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Re: ST 70 board components?

Posted by Bill Thomas (P) on January 8, 2004 at 21:26:07

In Reply to: Re: ST 70 board components? posted by Bambi B on December 30, 2003 at 10:14:44:

Bambi,

The "domino" capacitors are 390 pf 1000 Volt Mica capacitors. The reason to keep these is that it is pretty tough to find a dipped silver-mica at 1000 Volts, and it is important that this capacitor be rated at 1000 Volts to avoid breakdown during full-power signal swings.

As for the 7199s, realise that the original "Dynaco" branded 7199s were, in fact, made by Sylvania. A fairly exhaustive comparative analysis found that the Sylvanias had the lowest noise. The RCA's came in second.

One suggestion is to make sure and select resistor pairs where the schematic indicates a 1% match. The closer you can match them, the better the phase splitter and bias circuits will perform.

One VERY controversial area has to do with coupling capacitors. In stock configuration, the .1 uf capacitors cause a rolloff of the lowest frequencies. The problem is, the Stereo 70 uses a significant amount of negative feedback directly at the input of the driver board. With a lack of the lowest frequencies in the feedback signal, the input can wind up overdriven. The way to improve this is to increase the coupling capacitors to 1.0 uf, and then use a band limiting filter at the input of the amplifier. This has become known as "The Van Alstine Mod," and again, this is a VERY controversial area, but it might be worth a look. To read more about Van Alstine's rationale, try this link:

http://www.avahifi.com/root/equipment/amplifier/70_rebuild_plans.htm

I hope this helps you with your component selections.

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