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.

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

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


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.

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

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.