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SCA-35 Phono Board Modification

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Here are the SCA-35 phono stage mods that I promised a few weeks ago. It is really not much more than changing it to an improved PAS phono stage. A similar procedure was posted in the Tubes Asylum a few years ago by Adrian Chan. Also, I have seen several different recommended RIAA feedback values recommended over the years, all claiming to be the best. I have chosen the values that Joe recommended for the PAS a few months ago with what I think are excellent results. So here it is; you’ll need the schematic to follow along. Joe has it at the end of the Technical Support page at curcioaudio.com if you need it. Also, this procedure will disable the (now obsolete) tape head input.

Values given are for left channel, all resistors are ½ watt.

R1 – 1.2M, no change unless PEC is removed from phono input jacks, then change to 47K
R2 – 150K, no change
R3 – 5.6K, change to 47K
R4 – 120 ohm, change to 1K 5% carbon film or 1% metal film
R5 – 120 ohm, change to 1K 5% carbon film or 1% metal film
R6 – 1.2M, no change
R7 – 47K, change to 100K
R8 – 390 ohm, change to 1K 5% carbon film or 1% metal film
R9 – 18K, replace with jumper wire
R10 – 100K, remove, do not replace
R11 – 47K, replace with jumper wire
C1 – 0.1 mfd, replace with 10K resistor
C2 – 0.022/400V, replace with same value new cap
C3 – 0.1 mfd/400, change to jumper wire
C4 – 330 pF, remove, do not replace
Replace wire from eyelet 11 on circuit board to lug 2 on selector switch with 0.1/400V capacitor.
Remove wire from eyelet 3 to lug 4 on selector switch.
Make up a new feedback network consisting of 2 resistors and 2 caps. Solder a 95K resistor in series with a 2M resistor, both 1% metal film. Then solder an 820 pF/500V silver mica or polystyrene cap in parallel with the 95K resistor, solder a 2700 pF/500V cap in parallel with the 2M resistor. Solder the 95K end of this network to eyelet 11, and the 2M end to eyelet 3. Be aware that space is really at a premium. I soldered these networks on the bottom of the board, but in doing so, you must ensure that there is no chance of shorting against any other wiring. BTW, some of these are odd values; all are available from mouser.com if you don't find them elsewhere.

That’s it! You are now ready to enjoy a much better sounding phono stage. My concerns about hum were unfounded, even though the unit uses AC on the heaters. No hum with the volume fully up when using LS3/5a speakers. If you’re using very high-efficiency speakers, the Sovtek 12AX7LPS might be a worthwhile investment. But I have used 70’s Mullards and new production Chinese 12AX7s with excellent results. Gain is reduced by about 10db from the original, but is still adequate.

Beyond what I have already mentioned specifically, resistor and capacitor types are very subjective, so I’ll leave that issue alone other than to point out again that space is at a premium. I have used Illinois MWR metallized polyester and CDE metallized polyester coupling caps with good results, and they don’t take up a lot of space.

Any further questions, please feel free to ask.

Best regards,
Mike




Topic - SCA-35 Phono Board Modification - Flyquail56 20:10:25 03/5/06 ( 0)