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Old 4th September 2012, 04:21 AM   #661
bcarso is offline bcarso  United States
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Quote:
Originally Posted by kevinkr View Post
I read and reread bcarso's comments on the theoretical snr of the RN Marsh discrete op-amp and after thinking about it worked the same analysis, and conclude he got it correct - this is at least the way I analyze my own designs when not being too lazy... I understand the argument, but the actual feedback components are not improved by feedback so adding the feedback margin to the calculated open loop noise performance doesn't look right to me either. Scaling the resistor values in a downward direction which looks possible and reasonable here would help.

IMO the SNR is probably more than good enough, and I don't get me wrong I think this is a pretty cool design.
Well since Wave BLEW my door prize, it will have to be simply the pleasure of knowing how things work that motivates the understanding of how the discrepancy evolved.

I could tell you what I think happened, but I'd like to see the other answers. Simulations are interesting, but just reasoning about how the circuit works is even better. And again, I agree that it is a perfectly nice design and quite acceptably low noise.

Last edited by bcarso; 4th September 2012 at 04:25 AM. Reason: clarification of language
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Old 4th September 2012, 04:41 AM   #662
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First off this is for fun and exchanging some ideas...

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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. Weather it uses 50 cents or $1.25 in devices was not the point.
Agree on both counts. Can we continue on that path?

Good applications examples to show the other possible limitations of a discrete op amp as well. PSRR, noise, and offset in higher gain applications are interesting challenges with such simple circuits.

Dave
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Old 4th September 2012, 04:50 AM   #663
kevinkr is offline kevinkr  United States
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If we could get someone to do some pcb of the Marsh design and offer them as a group buy I'd be in for 4..
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Old 4th September 2012, 05:36 AM   #664
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Hi Scott,

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Originally Posted by scott wurcer View Post
I would like some suggestions for transistors,

Since no one has jumped in with suggestions I have a couple for the gain half–

The 2SK2145 looks very interesting. Noise performance still needs to be tested. Q1/Q2 and Q8/Q3 could be a matched NXP pair. The PMP4201 and PMP5201 look reasonable. For Q15/Q16 the BC847BVN pair might work. All have more gain and lower Ft than the 4401/4403. Still worth a look as they are duals and should work reasonably well for thermal balance. Reasonably matched pairs for the mirrors as well in a single package.

For SMT diodes or other small transistors see the NXP leaflet - http://www.nxp.com/documents/leaflet/75016734.pdf

I have not simmed it so I am shooting from the hip on the NXP duals.

How critical is matching for the output buffer?

Dave
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Old 4th September 2012, 05:54 AM   #665
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Originally Posted by bcarso View Post
Hey no fair Wave!
It's okey Brad, here is more: Why "Chaldean" Numerology is NOT Chaldean - The Mystery Explained
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Old 4th September 2012, 06:13 AM   #666
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Quote:
Originally Posted by bcarso View Post
Again, at best, the 1k feedback divider resistor alone limits S/N referred to 1V to a far smaller value than 135dB. I get at least 13.2nV/sqrtHz at the output, in a 30kHz BW, thus 2.29uV rms, which below 1V is an S/N 112.8dB. Not shabby and would trouble no one, but not the stupendous 135dB.
EDIT: OK I see where, as a computation , and based on the measurement of output noise of -85dBV, with an open-loop gain of 62dB and supposing an open-loop bandwidth of 25kHz, you could approximate the noise with closed-loop gain of 12dB as 135dB below 1V. However, this cannot be reconciled with the noise as described above.
I went today and dug up some equipment and got at the output (closed loop) more like -100db ref 1v. One channel a little less, one a little more. BW limited to 100hz to 10KHz is the best i could do towards an A weighted figure. No load and input shorted. -RNM
[should do better with Rf/Rg lowered and a couple other simple things. I'll work on it.]

Last edited by RNMarsh; 4th September 2012 at 06:21 AM.
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Old 4th September 2012, 06:15 AM   #667
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My next preamp design (if I ever get time for audio again ) would definitely be a CFA topology.
PMA,

I would be interested in hearing your (or anyone else's) take on why the CFA topology would be superior for audio? i'm not opposed just curious as to where the trades offs rate in "audio" performance.

Many thanks
-Antonio
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Old 4th September 2012, 06:24 AM   #668
RNMarsh is offline RNMarsh  United States
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Originally Posted by kevinkr View Post
If we could get someone to do some pcb of the Marsh design and offer them as a group buy I'd be in for 4..
They are done already... go over to the headphone section and see the one with Marsh in the subject line. I didnt make the pcb but they are available.
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Old 4th September 2012, 09:15 AM   #669
bcarso is offline bcarso  United States
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Originally Posted by fitzfish View Post
Hi Scott,


Since no one has jumped in with suggestions I have a couple for the gain half–

The 2SK2145 looks very interesting. Noise performance still needs to be tested. Q1/Q2 and Q8/Q3 could be a matched NXP pair. The PMP4201 and PMP5201 look reasonable. For Q15/Q16 the BC847BVN pair might work. All have more gain and lower Ft than the 4401/4403. Still worth a look as they are duals and should work reasonably well for thermal balance. Reasonably matched pairs for the mirrors as well in a single package.

For SMT diodes or other small transistors see the NXP leaflet - http://www.nxp.com/documents/leaflet/75016734.pdf

I have not simmed it so I am shooting from the hip on the NXP duals.

How critical is matching for the output buffer?

Dave
The 2145 is two 2SK117 chips, probably from adjacent locations on a wafer. The 117 is roughly 1/2 of a 2SK170, that is, the same channel length with about half the width, thus same figure-of-merit of gm/Cgs, except for parasitics. The gate leakage for a sample Wayne sent was below 1 pA at 9V G-S. I'll get a noise measurement of it soon, but according to Dimitri they manage about 1nV/sqrtHz. Not much not to like except the committed channels connection, which limits versatility just a bit, and of course the difficulty of breadboarding with a five pin SM package, and finally the dissipation limit, although I think at least the gate leads have fairly low thermal resistance to the chips.

Until NXP is persuaded to make a BF862 dual (and the latter was developed for AM radios, so the prospect may be unlikely) they are very attractive.

Brad
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Old 4th September 2012, 09:23 AM   #670
bcarso is offline bcarso  United States
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Originally Posted by RNMarsh View Post
I went today and dug up some equipment and got at the output (closed loop) more like -100db ref 1v. One channel a little less, one a little more. BW limited to 100hz to 10KHz is the best i could do towards an A weighted figure. No load and input shorted. -RNM
[should do better with Rf/Rg lowered and a couple other simple things. I'll work on it.]
Thanks for checking that out Richard!

I did some work with a 2SK170/2SJ74 pair in sim with a somewhat different topology, including a complementary DMOS output and more cascoding, necessarily a good deal more parts and higher rails, to explore the possibilities. Of course without SJ74 it is impractical I may show it anyway. I pushed the feedback divider down to 66.5/20 ohms, and the adjustable values in the sources to a net ~5 ohms. As mentioned, the low pinchoff FETs are close to their zero tempco operating points near Idss to begin with, which helps the offset stability concerns.
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