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Old 19th August 2011, 06:39 PM   #151
Elvee is offline Elvee  Belgium
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Here is a quick, informal test I improvised to check the effect of bias on E-lytics.

The capacitors were standard, run of the mill 470µ/16V, new and unused, 5~10 years old.

Prior to the test, they were reformed at 16V during half an hour.

They were then connected to the test set-up, with the switch in position 1.

The output was monitored on a digital scope, speed 20ms/div, vertical sensitivity 100mV/div.
When the trace was centered in a division, the trace never crossed the lines during 1~2 minutes observation.

The switch was then turned to position 2. After stabilization and offset adjustment to correct the effect of the leakage current (not shown on the schematic for clarity), the trace was observed again, in the same conditions.
This time, there were moderately frequent crossings of the 100mV limits.
As an unscientific estimation, I would say the amplitude was increased by ~25%.

The set-up was left in the same condition for 2 hours, and the trace observed again.
This time, crossings still occured, but very infrequently compared to two hours earlier.

My conclusion is that there is a minor but observable increase in the VLF noise when bias is applied to an electrolytic.

This of course is a one of observation, with one model of cap, and for a limited period of time, and it is partly qualitative.
It could be that differences vanish completely after two weeks, but in some conditions, there is a detectable effect (I hope this will not bring grist to the mill of "break-in adepts").
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File Type: gif Cap test.gif (10.4 KB, 588 views)
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Old 19th August 2011, 09:14 PM   #152
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Quote:
Originally Posted by Elvee View Post
What is the purpose of Q4?
Keep LTP collector voltages equal? Maybe useless, I dunno...
Not as-if I had any actual "thought process" going on.
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Old 19th August 2011, 09:22 PM   #153
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What happens to biased elko noise when bypassed with film cap?

Is the observed VLF noise random, or correlated?
Four elkos might be self bypassing, if there is no correlation.

Last edited by kenpeter; 19th August 2011 at 09:27 PM.
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Old 20th August 2011, 07:40 AM   #154
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by kenpeter View Post
What happens to biased elko noise when bypassed with film cap?

Is the observed VLF noise random, or correlated?
Four elkos might be self bypassing, if there is no correlation.
To the naked eye, it looks random, but I'd need to make a Fourier analysis to be positive.
Anyway, most of the noise is caused by the opamp, and this is random, and it certainly has the capacity to mask the characteristics of the capacitor's noise.

If I find some time next week, I'll try to make further tests and also use a lower noise amplifier than the LF411.
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Old 20th August 2011, 03:37 PM   #155
Bigun is offline Bigun  Canada
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Elvee, I knew I liked this design but as I start to realize how this works, well now I'm starting to consider it as the closest solution to a long search I've been on. I've been looking for a non-switching Class AB without switching x-over distortion (whatever that means). I've sworn off LTPs for various reasons but the way they are employed here with insensitivity to device matching gets my thumbs us. It also avoids all the limitations of matching complementary devices at the output too.

However, I was also interested in a hybrid - tube VAS up front. I wonder if it would be possible to use this (circlophone) approach to realize a unity gain power buffer without gnf (except for the bias servos).

edit: p.s. can you post the LTSpice file for this ?
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Last edited by Bigun; 20th August 2011 at 03:48 PM.
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Old 20th August 2011, 04:03 PM   #156
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Unity buffer is very simple with JLH+SRPP, merely change Q1 from NPN to PNP.
2n2905 or MJE350 might work...

Other methods described here return common mode feedback to the tail of a
LTP. Are gonna have a much harder time if a LTP isn't there (sworn off why?)...
Of course, if you made a LTP of a twin triode, you could still use any of the
other solutions. Its an autobias for the triodes as well.
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Old 20th August 2011, 04:38 PM   #157
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by kenpeter View Post
Unity buffer is very simple with JLH+SRPP, merely change Q1 from NPN to PNP.
2n2905 or MJE350 might work...
but this is Class A only though right ?

Quote:
Originally Posted by kenpeter View Post
gonna have a much harder time if a LTP isn't there (sworn off why?)....
well it started off because I needed a change, seems LTPs are everywhere. But Then there's the question about the accuracy of the LTP as an error amplifier relying on matching of two devices when a Singleton has no such limitation. And the LTP sacrifices balance between odd and even harmonics in favour of odd harmonics. Lastly, LTPs require more compensation capacitance than the Singleton and compensation is an ugly word most places. The 'series LTP' or Rush cascode seem a better option.

But I have to agree, the LTP does a couple of things really nicely - dc-offset and thermal drift are nicely handled.

Quote:
Originally Posted by kenpeter View Post
Of course, if you made a LTP of a twin triode, you could still use any of the
other solutions. Its an autobias for the triodes as well.
Unfortunately, no p-type triodes available...
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Old 20th August 2011, 07:00 PM   #158
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Bigun View Post
Elvee, I knew I liked this design but as I start to realize how this works, well now I'm starting to consider it as the closest solution to a long search I've been on. I've been looking for a non-switching Class AB without switching x-over distortion (whatever that means). I've sworn off LTPs for various reasons but the way they are employed here with insensitivity to device matching gets my thumbs us. It also avoids all the limitations of matching complementary devices at the output too.

However, I was also interested in a hybrid - tube VAS up front. I wonder if it would be possible to use this (circlophone) approach to realize a unity gain power buffer without gnf (except for the bias servos).

edit: p.s. can you post the LTSpice file for this ?
Ken and I have posted a number of circuits that might more or less fit your requirements ( have you looked at Unigabuf, that's a really quirky one).
I also have a huge number of other ones in my archives, I'll have to dig to see whether I can find something more appropriate.

But be aware that a good part of the "magics" of the Circlophone finds its origin in the association of same sex OP transistors with drivers having an horizontal symetry (LTP's), and a common current servo.
The result is transcends the mere sum of the ingredients, and if you remove one of them, the magic will not work.

With the LTP's, the current servo can act on both output devices simultaneously, and doesn't interfere at all with the signal path.
It would be difficult to achieve the same result otherwise (vertical [N-P] symetry could in principle do it but practical results are disappointing).

I'll think about it anyway.

Here is the .asc file
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File Type: txt circlophone.asc.txt (9.2 KB, 111 views)
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Old 20th August 2011, 09:06 PM   #159
Bigun is offline Bigun  Canada
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Thanks for the file Elvee.

Don't get me wrong - I'm not suggesting removing the LTP from this design - I don't see how it would work without it. What I like is that you don't find the device balance in the LTP to be critical - which I thought it was. Or do you mean it's not critical for operation of the common-mode servo but is still critical for good accuracy of gnf voltage feedback error amplifier ??
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Old 20th August 2011, 09:37 PM   #160
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Bigun View Post
Thanks for the file Elvee.

Don't get me wrong - I'm not suggesting removing the LTP from this design - I don't see how it would work without it. What I like is that you don't find the device balance in the LTP to be critical - which I thought it was. Or do you mean it's not critical for operation of the common-mode servo but is still critical for good accuracy of gnf voltage feedback error amplifier ??
Device balance in the driver (or VAS or predriver, call it what you like, it doesn't fit conventional classification, in short the lower LTP) is completely unimportant.

In the input stage, it is like any other amplifier for one thing: the offset voltage will be copied unchanged at the output, but unlike conventional amplifiers, the unbalance will not cause further degradation, the linearity will remain unaffected.
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