I am wondering if the hum is caused not by ground loops (as it remains when signal inputs are shorted) but by EMI pickup by the M2's signal transformer? The transformer is located as far away from the mains toroidal trafo as possible in this case. I will connect another amp in lieu of the M2 and see if hum remains.
I recall my AKSA Quasi amp had no audible hum (because I used as a headphone amp even) when using the Prasi PSU board. I still had audible hum on the M2 with the Prasi PSU.
I recall my AKSA Quasi amp had no audible hum (because I used as a headphone amp even) when using the Prasi PSU board. I still had audible hum on the M2 with the Prasi PSU.
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Audible hum is a very insensitive/inaccurate/unknown tolerances test.
Measuring output Hum+Noise with a 199.9mVac DMM tells one a lot more than listening with uncalibrated ears that mean nothing to other readers.
Measuring output Hum+Noise with a 199.9mVac DMM tells one a lot more than listening with uncalibrated ears that mean nothing to other readers.
IIRC, my Fluke 101 on AC mV scale reads 2.2mV at the outputs. Factory spec for M2 is 600uV, half this.
My head amps typically read 0.0mV to 0.1mV at output by comparison.
My head amps typically read 0.0mV to 0.1mV at output by comparison.
An externally hosted image should be here but it was not working when we last tested it.
This is how your amp should look like. Get the layout and PSU the best it can be.
It's EMI from the power trafo
Over in the Pass Forum, I asked what could be the problem because I suspected it was EMI pickup on the signal trafo.
Mr Pass encases his power trafo in a steel shield and puts the signal trafos in a mu-metal box. There is good reason for this...
ZenMod said move the trafo outside and see if it helps.
I found the easiest way to put the toroidal trafo outside was to unscrew the heatsinks from the bottom base panel of the amp case, the flip the heatsinks out, thereby doubling the distance of the signal traafo from the original position.
Drum roll....
It works now - helps immensely as the hum is barely audible with ear 3in away from woofer.
Measurement of output noise was 2.2mV before, now it is 0.7mV, very close to the factory spec of 600uV.
Further proof of EM field reduction was provided by measurement of the magnetic field (axis left to right horizontal looking at amp from front ot back) using a Lutron EMF 822A field tester. Original signal trafo location measured 34mGauss. With power trafo outside it is 0.3mGauss in same axis. On axis front to back it went from 12mGauss to 3.5mGauss.
So I think despite the naysayers who thought I had a ground loop due to poor PSU grounding topology, the problem has been the EMI field pumping the signal trafo full of line hum.
@Mark Whitney: So I doubt that just making my wiring neat like the above picture would solve anything.
Is this your amp? If so what is your output noise measurement in mV with inputs grounded?
Over in the Pass Forum, I asked what could be the problem because I suspected it was EMI pickup on the signal trafo.
Mr Pass encases his power trafo in a steel shield and puts the signal trafos in a mu-metal box. There is good reason for this...
ZenMod said move the trafo outside and see if it helps.
I found the easiest way to put the toroidal trafo outside was to unscrew the heatsinks from the bottom base panel of the amp case, the flip the heatsinks out, thereby doubling the distance of the signal traafo from the original position.
Drum roll....
It works now - helps immensely as the hum is barely audible with ear 3in away from woofer.
Measurement of output noise was 2.2mV before, now it is 0.7mV, very close to the factory spec of 600uV.
Further proof of EM field reduction was provided by measurement of the magnetic field (axis left to right horizontal looking at amp from front ot back) using a Lutron EMF 822A field tester. Original signal trafo location measured 34mGauss. With power trafo outside it is 0.3mGauss in same axis. On axis front to back it went from 12mGauss to 3.5mGauss.
So I think despite the naysayers who thought I had a ground loop due to poor PSU grounding topology, the problem has been the EMI field pumping the signal trafo full of line hum.
@Mark Whitney: So I doubt that just making my wiring neat like the above picture would solve anything.
Is this your amp? If so what is your output noise measurement in mV with inputs grounded?
That is infinitely more informative.
Based on the comparison of 0.1mVac and 2.2mVac you have two different outputs that are 26.8dB different.
2.2mVac is a terrible result for the H+N of any power amplifier.
It shows that you have a mistake in the assembly.
You are right to start again. But that should have been obvious on minute 1 when you tested and MEASURED the amp some weeks ago.
600uVac is not half of 2.2mVac, the ratio is 11.3dB
and 600uVac is in my view a bad result. I would expect a good power amplifier with a maximum output from 20W to 100W to have H+N measured on a cheap DMM to be <0.3mVac (300uVac) and preferably <0.1mVac
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Well I knew something was wrong which is why I started the thread to asks for help. Everyone kept saying it was the way I wired my PSU ground - perhaps partly true. But changing my grounding topology never improves the hum issue. The SMPS was tied directly to where the AC secondaries came from the trafo and the hum was gone. So maybe I should have realized that the ground loop was not an issue.
Not sure what you mean by outputs that are 26.8dB different? Even on the AKSA Quasi (circa 30dB gain) I was getting 0.1mV noise.
Flux leaks out of the transformer when the core saturates. So a buck transformer should lower that. Other option is more primary windings, but that is really a design issue.
Hum comes from many sources. Your grounding was wrong, and is still not as good as it could be - which is partly why changing it did not eliminate hum: you never got it right. This is not "naysaying" but sound advice. It was always likely that once the PSU was blameless then other sources of hum would become apparent.
I note that your first post in this thread clearly steered us towards the PSU.
I note that your first post in this thread clearly steered us towards the PSU.
This is how your amp should look like. Get the layout and PSU the best it can be.
That's indeed pretty. But it would be even prettier to my eyes if the secondaries wouldn't cross each other where they are coming out of the transformer. 🙂
http://www.diyaudio.com/forums/pass-labs/281520-official-m2-schematic-111.html#post4695411
Not my build, thought you would have recognised it. Sorry if you can't see why this would outperform your layout.
Not my build, thought you would have recognised it. Sorry if you can't see why this would outperform your layout.
I agree neat wiring is nice - but does not guarantee no hum. I do twist the wires but also notice Mr Pass doesn't bother to twist the AC wires coming from the trafo. This setup is a single shared CRC vs a dual CRC with independent bridges. I would expect my setup to have superior stereo separation - as a shared PSU for Class A leaves inter-modulation via rails.
No twisting here on trafo wires:
No twisting here on trafo wires:
You are still not seeing it. The amp boards are closer so that the ground connections are shorter. The AC and rectified DC leads have been routed away from the signal and ground wires.
I agree neat wiring is nice - but does not guarantee no hum. I do twist the wires but also notice Mr Pass doesn't bother to twist the AC wires coming from the trafo. This setup is a single shared CRC vs a dual CRC with independent bridges. I would expect my setup to have superior stereo separation - as a shared PSU for Class A leaves inter-modulation via rails.
No twisting here on trafo wires:
Never understood the horizontal and by definition unefficient cooling fins....🙁
It would make plenty of sense if it were to be installed in an industrial/commercial 19" rack cabinet.
But alas there aren't many Hi-Fi amplifiers that are, so the point is lost.
But alas there aren't many Hi-Fi amplifiers that are, so the point is lost.
I think NP found a bunch of these chassis / heatsinks at a good price, and now that's what he uses for FirstWatt. My F5 with these sinks does get pretty warm, but not too hot for my taste.
Gesendet von meinem D5803 mit Tapatalk
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