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In Reply to: RE: Volume Control posted by gilmorneau on June 19, 2015 at 07:25:03
Hello all,
Here is a schematic. I hope it helps. I have a NAD 1240 preamp with a few line in/line out's so I may try that also.
Thank you,
Jim
Follow Ups:
That setup uses AC heating. You need tubes with a hum bucking heater to have any chance at an acceptable residual hum level. The obvious choice is buying culled phono grade Sovtek 12AX7LPSes from a reliable dealer, like AA sponsor Jim McShane.The 0.01 μF. O/P coupling caps. combine with the downstream I/P impedance to form a high pass filter. FYI, those caps. combine with 100 Kohms to "corner" at (sic) 159 Hz. Working into 500 Kohms gets the "corner" down to a marginal 31.8 Hz. Don't connect the Lafayette unit to a SS line stage, unless modifications are made.
I hope I'm wrong, but it seems that unit was intended to be used with old GE carts., whose internal inductance contributes to the RIAA EQ. Another "gotcha" is the I/P impedance. Modern MM level carts need a 47 Kohm I/P impedance, not 6 Kohms.
Eli D.
Edits: 06/19/15
The high fidelity performance potential is great with these units...More info soon...Regards...
That the schematic provided is favorably wrong is good news. The factor of 10 is important.
IMO, aside from using 7025 equivalents, to control hum, biasing the filament winding's CT off B+, instead of a direct connection to ground, is indicated.
Speaking of B+, replacing the wretched 1/2 wave rectifier with a low noise full wave bridge seems obvious to me. Other than the routine replacement of dried out electrolytic caps., the CRC PSU filter seems quite adequate.
FWIW, I still would not connect the Lafayette preamp to a SS line stage, without adding buffering between the 2 units.
Eli D.
Thank you Eli. I will weigh my options. I am in fact using a vintage GE RPX cartridge.
Jim
That preamp needs a lot of help.
--------------------------
Buy Chinese. Bury freedom.
Both stages use grid leak, AKA contact, bias, which is OK at these low signal levels. Thorsten Lösch pointed out that grid leak bias in the 2nd stage lightly loads the EQ network, for better bass extension, in a passive EQ phono preamp setup. I exploited the idea in the tweaked RCA setup. However, I have my doubts about the 3M grid leak resistors Lafayette used, in providing a sufficient contact potential.
The Lafayette unit is a stereo rehash of GE's mono 6SC7 setup, which employs active EQ. Remember, GE was 1st with their moving iron cartridges.
When you get down to brass tacks, all tubed phono preamps are either RCA style (passive EQ) or GE style (active EQ).
Eli D.
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