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I don't have much in the way of inductors in my junk box, at least nothing that wouldn't melt. But so far no indications it's unhappy driving my two way (cross-over) floor-standers via fairly long hose pipe sized cables. I may be able to dig out something close to 300uH.
The next couple of days I'm going to draw a line under these tests and close out this stage of the project. It's been interesting, I have learned the value of making these things robust and will do some reading on this topic before I start my next project.
The next couple of days I'm going to draw a line under these tests and close out this stage of the project. It's been interesting, I have learned the value of making these things robust and will do some reading on this topic before I start my next project.
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My condolences Gareth. You can blame me, if that helps. 
The output inductor will only give so much protection and it is actually better to do these tests without one in place.
To get the compensation "just right" (from a 'sonics' perspective) may be a noble cause but it can mean the amp is teetering on the brink of oscillation. I have found simulation to be the best way to get to a close value and to check phase margin. You can even run these capacitive / inductive tests via simulation to get a hint as to how the amp will behave.
The output inductor will only give so much protection and it is actually better to do these tests without one in place.
To get the compensation "just right" (from a 'sonics' perspective) may be a noble cause but it can mean the amp is teetering on the brink of oscillation. I have found simulation to be the best way to get to a close value and to check phase margin. You can even run these capacitive / inductive tests via simulation to get a hint as to how the amp will behave.
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Of course it's your fault fortunately I bought 50 output pairs for TGM in the first place over a year ago - Digikey had an attractive price break. However, the pcb gets abused each time I replace them so I don't fancy doing this often.
Are the oscillations I provoked with the capacitive load an indication that I need to make further changes or does it simply mark the boundaries of where the amplifier can operate and I leave things as-is ?
Would changing Cdom actually help if the oscillations were confined to the EF output stage - I've seen base-emitter caps added to output stages before ?
fortunately I bought 50 output pairs for TGM in the first place over a year ago - Digikey had an attractive price break. However, the pcb gets abused each time I replace them so I don't fancy doing this often.Are the oscillations I provoked with the capacitive load an indication that I need to make further changes or does it simply mark the boundaries of where the amplifier can operate and I leave things as-is ?
Would changing Cdom actually help if the oscillations were confined to the EF output stage - I've seen base-emitter caps added to output stages before ?
Of course it's your fault
Would changing Cdom actually help if the oscillations were confined to the EF output stage - I've seen base-emitter caps added to output stages before ?
Sorry
The reason for compensation is to get the phase of the feedback signal correct, so that there is no positive feedback. Changing the value of cdom will be the 'dominant' factor. As far as I know, oscillation is not confined to any stage. The outputs will not withstand a lot of high frequency signal, regardless of the source - they will overheat and fail. The small signal stages before it will cope with HF no problem.
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John, I'll send you the bill then
I thought it was possible to have oscillations locally, anywhere there is a local feedback loop. The output stage is EF, so it has local feedback built-in via Re. Although some folk don't like to think of EF as having local feedback it's there all the same and can oscillate all by itself. I'm not sure if the front end of the amp can do much to help - it requires the zobel and output inductor for that ????
WuYit - a curse ! I've heard a lot of mumbo jumbo about amplifiers from the audio fraternity but never a curse !!!!!
I thought it was possible to have oscillations locally, anywhere there is a local feedback loop. The output stage is EF, so it has local feedback built-in via Re. Although some folk don't like to think of EF as having local feedback it's there all the same and can oscillate all by itself. I'm not sure if the front end of the amp can do much to help - it requires the zobel and output inductor for that ????
WuYit - a curse ! I've heard a lot of mumbo jumbo about amplifiers from the audio fraternity but never a curse !!!!!
If there are (guessing here, not an expert), they are probably not strong enough to damage your outputs. A low amplitude oscillation can exist and not have any real impact on the overall performance or sound quality.
Try a higher value for cdom and see how it goes.
For what it's worth, I have never had to use lead compensation in any of the amps I've built so far. Look at that too - it may be the cause.
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John,
I tried removing the phase lead and it's unconditionally unstable even with more Cdom
Reduced the phase lead to 10pF (it's pretty toxic so I prefer this to the 22pF I had before) and boosted Cdom from 68p to 90p and then to 112pF.
With 10p lead and 112pF lag the thing is stable into loads of 10nF, 20nF, 470nF, 940nF and 1.4uF. Ringing is non existent up to 20nF but appears at 470nF and gets slightly worse with higher capacitance.
Test duration kept very short - don't want to destroy it again.
So it seems some phase lead is needed and then it's a case of higher Cdom as you suggested.
Thinking back - I've increased rail voltage from 26V to 38V which would indicate a need to revisit compensation.
I will need to do another listening test - which is a bit of a faff at the moment.
A most interesting learning experience - although this amp will be going to somebody else it will help me enormously with my next project.
I tried removing the phase lead and it's unconditionally unstable even with more Cdom
Reduced the phase lead to 10pF (it's pretty toxic so I prefer this to the 22pF I had before) and boosted Cdom from 68p to 90p and then to 112pF.
