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Old 4th June 2011, 07:56 AM   #21
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I actually thought about that, I have a few 3886's, but using a more linear op amp for the volage gain and a class A output stage appealed to my DIY sense more. Next step willprobably be the full class A design I posted initially and a some stage a valve design. At some point I'll try the MOSFET buffer for comparison.
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Old 4th June 2011, 09:32 AM   #22
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Hi
Opamps I have lived with and recommend, every one of these needs a good pass transistor
power supply of 16v and don't forget to decouple the base of TO92's with at least 220nf
and 100n across emitters.

AD825 very good all round performance - very good low frequency reproduction
OPA 627 excellent
OPA1611 excellent

Cheers / Chris
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Old 4th June 2011, 10:37 AM   #23
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.
Quote:
A large heat sink is essential because there's no temperature compensation and below a certain current MOSFETS are positive just like a BJT (depending on type).
Forgot to mention this earlier, but the output device is thermally compensated by the feedback loop through the opamp. As you said, the opamp outputs will sit at the "on" voltage for the output device to maintain 0v on the output with the appropriate Iq as set by the CCS. For bipolars, if Vbe decreases due to increased device temperature, it will attempt to pass more current (through the load, not the CCS) and Vout will increase, which will be fed back to the opamp's -ve input. The opamp's output DC voltage will then decrease thereby applying thermal compensation.
I think that's the case anyway, feel free to dispute that.
In my case I've used a way undersized heatsink and although it gets very hot (I'd estimate 60-70degC), there seems to be no thermal stability problems as the DC output of the opamps decreases.

Also definitely post your old schematics if you can find them, I'd like to see some other interpretations of this type of amp!
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Old 4th June 2011, 03:00 PM   #24
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OK, I've gone through the moldy archives. Here are 3 circuits. The simple one is the one I built and listened to and was very impressed. Unfortunately I think I gave it away (like most projects) as I can't find it. The one with the opamp driven current sink I built and measured, but never did a board and chassis. I can't remember if it had any advantages or disadvantages, but the current was certainly more precise. The final circuit I've had to redact some of the schematic as I designed it for a commercial purpose, but its a blameless design of really superb performance. I did the board to allow experimenting with grounds, thus the notations of HCG and LCG for high current and low current ground paths. You'd want to change the input network. Or not- it might work fine as is. Don't wrap the output inductor around anything (people usually use the resistor) as that increases distortion. There are also two non-obvious transistors that should be in thermal contact (see D. Self book) and I lay them out next to each other flat to flat, grease 'em up and put a Ty-wrap around them.
Attached Images
File Type: jpg headamp1.jpg (26.4 KB, 1442 views)
File Type: jpg headamp2.jpg (26.3 KB, 1413 views)
File Type: jpg headamp3.jpg (100.1 KB, 1386 views)
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Last edited by Conrad Hoffman; 4th June 2011 at 03:18 PM.
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Old 5th June 2011, 01:56 PM   #25
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Silly me- I didn't even notice until now that those first two circuits are more different than I thought. One uses an N-ch fet and the other uses a P-ch. Note the input polarity on the opamps because of that.

FWIW, one can also go single supply, put the current source on top, bias the circuit half way up and AC couple both the input and output. That gives, I believe, a completely indestructible amplifier for any load condition.
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Old 5th June 2011, 05:20 PM   #26
Elvee is offline Elvee  Belgium
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Here is something all-discrete, class A, high performance and delightfully outlandish:
The tringlinator: http://www.diyaudio.com/forums/headp...amplifier.html

A chip version also exists: the chtringlunator:
What happens when you cross a Tringlotron with a Regulator-Chip Amplifier?
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Old 5th June 2011, 09:34 PM   #27
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I have several version of this topology that I will try to get posted tomorrow as I have several hundred designs that I have done in circuit maker but I will posted the ones that I have built and a few that I haven't built yet.

jer
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Old 6th June 2011, 04:45 AM   #28
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Here are just a few diiferent versions the first two I built and the rest have been simulated without errors.
I plan on trying these out.
the 1000v version I have also using opamps instead of transistors as I forget that I did both of that version.
I originaly design these circuits to power my esl's and esphones.

jer
Attached Images
File Type: jpg esltrialnewworking.jpg (30.6 KB, 1254 views)
File Type: jpg esltrial3irf510x3afinalworking.jpg (45.1 KB, 1190 views)
File Type: jpg esl6.jpg (38.2 KB, 559 views)
File Type: jpg newamp awesome 1ohm opamp type.jpg (127.6 KB, 600 views)
File Type: jpg newamp 4.01 100v bjt.jpg (69.9 KB, 617 views)
File Type: jpg newamp 1.05 1000v.jpg (63.0 KB, 544 views)

Last edited by geraldfryjr; 6th June 2011 at 04:58 AM.
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Old 6th June 2011, 06:52 AM   #29
jcx is online now jcx  United States
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A/DSL op amps can handle enough current to make great headphone drivers

wrap a good fet input op amp around it in a multiloop and you can't mach the performance in discretes

the TPA6120/THS6012 is my favorite for driving dymanic headphones
it can even be Class A biased AD8397-class A? show push-pull Class A output bais for op amps
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Old 6th June 2011, 11:17 AM   #30
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Thanks Conrad! Funny that we'd both tried almost identical circuits years apart.
Interesting schems Jer, I'm thinking about DIYing some ESL headphones with a direct drive amp.
I might have to go through the garage and find an old ADSL router to pull apart, sounds good.
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