I will study your changes @minek123
Have downloaded your schematic.
Why did you change the VAS current?
This is the squarewave with 200nF across load with the original Cello circuit
It is 2Vp-p and a Zobel filter in place.
It seems to be stable but with overshot.
It is 2Vp-p and a Zobel filter in place.
It seems to be stable but with overshot.
Reasons for lower LTP bias current are
- self heat up giving thermal drift
- lower base currents reduce dc offset errors
- decreased input noise current
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1) See D. Self's book page 163, 6th edition
Optimal VAS current is determined to be between 6-10mA
2) See this thread https://www.diyaudio.com/community/threads/vas-requirements-for-mosfet-ops.404529/post-7484758
Same conclusion as above
3) From my experience, higher VAS current USUALLY, with some limitations/exceptions, makes amps
a) more stable
b) faster
In most of my simulations, I was able to get the best results with VAS current 10mA and higher.
4) Most of amps I've seen, with 1 output pair (even laterals), have VAS current over 6mA
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Thanks to @minek123 I have redesigned Cello One.
It is among other things a completly new TMC compensation.
Also changed currect in LTP and VAS.
Added a Zobel filter.
The result is to find in my first post in this thread:
https://www.diyaudio.com/community/...att-with-tmc-and-laterals.406282/post-7526480
It is among other things a completly new TMC compensation.
Also changed currect in LTP and VAS.
Added a Zobel filter.
The result is to find in my first post in this thread:
https://www.diyaudio.com/community/...att-with-tmc-and-laterals.406282/post-7526480
Was the #62 waveform acquired at point T? If so, it may be a damped vibration (f=1/(2π√(5uH*200nF))=159kHz) due to LC resonance.
To check stability, it is necessary to observe the ”OUT” point.
If the output hits the positive rail and becomes saturated during excessive input, excessive current can flow through U10 and the bases of U6.
I recommend adding an overcurrent protection circuit.
The resistor added to the emitter of U6 should be about half the value of R3.
It might be disputable, but my impression was that all sims should be done without Thiele LR network (especially squares).
Some amps don't sim/behave well without LR, and designers say that LR is REQUIRED for proper behavior; I wouldn't trust such
amps. In my eyes LR is only for 'emergencies' (like protecting amp from voltage feedback from speaker under certain conditions).
Amp should sim well without it, especially with simple speaker models (R only, or RC only, no L).
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While at it, Baker clamp diode (BAV21) can be also added.
See https://www.diyaudio.com/community/threads/unusual-amp-from-1987.357369/post-7068145 for an example.
Please use scope time division like 10us/div or 20us/div. I do not understand why you use 200us/ div, such setting masks any details and shows nothing on stability.
In one amplifier of mine (on the drawing board), I have no degeneration on the VAS and therefore use another method to limit the base current during clipping: add collector resistance to U10 (buffer / beta enhancer).
What do you consider a safe limit for base current on a 2SA1209 for example? Datasheet does not specify it.
I was wrong here. Actually when testing capacitive loads, RL Thiele network helps a lot.
Of course it would be better if amp can drive 1uF load without it, but it's rather not viable proposition for most of amps.
https://www.diyaudio.com/community/...load-capacitance-how-much.145249/post-1845424
https://www.analog.com/en/analog-di...to-avoid-instability-capacitive-loading.html#
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H
HAYK
2m of low cost speaker wire I measure 400pF. PA amps are tested with 1uF, 100nF for HIFI.
