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Broadcast RIAA Blues
Broadcast RIAA Blues
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Old 7th September 2016, 04:11 AM   #11
wrenchone is offline wrenchone  United States
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1) I would not want the cartridge impedance interacting with the gain resistors

2) The noise was subjective. If you pot up the microphone sliders on the same mixing board to the same relative position, no hiss. Surface noise or no, I would have been snowed under with dreck by my peers unless the performance was the same. It was also maddening that the second quietest opamp on the planet was so noisy in that application.

3) The OPA134 sounded subjectively worse. The difference was immediately apparent, that "something doesn't sound right" that is the kiss of death. The AD745s from Mr. Wurcer didn't have those issues and are still in place to this day (thanks, Scott...)..

The second circuit I show in the first post, used in the master studio preamps, is fatally flawed in retrospect. It will be replaced in a few months by a much simper circuit that will likely kick its spotty behind re listenability.
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Old 11th January 2019, 10:22 PM   #12
tiefbassuebertr is offline tiefbassuebertr  Germany
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Quote:
Originally Posted by scott wurcer View Post
Someone needs to revive the old National Semi app note on the equivalent noise impedance of MM cartridges. If you look at the real part of the impedance at high frequencies the noise current of low noise bipolars is a real problem. Indeed the AD745 (or OPA627) would do the trick in this case. I designed the AD745 and I have trouble getting any, unfortunately they've been transfered to another business unit. One of our guys used to DJ for KLAX and he has 745's in his home system. Both of us are big supporters of college radio here in the Boston area, maybe we could find a couple of DIPs in our junk boxes.
Some users claim, that this model together with the AD743 is still the best available integrated operational amplifier for MM/MI RIAA phono preamps. LC-Audio also think so - go to
L C Audio Technology / RIAA/MC Amplifier
(last image)
Unfortunately no simplified schematic I can find in the datasheets - go to
https://www.analog.com/media/en/tech...eets/ad745.pdf
https://www.analog.com/media/en/tech...eets/ad743.pdf
Is it similar to those from AD825/AD829/AD817 (only one voltage gain stage as folded cascode and buffer stage) ?
AD817 is in use in the Graham Slee RIAA stage:
Vinyl Asylum
And what about with a AD797 with 2SK389 or other dual jFET at the inputs ? - go to page 9+10 under
http://www.cordellaudio.com/JFETs/LSK489appnote.pdf

Last edited by tiefbassuebertr; 11th January 2019 at 10:34 PM.
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