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Old 13th April 2011, 04:56 PM   #11
dadod is online now dadod  Croatia
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Question for Hugh and OS!
You probably notice tha I use local feedback on VAS, that bridge over TMC(1meg//5pF).
It decreases open loop gain at low frequences from more then 100dB to litle less then 60dB and kip it up to 30kHz.
Some says that there is no benefit in it, but I coud not resist but to tray it.
What is you oppinion??
dado
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Old 13th April 2011, 08:07 PM   #12
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I saw it , (1meg/5pF) . It's NOT lead compensation. It would have a minor effect on both the TMC transition and the unity gain point. Being INSIDE the miller loop it would be most likely overwhelmed by the main Cdom. ???

PS. It might not have much more effect than a couple very long traces on a PCB.

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Last edited by ostripper; 13th April 2011 at 08:12 PM.
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Old 13th April 2011, 09:28 PM   #13
dadod is online now dadod  Croatia
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First diagram is open loop gain without R24//C23 and second one is with R24 in place.
Influence is not so small. Resistor is VAS local feedback and capacitor to tame the 20dB pick at 28kHz.
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File Type: jpg DADO-TMC_NJFccs-OLG.jpg (120.2 KB, 493 views)
File Type: jpg DADO-TMC-Ronly_NJFccs-OLG.jpg (117.7 KB, 480 views)
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Old 13th April 2011, 10:07 PM   #14
jcx is online now jcx  United States
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I can't see the rational for cutting loop gain at audio frequencies - the very complete follow up analysis of Otala's ideas show his prescription of flat audio frequency loop gain is not in fact "the cure" for TIM/SID/FM IMD
the analysis and hardware measurements showed that high/"sloping" loop feedback can be used freely with other circuit modifications that do minimize these "time/phase" distortions
Walt Jung and Marshall Leach both published amp designs incorporating the flat audio frequency loop gain characteristic - and then later published that they had changed positions and determined that high feedback did not worsen Otala's proposed distortions

the sole "reason" to try flat audio frequency loop gain is the anecdotal comments by people who freely state that DBT is too "insensitive" to reveal differences in listening tests of amps designed according to their "principles"

I don't know of any reason to worry over loop gain peaking - although it is "inefficient" in the Bode Integral feedback limit analysis for a broad working band
but it is certainly minimized by allowing the gain to be large with multiple slopes in the audio frequency range

I would consider guarding/shielding the mirror out/VAS buffer end node just to maximize loop gain

Last edited by jcx; 13th April 2011 at 10:20 PM.
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Old 13th April 2011, 10:49 PM   #15
AKSA is offline AKSA  Australia
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Dadod,

Very high loop gains require more lag compensation to pull back to unity loop gain by the upper pole frequency. This gives a very clean sound, but can sound somewhat artificial. I find there are benefits in reducing loop gain with VAS local feedback too, but you can also use nested feedback to achieve the same thing. This will have the benefit of lowering VAS output impedance, which in turn reduces voltage distortion to the output stage. I designed an amplifier with only 16dB of global feedback using a double EF output stage and the sound was punchy, alive and powerful, quite the opposite of what you would expect.

I agree emphatically with OS about the complex interaction with the VAS, OPS and speaker giving good bass. It is not solely a function of damping factor. A speaker is a complex active load, and the back emf fed directly into the feedback loop causes a reducton in the image depth.

A good means of decoupling the VAS from the OPS is to use a small mosfet as the VAS, since these are fast devices, and completely isolate the gate from the drain and source.

If you are thoroughly adjusted to your source and speakers, then new amps will be easy to pick. While I agree with OS about a simple source, I do feel that you can easily 'hear' a new amp if the rest of your system remains unchanged and you are well used to it.

As JCX said, you should strive to keep loop gain consistent across the audio band so that distortion at the high frequencies is held to similar levels as at low frequencies. This is quite difficult but I get the feeling TMC helps.

Current swings in the VAS are not too troublesome, because they promote asymmetrical distortion, which is almost completely even order. It is odd order you have to be careful about. Moreover, bass sounds better with a little H2 mixed in; it wettens it, adding a bit of decay, which is musically more satisfying. Many people prefer tube bass for this reason; decay is important and militates against a super high damping factor.

