Lavaradin Amp and "Memory distortion"... - diyAudio
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Old 6th June 2001, 01:26 AM   #1
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First of all, in case you haven't seen it yet:

http://peufeu.free.fr/audio

Now, has anyone worked with these ideas or tried the famed Lavardin amp? The whole concept makes sense, and the article is backed by a bunch of measurments. Any comments?
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Old 6th June 2001, 03:47 AM   #2
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Mark Finnis discovered that one a while back and posted it in a thread called (I think) Son of OptiMOS. I read it, and found it fascinating, but to my recollection, no one had ever heard the amp, nor had they even heard of it. One of these days when I have nothing better to do (which means I won't get to it for another century or two at the rate I'm going...), I'm going to take a whack at a circuit incorporating those ideas, as it seems to make sense.
If anyone has fiddled with it since then, by all means 'fess up.

Grey
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Old 6th June 2001, 04:31 AM   #3
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Thanks, i'm checking the Opti-MOS already!
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Old 6th June 2001, 03:18 PM   #4
Asen is offline Asen  Bulgaria
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I hate critiques without explanations but take a look at this oppinion:
http://www.audioasylum.com/audio/gen...ges/21356.html
I myself came accross the Lavardin site long time ago and find it interesting. From what I've read so far in Audio magazines - Lavardin is the amp chosen from JM-Labs to represent their speakers.
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Old 6th June 2001, 06:22 PM   #5
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Thanks for this posting I read it with great interest.
This seems (to me anyway)to be an endorsement for Doug Self's work.
I'm looking forward to hearing more about it.
Phil.
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Old 6th June 2001, 08:21 PM   #6
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Default Memory

Discussed this with several people. Does make for a nice technical sounding piece to read.

Have nobody I know of, or even know of as a reliable source, who has compared anything built this way, so don't even have what I consider a "respected opinion" that this is making a difference. Is not unusual for person modifying to have very good opinion of changes. As feedback is MUCH, MUCH quicker than any thermal drift could be, should be corrected out with no problem at all. Would like to see and hear such an amp, but at this point don't have anything to judge by. Sounds great, but so does clamp on gizzmo that gives you 100 miles per gallon in your car.

Regards,
Greg

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Old 6th June 2001, 09:28 PM   #7
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Default re: memory

It may be of interest that Hugh Dean makes brief mention of Lavardin's work when discussing his AKSA design here:
http://www.printedelectronics.com/pe..._FAQ2.htm#Tten
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Old 7th June 2001, 12:46 PM   #8
jam is offline jam  United States
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I am sure that memory distortion is an important factor in determining the sound of an amplifier but in all fairness cascoding a circuit has other benefits such as lowering Miller capacitance and increasing the bandwidth of the circuit which in turn means lower distortion.
We should be careful not to attribute all the claimed benefits solely to the reduction of memory distortion and that it is the sole criteria for a good design.

Jam
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Old 9th June 2001, 11:06 AM   #9
ppl is offline ppl  United States
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I have not played with this theory eather but i know if you blow air around an Amp's input the DC off set will change rather quick and this is surly alott slower than any Audio so i give the Idea credit and will play with it wen done my other stuff but not a high priorty right now for me so have to content myself by keeping a thermaly stable envioment.
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Old 6th April 2002, 09:36 PM   #10
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Default review of peufeu site

I stumbled on this very old thread and followed the link. Fascinating what this Frech guy by the name of Pierre has put together. I have seen worse monographs, and some of those even passed for master's theses in EE!

- He maintains that THD and intermodulation distortion measurements are meaningless because of the use continuous signals whereas the power envelope of music changes continuously.
- Changes in the power envelope cause nonconstant heating in the input and gain stage transistors- As V_BE changes, so do the bias points of all transistors and hence their distortion spectrum. The human ear appears to be pretty immune to constant harmonic distortion, but will perceive a changing distortion spectrum as unnatural.
So far, so good. In my opinion this
might already explain why some almost traditional circuit concepts sound particularly good, even if the same THD levels can be achieved without these goodies:
- cascoding input and VAS transistors helps a great deal in minimizing both power dissipation and its changes with the input signal
- sacrificing gain and hence global feedback for more local feedback will stabilize bias points; in particular, and this is a point overlooked by Pierre, emitter degeneration resistors will alleviate the effect of V_BE changes, the more the higher the voltage drop across the resistor is
- in symmetrical designs, i.e. with an NPN and an PNP input pair and an NPN and PNP VAS, the effects should compensate in first approximation

Pierre analyzed a conventional amp design with degenerated input pair, current mirror on the input pair, single transistor VAS (without emitter degeneration!) and complementary emitter follower output. He models the heat dissipation by a series of resistors and capacitors driven by a current source (this an an absolutely correct way to model heat conduction, I did a similar simulation for laser optics). The neat thing is that this current is proportional to V_CE x I_C. I don't know how that is done in Spice. Neither do I know how to convert the voltage that corresponds to the die temperature back into V_BE in the transistor model. But then Pierre uses Microcap which I am not familiar with.

