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SCA-35 rebuild/mod -- some questions

Posted by kurt@bauer.org (A) on March 18, 2007 at 11:46:26

Hello Everyone,

I've embarked on the rebuild of a nearly-dead SCA35. It will power up with no smoke or angry red tubes but makes horrible howling sounds (through the speakers - quiet with no speakers). In preparing for this voyage, I got a reprint of James Lin's AudioXpress article and have read the Skaggs & Crawford Glass Audio article he references. I've also been on the Roger Stevens site, http://home.netcarrier.com/~rstevens/sca-35.html and had some helpful input from various forums. So I have a general plan on what I want to do but have some questions and need some clarifications (especially if Jim is around).

Here are the phase 1 plans...

1. Change out input connectors. In that process I plan to remove the extra low-level inputs so I'm left with one phone input. In the process the input equalization PEC goes away.

2. Upgrade the power supply. I've ordered Sheldon Stokes' PS board and will use fast recovery diodes will establish a star(ish) ground system. I'll also change the input power to a 3-prong, fused IEC.

3. Circuit/wiring upgrades. I have ordered new circuit boards from John Sullivan (mine are a little crispy) and will use carbon or metal film resistors and modern caps throughout. I'll also require the whole deal 22 teflon hook-up wire.

4. Bypass the stereo/mono, loudness, and filter switches. I'm trying to get a much out of the circuit as possible.

5. Bypass tone controls. I don't want to switch them in and out as Jim and Roger did. I just want them out as I've done with favorable results in my PAS3.

6. Upgrade volume and balance controls to stepped attenuators and maybe upgrade the selector switch. I can live with the cross-talk of the selector switch since I'll have one input live at a time but if there's a sonic improvement, I'd like to go for it -- opinions encouraged here.

The questions start with the removal of the tone controls. Jim had written that the tone control network introduced a 17dB loss in the midrange so when switched them out of the circuit, he added a voltage divider. This makes sense to me but what I can't match up is the loss: I get -15dB for power (proportional to R squared?) I know this is simple so I've got to be missing something -- please for forgive my ignorance, it's been a long time since I've studied this stuff.

Charging forward, in my desire to upgrade the volume and balance pots to stepped attenuators, I have discovered that acquiring anything over 100k Ohms is hard to source. A helpful soul over at diyAudio has pointed me in the right direction but, given the cost and the overall attentuation issue if I'm not better off redesiging this section and taking a more standard path. FYI, the pot values for the volume and balance pots are 250k and 750k, respectively.

I'm confused on how to move forward. On one hand, I think the selector-swith-input-to-power-amp-input circuit can be redesigned using 100k stepped attenuators for volume & balance. I think this is a modified voltage divider arrangement that may simplify the circuit I'm not sure what considerations are important to the input of the 7199 circuit.

On the other, will I get better sound by staying with stock values and add a 17dB (or whatever it is) attenuation between the volume and the balance controls. To further confuse the matter, Jim added a 500k Ohm (not 250k) volume control replacement and Roger added no attenuation at all when taking out the tone controls.

Any help clearing my fog would be greatly appreciated.

Many thanks.

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

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