Threshold S/500 Stasis amplifier in black. Late production with high serial number (2122).
Rated as 8/10 due to age.
Power output is 250 watts per channel into 8 ohms, and has enough current to drive a two ohm load with ease.
Reference the owners manual, the input impedance is 75K ohms and can be driven with most tube preamps. I used a Audio Research LS-25 and the sound was wonderful.
The S/500 is one of the very few amps made that has the smoothness of triode tube amplifiers with the addition of thunderous power and fast dynamics. In my opinion, the S/500 was the best sounding Threshold amplifiers that Nelson Pass ever made. the S/500 can play at loud levels without ever getting harsh or edgy. The sound does not change or degrade with increased levels. The S/500 amps sounds great from the lowest level to the highest level of output. The S/500 amplifier has a big following. Most owners don't part with the Threshold models. The Stasis design is truely the best design ever in amplifier design. I better stop bragging about this amp before I change my mind of selling it. Please grab it and give it a good home.
The stasis design is very unique. Instead of the common transistor amp designed with the output transistors connected as a emitter follower, (current buffer) the Stasis circuit is designed with the output transistors connected as a common collector output with current feedback to it's driver transistors. There are twenty of 20 amp rated outputs in each channel of the S/500 (Ten in the + rail, and ten in the - rail). Resulting in smooth 'tube like' sound and great dynamics. The sound is smooth at any sound level without getting bright or harsh as level increases. The stasis design is about the most linear output stage ever designed and can perform linear as a class A design in AB mode. As matter of fact, even all the Burmester amplifiers use the Stasis design!
This late production uses the 'OPTICAL BIAS' and is stated on the front panel. 'Optical bias' is a circuit that uses a opto IC in the bias circuit and coupled with a thermal sensor that is mounted to the heat sink. the concept of the design is to regulate the temperature of the output transistors.
The S/500 amp was part of my Threshold collection, and used them very little in a third system. the amp is in factory original condition. I am the second owner, and bought it from the original owner 10 years ago. I have the original box and manual.
The S/500'is hard to come by in this excellent condition with low hours. The flash makes the black anodize look a little purple, but in regular light, the metal is black, and not faded. Local pick up is preferred, but will ship, if you pay the amount for FEDEX Express air, and the cost for a larger outer box. I will not ship a quality audio piece ground cause it is too risky to get damaged. I highly prefer shipping FEDEX express air, since it does not involve conveyers, but the cost is high, if you want to pay it. FEDEX express air to CA would cost around $200. PayPal add 3%. Checks and money order must clear first, and cash is welcome on pick up.