He uses a 20 ms input transient that is constant at a high level slightly below the clipping level to offset the themal balance. The bias points need a couple of seconds to recover. He compares the voltage right after the end of the transient to the steady state voltage. This gives him an arbitrary measure for memory distortion. Switching the thermal model on and off for various transistors, he identifies the offenders (VAS, current mirror, input transistors, tail and VAS current sources in descending order).

This arbitrary measure is fine IMHO to optimize the circuit. But in order to really prove his point is valid in absolute figures, Pierre should show how the distortion spectrum is different right after the transient. First of all, he would need to get the absolute change in power dissipation right. Any musical signal, even a kick drum, will consist of a sum of damped sines and hence have no AC content. So the excursions to either side would probably be shorter than 50 ms (if the music is bandlimited to >20 Hz) and hence shorther than the thermal time constant of most transisor dies, resulting in some averaging.

In typical use with typical speakers, the music will be around 90 dB(A), so 1 W would be ok. This corresponds to 2 V_rms into 4 Ohms. So the distortion spectrum of a 1 or 10 kHz signal with 2 V at the output should be calculated. Then a loud drum or whatever could be simulated by a 500 ms 20 Hz sine burst at +20 dB relative to the 1 W signal. The distortion spectrum immeadiately after the sine burst should then be computed and compared to the original spectrum. If the 2nd harmonic changes from -105 to -107dB and the third goes up from -115 to -112 dB , then it will probably no be a relevant effect. But if there are 5 dB changes at -90 which might be possible in an average design, the whole idea of memory distortion would stand a high chance of being valid.

In terms of optimization, Pierre proposes to cascode all current sources and use pretty strong emitter degeneration on the current mirror (1k) - all fine with me.
For the input pair he compares a classic degenerated pair to cascoded, CFP and "new". For the cascode, he uses JFets to cascode bipolar input transistors - unusual enough! Also, he takes the input voltage for the Fets from the bases of the input transistors. I have never seen an input cascode like this and will have to think about it. I guess the gate-drain capacitance will result in a nice miller capacitance, completely defeating the tradidional reasons for using a cascode on the input, i.e. to get rid of nonlinear input capacitances. This nonwithstanding, the cascode helps to reduce memory distotion considerably by keeping the input transistors at constant voltage. This aim seems to be the reason for the weird cascode connection. In my eyes, it might be better to reference the cascode voltage to the emitters la Pass and Borbely.
Secondly, he tries to keep the input transistors at constant current by employing complementary feedback pairs as input devices. Again, I have never seen thelikes and will have to contemplate this. Reduction of memory effect is on par with the cascode.
Finally he combines CFP and cascode in the "new" circuit to keep voltage and power constant which results in yet higher reduction of memory distortion.
The simulations of input stage linearity (diff current as a function of diff input voltage) of both CFP and "new" look very good so I will probably use this in one of my designs once I have mulled over dynamic effects.

I don't find his improvement of the VAS too convincing. He takes the undegenerated (!) VAS and cascodes it - fine. Then he adds the "magic" resistor between VAS collector and cascode emitter. The basic idea is to compensate the parabolic power dissipation in the gain transistor by taking some of this power away to be dissipated in the resistor the value of which is chosen so that the power in the resistor becomes a tangent to the parabula. That is fine, only I think the resistor should be between emitter and rail which would help to linearize the VAS stage considerably.

Another point that one might derive from the fact that the time constants for memory distortion (if it is relevant) are on the order of 100 ms to 10 s is that it might be worthwhile to rethink the feedback and gain tailoring.

The GBW of a circuit is basically fixed by the output transistors and their drive circuit. One could opt for high open loop gain an begin to roll it off at very low frequencies. As this results in nonlinear phase of the amplifier with frequency, current wisdom has it that the open loop gain should be lowered so that the dominant pole can be at >20 kHz. If memory distortion is indeed relevant, the added open loop gain at LF might, however, be desirable to get rid of memory distortion through global feedback. I will have to think whether local emitter degeneration might even me more, i.e. doubly effective...

Hope this will start a lively discussion!


Eric
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