With 10p lead and 112pF lag the thing is stable into loads of 10nF, 20nF, 470nF, 940nF and 1.4uF. Ringing is non existent up to 20nF but appears at 470nF and gets slightly worse with higher capacitance.
Test duration kept very short - don't want to destroy it again.
So it seems some phase lead is needed and then it's a case of higher Cdom as you suggested.
Thinking back - I've increased rail voltage from 26V to 38V which would indicate a need to revisit compensation.
I will need to do another listening test - which is a bit of a faff at the moment.
A most interesting learning experience - although this amp will be going to somebody else it will help me enormously with my next project.
Some ringing is normal with a heavy capacitive load. Nice to do an inductive load too, just to be sure. Sometimes an amp can be more reactive with this kind of load.
It's all a learning process, for sure. I just spent probably 1000 work hours on a project that would cost maybe $500 to buy new. Working for 50 cents an hour is justified when you enjoy it and learn something. It doesn't pay the bills though...
Trying to clean my kitchen and make spaghetti sauce simultaneously. My kitchen can become the remote assembly area sometimes:
Ah, enthusiasm for the hobby (or total insanity).
It's all a learning process, for sure. I just spent probably 1000 work hours on a project that would cost maybe $500 to buy new. Working for 50 cents an hour is justified when you enjoy it and learn something. It doesn't pay the bills though...
Trying to clean my kitchen and make spaghetti sauce simultaneously. My kitchen can become the remote assembly area sometimes:
Ah, enthusiasm for the hobby (or total insanity).
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I like that photo, I can use that with my wife - she'll be happy that I'm doing all this in the basement and keeping my tools off the counter top afterall !!!
I agree, it ain't a worry to spend time when it's fun or educational. I did get a $250 home theatre amplifier as a gift recently - but I just couldn't bear to see my DIY chasis put aside so I returned it. The addiction continues....
I agree, it ain't a worry to spend time when it's fun or educational. I did get a $250 home theatre amplifier as a gift recently - but I just couldn't bear to see my DIY chasis put aside so I returned it. The addiction continues....
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I was looking for an excuse to keep the going with the DIY when the missus commented that the ready-made gear didn't sound as good. That may have been because I used the automatic set up feature to balance the channels and it didn't do a good job in my opinion - but who cares, it gave me a good reason to put the TGM box back into service. And now I have reason to rebuild it of course to make it better
I have a lot of 'next' projects in mind - you know how it is, amps, speakers and related things. The addiction runs free.
But I think first off will be another SS Class AB based on my TGM3, especially now that I've got it back up and running as a centre channel amp with a simple, single-device dc-servo sorting out the dc offset & thermal drift issue I had before. It may be an opportunity to go back to the HT project and start over, to do a better job and gain a bit more experience.
You playin' that guitar yet ?
I have a lot of 'next' projects in mind - you know how it is, amps, speakers and related things. The addiction runs free.
But I think first off will be another SS Class AB based on my TGM3, especially now that I've got it back up and running as a centre channel amp with a simple, single-device dc-servo sorting out the dc offset & thermal drift issue I had before. It may be an opportunity to go back to the HT project and start over, to do a better job and gain a bit more experience.
You playin' that guitar yet ?
I had a Yamaha HTR-5760 that has that setup thing. I didn't like the way it did it either. That Yamaha has been collecting dust for the last 2 years. Shame, it was ~$600 when I bought it, more than 5 years ago.
Does she read Cosmo? Cosmo has all kinds of tips on how to flatter men and puff them up. I had this girlfriend a few years back that considered that her bible and was trying some of it out on me. One in particular is memorable, regarding size. At 'some' point she said 'oh, is that 9 inches' ...reaaally, I'm going to fall for that?
I'm 'playing' a guitar:
Mine is not done yet. I'll finish it this summer.
Does she read Cosmo? Cosmo has all kinds of tips on how to flatter men and puff them up. I had this girlfriend a few years back that considered that her bible and was trying some of it out on me. One in particular is memorable, regarding size. At 'some' point she said 'oh, is that 9 inches' ...reaaally, I'm going to fall for that?
I'm 'playing' a guitar:
Mine is not done yet. I'll finish it this summer.
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Why not gut it and put your own amps inside ?
there's also the benefit of going metric
It looks superb - the guitar especially. I know I couldn't make that - maybe you enjoyed it so much you'd like to make another one
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Very nice guitar amp - not sure why you needed two ?
Well, I've done the listening test with the new compensation and to my ears it sounds slightly better - the hint of sibilance before is gone on most music, with a little doubt left when listening to specific tracks but it's hard to tell when relying on an iPod for the source.
Module is done.
Well, I've done the listening test with the new compensation and to my ears it sounds slightly better - the hint of sibilance before is gone on most music, with a little doubt left when listening to specific tracks but it's hard to tell when relying on an iPod for the source.
Module is done.
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