Hi James,
Gate resistors should solve that. However, matching is critical without source resistors. This is one of the biggest faults with the Counterpoint amps. No way could they match these tightly enough, and it was both stupid and commercial suicide to do things this way.
Now for the bad news. The Counterpoint design requires silly tight matching, far closer than any other amp I have ever seen. I honestly don't think you will be able to purchase matched sets like that. I did post how many I needed to go through to get an acceptable set. And that was from other matched sets!
-Chris
Gate resistors should solve that. However, matching is critical without source resistors. This is one of the biggest faults with the Counterpoint amps. No way could they match these tightly enough, and it was both stupid and commercial suicide to do things this way.
That's what I've always felt. However slight, some kind of bias control makes sense.
And you trust this matching? How tightly are they claiming a match between devices firstly? You must be able to confirm the match yourself.
Now for the bad news. The Counterpoint design requires silly tight matching, far closer than any other amp I have ever seen. I honestly don't think you will be able to purchase matched sets like that. I did post how many I needed to go through to get an acceptable set. And that was from other matched sets!
-Chris
Well, the seller is here in NJ, so I was thinking I might be able to pursuade him to make an extra-tight match.
I've been spending quite a bit of time trying to understand power MOSFETS - it seems some ot the vertical ones have a NTC and some a PTC after a certain point. I've also read (only on one page) that the 20N16/20P16 seem to be more prone to oscillation than the 10N16/10P16.
It looks like what I want to do can be done with a relatively minimum amount of effort. The only advantage I can see of really tight matching in this open-loop output circuit (with NTC devices) is to lower distortion. Am I missing something?
I'm not expecting a damping factor that is much better than a tube amp, and so far (when the amp is not oscillating) the distortion measurements are lower than the originl spec, and on par with a tube amp.
I'm going to try one more thing before I shelf the amp for another year - I attached a schematic that I found.
You are right, Chris - the more I look at this design (and the more I learn about design) the more goofy it looks. I'm starting to think it shoud be abandoned and maybe made into a MOSKido
Thanks,
I've been spending quite a bit of time trying to understand power MOSFETS - it seems some ot the vertical ones have a NTC and some a PTC after a certain point. I've also read (only on one page) that the 20N16/20P16 seem to be more prone to oscillation than the 10N16/10P16.
It looks like what I want to do can be done with a relatively minimum amount of effort. The only advantage I can see of really tight matching in this open-loop output circuit (with NTC devices) is to lower distortion. Am I missing something?
I'm not expecting a damping factor that is much better than a tube amp, and so far (when the amp is not oscillating) the distortion measurements are lower than the originl spec, and on par with a tube amp.
I'm going to try one more thing before I shelf the amp for another year - I attached a schematic that I found.
You are right, Chris - the more I look at this design (and the more I learn about design) the more goofy it looks. I'm starting to think it shoud be abandoned and maybe made into a MOSKido
Thanks,
Attachments
Hi James,
You have a good opportunity here. I don't own the prototypes and they are all currently out. Having an SA-100 or SA-220 chassis is very helpful for working out the problems on this model.
-Chris
That's if he is capable. Who knows how he is matching his stuff? Could be okay, might be acceptable for normal designs. This design requires very, very close matching.
Those issues aside, oscillation is a problem for all mosfets, so once you have it licked - no sweat from there on in.
After your parts are matched - yes.
Nope. Although, matching these outputs will probably reduce distortion as a side benefit.
Yes. Current sharing. If one of the pair works harder than the other, you may have that channel fail. Who wants to redo work already done. That just means your first bit of work is totally wasted. Most people would blame the design after this happens when it is really their own fault.
Again, not an issue. You have a tube VAS with an output stage outside the feedback loop. As long as the output stage is fairly linear, you will have what sounds like a tube amp.
Why not just finish it while the concepts are fresh in your mind. Counterpoint sold lots of these, and if you use better (for sound) outputs and watch the idle current you should be fine.
Look at it this way, you have a dead SA-100. If you repair it, it's worth more to sell and you might even like it. At the very least, it's a good learning exercise.
You have a good opportunity here. I don't own the prototypes and they are all currently out. Having an SA-100 or SA-220 chassis is very helpful for working out the problems on this model.
-Chris
Just a quick note - the Exicons seem to be working well despite not being perfectly matched!
Once I took care of the oscillation problem and swapped devices around for the best in-circuit match, the amp has been dead-nuts reliable for the past three months of daily use. While I can't say that the current sharing is perfect, I have not seen any current hogging in my measurements.
Temperature, offset, channel balance has been unwavering. This amp has been driving my NHT Classic 3 (not a particularly friendly load and low efficiency too) with no problems! I actually did not plan to keep this amp in the system for this long, but of the amps that I have, this one seems to sound best with these speakers overall. Sonically, it is not quite the equal of my other amps, but then again, I have done nothing to improve the sound yet. I just wanted the thing to work.
