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Old 29th March 2012, 01:35 PM   #11
Bonsai is offline Bonsai  Taiwan
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What makes you say that?
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Old 29th March 2012, 01:54 PM   #12
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The cascode reduces miller effect which is true but it doesnt reduce the intrinsic Cob of the vas, the capacitive effect is simply transfered to the cascode device where alas it is smaller but now you have darlington and twice the Cob in the vas. The lower output impedance of darlington will however have lower distortion but this only in the low frequencies, when Cob comes into play at high frequencies it will be inferior. I guess its a poison to pic. If your darlington has perhaps infinite hfe I guess at some point the impedance will be the determining factor. Maybe a darlington where the first transistor is a cfp would swing it in favour at high and low frequencies.
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Old 29th March 2012, 02:46 PM   #13
jcx is offline jcx  United States
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It is important to keep in mind there is no single “optimum VAS” – the rest of the amplifier circuit determines if some “enhancements” have any impact or are swamped by source/load effects

Self (latest edition), Cordell’s Audio Power Amplifier books are great resources

The Groner http://www.sg-acoustics.ch/analogue_...n_comments.pdf looks at a couple of Self’s book’s suggestions

http://www.essex.ac.uk/csee/research...%20cascode.pdf is essential to understand “cascode” variations – even if Hawksford didn’t invent or properly cite the origins of the circuits he shows

If you use current mirror on diff pair collectors, triple output stage for low VAS loading then VAS improvements like beta enhancement, cascoding can have visible effects

“beta enhancement” can be done with a follower between mirror and VAS Q, or similar effect can be had with small signal MOSFET VAS – which then needs cascoding

“proper” cascoding requires VAS degeneration R, returning/recycling the common base Q base current – well explained by Hawksford

really high gain, linearity VAS then deserve 2-pole compensation to make use of the gain to linearize more of the amplifier – “TMC” is an option but traditional 2-pole splitting of the Miller C gives largely equivalent results and greater improvement of input diff pair linearity

Cordell's recommended THD @ 20 kHz can be more revealing of AC linearity limitations than traditional 1 kHz test

Last edited by jcx; 29th March 2012 at 02:49 PM.
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Old 29th March 2012, 02:53 PM   #14
Bonsai is offline Bonsai  Taiwan
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jcx, yes I recognize quite a few of the valid points you raise.

Homodder, the beta enhancement devices are small signal and should be selected also for low Cob. However, since they are usually run into the cascode, their collector base voltages are held constant. Thus, I do not see the Cob issues you raise: only the cascode Cob remains the issue, for which there are a number of fine candidate devices.

With this construction, the very high local loop gain coupled to the miller (or TMC) comp around the VAS stage means you can get non- linearity down to very low levels in my view.
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Old 29th March 2012, 03:12 PM   #15
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I agree Jcx, its not such a straightforward comparison but with same outputstage beta enhanced vas offers more simply because it hits harder where its needed, its simple to have low distortion at low frequencies but where it matters most is what happens after 1 khz where in many designs the distortion soars. Some say it doesnt matter at these high frequencies but they dont take into consideration what happens to IM distortion when you mix a 10 Khz and a 300hz signal.

The TPC TMC comparison has been beaten to death, but Edmond is right, TMC has more benefits and it is more effective than TPC although by not much, however it reduces the distortion where its most needed, in the outputstage.
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Old 29th March 2012, 03:23 PM   #16
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Having the base collector voltage held constant doesnt remove the intrinsic Cob, this can not be removed in any way with circuitry, its device dependent. What cascoding does is remove the miller multiplying effect. With a darlington the two devices s intrinsic cob add up, not so when the beta enhancer is grounded.

You can try this with sim and see the effect.
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Old 29th March 2012, 03:40 PM   #17
jcx is offline jcx  United States
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I don’t think the proponents of TMC give the stability issue enough emphasis – in fact I suspect most are ignorant of the potential gotcha in TMC loop stability from the positive feedback around the output stage

TMC is potentially dangerous in inexperienced hands - the "single pole" global outer loop gain gives well damped step response - which is in this case insidious - since it doesn't give the expected stability margins we associate with the damped step response from dominant pole compensated amplifier most are familiar with

also the intercept frequency measured in the global loop is again not as indicative of stability margin as usual for the true single dominant pole case despite appearances - so eyeball or rule of thumb selection of TMC compensation frequency can lead to dangerously low stability margins

Bob Cordell's Power amplifier book

Last edited by jcx; 29th March 2012 at 03:43 PM.
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Old 29th March 2012, 07:23 PM   #18
tvrgeek is offline tvrgeek  United States
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I was playing with the spice model of a "mx-50" rip off from China. The e-bay schematic included a darlington VAS, what I got did not. Putting the darlington back in lowered distortion by 30 dB. I modeled a cascode but it was not as effective. This is only by simulation, not by ear. I bought these things so I would have a cheap amp to muck with. Cheap parts, cheap design, cheap price. I got the value I paid for, but I wish it had the darlington so one less hack to the boards.

What I have learned in the simulation, is small details make really big differences. Just topology does not guarantee performance. My Rotels have a simple single transistor VAS and sound just fine.

The gentlemen who have been helping my understand amplifiers would probably say something about a fully complementary cascode darlington with current mirror ccs and boosted supply rails, or that the input tube LTP can drive the KT88's just fine, thank you.
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Old 29th March 2012, 07:35 PM   #19
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I use a bog standard NPN with a resistor and preset in the base circuit.
However one trick I learned from blown up amps is to put a zener across the VAS to ensure the voltage cant go open circuit. Care needs to be taken tho, too small a zener and you could get crossover distortion and too high a value will blow up the output transistors.
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Old 29th March 2012, 07:45 PM   #20
godfrey is offline godfrey  South Africa
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@ Nigel:
It sounds like you're talking about Vbe multipliers for output stage bias, rather than voltage gain stages.
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