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Old 1st October 2012, 09:09 AM   #1401
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Quote:
Originally Posted by john curl View Post
I swear, Scott! Can't you design a SIMPLE but ELEGANT circuit? '-) Discrete devices are NOT FREE, you know.
...it might not be the most simple, but the diamond buffer with signal depending bias is an elegant beauty.
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Old 1st October 2012, 01:57 PM   #1402
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Quote:
Originally Posted by john curl View Post
I swear, Scott! Can't you design a SIMPLE but ELEGANT circuit? '-) Discrete devices are NOT FREE, you know.
The diodes are essentially free and the 12 bipolars are 5-10 cents each. The JFET's can vary but there are some < 25 cent options. This exercise was an attempt to also "make the numbers".

The noise is just under 2nV, if you really wanted 1 nV I would entertain the inductor trick and some paralleled FET's on the input. If you wanted 30 Ohm drive I would just use a good closed-loop buffer or high current op-amp as an output stage. Those MJE devices have big Cje's to drive and two of them are bigger than the op-amp would be anyway. Notice the JE990 drops to -75dB THD at 75 Ohms.
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Old 1st October 2012, 02:23 PM   #1403
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Note on those step responses, there was no input BW limiting or small cap in the feedback circuit to smooth the response.
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Old 1st October 2012, 06:05 PM   #1404
RNMarsh is offline RNMarsh  United States
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This is sophisicated and refined. Much needed and appreciated by all. Now its in the ether of the Internet for all time. The SW-OPA. You're a class act, Scott. Thx-Dick
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Old 1st October 2012, 06:14 PM   #1405
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Thanks, as said before all and in part in the creative commons. Pete B. had enough info that I think I could easily cobble together a FET JE990 with cheap available parts, but it will have to wait a few days.
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Old 1st October 2012, 06:19 PM   #1406
mlloyd1 is offline mlloyd1  United States
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please do. i, for one, would love to see such a version from you.

thx,
mlloyd1

Quote:
Originally Posted by scott wurcer View Post
Thanks, as said before all and in part in the creative commons. Pete B. had enough info that I think I could easily cobble together a FET JE990 with cheap available parts, but it will have to wait a few days.
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Old 1st October 2012, 07:02 PM   #1407
qusp is offline qusp  Australia
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Originally Posted by RNMarsh View Post
This is sophisicated and refined. Much needed and appreciated by all. Now its in the ether of the Internet for all time. The SW-OPA. You're a class act, Scott. Thx-Dick
I couldnt agree more, i've been waiting in the wings letting those who actually know what they are talking about speak, waiting for the inevitable mastery with this dreamteam and we havent been let down.

i'm in need of a slightly lower gain circuit though for my custom in ear headphones (under 10ohms at some frequencies, they are multidriver 28ohms nominal with 3 way WW-MM-TT 6 driver). previously actual opamps are some of the few things that can drive them properly.

Scott, I dont really like the idea of strapping an IC buffer to such a great discrete circuit; you call the last circuit a couple of pages ago a buffer, is that the buffer you are referring to in this last circuit for under 30ohms load impedance, or something else more standard?

is there any noise penalty for using actual diodes vs diode connected transistors? I was thinking of trying 2sk246/j103 or perhaps even some confused bisexual 2sk170 for both positions in the front end differential; any opinions?

Last edited by qusp; 1st October 2012 at 07:06 PM.
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Old 1st October 2012, 07:06 PM   #1408
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You can't divide anymore this amp on "input stage" and "buffer", the amp now is more like a tight nice knot.
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Old 1st October 2012, 07:17 PM   #1409
qusp is offline qusp  Australia
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Originally Posted by Wavebourn View Post
You can't divide anymore this amp on "input stage" and "buffer", the amp now is more like a tight nice knot.
was this in reply to me at all? the section of the circuit in post 1380 sure looks like a buffer to me, but I know what you mean, very neat and integrated. besides what do I know, I feel pretty humbled in this company

but its OK, dont worry Scott, I read that post more carefully and you were only referring to the current state of the circuit as you havent done enough tweaking to provide reliable operation and predicted performance at low gains. still not sure what the recommended route for driving low impedance might be with either circuit (preferably the second one), but i'm sure that will be covered in time. it will hopefully drive my 300ohms HD600 OK as is though

Last edited by qusp; 1st October 2012 at 07:21 PM.
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Old 1st October 2012, 09:31 PM   #1410
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Very elegant Scott! Are we to assume that these diode connected transistors are 1n4148 or 1n6263 or is there some new transistor when connected as a diode outperforms both the ones I mentioned. There is no transistor nomenclature. You mentioned the 2n5551 and the 2n5401. Did you model with these or are these 2n4403 and 2n4401s as in your first posted circuit. Also, forgive my ignorance in these matters but what is the quiescent current in the output devices and what procedure are we to use in setting bias. I'm an architect not a EE. Thanks a lot. Ray
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