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Old 18th June 2012, 01:47 AM   #281
rjm is offline rjm  Japan
Richard Murdey
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Much food for thought in that list. At first read, it strikes me as unduly - unfairly, even - restrictive. That's not a criticism - they are your opinions based on your (considerable) experience and I respect that - just an observation. Since you say it is do-able so it must be, though I do wonder just how many circuits in the wild not designed by Richard Marsh actually pass all 11... Given (3) I rather suspect not many.

And 4., coming from you, is somewhat ironic, no? (sorry, couldn't resist..)
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Old 18th June 2012, 10:28 AM   #282
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Originally Posted by RNMarsh View Post
Here are my own modest base requirements for a line stage (similar for power amp):

1. Open loop BW of 40KHz or more (-3dB)... 20KHz min.
2. IM and THD of less than .001% at 1v rms into 30 ohms for any frequency between 20Hz and 20KHz.
3. No coupling caps on input or output or in feedback path.
4. No use of dc servo circuits to track and correct dc offset and drift.
5. No more than 6-8 transistors (excluding power supply).
6. S/N ref 1 volt rms and without weighting of at least -130dB (input can be shorted or terminated).
7. No significant harmonics above the 2nd and 3rd.
8. Closed loop gain between 12 and 20 dBv
9. Low Zout (less than a fraction of an Ohm at any audio freq).
10. Distortion not be changed by source Z.
11. Transistors should be low cost and not be exotic, hard to obtain, very expensive or no longer manufactured.

There might be a thing or two I missed, off the top of my head, but this is do-able and should be done in all audio circuits IMHO. -RNM
Do we a get a prize to post such a circuit ??
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Old 18th June 2012, 12:33 PM   #283
RNMarsh is offline RNMarsh  United States
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Sure. What would you like... Glory or fame and fortune?
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Old 18th June 2012, 12:37 PM   #284
Telstar is offline Telstar  Italy
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4. No use of dc servo circuits to track and correct dc offset and drift.
Why not? That the circuit does not REQUIRE a DC servo is commendable, but what about zero GNFB and very low frequency DC-servo?
I'm definitely gonna compare your amp with such a circuit.

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8. Closed loop gain between 12 and 20 dBv
Can it be easily reduced? Low Z phones need 2-4x gain at best.
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Old 18th June 2012, 12:57 PM   #285
RNMarsh is offline RNMarsh  United States
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Much food for thought in that list. At first read, it strikes me as unduly - unfairly, even - restrictive. That's not a criticism - they are your opinions based on your (considerable) experience and I respect that - just an observation. Since you say it is do-able so it must be, though I do wonder just how many circuits in the wild not designed by Richard Marsh actually pass all 11... Given (3) I rather suspect not many.

And 4., coming from you, is somewhat ironic, no? (sorry, couldn't resist..)
The criteria for each of the 11 are reasonable and developed over decades but not easy for DIY. This little headphone amp is about meeting the criteria. It is amazing to me that with so many transistor part numbers, there are still so few good complements -- the jFETs I used are not that great complements, either. Would like to find better substitutes.... one's that are closer match right out of the box. Better complements would lower the harmonic distortion further.

#3 should not be hard with low dc gain circuits... phono stage would be a lot harder... Good place for a dc servo. [with downloading HD masters makes LP's a wee bit out of the party for me]

#4.... it was a solution to one person's circuit problem that got away from itself. It's downside is that designers feel they dont have to pay attention to dc drift/offset issues anymore.... just throw a servo on it. In many cases, that could be OK..... but not always. It didnt help MultiCap sales, either. :-) But, you got to do what is needed.... It isnt always about money.... at least not for me.

Last edited by RNMarsh; 18th June 2012 at 01:11 PM. Reason: I am back from Alaska !
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Old 18th June 2012, 01:13 PM   #286
forr is offline forr  France
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Hi RJM

Current conveyor

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forr - I hadn't heard the term before, but on searching, it certainly seems to be a variant thereof.
Look at Jan Didden's Pax amplifier for a very clever application.
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Old 18th June 2012, 01:16 PM   #287
RNMarsh is offline RNMarsh  United States
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Originally Posted by Telstar View Post

Can it be easily reduced? Low Z phones need 2-4x gain at best.
Sure, why not?
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Old 18th June 2012, 02:33 PM   #288
Telstar is offline Telstar  Italy
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Originally Posted by RNMarsh View Post
It is amazing to me that with so many transistor part numbers, there are still so few good complements -- the jFETs I used are not that great complements, either. Would like to find better substitutes.... one's that are closer match right out of the box. Better complements would lower the harmonic distortion further.
Currently the only option for close matching is to use monolithic dual jfets of a single polarity, such as LSK389, which are in production.

I hope jackinnj will offer matched input jfets for your amp *wink wink*
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Old 18th June 2012, 04:04 PM   #289
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Do we a get a prize to post such a circuit ??
Nah, I have those already, a bottle of Glenlivet XXV 25 year old Single Malt Whisky would be nice.
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Old 18th June 2012, 05:08 PM   #290
RNMarsh is offline RNMarsh  United States
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Nah, I have those already, a bottle of Glenlivet XXV 25 year old Single Malt Whisky would be nice.
Rats! All out of that one from my arsenal. I'll have to restock. But. If you dont come up with a working, different topology in, say, a week, then I'll take same type but from Rosebank (1979 - last year in production - Scotland).
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