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Old 5th March 2005, 11:18 AM   #91
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Default Graham, MikeB told me 100mA the quiescent used.

that's what i can remember....of course he may changed recently.

His amplifiers sounds good...very interesting amplifier

Carlos
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Old 1st April 2005, 06:18 PM   #92
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Hi Mike,

I'm wondering how the 'NoCap' circuit is working out for you

Through attempting to optimise my class-AABB output stage with a folded cascode splitter/driving circuit like yours, I have noted that matching the output stage to the high impedance common base cascode stage is most important. I did not copy your 330 ohm cascode resistor value, but found that this is best for open loop phase linearity and control.

I also noted that when setting output stage bias to minimise the levels of simulated high odd harmonic distortion when driving the virtual 'Ariel' loudspeaker, the resistor measured thd could subsequently not better 0.1% at all AF frequency/levels.

Many designers (though not all) aim for circa 0.002% at say 20kHz and use complex nfb arrangements to acheive this, but then nfb loop induced internal current draws due to asymmetrical music waveforms and crossovers/dynamic loudspeakers can simultaneously cause fractionally asymmetrical high speed storage/discharge current effects within an amplifier due to the additional 'stabilising' components that become necessary. This is why I recently showed some of the asymmetrical current draws in my AABB thread, and of course a 'NoCap' amplifier cannot fail in being less afflicted.

In my circuit I keep a higher 1A standing current so that the (low delay) nfb loop assisted output stage will more phase linearly resist crossover/loudspeaker back emf induced deflection wrt on-going signal amplification. The ratio of cascode to fixed bias at the output stage can be optimised for extremely low virtual loudspeaker induced harmonic generation, though still without a good resistor loaded specification being measurable !

I also found it necessary to use a 10uF/72k/10pF compensation network (values still need to be checked in real life) on the static negative driving 2N5401 cascode, to compensate for internal feedback within the dynamically operating positive driving 2N5401 cascode, this to balance internal cascode transistor losses and minimise harmonic distortion.
Something like this might further balance your own circuit; a check may be made for virtually identical output stage half current waveforms and peak +/- voltages at say 10kHz with a resistor load.


Hi Jan,

Re your Post#24 question and series output choke inclusion ... I prefer to NOT use them. I also prefer to site power amplifiers as monoblocs directly behind the loudspeaker using short low capacitance leads.
Choke inclusion at the ouput of a global nfb loop controlled amplifier can be audibly detectable, and via the X-Y Monitor test I posted in Wimms' Distortion Microscope I illustrated the electrically measurable effect from a 6uH component.


Mike, are you driving capacitive loads such that you must use a choke ?
2uH is not much, but it might still be worth trying short circuit switching it in and out of circuit to see if you can hear any change to transient and sibilant clarity. However the presence of a series output choke will not necessarily show up on all loudspeakers/crossover/cable systems, and maybe even less so on this low nfb delay design.

Look forwards to hearing of any new developments.


Cheers ............. Graham.
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Old 2nd April 2005, 03:05 PM   #93
MikeB is offline MikeB  Germany
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Hi Graham !

I already "discarded" this design, it had too much 3rd harmonic distortion
for my taste, giving some unpleasant sound. It was a very interesting
circuit, and prooved that "fast" amps with small phaseshift deliver
better dynamics and details. In this discipline that circuit was very
impressive. But the overall sounding was not ok.
The last weeks i have been busy with "sniffing" into ClassD, so this
amp just collected some dust.
Cany you show your "10uF/72k/10pF compensation network" in schematic ?

I have another circuit in mind, having even less phaseshift and
more simple, but having low thd, mainly 2nd harmonics.
I will try this one today... (A quasi-complementary)

My problem with achieving low thd is, that it is mostly done by
increasing feedback / openloopgain, handicapping the amp with
large feedbackstabilizingcaps, creating extra large phaseshifts, or
even worse, large dynamic phaseshifts, ruining the whole sound,
just to be able to reproduce the perfect sinewave.
I think that this is the reason why most good sounding amps have
not very low thd. They sound good inspite of thd, not because of... (IMHO)

Mike
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Old 2nd April 2005, 09:49 PM   #94
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Hi Mike,

That compensation circuit is on the circuit in my class-AABB thread. I have yet to investigate temperature compensation. PS. The top half choke should be 22uH and not 150uH.

In your second ( Darlington only ) circuit, the rightmost 2N5401 has degenerative internal feedback due to it having a swinging collector potential.
However the left 2N5401 has a fixed collector voltage and thus no internal feedback.
This may be compensated by a low 'C' of 4.7 to 10pF between the amplifier's output at the live end of the choke and the emitter of the left 2N5401. In parallel with this fit a resistor of probably about 120k in series with a 2.2 to 4.7uF dc blocking capacitor.
This will balance the dynamic characteristics of the inverting cascode transistors, and can further reduce low level harmonic distortion.
Maybe you could try simulating it first.
I've just tried simulating my circuit without the compensation, and the distortion goes from 0.02% at 80Vp-p 10kHz across the virtual loudspeaker, to 0.07%. (160kHz measuring bandwidth)

Simulation of my proposed circuit shows the distortion characteristic to be mostly third, and circa 0.01% when driving the virtual loudspeaker load at about 15Vrms. But third harmonic is what you get as soon as you go push-pull beyond class-A anyway, and this is why I am using a 1A quiescent for better quality at more normal listening levels.

I quite agree with your last paragraph, and like you I don't want to use too much nfb, but with class-D are you going to get the same imaging accuracy and transient clarity when the use of significantly valued series output chokes are so unavoidable, such that their effects vary independently on a per channel basis?

Another point regarding your 'NoCap' testbed relates to amplifier input resistance between the first base and the signal source.
I have for long now prefered low impedance amplifier drive via quality TV downlead coax and BNC connectors or filed out metal RCA phonos.. Your amplifier might well distort a little less if you do away with the 'input filter' which is also effectively in series with the first base as far as nfb loop control is concerned. The forward gain will not change, but the AF nfb response does improve slightly, and when compared to the 2k maybe by 2 to 3dB.

Another thing I prefer, is to use a resistor in the differential tail because its value tends to be so very high when compared to the generated differential emitter impedance. A current source can lead to unusual effects on transients and at high output.
I have just simulated my circuit with a current source in place of the passive biasing resistors; the simulated distortion went up from the 0.02% figure to 0.055% with some seriously undesirable cross-conduction on 10kHz sine, which in your circuit would attempt to raise the bias voltage on a per half cycle basis.
It should be very easy for you to work out the correct value for a fixed resistor and substitute it for listening tests.

Thus I am hoping that discarding your 'NoCap' design, has been only temporary.


Cheers ............. Graham.
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Old 23rd July 2007, 05:56 PM   #95
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Anybody looks at NAD 304 schematic (easy to find)?
It's very similar design.

And also NAD3400
(hard to find, it's similar topology, but more cascodes, and
R321, R327 are replaced by current sources. I found only
schematic of power amp in some polish electronic magazine)
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