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In Reply to: RE: Phono Section In A Vintage Sherwood Integrated. Any Good? posted by dls123 on December 15, 2013 at 17:31:07
Don makes good points about getting the critical parts quiet and inside tolerance.
If you are slightly adventurous, replace Sherwood's phono circuitry with the tweaked passive EQ RCA setup I've uploaded.
Simplifying source selection and elimination of switching in mV. level signal lines is something I find myself recommending over and over, again. Fifty plus years ago, tape head I/Ps and switchable RIAA/NAB EQ could be useful. That's passe, in today's primarily digital world. Hard wire 1 pair of RCA jacks directly to the phono preamp and make all the other inputs "line" level. A suitable replacement rotary switch, made by Lorlin, can be purchased from Mouser for about 5 $.
Eli D.
Follow Ups:
Hi Eli,
The blue values are if using the 12AX7, right?
While the 6H2П (6n2p) pins out like a 6922, the triodes inside are the "same" as those in a 12AX7.
The blue corrections fall into 2 categories. The change in interstage coupling parts values is needed to get RIAA EQ right. The changes in O/P coupling parts are needed to work correctly into the IHF "standard" 10 KOhm load.
FYI, I've uploaded the original RCA schematic. The original interstage coupling parts values were copied and found to be incorrect. The influence of the 20 MOhm 2nd stage grid leak resistor is why. That fix came from another "inmate".
BTW, the 7025 is a 12AX7 that incorporates a spiral wound, hum bucking, heater. That feature is absolutely essential, if AC heating is to be tried in a phono preamp. Stick to DC heating!
Eli D.
Thanks for the added info, Eli. I always use DC heating in phono stages. I have an RCA version working right now with 12SL7's, I think I'll use it as a platform to incorporate your changes and see how it goes.
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