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Old 14th March 2012, 01:52 AM   #771
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Good point, gfiandy.

Actually output was placed on the middle of the track but not the feedback joint. Thank you for notice.
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Last edited by terranigma; 14th March 2012 at 01:57 AM.
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Old 15th March 2012, 03:40 AM   #772
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Elvee, what's your take on a low parts count DC Tracker, DC Servo? I'd sure like to try an active circuit instead of C3.
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Old 15th March 2012, 01:31 PM   #773
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I do not see how such a circuit could be grafted to the Circlophone.

Anyway, a DC servo only makes sense when the DC performance/stability is very poor, as with some symetrical topologies, or at least to give an improvement over the existing situation.

In the Circlophone, the output DC offset is essentially that of the input LTP, unamplified, and to improve on that would require a monolithic pair, either just the pair itself like a 2SA798, or as part of an opamp that would perform a "classic" DC servo function to replace C3.

But frankly, I see no need for such a circuit, and if C3 annoys you, you can eliminate it completely and have a DC-coupled Circlophone, provided you choose (or hand pick) a low offset input pair.

Another possibility is to add an offset trimmer, but it would be a pity to depart from the completely adjustment-free philosophy of the Circlophone.
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Old 16th March 2012, 02:27 AM   #774
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Quote:
Originally Posted by Elvee View Post
. . .In the Circlophone, the output DC offset is essentially that of the input LTP, unamplified, and to improve on that would require a monolithic pair. . . But frankly, I see no need for such a circuit, and if C3 annoys you, you can eliminate it completely and have a DC-coupled Circlophone, provided you choose (or hand pick) a low offset input pair.
The need is. . . to try it for the purpose of quality leveling.
The case is when neither DC coupled nor the vast component manufacturing variety of the cap at C3. I'm near certain that we can do better than 2 diodes parallel with a random power cap, but there is only one way to know for sure.
DC servo:
Enhanced clarity just like a DC coupled amp, but without the caveats to dynamics and speaker safety.
DC servo:
Faster to build for the audiophile since there's no need to hunt for an optimized capacitor choice for C3 (quality control) if it is supplanted by an active circuit that promotes expected optimized results every time.
DC servo:
Not exactly have your cake and eat it too, but servo is excellent quality control.
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Originally Posted by Elvee View Post
Another possibility is to add an offset trimmer, but it would be a pity to depart from the completely adjustment-free philosophy of the Circlophone.
Although it isn't particularly elegant and despite various reported possibilities from trimmers, it would be nifty to try it and find out its worth via actual performance. Due to wide variety of capacitor quality, for high fi use, the C3 area wasn't particularly adjustment free anyway and turning a dial is more efficient than swapping caps for quality control.

P.S.
For the record, I'd rather have an elegant servo to compare to trimmer to compare to C3. I seek your excellent guidance on how to best implement the servo and trimmer options, so they can be tried correctly.
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Old 17th March 2012, 08:23 AM   #775
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by danielwritesbac View Post
The case is when neither DC coupled nor the vast component manufacturing variety of the cap at C3. I'm near certain that we can do better than 2 diodes parallel with a random power cap, but there is only one way to know for sure.
That's the opposite: C3 works at the mA/mV level, and under such conditions even the crappiest chinese cap you could find on ebay would emulate a piece of wire, with degradation absolutely undetectable, even with the best instruments available.
By contrast, even a very good servo made with the best capacitors, opamps, etc, will introduce measurable impairments, noise, non-linearity, phase issues, etc.
Something else has to be considered: complexity. With a simple cap, nothing can go wrong, and to be absolutely sure the circuit can recover from any unforseen situation, diodes have been included to add a supplementary layer of safety.
They need not be implemented, this will change nothing.
But with opamps, an auxiliairy supply, etc, there is much more room for things to go wrong, and that could in the end easily cost a speaker.

Here is an offset adjustment circuit. It can be used with C3 present or shorted.
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Old 17th March 2012, 10:22 AM   #776
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Awesome!

And thank you for the explanation. Apparently, the effort of selecting optimized parts for a servo is not easier than effort of selecting a most pleasant model of capacitor and the audio quality results either way is the same at best, with other conditions probably favoring the cap as higher quality. Well, that certainly busts a few myths. Thanks!
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Old 17th March 2012, 11:45 AM   #777
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If you combine the input capacitor (which can be a high quality one) with a shorted C3 and a low offset pair, or an offset adjustment, you get the best of both worlds: safety of an AC coupled amplifier, without servo or feedback cap issues.
That's the most reasonable and effective solution if you want to get rid of C3
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Old 17th March 2012, 11:54 AM   #778
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Currently working on miniature size stereo Circlophone
Fairchild's KSA1220AYS for driver?
ST's BD911 for output?

These look somehow very interesting. That driver looks almost as good as a Philips? And that little TO220 output looks surprisingly durable for its size.

Last edited by danielwritesbac; 17th March 2012 at 12:06 PM.
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Old 17th March 2012, 06:46 PM   #779
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Quote:
Originally Posted by Elvee View Post
If you combine the input capacitor (which can be a high quality one) with a shorted C3 and a low offset pair, or an offset adjustment, you get the best of both worlds: safety of an AC coupled amplifier, without servo or feedback cap issues.
That's the most reasonable and effective solution if you want to get rid of C3
Thank you!
The point wasn't exactly to get rid of C3, but rather to make tests and comparisons both with and without C3.
With matched input, and temporarily shorted C3, it should be possible to choose reasonable but somewhat adjusted values for both R16 and R17 to result in zero offset. Your 10k input load spec makes this easier than usual. After zero offset, a wider variety of capacitors will make good results for C3 and then D11 won't misfire when the starting point is zero. For the record, I wouldn't promote a going without an NFB cap, on a high gain amplifier, except as a useful testing condition.

Some people say that all caps are good (myth), and some people say that all caps are bad (myth); But, the truth is that all caps are different. I believe that we should try about 5 varieties for finding favorable signal caps if they are in areas that also have gain applied. Optimizing signal cap selection is usually a task of choosing from amongst peers (not different than the task of choosing from amongst currently produced transistors via "test drive"), keeping the best results in circuit; however, in trade for slightly increased risk, the trimmer method is faster, easier and not component centric. The trimmer method could work for people who suffer capacitor myths or are in too much of a hurry to perform an optimized selection, and it could work fine for lower gain amplifiers.
Thanks again!

-----------
unrelated questions
For which rail of circlophone would it be most useful to add one 10uF cap?
I guess v-?
Which rail of circlophone would you choose to power just one LED+R?
I guess whichever rail drains slowest when the amp is powered off.

Last edited by danielwritesbac; 17th March 2012 at 07:05 PM.
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Old 17th March 2012, 11:46 PM   #780
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Default PCB status

I tested a build depends on my actual layout so far.. and it suffered by that enormous offset problem again. I thought there is no reason to facing with same problem with a new layout except that if I didn't repeat same mistake again. Then I tracked pcb layout again and I detected a very annoying faulty track at input pair alignment. I thought that I didn't touch there but I remember I have cancelled alternative connection of 270p at post #708 but without removing that extra track completely. I fixed it but It didn't help for offset problem. I think I injured some place on circuit permanently.. And I'm about to rebuild it from scratch again despite that all of time consuming cost. Sorry for who waiting for good news again.
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Last edited by terranigma; 17th March 2012 at 11:50 PM.
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