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Old 3rd September 2012, 01:11 PM   #631
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by coluke View Post
[...]

I think the Ayre KX-R is based on the same concept.

L.
Input is J-Fets, gain stage is cascoded current mirrors. I don't know the output stage architecture though, but it seems to be a complementary JFetfollower. And balanced from input to output.
Charles Hansen in his posts leaved enough information to second guess, together with some magazine / website information ans pictures.

Check out the simplified (and strangely drawn) schematic here.
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Old 3rd September 2012, 02:04 PM   #632
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We're back to "this probably sounds better" and the sound of feedback vs no feedback, amply covered in other in other threads.
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Old 3rd September 2012, 02:19 PM   #633
coluke is offline coluke  Italy
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Originally Posted by scott wurcer View Post
We're back to "this probably sounds better" and the sound of feedback vs no feedback, amply covered in other in other threads.
Nope, Scott - didn't mean to cover once more the 'sound of feedback' topic, just described the reason why I tried that specific topology. Actually I've never been able to hear any difference between CL and OL version of the amplifier, provided the output impedance of the OL version is very low (say < 1 ohm).

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Old 3rd September 2012, 02:40 PM   #634
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Originally Posted by coluke View Post
Nope, Scott - didn't mean to cover once more the 'sound of feedback' topic, just described the reason why I tried that specific topology. Actually I've never been able to hear any difference between CL and OL version of the amplifier, provided the output impedance of the OL version is very low (say < 1 ohm).

L.
That was meant in a more general sense. My original thought was along different lines like something that could be droped into a standard op-amp I/V or the Pacic-Pre phono. Keep it simple enough anyone can try it, nothing ventured nothing gained. Weather it uses 50 cents or $1.25 in devices was not the point.
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Old 3rd September 2012, 03:52 PM   #635
bcarso is offline bcarso  United States
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Never say never - current-mode open-loop topologies are indeed capable of very low THD: I've built and tested a HP amplifier based on a pretty standard architecture (diamond in - transimpedance gain stage - high current diamond out) whose THD is in the -100 dB range (low order only) @ 1 kHz, 0 dBV into 30 ohm. And it sounds very good, too , but it is by far too much complex to be mentioned here (about 20 BJTs - basically a discrete CFA). You can easily convert it to closed-loop, and that's the reason why I gave a try to this architecture.

I think the Ayre KX-R is based on the same concept.

L.
Many of the topologies developed over the years for oscilloscope vertical amplifiers have very low distortion without global feedback. Feedback would have limited the frequency response. For a good coverage of many see Doug Feucht's book Handbook of Analog Circuit Design. The material has appeared here and there, including John Addis' contribution to one of the Jim Williams compendia, but afaik not as extensively in book form as Feucht's.

Translinear circuits with bipolars, which rely upon their very predictable characteristics, are routinely capable of order -80dB performance over a useful range of currents, and without additional local feedback (which improves things but slows them down). Barrie Gilbert is the foremost authority on these techniques, exploited in his eponymous multiplier.

I presented a current mirror in another thread that shows how to use cascoded FETs and floating bias sources with local feedback to realize low noise and distortion. It costs volts and power dissipation, but works well. It was to some extent offered for those who spurn bipolars, believing that there is some intrinsic inescapable "sound" there that they don't like.
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Old 3rd September 2012, 03:57 PM   #636
PMA is offline PMA  Europe
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My next preamp design (if I ever get time for audio again ) would definitely be a CFA topology.
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Old 3rd September 2012, 05:08 PM   #637
bcarso is offline bcarso  United States
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For a good coverage of many see Doug Feucht's book Handbook of Analog Circuit Design. The material has appeared here and there, including John Addis' contribution to one of the Jim Williams compendia, but afaik not as extensively in book form as Feucht's.
Correction: Dennis Feucht, not Doug.
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Old 3rd September 2012, 05:46 PM   #638
RNMarsh is offline RNMarsh  United States
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Default OP-Amp design

Quote:
Originally Posted by scott wurcer View Post
My original thought was along different lines like something that could be droped into a standard op-amp I/V or the Pacic-Pre phono. Keep it simple enough anyone can try it, nothing ventured nothing gained.
I also thought that was the point of coming to this forum as it was your input to do a new design (other than JC's) we (I) waited for and you delivered a very good one.
Has it now morphed into something else? Or, is it when a forum is done, it doesnt know how to end so it rambles on?

There are now plenty of Current-mode feedback designs being made on this DIYAUDIO and other sites. So there needs to be a goal for us to shoot for and not just ramble on about this or that circuit.

What are the goals for a new design?

Personally, I would now like us to try matching the performance of an opamp's high feedback with one with no gnfb. There are not many out there. Make it popular and easy as possible to make as the new goal.

Last edited by RNMarsh; 3rd September 2012 at 05:56 PM.
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Old 3rd September 2012, 06:36 PM   #639
RNMarsh is offline RNMarsh  United States
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PS - On the mid-level of gnfb circuit without dc servo... some have wondered just how stable over the long term it could be. After a year of service, I measured the dc offset at the output again and it is exactly where i left it a year ago..... .00x + or - a couple mV. -RNM
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Old 3rd September 2012, 06:49 PM   #640
bcarso is offline bcarso  United States
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PS - On the mid-level of gnfb circuit without dc servo... some have wondered just how stable over the long term it could be. After a year of service, I measured the dc offset at the output again and it is exactly where i left it a year ago..... .00x + or - a couple mV. -RNM
Richard, in your Linear Audio article, Table 1 shows an entry for signal-to-noise ratio with the input shorted of - [sic] 135dB. Is this for a reference level of 1V rms at the output, into 30 ohms?
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