Thank you for sharing your design,

Hugh
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Old 14th April 2011, 10:51 AM   #16
AndrewT is offline AndrewT  Scotland
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This is the first time I can recall seeing bootstrap+CCS feeding the VAS.
I will be following developments.
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Old 14th April 2011, 11:14 AM   #17
jcx is online now jcx  United States
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Default flat loop gain == rising distortion with frequency

Quote:
Originally Posted by AKSA View Post
Dadod,

...As JCX said, you should strive to keep loop gain consistent across the audio band so that distortion at the high frequencies is held to similar levels as at low frequencies. This is quite difficult but I get the feeling TMC helps.

...Hugh

I must have been too indirect - that is nearly the opposite conclusion from how I interpret the research, theory, practice

there are several distortion mechanisms that rise with frequency - in fact they cause rising phase shift nonlinearity with frequency too - independent of loop gain slope - so for flat loop gain amps the distortion does rise with frequency
with higher loop gain the distortion rises to the same level that it gets to with the same amp "flat loop gain compensated" - but the distortion at all frequencies below the corner is lower

I would point to Self, Cordell and even our low/"no" feedback designers like Pass and Curl here to suggest that the output stage looks pretty feeble - even at 100 W parallel output devices are a good idea for multidriver dynamic loudspeaker with possible 2 Ohm impedance dips due to bad XO design

and triple EF seems like a requirement to avoid load impedance and Q beta nonlinearity from limiting VAS performance

when those measures are in place I think the "back emf"/complex load arguments are way less relevant
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Old 14th April 2011, 11:22 AM   #18
Dave S is offline Dave S  United Kingdom
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Quote:
Originally Posted by AKSA View Post
Moreover, bass sounds better with a little H2 mixed in; it wettens it, adding a bit of decay, which is musically more satisfying. Many people prefer tube bass for this reason; decay is important and militates against a super high damping factor.

Thank you for sharing your design,

Hugh
There are two points here:
1) "High damping factor" is a bit a misnomer IMO, low output impedance is a better description. The problem with low output impedance is potentially an under-damped response to reactive loads and back EMF. Poor reactive load transient response combined with poor transient response from ported speakers systems means poor bass and timing. A bit of damping achieved by adding some output resistance can help here.

2) "a little H2 mixed in":- I cannot agree with this. If this does indeed add "decay" as you suggest, then surely it's going to mess up the timing (which incidentaly is the reason I sold my AKSAs). I would advocate getting some speakers with accurate bass.
Or maybe people don't actually want bass that starts and stops properly?
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Old 14th April 2011, 11:47 AM   #19
AKSA is offline AKSA  Australia
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Quote:
"High damping factor" is a bit a misnomer IMO, low output impedance is a better description.
David, this is semantics, superficial nonsense. You know as well as I do that damping factor is the ratio of load to source impedance.

Quote:
"a little H2 mixed in":- I cannot agree with this.
Again, you know well that SETs inject a great deal of H2 and that this is indeed enjoyed by many audiophiles. Adding some output resistance simply hikes up output impedance, lowering damping factor and achieving precisely what I was alluding to.

Quote:
(which incidentaly is the reason I sold my AKSAs).
You mustn't kick yourself for your apparent 'poor judgement' buying the AKSA, which you obtained by crafty negotation anyway. I would hope you sold it for a good price! This is a cheap shot, and completely unsolicited.

Quote:
I would advocate getting some speakers with accurate bass.
More gratuitous criticism, really, why don't you use the forum for its intended purpose, to swap information of genuine use to other people?

Hugh
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Old 14th April 2011, 11:49 AM   #20
wahab is offline wahab  Algeria
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Quote:
Originally Posted by jcx View Post
I would point to Self, Cordell and even our low/"no" feedback designers like Pass and Curl here to suggest that the output stage looks pretty feeble - even at 100 W parallel output devices are a good idea for multidriver dynamic loudspeaker with possible 2 Ohm impedance dips due to bad XO design

and triple EF seems like a requirement to avoid load impedance and Q beta nonlinearity from limiting VAS performance
Agree for the necessity of a beefed OS , but triple EF
is quite annoying on the stability front...

As a cure , lateral fets OPS bring lots of benefits and very few
drawbacks if any...
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