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Official M2 schematic
Official M2 schematic
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Old 24th August 2018, 07:42 PM   #3031
kbergsson is offline kbergsson  Iceland
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Im in no hurry, my Aikido Is a good linestage ( 4x 6CG7 ), has sober amount of gain and is quite dynamic and transparent. Adds just a little of own colour to the sound. The autoformer excercise was more to hear M2 without additional colour and mud.

Last edited by kbergsson; 24th August 2018 at 07:45 PM.
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Old 13th September 2018, 10:57 PM   #3032
Mark Johnson is offline Mark Johnson  United States
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Official M2 schematic
M2 PCBoards are now available in the diyAudio store, FYI

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Old 14th September 2018, 12:41 AM   #3033
Holzarbeiter is offline Holzarbeiter  United States
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Ordered mine. Thanks for doing this. Next month another...
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Old Yesterday, 11:59 AM   #3034
qwertyl is offline qwertyl  United Kingdom
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Default Right channel is running away

I have an issue with my M2 bulid on TeaBag's boards. Left channel is fine 0.62V across R13/R14, but right channel goes to 0.75V, stays there for a few minutes and then runs 1V...1.5V...2V. Any ideas? I have replaced 4N35, but no difference...
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Old Yesterday, 12:53 PM   #3035
Mark Johnson is offline Mark Johnson  United States
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Official M2 schematic
Default re: Right channel is running away

Maybe a promising first line of inquiry, might be to learn whether the "input side" of the bias control loop is operating correctly. That's R10-R14 plus Q5-Q7, the complete circuit which includes the LED portion of the optoisolator.

It might be extremely revealing, to measure the LED current for the good channel and also for the bad channel. You can get this by measuring the voltage across R11, and then use Ohm's Law: Current = Voltage/Resistance.

In this case, LED current = R11 current = R11_voltage / R11_resistance and the M2 schematic tells us that R11 = 221 ohms.

Are the LED currents wildly different between the good channel and the bad channel? If so that's a giant clue.

It could also provide more insight, to directly measure the LED voltage (positive DMM lead on Q5 pin1, negative DMM lead on Q5 pin2) for the good channel and also for the bad channel. A screwy LED voltage on the bad channel, provides more clues.

BTW you'll want to ask for debug suggestions & recommendations from people who have successfully built M2s using the Tea-bag PCBs. They may have encountered this exact problem already, and can hand you a solution straightaway.

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Old Yesterday, 01:54 PM   #3036
qwertyl is offline qwertyl  United Kingdom
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Sorry, false alarm. It was just battery going flat in one of my DMMs... Battery replaced and now I have a stable 0.62V on both channels :-)
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Old Yesterday, 02:08 PM   #3037
qwertyl is offline qwertyl  United Kingdom
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My Jfets may not be matched very well - I have about 6mV on Pin 1 of Edcor (both channels). Is that acceptable?

Last edited by qwertyl; Yesterday at 02:13 PM.
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Old Yesterday, 04:11 PM   #3038
Russellc is offline Russellc  United States
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Official M2 schematic
Somewhere, and I'm not finding it right now, someone showed a nifty trick to allow zeroing that figure. It was in one of these 2 M2 threads....

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Old Yesterday, 04:16 PM   #3039
Zen Mod is offline Zen Mod  Serbia
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Official M2 schematic
20R trimpot in sources

wiper to Edcor
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Old Yesterday, 04:28 PM   #3040
Mark Johnson is offline Mark Johnson  United States
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Official M2 schematic
M2x offers the PCB option of installing a 220uF 10V bipolar coupling capacitor (Nichicon UES) in series between JFET buffer and Edcor transformer primary. The M2 circuitry already includes one coupling capacitor, it's called "C2" in the official schematic attached to post #1 of this thread. If you don't mind one coupling capacitor, surely you won't mind two of them??

It also offers the possibility of adjusting the trimpot for just-the-right-amount of 2nd order harmonic distortion (if you go for that sort of thing), having just the right phase, without worrying about the resulting output offset voltage. The new 220uF removes every trace of output offset voltage. Twirl the trimmer however you want; output offset remains zero.
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