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Re: SCA-35 rebuild/mod -- some questions

Posted by JimL (A) on March 18, 2007 at 20:18:21

In Reply to: Re: SCA-35 rebuild/mod -- some questions posted by Eli Duttman on March 18, 2007 at 16:23:33:

Eli,

I like the RCA circuit for this particular application because it's simple, classic, has much more accurate RIAA EQ than the original, and uses the same tubes as the stock Dyna, but agree that high output impedance is its Achillies heel. Not familiar the ZVN0545A source follower tweak but it sounds interesting. Only problem is that space is very limited (open board space measures appoximately 3" x 3 1/2" and the selector switch overhangs about 1/2 of it) in the SCA-35 - the basic RCA circuit barely fits without additions, and in fact I had to omit the first stage RC decoupling cap in the power supply due to space limitations, and also reduce the output capacitor to a mere 0.1 uf for the same reason. Squeezing in another output cap for the source follower would probably be very difficult if not impossible, but it sounds like an excellent idea in an application with a bit more space.

Changing the 22 kilohm resistor in the original circuit to 24 kilohms improves the RIAA EQ, and I have so posted in the past - the accuracy improves from approximately plus or minus 0.4 dB to plus or minus 0.2 dB, which shows I think that the RCA engineers knew what they were doing. They did as well as could be expected using 10% resistor values. Both are much more accurate than the stock Dyna circuit, which in my sample measured plus or minus 2 dB! Similar results were reported in Hi Fidelity and Audio in reviews of new SCA-35 so it's not just due to aging components.

The 680 kilohm resistor is not only a grid leak resistor but also part of the RIAA EQ, changing that will alter the RIAA accuracy very slightly in the highs but significantly in the lows - the nominal low frequency rolloff will go from 50 Hz to 30 Hz. In theory the values for the cathode bias cap should result in a roll-off of the second stage around 2-3 Hz, which I believe is low enough, considering that we are dealing with an integrated amp not likely to be used with speakers that can get anywhere close to 20 Hz. Not to mention that the low frequency response is further limited in my version by the small output cap. Of course I can't answer for the subjective impression of the changes you suggest. I do wonder though whether the improvement in bass extension is due to the change in RIAA EQ rather than change in the bias method.

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