REVIEW / Threshold SA/1
Class A Stasis mono amplifiers
Together with Mark Levinson, the Threshold company built high power solid state amplifiers before Krell Industries – the name we often associate with highpower Class A amps – even existed. The principal designer of Threshold, Nelson Pass, grounded his firm in 1974 together with Rene Besné who took care of the PR, sales and beautiful multi-layered frontplates and stylish logo.
In 1975 their first product was the Threshold 800A a so called sliding bias Class A amp. To his dismay this topology was shamefully copied by lots of other manufacturers that thought you could produce amplifiers with mediocre powersupplies a handfull of active components and a badge reading something like ‘New Class A’ or ‘Class AA’ but they never sounded as good as the original.
The 800A idled at a fourth of its RMS rating while most competing sliding Class A designs only were biased for a few Watts and thereby could get away with far less hardware. After designing a very successful smaller unit the 400A Pass moved on and designed a mixed topology with the 4000 (sliding Class A and Cascode technologies) which was in a sense the mold for all his successive designs. The 4000 weight in at 80 lbs. had a huge powersupply and twenty 150 Watts powertransistors to deliver 200 watts per channel. The unit could be bridged for 6 or 700 Watts mono.
At the winter CES of 1980 Pass revealed his ‘piece the resistance’ the Stasis 1 mono-amplifier. The Stasis topology was already patented in 1975 by Pass but it took a lot of groundwork to finalize this theoretical concept in a commercial available package. The first two units traveled around the world and were highly praised by the German magazine ‘Audio Exklusiv’ and the Japanese ‘Stereo’ where it even got the prize of the year. The Telarc recording company that made high quality vinyl pressings used a pair to drive their vinyl cuttermachines.
After the first series of Stasis amps (Stasis 1 , 2 and 3) Nelson Pass came with the next evolution starting with the S/1000 mono amp. Like the Stasis 1 it was graced with large VU meters but it was a bridged amplifier to get 500 watts at 8 Ohm. Two years later the S/1000 came in a massive parallel version that could boast 160 watts of Class A power and was rebadged to the SA/1. Quite a couple of S/1000’s were converted to SA/1’s which meant you lost one frontend board! This construction of mono-amps that come in two flavors, that is bridged and parallel configured, is still being practiced by Nelson Pass nowadays with his Passlabs amplifiers. You have the X-350.8 Class AB stereo or the XA-160.8 Class A mono with basically the same hardware in each chassis.and the X-1000 and XA-200 which use the same hardware but a different setup for the last amplifying stage and another setting for the railvoltages. The Thresholds S and SA series have become the Passlabs X and XA series in a way.
The SA/1 amp received even more praise worldwide then the Stasis 1 did and Threshold sold a lot more of them. Even in the US magazines like ‘Audio’ and Stereophile liked it instantly. The chief editor of the last magazine J. Gordon Holt made it his reference amp to power his extremely low efficient (77 dB or so) SoundLab A1 electrostatics1. The Stasis topology mainly centers around the fact the one should keep the active components in near state of stasis so that they can operate in their most linear fashion and will have the least distortion. With Threshold this mend that a handful of these transistors (the Stasis section) is kept in this state although they are directly connected with the load. An array of current dumping transistors are in a ready state to supply as much current to the load as the VA wants to deliver to keep the Stasis section out of trouble because it sees an almost infinitive resistance presented to it.
The stasis section in a SA/1 counts four TO3 transistors the other thirty six participate in the current dumping affair. In the ‘SA series’ both the Stasis and current delivering section are Class A biased to their full ratings at eight Ohm. In the ‘S’ Series the Stasis section is Class A but the current delivering section is not but is characterized as a highly biased Class AB design. So the SA/1 is expected to deliver 160 watts in Class A and that means it sucks around three to four amps an hour from your utility just minding its own business. The heatsinks become pretty hot at 50 degrees centigrade but according to Nelson Pass it’s no problem as long as they stay under 55 degrees. In 1985 the SA/1 became the reference amplifier of the German based magazine ‘Audio’ and kept that status for almost four years which is quite an achievement because the competition did not sit still in those years.
Together with the Rowland Model 7 and Levinson ML-2 I found these solid-state amplifiers the most rewarding sound wise in my setup. In fact I would opt for having these amps in such an arrangement that I could listen to one or the other with the flick of a switch so I could choose the best amp in specific circumstances like the kind of music I’m in the mood for listening to and the level I desire. Le Sacre du Printemps with the Model 7 and the delicacy of Nick Drake would call for the Levinson whereas Suzanne Vega’s first album would benefit the most with the Thresholds. We all know that this is not possible with only one listening room and limited resources. The Threshold does not possess the warmth of the Model 7 that can sound sometimes to closed in, or the magical clean sound of the ML-2. It falls somewhere in between and therefore maybe is the most all-round of the three. It can however clip when driven insanely loud which I only accomplish with the Krell KMA-200’s, Rowland 7’s or Levinson ML-3 for that matter. When it’s fully warmed up which takes approximately half an hour it develops a very liquid and seducing sound that you’re driven into. Even at very low levels it sounds full and many highpower amps falter at that discipline, that is the first important Watt.
In the US the introduction price was 6,000 dollars a pair and in Germany 30,000 Deutschmark. These are very sought after products and expect to pay the new price at least for a mint pair in the US. In Germany the going price nowadays is around 5 or 6000 Euro’s. The latest version of the two versions that were manufactured is the ‘optical bias’ (it states this explicitly on the front and back of the amp) one which will be slightly more expensive. The black versions are pretty rare but seem to be not commanding an higher price. The converted S/1000’s go around 4,000 USD and Euro’s.
Nelson Pass states that the electrolytics will probably be the first parts that need to be replaced. When he or Jon Soderbergh (former Threshold technician and now active with repairing and servicing Threshold gear) have to service these Threshold amps they replace the main large caps (four of them in every SA/1) as a standard practice. The on/off switch (which is in fact a magnetic circuitbreaker) is a possible weak spot also because Threshold never adopted a soft start delay in these amps so the influx of current is really huge at startup and stresses the internal contacts of the switch. They are Heinemann’s but can be replaced by Airpax circuitbreakers. Apart from the railfuses and AC fuse there’s no further protection then this circuitbreaker so do not fool around with these amps because they can damage your speakers when you pull maybe inadvertently a signal lead or connect a source that has DC at his outputs.
The Levinson’s and Rowlands had much better protection in that time then the Thresholds. There are four incandescent small bulbs behind the frontplates to illuminate the meter. These will burn out in time but are easy to replace (14V, 80mA). The meter itself is a Modutec – the Mustang Series according to NP (now Jewel Instruments) and is not an item at their current range so be careful with it. There are no internal adjustments to align it at 0 dB when the amp is off. The only alignment that is available is the bias-pot on the frontendboard. It’s a one turn device so a small turn will have great consequences in quiescent current level. Remember that when the amp has settled after two hours the temperature at the last insert at the back for a screw that hold the topplate should read something between 49 and 53 degrees Celcius. You can always ask Nelson Pass himself about it because he’s very active at his own forum: http://www.diyaudio.com/forums/pass-labs/. My experience is that he will address your problem within a couple of days and that’s another reason why these old Thresholds are so great.
1 Roger West of West Laboratories the principal designer of SoundLab asked in the early eighties Jeff Rowland to build him an amplifier that would sound great and could cope with the load and low efficiency of his electrostatics. Jeff designed the WL-500 the predecessor of the famous Model 7.
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