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Old 5th August 2010, 10:36 AM   #1501
Salas is offline Salas  Greece
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Quote:
Originally Posted by Werner View Post
I built the 2SK369 folded cascode / OPA637 design I posted early on in this thread.
That one?

Nice build. Are the LEDS a dedicated Vref instead of the resistive voltage divider?
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Old 5th August 2010, 11:05 AM   #1502
Werner is offline Werner  Europe
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Quote:
Originally Posted by Salas View Post
That one?
You're a fast searcher. Yes, that one. At least, close to it.

The LEDs bias the cascode transistors. Inspired by this here thread.

All six regulators are emitter followers. All supply currents are nominally constant, the front-end innately, the opamp too: the 637 drives its normal load and at the same time an inverting opamp with the same load, both in class A.

The supply itself is another box, in fact a commercial Michell/Trichord Never Connected with regulated +/-22V.
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Old 5th August 2010, 11:44 AM   #1503
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Hi Werner,
nice build!

Regarding layout of the input stage: do you have seperate grounds or is the thick silver stripe at the psu input V+? If seperate, which resistor goes straight to the psu ground?
What are the golden/black boxes above the 637's? I did not understand your comment what exactly the second opamp does, do you like to elaborate?

I build both versions of your phono (on protoboards, no 'builds-to-stay') with minor variations and experimented a bit further.


I had to make the load resistor for the input fet trimmable to get the best working conditions for it. I second your comment regarding trebles. It offers a lot of resolution too, very valuable for classical music. However, my version tended to be a bit foggy on very complex material, and so far I have not found a remedy. It would be interesting if your version differs in this aspect.


Rüdiger
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Old 5th August 2010, 02:24 PM   #1504
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Hi Werner ! Could you post the circuit diagram of your latest version ?
I am building a Phono Amp for Martina Schöner ( L´Art du Son, Lorycraft-Garrard ) that i call UPE ( Universal Phono Equalizer ). Martina likes to listen to pre RIAA opera, jazz and pop so she needs alternative equalization curves. My new topology: Transconductance Input Stage, Programm Module, Buffer allows that by interchanging the program module.
It also allows to bypath the EQ totally and then the UPE is a Pre-Pre ( Headamp ).
Here are some photos of the prototype. More explanations will come.
Attached Images
File Type: jpg L´Art du Son UPE inside 1.jpg (90.6 KB, 454 views)
File Type: jpg L´Art du Son UPE inside 2.jpg (108.3 KB, 440 views)
File Type: jpg L´Art du Son UPE Program Module 1.jpg (90.3 KB, 436 views)
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Old 5th August 2010, 07:24 PM   #1505
Werner is offline Werner  Europe
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Sorry, I don't have a complete circuit diagram.

do you have seperate grounds

Yes. No naughty currents are mixed up with the nice ones.

What are the golden/black boxes above the 637's?

NOS styroflex caps.

I did not understand your comment what exactly the second opamp does, do you like to elaborate?

An inverting amplifier (TL071) hangs off the output of the OPA637. It is so configured that, for a given load impedance (my ADC), it pulls a current from the supplies that is the inverse of the 637's current. So the net supply current is zero.

I had to make the load resistor for the input fet trimmable to get the best working conditions for it.

I have a second identical board, with sockets everywhere. Prior to building the thing I characterised all my 2sk369s, bipolars, and bias LEDs, in circuits similar/the same as in the actual preamp. Not easy, as the front-end is, let's say, thermally full of temperament ...
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Old 5th August 2010, 09:34 PM   #1506
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Thanks Werner.

Nice ideas as always!

Rüdiger
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Old 6th August 2010, 04:02 AM   #1507
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The Transconductance-Transimpedance stages i am working on get more and more gain by raising the output impedance of the current mirrors. The cascoded current mirrors i have shown here are only a beginning. You can imagine that DC offset gets a real problem when you feed the output current into a high impedance network like an RIAA shaper. As a consequence i am looking together with another engineer at novel ways of DC management without the use of Servos or big coupling caps. In fact we got a prototype working. What we need is a high speed buffer with very low input capacitance, exactly a gain of one and ideally zero output impedance plus low intrinsic self DC output. This can be made near perfectly with a good OP amp used as voltage follower and that is what we use in the prototype. Somehow this was too easy and the better i get at designing discrete circuits the more uncomfortable it feels to go the easy road by restoring to Op amps. Unfortunately my usual Fet-Darlington buffer that was based on a JLH design is not good enough for that task. It sounds very well but it has still some input capacitance, an non zero output impedance because of the presence of resistors in the collector chain and as a consequence a gain of less then one.
So i redesigned it. First i cascoded the input Fet´s. I use the Hawksford cascode here, a rare sight and i have never seen it being used in a buffer. It feeds back the base current of the cascode transistors into a low impedance joint of the amplification chain and gives an improvement in speed and distortion at the expense of some gain. Gain i have a lot in my design so i thought it could be beneficial here. The second change is putting the resistors into the emitter feed of the output darlingtons and not into the collector feed. This way i make a kind of "Super Szikley". This whole "thing" is theoretically damn fast and the old buffer with a -3dB point of 16MHz suddenly seems old and tired. How much of this speed i can rescue into a stable circuit has to be seen because Szikleys are famous for oscilation. Of cause i have and idea how to fix it when it ocures, so let´s wait for tomorrow when i put this circuit through it´s paces.
Attached Files
File Type: pdf Unity Gain Buffer Hawksford Cascode UGBHC.TSC - TINA.pdf (44.3 KB, 169 views)
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Old 6th August 2010, 07:36 AM   #1508
masag1 is offline masag1  Germany
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Hello Joachim,

The buffer stage looks similar to the 8 Watt Hiraga design, of course no output Transistors and you are using the Darlingtons and not a driver stage. Cool!

For the FF2010 Linestage I received several high quality PSU from a fellow DIY'er to test the sound influence of the psu.

Regards

SAm
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Old 6th August 2010, 10:13 AM   #1509
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Joachim,
here is my all-out buffer stage. You don't need the servo if you don't have DC at the input. It's rock stable if build properly, DC and AC wise. Normally, R7 can be omitted. You can make it tending against zero output impedance if you enclose it in a loop with a gain stage, and use beefier out put Q's.

A tad complex, maybe

Rüdiger
Attached Images
File Type: jpg servobuffer.jpg (49.2 KB, 392 views)
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Old 6th August 2010, 11:49 AM   #1510
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Hi Sam ! You are right, the Le Monstre looks very similiar. I was not aware of that. Of cause my stage does not have voltage gain and has the Hawksford cascode. By the way i build the first original Le Monstre in Germany for Kurt Hecker in 1983. A friend of mine bought it some years ago from a customer that had disasambled it. It was in terible shape but my friend is a very good craftsmen and he has restored it perfectly. It playes again perfectly well and it is only 5km away from my house, so somehow it came back to Papa. I will make a photo and post it.
Rüdiger, your buffer looks good and i think it is not that complicated when you boil it down without the "helper" transistors. I really like the way you implemented the servo.
Filtered with capacitance multipliers and working on the current mirror the sonic impact of it should be minimal.
Today i will finish my buffer and look if it is stable.
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