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Old 1st July 2015, 06:43 AM   #1
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Default Seek advice on simple & very good sounding class A/B output stage.

Hi All,

I have what I hope is a quick question for the right person(s) here ...

As it is I'm looking for a simple & very good sounding class A/B output stage to be used for a low to medium power amplifier.

The amplifier will be driven from a single-ended voltage amplification stage.

Some specs could be:

- output power ~ 20 watts/8 ohms
- Non-global-feedback
- PSU voltages up to +/- 28 VDC
- preferably doesn't need a servo (voltage amplification stage should be DC stable)
- Preferably an emitter follower solution with proper output biasing scheme ...
- a stable and worry-free solution

Might one of you help with a link or a small schematic of such an output stage?

Thanks for any help you may give

Jesper
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Last edited by gentlevoice; 1st July 2015 at 08:17 AM.
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Old 1st July 2015, 06:56 AM   #2
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Based on my experience, for good sound you'll want an OPS which has good PSRR. A diamond buffer gives about the best PSRR I've seen of the more traditional OPSs but I can't help thinking it must be possible to beat the PSRR of a diamond by cascoding the output devices. Maybe in the style used in the beta22 amplifier (which apparently sounds very good). The schematic of that is here - The β22 stereo amplifier
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Old 1st July 2015, 08:17 AM   #3
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Hi abraxalito,

... thanks for your suggestion - which also made me realize that I need to add to my first post that it should also be NGFB ... sorry for the confusion

BTW, the stage you suggest isn't single-ended as far as I can see ... or might I have missed something here?

Have a good day

Jesper
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Old 1st July 2015, 10:40 AM   #4
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Not sure what you mean by single ended VAS. Do you mean bootstrapped, rather than using a constant current source or single transistor rather than so called "balanced VAS" configuration?
Here's link to a well known catalogue of basic types that I would call "single ended" if you needed to distinguish from anything with differential outputs etc: Distortion In Power Amplifiers. See section 5.2.2
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Old 1st July 2015, 11:12 AM   #5
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Quote:
Originally Posted by gentlevoice View Post
i
BTW, the stage you suggest isn't single-ended as far as I can see ...
Now its my turn to be confused. You asked for 'classA/B' but that can only be push-pull, not SE. SE would be pure classA.

So did you really want a single-ended output stage?
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Old 1st July 2015, 03:36 PM   #6
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@abraxalito:

Quote:
Now its my turn to be confused. You asked for 'classA/B' but that can only be push-pull, not SE. SE would be pure classA.

So did you really want a single-ended output stage?
:-) ... this communication ...

No, as I write in #1 the VAS will be single-ended whereas I would like the output stage to be push-pull (or the like) and class A/B. So what I need preferably is a schematic for a SE-to-PP configuration ...

@Ian Finch: It's are very basic VAS stage with an NPN & a PNP transistor working counter to eachother voltage-wise. I just need to connect it to an output stage - basically it's a good biasing configuration & SE-to-PP I'm looking for.

Hope this makes it clearer ...

Cheers,

Jesper
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Last edited by gentlevoice; 1st July 2015 at 03:40 PM.
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Old Yesterday, 09:14 AM   #7
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OK so, it's an AB class amp with typical Current Source VAS. This may simply mean a generic amplifier but NGBF would make it very rare, if it could perform with acceptably low noise and distortion whilst sounding as good as people may expect. However, if it were that simple, it would have been done to great commercial success long ago.

It would be helpful to know what the input stage was to be too. The gain and local feedback in the front end play a large part in determining what is possible here.
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Old Yesterday, 02:01 PM   #8
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Hi Guys

The standard Lin-Thompson topology used by Self and everyone else falls into the description Jesper provided: asymmetric front-end with single-ended VAS driving a push-pull output stage. The VAS is single-ended by virtue of the fact that only one side is signal-driven - the current-source load for it is not signal-driven.

As far as an output stage in isolation, used in a non-global-feedback amplifier, check out Lineup's power buffer. It uses a diamond internally for gain correction and offset control. The thread shows the use of mosfets or BJTs for the output. In his amp, he has a separate front-end that has feedback around it, driving the separate power buffer. New Lineup IDEA - Power Follower/Output stage

Note that the lowest THD will be attained if the output stage is within the feedback loop. Self's Compact blameless has single-digit ppm THD using output-inclusive compensation, aka transitional Miller comp. Just 17 devices including protection, which he counts as 13 devices... You can see the circuit and data for that on his site Signal Transfer Company, or in his book.

Personally I prefer complementary-symmetry front-ends and outputs.

Have fun
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Old Today, 08:16 AM   #9
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Hi Ian & Struth (remember we've met here before but don't right now have your name present) ...

Thanks for your feedback - I won't, however, have time to look at it today but will get back on Monday ...

Have a good weekend when you get to there ...

Cheers ;-)

Jesper
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