Bryston SST Series Power Amplifiers Biasing Instructions
The following instructions are intended only for qualified service personnel with the necessary tools, training, and experience with Bryston power amplifiers. Re-biasing of any Bryston amplifier is only necessary after replacement of critical components and should NEVER be attempted by end users.
Biasing Procedure
The BIAS in SST amplifiers is set by measuring the voltage drop across the output stage emitter resistors. Test Points from these resistors are brought out to square pin (or socket) headers labelled TP1 & TP2 that are located at the top edge of the amplifier PCB's, and are thus accessible from the top of the amplifier (after the top covers have been removed).
The BIAS trim pot is adjusted until the total of both measurements (from TP1 and TP2) add up to approximately 12mVdc. First, adjust the BIAS trim pot until the voltage at one of the test points is roughly half of the total (0.006V) and then measure the voltage across the other test point. Switching back and forth between measuring TP1 and TP2, adjust the BIAS trim pot until the combined total is approximately 12mV.
Next, let the amplifiers warm up, preferably with the top cover on the amplifier to more closely match actual operating conditions. The amplifier's heat sinks should be warm, but not hot, to touch. Adjust the BIAS trim pot until the combined measurements of TP1 and TP2 equal approximately 25mV. Allow the amplifier to run for the next hour or two, adjusting the bias, if necessary, roughly every half hour until it has stabilized.
Visual Guide to Test Points and Bias Trim Pots
14B SST (Example)
Description of the first image: A section of a 14B SST amplifier's printed circuit board (PCB) showing test points labeled "TP1" and "TP2", and a "BIAS TRIM POT" for adjustment.
3B, 4B & 7B SST Models
Description of the second image: The PCB layout for 3B, 4B, and 7B SST models, highlighting "TEST POINTS" labeled "TP1" and "TP2", and a "BIAS TRIM POT".
14B SST - Bridged Channels
Description of the third image: A bridged channel of the 14B SST amplifier, labeled "Channel '1A'" and "Channel '1B'". It shows two pairs of test points ("TP1", "TP2") and two "BIAS TRIM POT"s, indicating that each bridged channel requires two bias adjustments. The text explains that the 14B SST has four channels combined into two bridged channels, and each bridged channel (A & B) has its own test points and bias trim pots, which must be set to the same levels. The same setup is present for the other bridged channel.
6B SST and 9B SST Models
Description of the fourth image: The PCBs for the 6B SST and 9B SST models. The 6B SST image displays "TP1", "TP2", and a "BIAS TRIM POT". The 9B SST image also shows "TP1", "TP2", and a "BIAS TRIM POT", along with other components and labels like "OFFSET", "LED BRT".
Document Source: SST-BIASING-INSTRUCTIONS-2.cdr, dated 2005-11-29, prepared by WRP.
