Also prevent unwanted DC current loading through the transformer while doing so.
Post #2592 shows the 20 ohm pot mod....
https://www.diyaudio.com/forums/pass-labs/281520-official-m2-schematic-52.html#post5420988
Russellc
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.[/QUOTE]
Hi Mark, just wanted to clarify your response. If the 220uf cap is installed then the trimpot isn’t needed to zero the offset? That’s great to hear as I’ve been reading through the M2x guide again in preparation for when I get my build started. I don’t think that point is understood in the M2x thread. Maybe that can be added in? I may have read it incorrectly but I would have ended up installing both the cap and the trim pot to help with that offset.
Hi Mark, just wanted to clarify your response. If the 220uf cap is installed then the trimpot isn’t needed to zero the offset? That’s great to hear as I’ve been reading through the M2x guide again in preparation for when I get my build started. I don’t think that point is understood in the M2x thread. Maybe that can be added in? I may have read it incorrectly but I would have ended up installing both the cap and the trim pot to help with that offset.
If you are referring to the above 20 ohm pot mod, it has absolutely nothing to do with offset measurement, nor 2nd order distortion, it merely prevents DC from getting to the Edcors by balancing the difference from less than ideally matched Jfets, as does Marks option for 220 uf cap.
There is in one of these M2 threads, talk of using pots to zero the offset instead of trading resistors, but Marks 220 uf cap wont stop that either, it, like the 20 ohm mod above, merely prevents DC at the Edcor.
If you are responding to something else, nevermind!
Russellc
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Same here, one of mine was at 4.5 or so IIRC, not worth the teardown to replace, but for future M2 builds or if I tear into for another reason, I will do the 20 ohm pot thing.
Russellc
I think you have confused a couple of things...M2 has a pot that is used to zero offset. Sometimes, it cant, so certain resistor changes are made to allow zero. There is somewhere in these threads where those resistors are combined with pots to adjust offset without resistor change.
The 20 ohm pot thing I am talking about, as well as Marks addition of the 220uf cap, is to prevent DC at the input of the edcor. 5 millivolts is the suggested max, but less than ideally matched Jfets can raise that number above what the Edcor likes... Hope this helps.
There is no pot to adjust 2nd H in the M2. In fact, it is a 3rd H amp....as I understand it anyway, for whats that is worth, I may have that botched...
Russellc
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Hi Russel, I’m referring to this blurb on page 1 of the M2x thread.
3. WARNING: BLASPHEMOUS HERESY! DO NOT READ THIS! Some DIY builders of the M2 amplifier, using the very fine “Tea‐Bag” circuit board, have reported a problem to the diyAudio forums. Their M2 amplifier’s output offset voltage is negative, and no setting of trimmer resistor RV1 removes this negative offset. I would like to gently mention a possible fix: leave R7=47K, but change R6 to 37K and change RV1 to 20K. Now (R6+RV1) can vary from 37K to 57K, in other words, from (10K less than R7) to (10K more than R7). This lets you null out either polarity of offset voltage. However, to faithfully reproduce Nelson Pass’s original M2 design, the M2X schematic and PCB silkscreen do not include this modification. M2X has R6=47K and RV1=5K. If you decide to make this R6,RV1 modification on your M2X, don’t tell anyone. And don’t quote me.
If I’m missinterpretting something please explain. I’m still learning as I go and want to be sure I’m getting this all correct.
Thanks!
3. WARNING: BLASPHEMOUS HERESY! DO NOT READ THIS! Some DIY builders of the M2 amplifier, using the very fine “Tea‐Bag” circuit board, have reported a problem to the diyAudio forums. Their M2 amplifier’s output offset voltage is negative, and no setting of trimmer resistor RV1 removes this negative offset. I would like to gently mention a possible fix: leave R7=47K, but change R6 to 37K and change RV1 to 20K. Now (R6+RV1) can vary from 37K to 57K, in other words, from (10K less than R7) to (10K more than R7). This lets you null out either polarity of offset voltage. However, to faithfully reproduce Nelson Pass’s original M2 design, the M2X schematic and PCB silkscreen do not include this modification. M2X has R6=47K and RV1=5K. If you decide to make this R6,RV1 modification on your M2X, don’t tell anyone. And don’t quote me.