THD does not go above .035% (no LP filters either) at any output power, which is way better than spec!
I hope to have time within the next month to go back and finish up/optimize the output stage. My original plan was to repair and sell this amp. I do feel confident that it will not fail in the field, but now that it seems to be working reliably, I might have to keep it
I also now have the means to output data from my Boonton via GPIB to a spreadsheet so I can capture freq resp, THD vs Freq, etc. I'll try to post some specs when I get around to it.
Thanks & Cheers,
Once I took care of the oscillation problem and swapped devices around for the best in-circuit match, the amp has been dead-nuts reliable for the past three months of daily use. While I can't say that the current sharing is perfect, I have not seen any current hogging in my measurements.
Temperature, offset, channel balance has been unwavering. This amp has been driving my NHT Classic 3 (not a particularly friendly load and low efficiency too) with no problems! I actually did not plan to keep this amp in the system for this long, but of the amps that I have, this one seems to sound best with these speakers overall. Sonically, it is not quite the equal of my other amps, but then again, I have done nothing to improve the sound yet. I just wanted the thing to work.
THD does not go above .035% (no LP filters either) at any output power, which is way better than spec!
I hope to have time within the next month to go back and finish up/optimize the output stage. My original plan was to repair and sell this amp. I do feel confident that it will not fail in the field, but now that it seems to be working reliably, I might have to keep it
I also now have the means to output data from my Boonton via GPIB to a spreadsheet so I can capture freq resp, THD vs Freq, etc. I'll try to post some specs when I get around to it.
Thanks & Cheers,
Hi James,
I think you should redo your measurements. What are you using to get your distortion figures?
-Chris
That's excellent news. I'm very happy it worked out well for you.
Now that is not normal. Bias will drift as it has a positive tempco. Likewise, your DC offset never settles down. Channel balance is more a factor of the voltage amp circuit.
The output stage is not included in the feedback loop. It is taken through a tube (from plate to cathode). Not the most linear path if you ask me!
I think you should redo your measurements. What are you using to get your distortion figures?
-Chris
Hello James,
I am hoping you will check this thread again. I really am grateful for the time you spent documenting the process you went through to get this to work. I am a lover of these amps and am self taught in electronics. I would love to get my SA-12s running again, but my design skills are still a little too weak to pull it off. Your design skills are clearly on a higher plane than mine. I was hoping you could further my education by sketching your modifications on the schematic posted earlier in the thread. I have an idea what you did and the schematic would fill in the gaps.
Thanks
David.
Hi Rickor,
You might have to buy another SA-100 with a blown output stage and make one out of two. At least you'll know those outputs are both real and factory matched.
I might have to continue my development using another SA-100. It might involve a set of good devices. Right now it hasn't materialized, but maybe in 2018 ...
-Chris
You might have to buy another SA-100 with a blown output stage and make one out of two. At least you'll know those outputs are both real and factory matched.
I might have to continue my development using another SA-100. It might involve a set of good devices. Right now it hasn't materialized, but maybe in 2018 ...
-Chris
I have just sold my SA-100, and looking back I have been asked by several folks to share my modification of the SA-100, so here it is.
Now, a few things - this is not exactly original thought on my part, so some credit to those who have done this before me:
I used the George Kay's Moscode 300 and Erno Borberly's Hafler DH-200 schematics as guides, as well as designs from a japanese site that I can no longer find. I also used the SA-100 schematic drawn by Federico Paoletti as a reference for my drawing (which is actually part of an LTSpice simulation). Whatever anyone may think of Mike Elliot's Counterpoint designs, I sincerely appreciate and admire what he has done in audio.
This was my first attempt at design - building/modding this circuit was fairly straightforward, but getiing it to be stable was not. What I learned is that the Exicon MOSFETS will oscillate despite what the app notes indicate, so always use a pf cap between the gate and source of your N-channel devices. The instructions were written long after the mod was made - be aware that I may have left something out as I did not take pictures of the bottom of the board - try at your own risk.
I don't post very much, so I would like to thank the diyaudio.com community as I have gleaned much good information and advice just from lurking and searching. This resource truly is a treasure - I am happy to give something back.
I have attached schematics and pictures of the mods.
Now, a few things - this is not exactly original thought on my part, so some credit to those who have done this before me:
I used the George Kay's Moscode 300 and Erno Borberly's Hafler DH-200 schematics as guides, as well as designs from a japanese site that I can no longer find. I also used the SA-100 schematic drawn by Federico Paoletti as a reference for my drawing (which is actually part of an LTSpice simulation). Whatever anyone may think of Mike Elliot's Counterpoint designs, I sincerely appreciate and admire what he has done in audio.
This was my first attempt at design - building/modding this circuit was fairly straightforward, but getiing it to be stable was not. What I learned is that the Exicon MOSFETS will oscillate despite what the app notes indicate, so always use a pf cap between the gate and source of your N-channel devices. The instructions were written long after the mod was made - be aware that I may have left something out as I did not take pictures of the bottom of the board - try at your own risk.
