Threshold S/500 Series II optical bias and Wolcott Presence on Sound Labs

Hi, can anyone please provide listening impressions on the Threshold S/500 and Wolcott Presence amps driving a pair of Sound Lab A-1’s or A-3’s. I was a long time admirer of Henry Wolcott’s engineering that went into the reputable Presence P220M and P280/S. The Presence apparently mated incredibly well with Sound Labs.

I currently use a Threshold S/500 to drive a pair of A-3’s. Though the Threshold performs excellent, just wondering if energy spent searching for Wolcotts is a futile attempt both in context of their rarity on the market nowadays and also the potentially marginal returns on performance in comparison. It would have been great to have afforded a pair of Wolcotts when they were more prevalent but I can’t find them anywhere. Thanks for your input.
Once again thank you.  The Stereophile article is really thorough on the Sound Lab load characteristics.  I am not well versed in engineering, physics, and associated acoustical metrics: so, it will likely take me a couple of months to really understand the article.  

I visited less than a handful of audio shops over the past 15 years that had Sound Labs(two with a pair of A-1's and another with a pair of U-1's) on hand to audition:  none had amplification that was able to successfully drive the Sound Labs (They complained that there was some fluttering noise/distortion in the diaphragm or that the amps' overload lights were triggered even at low volumes: They didn't want to sacrifice the amplifiers, as they expressed, and Sound Labs sadly sat there unplugged).

Our Sound Labs just came in a little over a week ago:  The S/500 opened up a new acoustical dimension by bringing them to life in a way that I was never able to experience before.  Piano reproduction -- attack and decay -- and the immediacy(speed?) of instruments like cymbals, hi-hats, is simply uncanny.

Well if Soderberg serviced it last it should be fine to go. I would keep it.

The sound labs have an evil load up high also with a nasty negative phase angle, which could be seen to the amp as low as 1 or 1/2 an ohm, you want the Threshold for this, the tube amp could sound very distant in the highs with this type of load.

Cheers George

Thanks for the suggestion George:  The amplifier had the renown Jon Soderberg updates done to it.  It really is a great and legendary piece of engineering, period.

PS: Should you want to keep it and get a service job done on it to bring it back to life, I have all the schematics for it, if you or your service tech should need them.

I would say if original it would want all electro caps on the input/driver boards done, and maybe the PS caps as well if they are leaking. 

Cheers George


I owned an S500 optical for a while. Nice amp, it's a keeper, that is 250w and for memory used 20 bi-polar outputs per ch, and it has a big power supply, so it should have no trouble driving anything.

For optimum sweetness the heat sinks should be around 40-50c on a 30c day for optimum classA bias current. After a couple of hours warm up, that equates to NP’s palm of hand test of about 5 seconds before you have to remove them, so long as you don’t have hands like a brickie. Both side the same naturally.

Cheers George