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A simplified universal differential or single ended phono preamp
A simplified universal differential or single ended phono preamp
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Old 6th April 2017, 05:28 PM   #11
DPH is offline DPH  United States
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Quote:
Originally Posted by scott wurcer View Post
still no 7th's
Pity, sometimes a minor seventh really helps a chord out.

Nice to hear it's well-behaved into overload conditions. Or, well, see.
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Old 6th April 2017, 09:36 PM   #12
scott wurcer is online now scott wurcer  United States
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Originally Posted by RNMarsh View Post
What is the CMR vs freq and the +/- PSRR vs freq?

THx-RNMarsh
CMRR was -86dB at 1kHz, pretty much follows the RIAA curve so I would say fairly constant with frequency. I was scared for a minute because I am using two channels of a sound card to make a true differential input and when I set the phase to 0 the CMRR didn't look so good. The DAC's turn out to have 0.5% gain mismatch, had to short the inputs and use one DAC.

I removed the bypassing and nothing oscillates so I was thinking I would try to measure PSRR by putting an old output transformer backwards to inject some AC in series with the supply with the 4 Ohm tap. Anyone try this before?
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Old 6th April 2017, 09:55 PM   #13
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About 7th harmonic:
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Old 6th April 2017, 10:02 PM   #14
john curl is offline john curl  United States
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Congratulations Scott, you finally found the audio usefulness of using all fet designs! Can't make 7th harmonic? Good!
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Old 6th April 2017, 11:18 PM   #15
gerhard is offline gerhard  Germany
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Quote:
Originally Posted by scott wurcer View Post
I removed the bypassing and nothing oscillates so I was thinking I would try to measure PSRR by putting an old output transformer backwards to inject some AC in series with the supply with the 4 Ohm tap. Anyone try this before?
I have made a box/little board with 2 BB/TI BUF634( 5pin-TO220) for Vcc/vee + noise / sweeper input but never really used it at all. Seriously, I need more time.

And I insist that the raw supplies are reasonably clean; if someone needs 110 dB PSSR
that means that he already has all the dirt on his board.

cheers, Gerhard
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Old 7th April 2017, 12:29 AM   #16
scott wurcer is online now scott wurcer  United States
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That's what we do with VI's in the testers, looking to avoid building something I will never use again. I had a tech once that floated a whole Kronhite but made a miss-connection that let lots of magic smoke out.
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Old 7th April 2017, 01:35 AM   #17
gerhard is offline gerhard  Germany
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Hey, I once worked for Verigy (ex HP) and I had a medium sized mixed signal tester just for my work; I was the only user, including somewhere an AP 2732(?, like an early eprom) which was a major PITA in the system and was floated. They needed someone who understood Unix/Linux drivers as well as TDR ... For me, it was too software-centric in the long run. Especially if they had the newest 50 GHz VNAs in the next cubicle and I had to ignore them. Yes, more than one or two.
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Old 7th April 2017, 07:08 PM   #18
EUVL is offline EUVL  Europe
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Almost missed the post of Scott's RIAA.
Thanks for sharing with us, and look forward to a high(er) gain version before too long.




Best regards,
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Old 7th April 2017, 07:42 PM   #19
scott wurcer is online now scott wurcer  United States
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Using the same philosophy you might need 6DJ8's at the input rather than JFET's. The gain structure for open-loop amplifiers is a difficult trade-off unless you punt the THD and say -60db is fine or something like that.

OTOH I have a third board and lots of FET's from Paul Norton there might be a sweet spot with an LSK389 and more bias current to get the 14dB of gain back.
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Old 7th April 2017, 07:50 PM   #20
EUVL is offline EUVL  Europe
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Remember the spice files I sent you a while ago for the version with 300x gain at 1kHz ?
In spice at least, THD is quite decent at 2Vrms out, using 4x 2SK372 at the input.
Maybe you would have another look at see if I have done something really silly......


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