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Topic: Subwoofer Hum from DIY Klipsch Sub (Read 5517 times) previous topic - next topic
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Subwoofer Hum from DIY Klipsch Sub

Hi all.
 
Before I start, let me make it clear that I am quite a n00b when it comes to all this.

My Klipsch SW308 subwoofer recently died on me, and despite a fairly knowledgeable friend taking a look at it, it appeared that the plate amp had gone. Not an obvious fix, and would probably take more time and expense to work out what had fried than it would to replace the thing.

Although I paid what was quite a small sum of money for the sub (compared to what it's RRP appears to be: $900!!), I was very pleased with it, particularly the amount of output for such a small size cabinet. I therefore decided that replacing the amp might be the best course of action - far cheaper than replacing the whole thing. After all, the sub is essentially two simple elements (AFAIK): an amp, to amplify the line level signal from the AVR, and the speaker / enclosure, which is just a simple magnet and cone in a rigid box.

The cheapest 300W amp I could find was a amp module from BK, a well regarded British outfit. Being just an amp module, it had no crossover or level adjustment, but I figured these would be better controlled on the AVR anyway.

Last night I wired it all up, and it worked straight off (to my surprise!). The new amp is sitting outside the enclosure, with some neatly drilled holes allowing connection of the old speaker to the new amp. The enclosure is air tight.

A couple of things I have noticed:

1. When the AVR is turned off there is now a hum from the subwoofer. Can i eliminated this somehow?
2. Probably too late to ask this now, but is my above logic correct. Can i simply replace the amp module an expect to get a similar sound from the sub to what I had previously (assuming it is of a capable output)? The tone from the subwoofer on the AVR auto-calibration setup seems quite different to how it was before it died (a higher note), and after this is complete, it's a little quieter.

Sorry, long post! But any thoughts welcome!

Subwoofer Hum from DIY Klipsch Sub

Reply #1 –
Quote from: tdansmith on
1. When the AVR is turned off there is now a hum from the subwoofer. Can i eliminated this somehow?


Maybe, maybe not.

It's an engineering question that needs to be addressed by an on-site engineer. Got one?

Quote
2. Probably too late to ask this now, but is my above logic correct. Can i simply replace the amp module an expect to get a similar sound from the sub to what I had previously (assuming it is of a capable output)? The tone from the subwoofer on the AVR auto-calibration setup seems quite different to how it was before it died (a higher note), and after this is complete, it's a little quieter.


Your original logic may be seriously flawed.

Plate amps in subwoofers are not all the same at a given power ratings level, and there is a good possibility that the original amp was specifically tailored for the rest of the subwoofer in terms of frequency response for example.

Subwoofer Hum from DIY Klipsch Sub

Reply #2 –
Thanks for the response.

On the first point, it looks like the cable from AVR to subwoofer amp could be the source of my hum. It's a cheap RCA audio cable, and I understand that they can readily pick up interference. I can, as a first step, replace this with an inexpensive but superior sheilded cable.

On the second point, could you elaborate. *serious n00b alert*. I don't really understand this 'tailoring' you mention.

My understanding was that amps are effectively interchangeable (assuming that they are powerful enough to drive the speaker without cliping) and 'transparent'. As an example, switching out my low cost AVR for a more expensive model shouldn't affect the sound, should it? Assuming that both are capable of driving the speakers, and are set up similarly.

I'm concerned that my lack of knowledge may upset the more clued in members, but I am genuinely interested to learn. Please bear with me!

Subwoofer Hum from DIY Klipsch Sub

Reply #3 –
Without more information this is all essentially guesswork. Method is what is needed.

Leave the new amp where it is and plug one of your other speakers into it. Does it still hum?
Take the modified sub and plug it into your mains amp directly. Does it still hum?


Subwoofer Hum from DIY Klipsch Sub

Reply #4 –
Quote from: tdansmith on
On the first point, it looks like the cable from AVR to subwoofer amp could be the source of my hum. It's a cheap RCA audio cable, and I understand that they can readily pick up interference. I can, as a first step, replace this with an inexpensive but superior sheilded cable.


Again, you are getting bad advice. Hum pickup in a subwoofer cable is usually not due to poor shielding. It usually comes from an entirely different source - ground loops. Ground loops are a consequence of the fact that the cable conducts electricity which of course it must do.  It  is really an environmental problem.

Quote
On the second point, could you elaborate. *serious n00b alert*. I don't really understand this 'tailoring' you mention.



Which makes my point that a noob who is trying to re-engineer a subwoofer is way over his head.

Quote
My understanding was that amps are effectively interchangeable (assuming that they are powerful enough to drive the speaker without cliping) and 'transparent'. As an example, switching out my low cost AVR for a more expensive model shouldn't affect the sound, should it? Assuming that both are capable of driving the speakers, and are set up similarly.


