So, I decided to jump into the wonderful world of the SFM (Sound Filed Management) to finally fix the lower end of my audio system. Now I have 3 subs with a miniDSP running the digital correction computed by MSO (MultiSubOptimizer). The result is great - very flat FR and as an added bonus I was able to achieve almost perfect power utilization, I get amazing TDH figures on the lower end even when pushing 110db of SPL in all seating positions.
However, now I can hear the 3 subwoofers humming! I mean, when it's really quiet in the house and when I am just sitting in the media room, I can hear it. I did not hear it with one sub. All the subs are exactly the same - I have measured the idle noise coming from them, indeed I see 60, 120 and 100 (which I cannot explain) Hz peaks in the REW FFT chart. It looks like the ~10dB gain that I get from 3 subwoofers vs one happens to be just enough for the hum to be audible and annoying.
It's not the ground loops - everything is isolated in addition to the link between the outputs of miniDPS and the subs being digital wireless.
So, it appears like the internal PS/amps of the subwoofers just have enough ripple so that the 3 of them produce audible effect in my room with the particular placement (that was optimized in order to maximize the output).
Any suggestions what to do?
Thanks!
However, now I can hear the 3 subwoofers humming! I mean, when it's really quiet in the house and when I am just sitting in the media room, I can hear it. I did not hear it with one sub. All the subs are exactly the same - I have measured the idle noise coming from them, indeed I see 60, 120 and 100 (which I cannot explain) Hz peaks in the REW FFT chart. It looks like the ~10dB gain that I get from 3 subwoofers vs one happens to be just enough for the hum to be audible and annoying.
It's not the ground loops - everything is isolated in addition to the link between the outputs of miniDPS and the subs being digital wireless.
So, it appears like the internal PS/amps of the subwoofers just have enough ripple so that the 3 of them produce audible effect in my room with the particular placement (that was optimized in order to maximize the output).
Any suggestions what to do?
Thanks!
I think you have just got revisit your amp.
Screen input wiring. Keep inputs short.
Make sure smoothing caps are adequate.
Keep audio away from transformers.
Keep audio away from mains.
If you are doing wireless to the subs what is the power supply to the wireless receivers? Maybe ground loop between the receiver and sub?
These are MartinLogan subwoofer, I am pretty sure they design them properly. But, if it comes to it I will open them up to see if I can pinpoint it.
The wireless receiver is onboard on the plate amplifier, so there is no ground loop there.
The wireless transmitters are connected to the source via isolating transformers.
I don't see any possibility for ground loops.
I can understand if they designed the subs with the aim to barely satisfy minimal requirements and if you have just one of them than it's not audible in a reasonable setup. But it kind of sucks if it's the case 🙁
The wireless receiver is onboard on the plate amplifier, so there is no ground loop there.
The wireless transmitters are connected to the source via isolating transformers.
I don't see any possibility for ground loops.
I can understand if they designed the subs with the aim to barely satisfy minimal requirements and if you have just one of them than it's not audible in a reasonable setup. But it kind of sucks if it's the case 🙁
Have you tried highpass filter in minidsp? to check if the hum is generated from before or after the minidsp.
I tried disconnecting miniDSP, the wireless transmitters from power, before that I tried disconnecting all signal cables and shorting the inputs of the subs and of the wireless transmitters - does not make any difference. It's the subs themselves.
It MIGHT be the case that the hum was always there, but now your EQing with MultiSubOptimizer has created gain at 60Hz to compensate for room modes or something.
If the hum was not there before using MultiSubOptimizer, you could try to undo the EQ process step by step and even go as far as removing the minDSP (my older 2x4s often suffered from noise and hum pickup depending on the location of input and output cable runs, etc.). See if at any point the hum disappears. Since you are using wireless connections between source and subs, the problem is definitely not a ground loop.
EDIT: Just read the last post, so maybe my ideas will go no where. For a ground loop to occur, there must be a LOOP formed by grounds. I can't see how this can be formed by your subs (alone) without other equipment connected to them via ground or shield run. You said all sub connections are wireless, right?
Is the gain on the subs turned up to "max"? If you can reduce the amp gain and instead use a hotter signal feed you can reduce noise and hum.
If the hum was not there before using MultiSubOptimizer, you could try to undo the EQ process step by step and even go as far as removing the minDSP (my older 2x4s often suffered from noise and hum pickup depending on the location of input and output cable runs, etc.). See if at any point the hum disappears. Since you are using wireless connections between source and subs, the problem is definitely not a ground loop.
EDIT: Just read the last post, so maybe my ideas will go no where. For a ground loop to occur, there must be a LOOP formed by grounds. I can't see how this can be formed by your subs (alone) without other equipment connected to them via ground or shield run. You said all sub connections are wireless, right?
Is the gain on the subs turned up to "max"? If you can reduce the amp gain and instead use a hotter signal feed you can reduce noise and hum.
