This potentiometer is supposed to reset the offset at the input of the edcore transformer in case the jfets are not well matched.
Yup, I know what the purpose of the trimpot is. I'm just not sure what to look for when I adjust it.
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Thank you darrr. This is exactly what I was looking for. I know it was described before, but I didn't understand it.
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Just a quick update. Thank you 6L6. Changing r1 to 20k and changing out r6 to 39k(they didnt have 37k in stock at digikey) ohms, allowed my amp to get to 1-3 mV of DC offset. Just letting it cook then I will do a sound test.
That's fantastic news!
Once it's sat for 30min thats long enough, check the bias/offset again (although it's almost certainly going to be ok) and grab some speakers.
You are going to love it, M2 is genuinely special.
Once it's sat for 30min thats long enough, check the bias/offset again (although it's almost certainly going to be ok) and grab some speakers.
You are going to love it, M2 is genuinely special.
Finally completed the M2 with Tea-bag's boards. Managed to set the trimpot that replaces R3 and R4 to zero volts or thereabouts. It drifts a bit with heat. Also managed to get virtually zero DC offset after decent warmup.
The amp worked first time, but the volume of the right channel is much lower than the left one. I've checked input levels on the PC, swapped everything that I can think of that could make a difference, like interconnect between pre and amp, speakers, speaker cable, etc.
All components were measured before populating the boards, except the Edcor transformers.
I have to set the left channel at 36% to balance the volume. Any ideas what it could be?
The amp worked first time, but the volume of the right channel is much lower than the left one. I've checked input levels on the PC, swapped everything that I can think of that could make a difference, like interconnect between pre and amp, speakers, speaker cable, etc.
All components were measured before populating the boards, except the Edcor transformers.
I have to set the left channel at 36% to balance the volume. Any ideas what it could be?
One of the transformer termination pins that stick out the side was bent and pretty much touching the iron laminations. 😱
I will run a sound test tomorrow morning when I can make some noise again.
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Tested and the low volume problem is still present in the right channel.
Will I be able to measure the transformer impedance in-circuit? How about continuity? Removing the Edcor seems like almost impossible without destroying the pins.
I drew up this circuit for the Edcor from the datasheet to help me determine where to measure:
Will I be able to measure the transformer impedance in-circuit? How about continuity? Removing the Edcor seems like almost impossible without destroying the pins.
I drew up this circuit for the Edcor from the datasheet to help me determine where to measure:
Maybe you swapped R1 and R2..? Also, is it possible to insert the transformer back to front (I'm not familiar with this amp...)? Compare the R3 and R4 voltage drops between good and bad channels, and let us know.
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measure resistance between JFet sources and GND
measure resistance between JFet sources and Silmic positive pin
do the same for both channels
ZM your first instinct was correct. There was a broken winding on the one pin that was bent. When I bent it back, I didn't look close enough to see the winding had broken loose from the pin. It was an easy fix. See bottom picture.
Now both channels sing together. Thanks so much for your help guys.
Now both channels sing together. Thanks so much for your help guys.
Skylar88,
Impressive you could see that! Good sleuthing!
This reminds me a time last year when I first got my Keysight scope (which has FRA/Bode plot capability) and had some output transformers to measure from Toroidy, Transcendar and Edcor. The Edcor single ended transformers measured the worst and were very far apart in linearity of the frequency response. I sent them back to Edcor and indeed they were faulty. The FR on one of the pair was 3dB down at 11 khz and 10 dB down at 20khz while the other was nearly flat to 19khz.
They admitted that something slipped in the QC process...
Best,
Anand.
Impressive you could see that! Good sleuthing!
This reminds me a time last year when I first got my Keysight scope (which has FRA/Bode plot capability) and had some output transformers to measure from Toroidy, Transcendar and Edcor. The Edcor single ended transformers measured the worst and were very far apart in linearity of the frequency response. I sent them back to Edcor and indeed they were faulty. The FR on one of the pair was 3dB down at 11 khz and 10 dB down at 20khz while the other was nearly flat to 19khz.
They admitted that something slipped in the QC process...
Best,
Anand.
Thanks Anand. Finding the problem was made easier with Zen Mod pointing the way to the transformer. So, that's where I looked first. There's a lot to be said for experience. 🙂
What a nice sounding amp this is. I bought the boards three years ago about an hour before Mark Johnson announced the M2x, but only completed it now. Maybe I'll build an M2x in future, so that I can try out the different input boards, but for now I'll just enjoy this amazing amp. I'll post a pic once the enclosure is done.
And btw,the PSU is external: Two 150W Meanwell SMPS feeding power through two Mark Johnson/Gtose cap multipliers and the result is zero noise with my ear pressed up against the speakers. No hum or hissing. Nothing! That's without mumetal shields around the autoformers!
What a nice sounding amp this is. I bought the boards three years ago about an hour before Mark Johnson announced the M2x, but only completed it now. Maybe I'll build an M2x in future, so that I can try out the different input boards, but for now I'll just enjoy this amazing amp. I'll post a pic once the enclosure is done.
And btw,the PSU is external: Two 150W Meanwell SMPS feeding power through two Mark Johnson/Gtose cap multipliers and the result is zero noise with my ear pressed up against the speakers. No hum or hissing. Nothing! That's without mumetal shields around the autoformers!
Great bit of information, thanks for sharing. I always stayed away from the transformers (and sound-coupling/DC-blocking caps) in the signal chain. I'm thinking here.... the time I would spend fault-finding the channel with that out-of-spec transformer... 😱
EB,
Yeah. When I sent those SE output transformers back to Edcor, they stated that one of them had an incorrect interleaving of the windings hence the tremendous difference in the FR. From my understanding some of their transformers are wound overseas. Oversights and slip ups occur from time to time but if we as diy hobbyists don't double check we'll be chasing our tail in the end product. Given that a few of Pass' designs, like F6, M2, etc...use transformers within the signal chain, we can't assume that said transformer...although reliable is necessarily...functional! So test, test, test! Then complain!
Best,
Anand.
Yeah. When I sent those SE output transformers back to Edcor, they stated that one of them had an incorrect interleaving of the windings hence the tremendous difference in the FR. From my understanding some of their transformers are wound overseas. Oversights and slip ups occur from time to time but if we as diy hobbyists don't double check we'll be chasing our tail in the end product. Given that a few of Pass' designs, like F6, M2, etc...use transformers within the signal chain, we can't assume that said transformer...although reliable is necessarily...functional! So test, test, test! Then complain!
Best,
Anand.
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