That's all true but it only applies to amps that are  sold as stand-alone products, and are not tightly integrated into a self-powered loudspeaker.


Quote
I'm concerned that my lack of knowledge may upset the more clued in members, but I am genuinely interested to learn. Please bear with me!


If you want to learn, take the time to educate yourself at your own expense on your own time. There are online basic electronics courses. There are good books about basic electronics and audio.

Asking people to take the time to write personalized course material is a huge imposition. 

Subwoofer Hum from DIY Klipsch Sub

Reply #5 –
Ok, so setting aside the humming issue, which I have researched and might be either ground looping, as you say (though I had discounted this as there is no ground on the new amp, as it currently stands) or interference through a cheap RCA cable...

Could you explain one thing: I now understand that an actively powered speaker may have an amp that is tailored in some way to the speaker to which it is linked - that makes sense. It follows that replacing this amplifier with a standard / non-tailored amplifier may therefore generate a different sound. Is the new sound necessarily worse than the original? Am I not doing what a lot of DIY subwoofer builders doing by pairing up an off the shelf amp with a speaker?

The comparison to the AVR and bookshelf (passive) speakers still stands, surely. That the passive speaker in this case used to be an active speaker with a 'made to measure' amp doesn't make it unsuitable for putting to use with a replacment amp, does it?



Subwoofer Hum from DIY Klipsch Sub

Reply #6 –
What you are overlooking is that BOTH the amplifier and speaker were custom matched. This is very different from a speaker that is designed to work with any amp, and an amp that is designed to work with any speaker.

As to whether the sound will be better or worse, only you can judge.

Subwoofer Hum from DIY Klipsch Sub

Reply #7 –
Quote from: tdansmith on
Ok, so setting aside the humming issue, which I have researched and might be either ground looping, as you say (though I had discounted this as there is no ground on the new amp, as it currently stands) or interference through a cheap RCA cable...


If your amp is connected to the main amplifier it *is* grounded at the amp, which is in turn grounded at the wall outlet.  If your amp is plugged into the wall it *is* grounded at the wall outlet. There *must* be a ground (return path) for current to flow at all, by the laws of physics.

If the amp ground sits at a different potential than the wall socket ground that causes ground loops and hums.  In most houses this is likely to be so unless both amplifiers are plugged in at the same wall socket.

Ed Seedhouse
VA7SDH

Subwoofer Hum from DIY Klipsch Sub

Reply #8 –
Quote from: Ed Seedhouse on
Quote from: tdansmith on
Ok, so setting aside the humming issue, which I have researched and might be either ground looping, as you say (though I had discounted this as there is no ground on the new amp, as it currently stands) or interference through a cheap RCA cable...


If your amp is connected to the main amplifier it *is* grounded at the amp, which is in turn grounded at the wall outlet.  If your amp is plugged into the wall it *is* grounded at the wall outlet. There *must* be a ground (return path) for current to flow at all, by the laws of physics.

If the amp ground sits at a different potential than the wall socket ground that causes ground loops and hums.  In most houses this is likely to be so unless both amplifiers are plugged in at the same wall socket.

You are confusing ground with neutral. In many parts of the world ground and neutral connect to the same place in the breaker box, but serve very different purposes. In other parts of the world neither of the "hot" connections is ground.

If the amplifier has a 3-prong plug then it is grounded. Otherwise it is not, but may be grounded through its connections with other components.

Subwoofer Hum from DIY Klipsch Sub

Reply #9 –
For the hum, try turning-down the volume/gain knob on the subwoofer amp (which will hopefully reduce the hum), then adjust-up the AVR subwoofer volume to compensate.  Or, do it the other way around and turn-up the AVR's sub-output, and then turn the sub's gain volume down to to bring the bass back into balance.    But first, check to see if tuning down the sub's gain helps with the hum.

Quote
that makes sense. It follows that replacing this amplifier with a standard / non-tailored amplifier may therefore generate a different sound. Is the new sound necessarily worse than the original?
With a $900 Klipsch, I gotta' assume it was pretty-well optimized and any changes are likely to degrade it.  With a cheaper sub, maybe that wouldn't be the case.

That doesn't mean your results are going to horrible or unacceptable.

BTW - There are plate amplifiers with adjustable EQ, but it would be difficult to exactly match the original performance, and maybe no better than your AVR's auto calibration.


P.S.
The Internet says your sub uses a Bash amplifier.    Is it a Bash 300S?  Or did you search the Net for a matching model number?  If you can find the same model number off-the-shelf, then the amp wasn't customized for the Klipsch.