If I’m missinterpretting something please explain. I’m still learning as I go and want to be sure I’m getting this all correct.
Thanks!
To remove negative offset at the output of the amp I had to increase R7 to 52.3k.
Yes, they are referring to offset voltage at the outputs. Sometimes the pot will not allow you to zero the offset. The HERESY spoken of is to make the change to one resistor to 37K, and add the pot. This prevents having to trade out resistor, just adjust the pot to trim.
The problem I am talking about, and Marks 220 uf cap addresses is a measurement taken between ground and the input of the Edcor. If your Jfets are ideally matched, you should get a figure under 5 millivolts, which is deemed good. If you are getting more than 5, the pot will allow you to bring that number down to zero. The Edcor does not like much DC on its input.
I believe it was member Eric who built M2 with a less than ideally matched Jfets and was able to zero with the addition of the 20 ohm pot.
See? the two problems are separate from each other?
Russellc
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Yes, I had to make a couple changes, I guess the trim pot would have been handy, but once you can zero, its done...really do like this amp, mine is Teabag version too, I bought Marks boards as well, still have a couple extra stuffed Teabag boards as well...
I see FetAudio (spencer) is reaching the end of his supplies, Punkydawgs and the Israel source are the last I know for genuine Toshibas. I laid in a small stash that should last me in the idss ranges I preferred.
Marks boards relieve that anxiety, front end cards that are allowing builds with other buffer set ups without Toshibas. They are also receiving positive reviews from those who know, which is great news.
Russellc
My own personal feeling is: I don't want any DC current flowing in the primary of the Edcor transformer. The resistance of the primary winding is only 42 ohms, according to Nelson Pass's own measurements (link). Thus Ohm's Law tells us that a 10 millivolt offset voltage at the JFET buffer output, results in a DC current of (0.010 / 42) = 0.00024 amperes. That's 0.24 milliamps. I don't want that much DC current flowing in the primary; it might magnetize the core, or it might push the operating point well away from the middle of the B-H curve, making the core susceptible to asymmetric clipping.
So that's why I prefer to use a 220uF blocking capacitor between the input stage (all FIVE input stages in the case of M2x), and the transformer. It's my own nervous fear. Let's call it "design conservatism" instead.
I included the adjustable trimmer potentiometer in the PCB of the Toshiba JFET input stage "Ishikawa", just because this pot seemed to be present in many other Pass Labs & First Watt designs. I figured: give builders the option to include it or to leave it out, as they see fit. Some people will slavishly copy Nelson Pass's Official M2 Schematic (post #1 of this thread) and decide that if N.P. doesn't need a trimmer then neither do they.
Other builders may want the potentiometer, so they can null out the output offset without installing the 220uF coupling capacitor.
Other builders may want the potentiometer because it's the notorious component "P3" from the very famous First Watt F5 amplifier. Jockeying around with "P3" on an F5 amplifier, lets you dial in as much or as little 2nd harmonic distortion as you like. Some M2 builders may want this. It certainly is entertaining, and entertainment is the final end-goal of many First Watt projects.
Member 6L6 actually made a YouTube video about potentiometer P3, showing what happens when you twist it one way and then the other way, in a First Watt amplifier. The video is here:
All of these options are included on the Toshiba JFET input stage PCB, and you the builder can choose what you do or don't implement in your build. The FirstWatt original M2 did not include a potentiometer and it did not include a 220uF coupling capacitor between the JFET stage and the Edcor transformer's primary. But YOU may want one or both of them, anyway.
So that's why I prefer to use a 220uF blocking capacitor between the input stage (all FIVE input stages in the case of M2x), and the transformer. It's my own nervous fear. Let's call it "design conservatism" instead.
