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Old 9th August 2005, 07:42 PM   #1
Tim__x is offline Tim__x  Canada
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Default LTP+CFB Headphone Amp (spiced)

My idea is fairly simple, Q1-6 plus CCS's Q11 and Q12 form a fairly standard LTP. What makes this amp different is that instead of the standard EF or CFP output stage, Q7-10 plus CCS Q13 form a CFB amplifier canceling most of the crossover distortion before global feedback.

Sims give distortion below 0.000001% (mostly 2nd) THD at 1Khz, 1V into 32 ohm load. I know sims can give distortion several orders of magnitude better than real life, but even if that is the case performance should be good.

The design could be further improved with better CCS's, R6 could be replaced with a CCS, better biasing, maybe even a cascoded LTP.

It's fairly stable, though in real life it would probably need heftier miller comp.

Thoughts? Critiques?
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Old 9th August 2005, 07:43 PM   #2
Tim__x is offline Tim__x  Canada
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Default THD FFT

.
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Old 9th August 2005, 07:45 PM   #3
Tim__x is offline Tim__x  Canada
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Default IMD FFT

4Khz 1V 7Khz 4V
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Old 9th August 2005, 07:57 PM   #4
Allexx is offline Allexx  Ukraine
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Gain=1?
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Old 9th August 2005, 08:15 PM   #5
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Quote:
Originally posted by Allexx
Gain=1?
Achieved with six stages

What is biasing of output stage?
I guess it is class AB, please tell me why don't you run it in class A?
I never understood why one should run headphone discrete amp in class AB.
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Old 9th August 2005, 08:22 PM   #6
Tim__x is offline Tim__x  Canada
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For now, yes. It's a trivial change to increase gain, simmed THD with 40db of gain (again, 1khz 1V into 32) is 0.000012%.

AC and DC impedances as seen by the LTP should be balanced as much as possible, because at this distortion level those have a significant effect.

Q1-Q2 should be matched, as should Q5-Q6 , R1-R2 and R4-R5.

Lowering Iq does not raise distortion measurably until below 1ma.

Please keep in mind that this is still all just simmed.

It's in AB for two reasons
A: so it could be battery powered
B: I think the topology could be used for power amps where class AB does have significant benifits.

P.S. Come on, six stages is nothing, people use opamps as buffers all the time.
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Old 9th August 2005, 08:28 PM   #7
Duo is offline Duo  Canada
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Lol, aside from gain being 1, which is easy to change, I like this amplifier idea.

It has given me some design ideas of my own for reduced output stage distortion using more complex ideas.
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Old 9th August 2005, 08:39 PM   #8
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nice than



for one 6 stages are nothing, another will use cathode follower as only active device, just a matter of taste.

I think proper compensation might be an interesting issue here.
You use two common emitter CCS-loaded stages. I think the first one is pretty fast (without Cmiller) because is driven by a low output Z buffer, so natural miller efeect is redused here, it is maybe not the best place for dominant pole.
The second local loop probably does not need any inner compensaion I think.
Just thoughts...
regards
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Old 9th August 2005, 08:42 PM   #9
Tim__x is offline Tim__x  Canada
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I must admit, it's not that fast, GBW is around 10Mhz, but the the slew rate is only a few v/usec, adequate for audio, but it does keep the design in it's current form from getting spectacular IMD numbers.

Edit: removing the CFB compensation causes that loop to oscillate.
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Old 10th August 2005, 03:35 AM   #10
thanh is offline thanh  Viet Nam
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Hi Tim_x ! I like low distortion . your numbers is very interesting
What is the function of Q7 ? how does it operate ?
Thanks !
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