Subwoofer Hum from DIY Klipsch Sub

Reply #10 –
Quote from: tdansmith on
Could you explain one thing: I now understand that an actively powered speaker may have an amp that is tailored in some way to the speaker to which it is linked - that makes sense. It follows that replacing this amplifier with a standard / non-tailored amplifier may therefore generate a different sound. Is the new sound necessarily worse than the original? Am I not doing what a lot of DIY subwoofer builders doing by pairing up an off the shelf amp with a speaker?


It is well known that tightly integrating the design of speakers and amplifier can have huge sound quality and cost effectiveness advantages.  In principle there is little that happens inside the box that couldn't be more-or-less duplicated outside the box except things like servo control which some modern subwoofers do use, but apparently not this one. 

The big difference is the skill and effort levels of the people doing the system integration.  This varies and just because someone works as an engineer for a big speaker company does not mean their their accomplishments are either trivial or unassailable.

How much effort do you want to put into educating yourself to duplicate the performance of a sub $1K subwoofer? In the end just replacing the old sub could be the cheap way out.

Quote
The comparison to the AVR and bookshelf (passive) speakers still stands, surely. That the passive speaker in this case used to be an active speaker with a 'made to measure' amp doesn't make it unsuitable for putting to use with a replacement amp, does it?


It all depends...

Subwoofer Hum from DIY Klipsch Sub

Reply #11 –
Thanks for all the feedback, I really appreciate it.

A lesson for the future - perhaps get advice before starting on a project like this!

Still, I am where I am. I actually played around with subwoofer placement over the weekend, and have found that this has a massive effect on the sound. I was aware that placement would affect the sound, but the difference blew me away. I am now very happy with the sound that the subwoofer is making. Whether it sounds better than the old amp? Well, it's a moot point - I'm not about to pay $900 for a new one!

Regarding the hum, I have done further testing. There is no hum when both AVR and subwoofer amp are on. The hum comes about when the AVR is turned off. However, it is also present with or without the coax connection to the AVR, which surely rules out ground loop, being that in the latter scenario, there is only a ground at the subwoofer amp. If i prod the wires connecting the amp module to the RCA socket that I have soldered in I can slightly increase the volume of the hum. No other wires appear to be sensitive.

At this stage, I'm willing to give the amp module itself the benefit of the doubt, and accept that it is probably not faulty, which would indicate that the hum is being generated in the RCA socket or wiring. If this is the case, should I look at better insulated wiring at this location? Currently, the wire is of a generous guage, and has a flexible silicone sheath - basically some leftover wire from an auto wiring fix.

Is the fact that when the AVR is on (and connected) the hum dissapears relevant to diagnosing the source?

Subwoofer Hum from DIY Klipsch Sub

Reply #12 –
Quote from: tdansmith on
Thanks for all the feedback, I really appreciate it.

A lesson for the future - perhaps get advice before starting on a project like this!

Still, I am where I am. I actually played around with subwoofer placement over the weekend, and have found that this has a massive effect on the sound. I was aware that placement would affect the sound, but the difference blew me away. I am now very happy with the sound that the subwoofer is making. Whether it sounds better than the old amp? Well, it's a moot point - I'm not about to pay $900 for a new one!

Regarding the hum, I have done further testing. There is no hum when both AVR and subwoofer amp are on. The hum comes about when the AVR is turned off. However, it is also present with or without the coax connection to the AVR, which surely rules out ground loop, being that in the latter scenario, there is only a ground at the subwoofer amp. If i prod the wires connecting the amp module to the RCA socket that I have soldered in I can slightly increase the volume of the hum. No other wires appear to be sensitive.

At this stage, I'm willing to give the amp module itself the benefit of the doubt, and accept that it is probably not faulty, which would indicate that the hum is being generated in the RCA socket or wiring. If this is the case, should I look at better insulated wiring at this location? Currently, the wire is of a generous guage, and has a flexible silicone sheath - basically some leftover wire from an auto wiring fix.

Is the fact that when the AVR is on (and connected) the hum dissapears relevant to diagnosing the source?


There is a kind of hum that fits your description of the problem that I have personally experienced. It only happens when a subwoofer or other active speaker is hooked to the speaker terminals of an AVR. Is that what you are doing?

The problem comes about because most AVRs and power amps have speaker protection relays that go to the open circuit state when the amp is turned off.  The speaker leads then act like antennas for hum, and the subwoofer or other active speaker starts humming.  One easy circumvention is to connect 1,000 ohm 2 watt resistors across the speaker inputs to the inputs to the amp that drives the humming speaker.  This can often be done easily by connecting the resistor to the screw terminals on a standard dual banana plug, and plugging it into one  of the speaker-level inputs or outputs.