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Old 27th October 2004, 09:25 PM   #1
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Default class A single ended

I have a project in my mind...
it is a single ended pure class A mosfet amp with DC-feedback- no transformer then...
I do fear about output impedance, not very much sure about ocsilation issue.
Is it a good idea to avoid DC at output using an integrator???
What do you think??
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Old 27th October 2004, 09:31 PM   #2
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simulation results seem stange to me - very low distortion in an amp without global feedback ???

by the way , i saw once an amp project with a huge cap (4700u or someyhing) in serial to speaker- can this be stable???

... much confusion...
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Old 27th October 2004, 09:40 PM   #3
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schematic not zipped
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Old 31st October 2004, 11:31 AM   #4
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anyone??
I feel ignored....
considering unsubscribing this forum

no suggestions nor comments
of just antypathics...??
you make me sad
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Old 31st October 2004, 11:44 AM   #5
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No, you are not being ignored.

As I post this, 223 people have seen this thread.

I know very little of solid state to speak of so I cannot comment, though I do want to learn.

Perhaps you could ask for some more specific critique of your design to elicit a response?

As for a large electrolytic 4.7mF (4.7milli-farad) capacitor in series with the output, it is used to keep DC from the speaker in some designs where the offset is not likely to be stable, or there is a large DC offset at the output as part of the design e.g. original JLH, ESP DOZ etc. As you made that schematic you probably already knew this. In regards to stability, I have absolutely no idea, apart from the fact that they have, and are used in established designs.
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Old 31st October 2004, 12:45 PM   #6
MikeB is offline MikeB  Germany
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Quote:
Originally posted by darkfenriz
simulation results seem stange to me - very low distortion in an amp without global feedback ???

by the way , i saw once an amp project with a huge cap (4700u or someyhing) in serial to speaker- can this be stable???

... much confusion...

I have no experience with singleended classa, how low are your
distortions ? Having no global feedback does not necessarily mean
high distortion. There is a reason why singleended classa is a good thing.

About the huge cap in serial to the speaker, as it is serial it does not
change stability. It does not create a capacitive load, and as it is
very big it's only blocking very low freqs. Don't forget, the crossover
in speakers have "big" cap in series with the tweeter (~4.7uf).

Mike
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Old 31st October 2004, 04:53 PM   #7
johnnyx is offline johnnyx  United Kingdom
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A cap to ground on the inverting input, C5, will cause trouble. It makes an oscillator. Replace it with a resistor, and use C4, forming an integrator, to remove the signal from the servo control.

Why don't you build it and see?
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Old 31st October 2004, 08:02 PM   #8
markp is offline markp  United States
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Quote:
Originally posted by darkfenriz
simulation results seem stange to me - very low distortion in an amp without global feedback ???

by the way , i saw once an amp project with a huge cap (4700u or someyhing) in serial to speaker- can this be stable???

... much confusion...
I've built single ended amps with caps that big on the output and have had no problems with stability. They were just single stage monsters with giant 300watt resistors in the collector and 50watt in the emitter. Full class-A room heaters!
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Old 31st October 2004, 09:09 PM   #9
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Quote:
Originally posted by markp

I've built single ended amps with caps that big on the output and have had no problems with stability. They were just single stage monsters with giant 300watt resistors in the collector and 50watt in the emitter. Full class-A room heaters!
That's how we like them.

To address the original question, one of the reasons we bother
with single-ended class A is that it allows such good performance
with such simple circuits. In your case I see three gain stages
where you could conceivably get away with one, and lots of
current source regulation where a simple voltage reference
driving the gate of the Mosfet might do.

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Old 1st November 2004, 05:54 AM   #10
Ori is offline Ori  United States
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Default Comments on the amp circuit

First stage is statically biased. This may simulate well, but will change quite a bit with operating conditions.
You buffered the input with the follower stage, but it represents 29K impedance - not exactly high input Z.
Same comments about the bias of the voltage-gain stage. Idealy you will DC-couple the first two stages and work out a DC-servo to maintain the bias, or, as Nelson Pass said, drop one stage.
The output stage is again capacitively coupled, when you could DC-couple it (and change the DC-servo to global).
Q1/M0 circuit is again staticaly biased. I suggest that you check Mr. Pass' Zen (DIY project) and Aleph (commercial amp) schematics for a good starting point. You may not like the (harmonic behavior of the) differential input of the Aleph, but check it out anyway for the simplistic DC-coupling and look at the original Zen amp for a more predictable current source. With the Zen output stage you'll need a DC-servo, so that would be a nice contribution, which many people may like to clone!
Have Fun!!!
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