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Old 12th September 2012, 10:56 PM   #901
bcarso is offline bcarso  United States
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Originally Posted by fitzfish View Post
Can we then use /-15 and +/-24 volts as rails for simulation and circuit performance reporting then? If someone has a desire to see low voltage performance perhaps +/-8V would be reasonable?

This is not to limit any rail voltage usage or application but to make compairsons easy between designs for conversation sake. These two nicely cover integrated op amps and the pro audio modules so they are typical usage.

Dave
I'd move to some higher-voltage bipolars for the cascode devices of the gm output stage, and the output buffer, as 4403 is stated as a -40V Vce or Vcb device, thus o.k. with +/- 15 but pushing it for +/- 24. And as I mentioned, cascoding of each BF862 is desirable, although they would be fine at +/- 8.
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Old 13th September 2012, 01:05 AM   #902
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Default Ready to build Scott's circuit?

I'm ready to build Scotts circuit just to be able to attempt measurement and to listen to such a distortion-less circuit. Are we ready -- seems just output stage isnt finalized but Scotts is more than fine. A better one?

I got to test my simple circuit with Demian Martin's ShibaSoku gear today as a back up measurement to my AP: At 1volt ref output/input shorted - THD = .0005%... 2nd and 3rd in approx equal amounts... other harmonics are -20+ further down from these two. [set at zero dc output]

S/N = -106db with 30KHz cutoff. Imagine cleaner supply lines might help a little as would lower R values in fb and gain. Should be easy (?) to get -110db. But since it wasnt designed for low noise, it isnt too bad for a real built circuit with modest feedback.

Let's build Scotts and see what real world numbers can be obtained vs SIM? I'm collecting parts now. Good ideas and info here - five stars. Who is doing the pcb?
Thx - RNMarsh
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Old 13th September 2012, 05:57 AM   #903
PMA is offline PMA  Europe
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Quote:
Originally Posted by RNMarsh View Post
At 1volt ref output/input shorted - THD = .0005%... 2nd and 3rd in approx equal amounts... other harmonics are -20+ further down from these two. [set at zero dc output]

S/N = -106db with 30KHz cutoff.
Hello, these parameters are not so difficult to get. Let me consider them and let me add time response. Blue is input signal, yellow pre output.
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Old 13th September 2012, 06:43 AM   #904
RNMarsh is offline RNMarsh  United States
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Its something for me to shoot for.... find the circuit that uses the fewest number of transistors that can drive 30 ohms and be dc coupled/stable as well. While thd and s/n is under -100db. Doing it with open loop bandwidth of 20Khz (40KHz preferred). Now if I use different low noise transistors and cascode and the like, then what? But then I have made the circuit a lot more complex and not minimal number of transistors. I met my goal. Would like to see equal or better in all same areas. So far only Scott has risen to the challenge with a great opamp circuit. Lets see your minimalist super circuits. Final data has to be from working circuit not sim. In case of tie (?) the circuit with the fewest transistors wins the gold metal. Thx - RNM

Last edited by RNMarsh; 13th September 2012 at 06:52 AM.
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Old 13th September 2012, 12:28 PM   #905
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Why not try a variation of this circuit as well, Richard? A few resistor changes, and it should be OK.
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Old 13th September 2012, 02:52 PM   #906
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John,
And anyone else who wants to answer this question. My question is are any of the proposed circuits that you are showing here actually circuits that you would use in anything but a challenge here to use the least amount or limited amount of discrete transistors. Or is this more like reading Cordell's book on amplifier design and this will technically meet the challenge but in reality to make it a viable product you would have to start adding more devices to make it actually functional for the purpose? I am just curious as to the intent for the design, if it is more than an intellectual challenge.

Steven
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Old 13th September 2012, 03:03 PM   #907
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Its something for me to shoot for.... find the circuit that uses the fewest number of transistors that can drive 30 ohms and be dc coupled/stable as well. While thd and s/n is under -100db.
Dick, I'm afraid I have to stick with +24dBu output. There are no general purpose amplifiers specified at 1V out only. The JE990 and all available discrete op-amps I can find are specified at 20V p-p out. Professional mixing boards in general use 24V rails and the amplifiers are expected to drive 600 Ohms to near the rails.

There's nothing wrong with a headphone amp that's as loud as you would ever want at 1V, but it is fairly narrow in scope.

Before we build I have one more set of suggestions, that I will get to tonight.
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Old 13th September 2012, 03:05 PM   #908
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Why not try a variation of this circuit as well, Richard? A few resistor changes, and it should be OK.

Lots of junction capacitance at the Vas output, wouldn't a buffer on the MOSFET gates help?
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Old 13th September 2012, 03:21 PM   #909
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John,
And anyone else who wants to answer this question. My question is are any of the proposed circuits that you are showing here actually circuits that you would use in anything but a challenge here to use the least amount or limited amount of discrete transistors. Or is this more like reading Cordell's book on amplifier design and this will technically meet the challenge but in reality to make it a viable product you would have to start adding more devices to make it actually functional for the purpose? I am just curious as to the intent for the design, if it is more than an intellectual challenge.

Steven
This was not intended as an intellectual exercise in the beginning. After some output stage work I think I can reach (at least) my original goal, a discrete FET input general purpose op-amp with performance as close as possible to the JE990/OPA627/37 when used in the same circuits using almost as few transistors as the JE990 so it fits on a small card. A plus would be an all tiny SMT version.

I would also hope to build and measure a prototype.
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Old 13th September 2012, 03:26 PM   #910
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Scott,
Thank you for your answer. I was beginning to think this had just become an intellectual pursuit of strange topologies.

Steven
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