signal path just before the main power amplifier to roll off the frequency response of the main amplifier and speakers. However, the 2WQ does not take its input from the corssoover BUT SAMPLES THE OUTPUT FROM THE MAIN AMPLIFIER DRIVING THE MAIN SPEAKERS. To compensate the LF roll off, the response of the 2WQ is contoured to restore the LF to proper level. THE 2WQ'S INPUT IMPEDANCE IS HIGH ENOUGH THAT IT HAS NO EFFECT ON THE OUTPUT OF THE MAIN AMPLIFIER."
I hope you will not claim that all this is rubbish and companies like Rel, Vandersteen etc make P/A system and not high fidelity systems of which you claim to be a proponent. Or, if you believe that your knowledge is far superior to all these companies, please continue your postings as you wish and I am not responding anymore.
Thank you very much and wish you all success in your quest, whatever it may be.
Regards.
> murali
 

flanker.r

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I have great regard for REL and they are in my opinion very very good sub-woofers for music.

This is exactly what I was saying. Rel and a few other manufacturers have special impedance matching circuitry in them so that their amplifier accept a speaker level voltage of almost 0 impedance (say 0 because compared to the input impedance, this is small enough to be ignored) and convert it very high Z in this case, 10K.

Unless this special circuitry is provided, we cannot connect an amplified output to another amplifier's line level input. This is the point I have been stressing all along and pointed out that this method of connection in a general /generic active subwoofer that does not have this circuitry is not possible or is incorrect.

I raised the questions in the initial post simply because the thread was named 'Subwoofers - A Primer' which meant a generic way of connecting an active sub. If that was the case, my questions remain unanswered by your suggested method of connection.
 

Shuvc

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I have a Q. I'm not sure if it relates directly to the discussion here. But anyway, here goes.

I have a 10yr old low end sony sub-sat-recvr HT. The sub is active. The rcvr has no crossover settings. The sub and fronts can be connected thus:
1. Full range pre-out from rcvr to sub. Set LF crossover at sub. Full range speaker level out from rcvr to front spkrs.
2. Full range front channel speaker level out from rcvr to sub. Sub has 2 pairs of spkr level ins and outs. Spkr level out from sub connected to front spkrs. Set crossover in sub. Not sure if the crossover splits the signal, acting as a LPF for the sub and a HPF for the fronts.

What could be the difference in these 2 cases?
 

Muthusami S

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I am sure Sri Muthusami thanks you too as always.
Happy listening.
> murali

Yes Murali. You are right. I thank Flanker always since his posts and responses are useful to me. Thanks a lot for your observation.

Regards,
Muthusami. S
 

arj

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Rel and a few other manufacturers have special impedance matching circuitry in them so that their amplifier accept a speaker level voltage of almost 0 impedance (say 0 because compared to the input impedance, this is small enough to be ignored) and convert it very high Z in this case, 10K.

.


thats interesting info..Thanks !

I think MJ acoustics also have this..
 
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