Threshold SA-1 Class A Monoblocks

Own a Classic Pair - Completely restored/rebuilt by Threshold Audio in June of 2010 - see $1,900 receipt - Original Shipping boxes and packing

Description: Solid-state monoblock power amplifier. Rated power: 160W into 8 ohms, 300W into 4 ohms. Current capability: 40A continuous, 60A peak. Frequency response: 7Hzצ100kHz, +0/–3dB. Voltage gain: 26.5dB. Input impedance 75k ohms. Dimensions: 19" W by 8¾" H by 19" D. Weight: 78½ lbs.
Price: $6000/pair (1986); no longer available (2006). Manufacturer: Threshold Corp., Sacramento CA 95815 (1986); Threshold Audio, Inc. PO Box 41736, Houston, TX 77241. Tel: (713) 466-1411. Fax: (713) 856-0278. Web: www.threshold-audio.com.

The SA-1's input stage consists of four matched junction-type FETs, with the first pair operating as a differential pair and the second pair in a cascode arrangement. Because JFETS have an inherently very high input impedance (unlike bipolars, which draw current from the input signal), it is unnecessary to play games with feedback or "bootstrapping" (footnote 2) to minimize the load across the preamp outputs. There is consequently less chance for adverse interactions between the preamp and the power amp. In addition, the front-end elements are so arranged as to behave complementarily in response to power-supply voltage variations, making the front end virtually immune to signal-induced supply fluctuations stemming from the high-current output stage. Not that such fluctuations should be that much of a problem here: The SA-1 has a humongous toroidal power transformer and an incredible 120,000µF of power-supply storage, suitably bypassed with film resistors.

The output stages look like overkill, too. Threshold claims that each output device is rated by its manufacturer at 20 amps at 200 volts, which gives each one a power capability of 20 watts for every watt of rated power! That's called conservative design; it allows the designer to dispense with "protection" circuits, which nearly always befoul the sound when they're not, occasionally, saving the amplifier.

The SA-1 has no overall feedback, no output fuses, and no inductor in series with the output (to protect it against very low impedances at very high frequencies). These things allow it to present an extremely low source impedance (high damping factor) to the speaker, which no doubt contributes—along with its very high current capability—to its remarkable bass performance, about which more later.