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Re: SCA-35 - slow moving train

Posted by JimL (A) on April 1, 2007 at 21:53:14

In Reply to: SCA-35 - slow moving train posted by kurt@bauer.org on April 1, 2007 at 17:20:49:

The phono section looks good, but as I've mentioned before I would substitute a 24 k resistor for the 22 k resistor in the RIAA network - it measurably improves the EQ. By my spreadsheel calculations, using the nominal resistor and capacitor values, the variation from ideal RIAA EQ decreases from plus or minus 0.4 dB with the 22 k resistor to plus or minus 0.2 dB with the 24 k resistor. Use 1% components for the RIAA if possible. The addition of the FET transistor source follower at the output and the extra power supply cap for the first section of the phono section are good additions, if you can fit everything into the small space allowed.

In terms of R1, R2, C1 and C2, I assume you're talking about the amp section. C1 plus R1 and R2 serve as a high pass filter to prevent DC from getting into the amp. If your sources don't have any significant DC on them or are capacitor coupled, you can get rid of C1. C2 serves as a high pass filter in conjunction with R1 and R2. Again, if your source doesn't have ultrasonics you can remove C2. I wouldn't remove R2 because it keeps the grid of the input resistor at ground potential which is needed for the circuit to function properly. If you remove C1 and C2, then the only function of R2 is as a grid stop resistor.

It would be difficult if not impossible to get DC to the heaters without using a different power transformer, because in order to convert AC to DC for the heaters you need voltage and current headroom which the stock transformer does not provide. In other words, if you rectify the AC heater windings you won't have enough extra voltage to get to 12 volts DC due to obligate voltage losses through the bridge rectifier, and in addition, you'll need extra current capacity which the stock transformer does not have. For a bridge rectifier, which you would need because the heater winding is not center tapped, you'll need the AC rating to be 1.8 times the DC current needed for the heaters assuming a capacitor input, which is the only input that will get you anywhere near the voltage needed. In fact a number of people have commented on AA that the stock transformer is just barely capable of supplying the amount of juice the SCA-35 needs.

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