I know. I’m considering buying the M2X boards.
So...you have been assimilated.
But seriously, the Vfet amp is one heck of a way to get into the Pass world.
You'll love it.
Dennis
HUM PROBLEMS.
REf my previous posts about hum and noise problems I switched the transformer to the other side to decrease magnetic coupling with the sensitive input circuits. However, although better, this did not completely solve the problem. I also did some scope measurements. This is how it is now.
REf my previous posts about hum and noise problems I switched the transformer to the other side to decrease magnetic coupling with the sensitive input circuits. However, although better, this did not completely solve the problem. I also did some scope measurements. This is how it is now.
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So the hum/noise is around 2mv with input shorted although the left channel has more HF components and this makes it more audible. This would be around -60db ref. 1 watt? With preamp connected the problem magnifies.
When I was repositioning the transformer I experimented moving it around as far away from the circuit as it would go but after a certain point the noise would not go down any further so I guess it is grounding issues. The earth mains is not connected in any way to the PS only to the chassis.
When I was repositioning the transformer I experimented moving it around as far away from the circuit as it would go but after a certain point the noise would not go down any further so I guess it is grounding issues. The earth mains is not connected in any way to the PS only to the chassis.
First i would separate the ground from the +- twist running to the board.
Second i would rotate the transformer with the inputs open to minimize the magnetic interference. It should be quiet with the speakers attached and no inputs.
Then if it still hums with the preamp attached, i would replace the CL-60 to ground with Nelsons Ground breaker from the A75 amp.
Hope this helps,
Second i would rotate the transformer with the inputs open to minimize the magnetic interference. It should be quiet with the speakers attached and no inputs.
Then if it still hums with the preamp attached, i would replace the CL-60 to ground with Nelsons Ground breaker from the A75 amp.
Hope this helps,
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Pass DIY Addict
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Some of my amps prefer a CL-60 to ground, some needed a direct line to ground, others needed one of the diodes from a bridge rectifier in order to eliminate the hum. Experiment with different arrangements. Whichever you choose, be sure that your connection to earth can handle the full voltage and current of your PSU rails. When a problem happens (a PSU cap shorting due to age/heat exhaustion) you want the fuse to blow, not your connection to ground.
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This is what I meant - some of my amps hummed with no connection between PSU and earth. Removing this hum required different grounding schemes for different amps. It's a quick and easy experiment to conduct.
I have experimented with this by grounding the 0v but the difference is minimal. Actually when you measure with the scope essentially you connect PS 0v to mains earth through the ground lead of the probe which directly connected to scope chassis and earth. I tried differential measurements but they were not much better.
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I looked up some old measurements.
I fed -20dBV @ 100Hz into the input, resulting in an output of -3.67dBV. The amp thus has a gain of 16.3dB.
With -20dB at the input, the output measured 78dB S/N. If I didn't screw up the math, that's 0.2mV peak-peak residual noise (200uV peak-peak).
Distortion apparently was at 0.012%.
Edit: That's like 83uV RMS noise, which is okay I guess.
I fed -20dBV @ 100Hz into the input, resulting in an output of -3.67dBV. The amp thus has a gain of 16.3dB.
With -20dB at the input, the output measured 78dB S/N. If I didn't screw up the math, that's 0.2mV peak-peak residual noise (200uV peak-peak).
Distortion apparently was at 0.012%.
Edit: That's like 83uV RMS noise, which is okay I guess.
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It is not as close as it seems from that viewpoint. There is a gap of 7or 8 cm. And of course any very small EM coupling on the DC supply should be taken care of by the PSRR.By the way I measured the PS ripple under load and it was about 75mv so it cannot be a problem from that side of the amp.
Which is the approximate noise level most Pass projects have.
I don't mind a bit of hum/noise as far as it is not audible from the listening seat but with 96db speakers I can hear this noise.
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Several months ago I was having problems setting the bias of my right channel and decided to Step away for some time. Before I did I had pulled the VFET's to start the bias procedure again from the beginning. Recently I returned to the right channel to start over and saw that the heat sink grease was all over the front of the VFET's. After cleaning them up I found that I no longer had the Vgs ratings. I don't have a power supply to use to test them can anyone suggest how I should address this?