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Any experience with unbalanced channels--7B-ST?

One panel of my SoundLab A3s is at the factory for diaphragm repair, so I've put my Maggie 3.5Rs back in service in the interim.

Last night I noticed that the three players of the Florestan Trio appeared to have moved stage left, a situation I hadn't really noticed before. (Much of my recent listening has involved less formally structured music, such as several bluegrass groups, including primarily AKUS, in which right/left balances are less noticeable.)

I then dug out Stereophile's test record and my Radio Shack Sound Level Meter to see if I could find the cause of this apparent unbalance. Sure enough, the left channel read 70dB, while the right ("stage left") read a solid 76dB on the track with white noise. After swapping inputs (XLRs from my Burmester 001) to the Brystons, I got the same result. Same after swapping speaker positions. So the problem appears to be in the amps. As a Hail Mary, I then switched the amps from series to parallel operation and the problem disappeared!

Perusing the final inspection test specs that came with the amps, I noticed that the left channel (Ser. No. 774380) clipped at 631 watts, right (Ser. No. 774379) at 689 in series operation. In parallel, left clipped at 608 watts, right at 589. Am I correct in attributing the channel unbalance to the greater disparity--left vs. right-- in series operation test results, or is there another problem that should be corrected? Can internal adjustments be made to balance the amps for series operation? Of course I'd expect to get balanced channels regardless of series/parallel settings.

I plan to contact Bryston, but in the meantime I thought I would consult the many knowledgeable members of this website
to determine if this situation has been observed before.
Jim
914nut

Showing 1 response by jameswei

jameswei
883 posts
 
I'm concerned that your right channel 7B ST has a bad amplifier module.

Per the owner's manual available on the Bryston web site, each 7B ST monobloc consists of two amplifier modules. They can be connected in series or parallel, depending on your choice using a switch on the back. When they are in series, the modules are in bridged mode. Normally, bridging two amps of equal power results in four times as much power output (or gain, in this case). Four times as much power corresponds to 6 db of audible volume. If one of the two modules in your right monoblock failed in certain ways (partial versus complete failure), you might get 6 db less gain.

When you switch to parallel operation, the right channel monobloc will output the correct voltage swing but it won't have the extra amp module to tap for the increase in current availability that the parallel configuration is supposed to bring. However, unless you run way up in volume or have an extraordinarily difficult load, you won't notice the absence of additional current availability. The audible volume should sound correct at normal volumes.

Obviously, Bryston will provide the definitive answer. Good luck.
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