I included the adjustable trimmer potentiometer in the PCB of the Toshiba JFET input stage "Ishikawa", just because this pot seemed to be present in many other Pass Labs & First Watt designs. I figured: give builders the option to include it or to leave it out, as they see fit. Some people will slavishly copy Nelson Pass's Official M2 Schematic (post #1 of this thread) and decide that if N.P. doesn't need a trimmer then neither do they.
Other builders may want the potentiometer, so they can null out the output offset without installing the 220uF coupling capacitor.
Other builders may want the potentiometer because it's the notorious component "P3" from the very famous First Watt F5 amplifier. Jockeying around with "P3" on an F5 amplifier, lets you dial in as much or as little 2nd harmonic distortion as you like. Some M2 builders may want this. It certainly is entertaining, and entertainment is the final end-goal of many First Watt projects.
Member 6L6 actually made a YouTube video about potentiometer P3, showing what happens when you twist it one way and then the other way, in a First Watt amplifier. The video is here:
All of these options are included on the Toshiba JFET input stage PCB, and you the builder can choose what you do or don't implement in your build. The FirstWatt original M2 did not include a potentiometer and it did not include a 220uF coupling capacitor between the JFET stage and the Edcor transformer's primary. But YOU may want one or both of them, anyway.
Let me make sure I have this right. By "Other builders may want....(above) do you mean this cap not only prevents DC at the input of the Edcor, but also "nulls out the output offset" ( At the amplifiers output) or just before the Edcor?
Is your pot on the daughter board of the "Ishikawa" version only set DC at the input of the Edcor, like https://www.diyaudio.com/forums/pass-labs/281520-official-m2-schematic-52.html#post5420988, or is this something that also sets output offset at the amp's output?
Also another question about that pot on the "Ishikawa" version, it also adjusts the harmonic distortion spectra as well? (like P3 on some amps ) So (assuming it is also used to cancel DC at the Edcor's input) using it to zero DC at the Edcors input would also shift said harmonic spectra? If so this may be a good reason to use very tightly matched Jfets to eliminate DC at Edcor input...and not rely on the pot to do so....I understand M2 is a 3rd harmonic sort of amp and injecting 2nd order harmonic not a good idea? please school me here.
Thanks,
Russellc
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These are some of the threads Im talking about as to H2 and H3 with the M2 concerning P3..
https://www.diyaudio.com/forums/pass-labs/321925-diyaudio-watt-m2x-12.html#post5542294
https://www.diyaudio.com/forums/pass-labs/321925-diyaudio-watt-m2x-12.html#post5542611
https://www.diyaudio.com/forums/pass-labs/321925-diyaudio-watt-m2x-12.html#post5542785
Russellc
https://www.diyaudio.com/forums/pass-labs/321925-diyaudio-watt-m2x-12.html#post5542294
https://www.diyaudio.com/forums/pass-labs/321925-diyaudio-watt-m2x-12.html#post5542611
https://www.diyaudio.com/forums/pass-labs/321925-diyaudio-watt-m2x-12.html#post5542785
Russellc
That was a surprise to read. If we adjust the pot to increase NJFET source resistance to output, and simultaneously decrease PJFET source resistance to output, then we've decreased the gm of the pullup circuit and we've increased the gm of the pulldown circuit. The gm's are no longer balanced. Are you certain this has no effect upon the 2nd harmonic, the harmonic which is present when the top half of the waveform is not a perfect match for the bottom half of the waveform? No effect at all? Think about Douglas Self's wingspread plots.
Naturally the effect will be greater in a common source configuration, where twirling the pot decreases the pullup gain ratio (Rd/(Rs+delta)) and simultaneously increases the pulldown gain ratio (Rd/(Rs-delta)) .
But why not encourage people to horse around with it and entertain themselves? There's no harm done and plenty of fun and learning. Experimenters may even discover a pot setting that gives greater listening enjoyment than all other settings. You've probably heard the expression, "It's real to ME, damn it!" which may apply to a great many areas of audio entertainment, not just pot twirling on a JFET unity gain buffer.
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Wow, I might have had one thing right, so I assume using a 20 ohm pot to null out DC at Edcores input will zero the DC there, but also at amps outputs? I don't get that, and a million other things!
Thanks,
Russellc