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Old 26th November 2008, 11:16 AM   #311
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To get rid of memory distortion in CM try using cascoded mirror, I havent used this in self type circuit but it works and sounds great in a different current feedback design Ive tried.
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Old 26th November 2008, 01:54 PM   #312
jam is offline jam  United States
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homemodder,

Have you tried the additional resistor in series with the cascode as suggested in the memory distortion page?

Jam
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Old 26th November 2008, 06:22 PM   #313
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Jam do you mean in a current mirror or the resistor in the vas. The resister in the vas is interesting, I havent tried it, but in general I have disliked the sound using cascode in vas and ltp in self type amps. I use very low cob , high speed trannies, and I like the warmer sound without cascode. Maybe this resistor does something to the sound, anyone here that can comment on this ???

Cascoding Jfets works good but now I got hold of some sanyo Jfets and in a headamp I tried them in, Ive gotton rid of the cascode too.

Jam take a look at the buffer thread for my comments on the sound of mirrors with that buffer design of mine.
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Old 26th November 2008, 09:31 PM   #314
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Quote:
Originally posted by AKSA
VHF,

Lavardin's technique for reducing 'memory distortion' in the VAS was simply to cascode, removing Early effect in the lower device; but I found years ago that unless you inserted a resistor between the lower and upper device between collector/emitter the clip performance became very ugly.... some mention is made of this by peufeu here where he addresses it as 'the magic resistor'.

Notice the ultimate version cascodes the cascode. I'm not convinced this is the best way to do it, too much cost of rail efficiency, but a simple cascode reduces memory distortion almost ten fold, obviously worth it.

This is a guy who has it all; deep maths insights, clearly a gifted engineer, but one who listens, is not didactic or opinionated, and is not afraid to describe the sound in heavily subjective terms.

I guess this is the engineer's equivalent of dancing.....

Good to see, restores the faith.....

Cheers,

Hugh

Memory distortion is an interesting matter, but bear in mind that the right measurements will often bring it out. Even very low frequency THD can be useful in this regard. Some take THD at 20 Hz for granted and don't even measure it, as it should be "easy" for an amplifier to faithfully reproduce such a slowly changing signal (especially with all of the loop gain often available at those low frequencies). But don't take it for granted, and do be suspicious if it rises above, say, 1 kHz THD. Such "memory distortion" even exists in the feedback resistor as a result of its thermal variations when it is subjected to substantial power.

However, some of these effects are buried in amplifiers that have otherwise high levels of THD.

Another interesting test is to do SMPTE with a low frequency component much lower than the usual 60 Hz.

Another interesting test would be low-frequency CCIF, where 19Hz +20Hz signals are combined, resulting in a 1 Hz signal power envelope.

Cheers,
Bob
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Old 26th November 2008, 09:48 PM   #315
AKSA is offline AKSA  Australia
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Thanks Bob,

Interesting comments. I guess you would expect memory distortion to decrease at high frequencies, but it should still affect the midrange where human voice lies.

No one talks of dielectric absorption in power amps, presumably at C1, do you have a view on this?

Hugh
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Old 26th November 2008, 11:11 PM   #316
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Quote:
Originally posted by AKSA
Thanks Bob,

Interesting comments. I guess you would expect memory distortion to decrease at high frequencies

Hugh
On IC's easily out to 50-100kHz.

BTW I'm fairly surprised that no one picked up on the fact that the forward gain path on that "discrete OPA627" I posted is determined to the first order only on pair-wise device match i.e. first order rejection of all thermal distortion.
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Old 27th November 2008, 03:06 AM   #317
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Quote:
Originally posted by AKSA
Thanks Bob,

Interesting comments. I guess you would expect memory distortion to decrease at high frequencies, but it should still affect the midrange where human voice lies.

No one talks of dielectric absorption in power amps, presumably at C1, do you have a view on this?

Hugh

Depending on the thermal time constants of the elements involved, I suppose thermally-induced memory distortion could have an influence even when high frequencies are involved, since the time variation of the power envelope of the high-frequency signals could play a role. Consider, for example a test tone consisting of 20 kHz and 19,990 Hz, with a power envelope changing at a 10Hz rate.

Not sure about DA by itself, but I do believe in using a high-quality polypropylene capacitor for input coupling.

Cheers,
Bob
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Old 26th December 2008, 03:50 AM   #318
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Hello good evening to all friends and have a happy Christmas hohoho
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Old 31st December 2008, 05:37 AM   #319
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Happy New Year everybody
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