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It was one of the first things that I tried - it did help me find and eliminate bona fide ground loops - as a result I inserted isolating transforms between the miniDSP and the wireless transmitters. So, since it's been done the gain control on the subs has no effect on hum level. And the gain is near the "MIN" now anyway for other reasons.
That's why I believe it's in the subs, not in the interconnects or the sources. Unless I am missing something, of course. It could also be a quality of the AC power in theory (although the harmonics, while noticeable, still are pretty low and most of the hum is the 60Hz fundamental per FFT), but in general I don't believe that properly designed audio equipment should be that sensitive to such minor disturbances on the mains.
Not sure I if understand - but yes, it comes from the subwoofer transducers. I checked it with a measuring mike and the hum is gone when I power the subs down. And yes, they are close to the walls.
OK, well the hum has to be picked up somewhere along the line, likely in a component but possibly in cabling. You need to go through the system step by step, disconnecting input or using a grounding RCA plug, to find the source of the problem. If you are not familiar with it, a grounding plug is just an RCA plug with signal and ground connected in the plug and no other wires. For example, you could put grounding plugs on all the miniDSP inputs coming from the rest of the audio system. If the hum goes away you know the source if upstream from that point. If the hum is still there, you know the source is downstream. Assume the hum continues - you then move the grounding plugs to the input of the next component downstream: the wireless transmitter inputs. Move up or downstream until you identify where the hum is injected.
From what you have tested and reported ....
The system with 1 sub does not hum.
The same system with 3 subs does hum.
Are the subs all plugged into the same mains outlet or different outlets in the room..? If different this can be a cause..
With everything powered off but 1 sub is there a hum ?
If not power on a second sub... is there a hum ?
If not then repeat for the 3rd sub... is there a hum.?
The system with 1 sub does not hum.
The same system with 3 subs does hum.
Are the subs all plugged into the same mains outlet or different outlets in the room..? If different this can be a cause..
With everything powered off but 1 sub is there a hum ?
If not power on a second sub... is there a hum ?
If not then repeat for the 3rd sub... is there a hum.?
I did that already, including grounding the inputs on each stage. The hum is present with the subs' inputs grounded and the internal wireless links disconnected. It's just the subs alone.
Do these amps have switched mode power supplies or switching regulators? One can get strange beat phenomenon with similar switching regulators on the same supply. It's a recognised problem on DC supplies and might be with identical kit on ac too.
A filter between each and the supply might do it. Try two amps, fit a mains filter (LC type) to one and see.
A filter between each and the supply might do it. Try two amps, fit a mains filter (LC type) to one and see.
Yes, SMPS + Class D. But then again, the very stable beat phenomenon that I see I can only understand if the frequencies are correlated and just happen to be 60Hz apart or something like that. On the other hand the switching frequency can be modulated by the 60 Hz ripple - I need to think how to experiment with it - so thank you for the suggestion. I already tried to plug one of the subs into a back UPS with an inverter, but the one that I have has a horrible AC quality that by itself is audible.
My little thoughts...
Assuming a 100 dB speaker. When You add two more speakers, the sum is increased by 104.77 dB according to online calculators. With one sub hum was there but was masked by background "silence". Now You quasi doubled the amplitude (near 6dB) and You can hear the hum because it's above the "silence" threshold, and perhaps the wall reflections also contribute for it. Are You sure it's the speakers and not the walls or floor that are humming ?
Assuming a 100 dB speaker. When You add two more speakers, the sum is increased by 104.77 dB according to online calculators. With one sub hum was there but was masked by background "silence". Now You quasi doubled the amplitude (near 6dB) and You can hear the hum because it's above the "silence" threshold, and perhaps the wall reflections also contribute for it. Are You sure it's the speakers and not the walls or floor that are humming ?
Sounds like your MartinLogan sub amps have too much gain for their PSU design 😉.
A relatively easy work around would be to reverse polarity on both the speaker output and input of one amp, which would (mostly) acoustically cancel the common hum and harmonics in the room, yet still allow the music content to play "in phase" with the other two.
That said, with three subs,(one reversed in/out as above) there will still be +6 dB more hum than with one, but you might move most of the noise away from the listening position by reversing the "right" one of the three.
Working live sound, I have come across far too many amplifiers that radiate noise, it is also possible the isolation transformer is too close to the MartinLogan power supplies, minor angle adjustments or a few more inches can make large differences in radiated pick up.
Good luck, have fun whacking the mole!
Art
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Another possibility- try reversing your isolating transformers direction, they may not be 1 to 1, in which case they may be increasing gain depending on input/output loading.
"A relatively easy work around would be to reverse polarity on both the speaker output and input of one amp, which would (mostly) acoustically cancel the common hum and harmonics in the room, yet still allow the music content to play "in phase" with the other two."
That is actually a great idea and genuine "out of the box" thinking.
That is actually a great idea and genuine "out of the box" thinking.
Troy,
Thanks!
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Inverted thinking can sometimes better the signal to noise ratio, if not the person..
Art
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