I don't post very much, so I would like to thank the diyaudio.com community as I have gleaned much good information and advice just from lurking and searching. This resource truly is a treasure - I am happy to give something back.
I have attached schematics and pictures of the mods.
Just a cheap transistor matcher from AliExpress. No heating in the oven.
I still have 2 SA220s in the basement with output stages repaired and playing (1 for about 5 years and another for about 8 months) using IRF9241 and RFM10N15 I got from Chipsgate (ebay seller). I just buy 20 each and was able to get 3 sets of 4. Now the latest batches I got from a domestic ebay seller and Rochester was a disaster. They match OK on the cheapo tester but they pop after 1-2 hours warm up. I was barely able to get them to stay alive with 0.25 ohm source resistors but one channel lasted 3 months on an SA12.
I am not wiring these 240/9240s right against each other like stock SA12/100/20/220 but again I am using 0.25 ohm source resistors. So far all the reference schematics I have seen (except for 1 which I think was drawn wrong) on the interwebs have pointed to using 0.2-0.47 ohm resistors so I dare not even try not using them.
I do suspect Mike Elliott's 0 source resistors idea was because of the lack of NFB wich is tricky to implement in the hybrid circuit. Or was it because of the high output Z of those old mosfets?
I still have 2 SA220s in the basement with output stages repaired and playing (1 for about 5 years and another for about 8 months) using IRF9241 and RFM10N15 I got from Chipsgate (ebay seller). I just buy 20 each and was able to get 3 sets of 4. Now the latest batches I got from a domestic ebay seller and Rochester was a disaster. They match OK on the cheapo tester but they pop after 1-2 hours warm up. I was barely able to get them to stay alive with 0.25 ohm source resistors but one channel lasted 3 months on an SA12.
I am not wiring these 240/9240s right against each other like stock SA12/100/20/220 but again I am using 0.25 ohm source resistors. So far all the reference schematics I have seen (except for 1 which I think was drawn wrong) on the interwebs have pointed to using 0.2-0.47 ohm resistors so I dare not even try not using them.
I do suspect Mike Elliott's 0 source resistors idea was because of the lack of NFB wich is tricky to implement in the hybrid circuit. Or was it because of the high output Z of those old mosfets?
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Hi phi70,
It is critical to have zero in the way of source resistors to get low distortion output. For that the mosfets must be matched crazy tightly.
No offense intended, but what you are doing is completely unacceptable for a proper repair. Please, do not repair anyone else's unit this way - and do not accept money for a job like that. You did get them functioning, but they fail their distortion and power specs. You find this acceptable, but they are no longer SA-100 or SA-220 amplifiers. Just to be clear.
Also, please do not encourage anyone else to do this. The goal should always be "equal or better performance" and higher reliability. Should these amplifiers ever been released (sold) on the market? No! Especially since a couple of us have shown that good performance was possible with high reliability. Some times a repair just isn't a DIY effort that is possible. Don't feel bad, Carver amplifiers (well designed) are not a DIY repair either.
-Chris
It is critical to have zero in the way of source resistors to get low distortion output. For that the mosfets must be matched crazy tightly.
No offense intended, but what you are doing is completely unacceptable for a proper repair. Please, do not repair anyone else's unit this way - and do not accept money for a job like that. You did get them functioning, but they fail their distortion and power specs. You find this acceptable, but they are no longer SA-100 or SA-220 amplifiers. Just to be clear.
Also, please do not encourage anyone else to do this. The goal should always be "equal or better performance" and higher reliability. Should these amplifiers ever been released (sold) on the market? No! Especially since a couple of us have shown that good performance was possible with high reliability. Some times a repair just isn't a DIY effort that is possible. Don't feel bad, Carver amplifiers (well designed) are not a DIY repair either.
-Chris
The guys seeking my help often have a very tight budget and I charge them only parts $ to revive an otherwise junked unit sold as trash on ebay or craigslist or straight to Goodwill.
I am assuming this forum is DIY audio, not commercial modder blog (?).
Eg I completely recapped an SA12 (all caps leaked goo) and both channels output stage for $100. The owner got to hear the results on 2 sets of speakers before he said "wow thank you you revived this doorstop sitting in my closet for 10 years" Go figure how much I lost on parts. It's aDIY audio hobby. I just dont have the b/gall to quote them $2500 for a rebuild of a hybrid amp.
I am assuming this forum is DIY audio, not commercial modder blog (?).
Eg I completely recapped an SA12 (all caps leaked goo) and both channels output stage for $100. The owner got to hear the results on 2 sets of speakers before he said "wow thank you you revived this doorstop sitting in my closet for 10 years" Go figure how much I lost on parts. It's aDIY audio hobby. I just dont have the b/gall to quote them $2500 for a rebuild of a hybrid amp.
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