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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
jafox
1,466 posts
 
Easiest way to eliminate the speakers is to swap the amps to each drive the other speaker. Depending on where the "problem" goes, you will know if it is a speaker or one of the amps. Did you also check to make sure the Midrange and/or Tweeter fuses in the Maggies are not blown? When the mid fuse goes, it is not always so obvious.
John
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.
914nut OP
100 posts
 
Thanks for all the suggestions so far. Audiogon is a wonderful resource, thanks especially to respondents such as you.

I must say, however, that the suggestion that the Maggies may be at fault doesn't hold water. Please note that I've gone through all possible left/right combinations of inputs, speaker swaps, etc. with the same 6dB drop in the left channel in series operation (same amp in all cases).

The analysis regarding the effects of module bridging makes a lot of sense, and I wouldn't be surprised by the same response from Bryston when/if they answer my query of this morning.

Jim
ozfly
887 posts
 
Right you are Jim. I suspect Jameswei has it nailed. I definately answered too hastily on this one.
914nut OP
100 posts
 
Update:

I've heard from Chris Russell at Bryston who also feels that a module in the left channel amp has gone south. He suggested I check the two fuses adjacent to the IEC power connector. I did so and they checked OK.

Beyond that, he gave me the phone number/email address of the Bryston Service Center in VT. I called them and spoke with "Dennis", who gave me the shipping address for the return of the amp.

It will be in the capable hands of Fedex tomorrow. If what I've heard/read about Bryston, I might have it back next week. That would be nice!

Thanks again for all the suggestions and kind words of encouragement.

Jim
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