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In Reply to: RE: Volume Control posted by Eli Duttman on June 18, 2015 at 10:38:25
just to clarify, I was suggesting the *line* stage from the back of the RCA tube manual, which uses a single 12AU7, not the phono stage, which uses the 12AX7 and requires a load of 220k or more. The OP's mono system would then be: Lafayette phono preamp---->RCA line stage---->Quad II amp. I've used this line stage with Quad II's (though I used octal tubes), and it works very well. The Quad II has an input impedance of 1.5meg ohms IIRC, which is easily driven by a 12AU7 (or 6SN7) cathode follower. The input impedance of the line stage is set by the volume pot, which is spec'd at 100k in the RCA tube manual. I don't have a schematic of the Lafayette phono pre, so I couldn't say whether it would drive 100k. If it takes the output off the plate of the 12AX7, probably not, but if it has a CF output, then it shouldn't pose a problem.
The easiest and most elegant solution would still be to source an old mono Quad preamp. They show up on auction sites once in a while (hint: try in the U.K.), and don't seem to go for a lot of money.
Follow Ups:
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
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.
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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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