[Don Sutherland]

I recently purchased what I thought was a 0.4B preamp with balanced outputs (which should make it a BP4). The rear panel is silk screened with 0.4B and the serial number is 504974. The preamp seemed to work fine except for no output from the balanced outputs. When I opened it up, the reason was clear. The cable for the balanced outputs is not connected to anything. I also found that one of the op-amp modules on the board was missing a 22 pF capacitor. The leads were still soldered to the board, but the capacitor was not there. During testing, I heard some rude noises (high frequency oscillation?) a couple of times briefly. I installed a 22 pF ceramic capacitor (the blue one in the photo) and so far no bad noises. The circuit board is marked Bryston 0.5B, Revision 5-85, so I am wondering if the original board was replaced without providing any means for using the balanced outputs. From the diagram in the manual for the 0.4B, it would appear that 4 op-amp modules would be required for balanced outputs, 2 as line drivers and 2 as inverters. The board in the unit has 4 more op-amp modules which have no parts installed, so possibly 2 of them could be used to provide balanced outputs. If anyone could shed some light on this, it would be very much appreciated.

[Mike Pickett]

Hi Don;

Not only is the output cable not attached to anything, the balanced output board is missing!  This board would normally sit above the output amplifiers, and plugs into those two 6 pin connectors on either side of the output amps.  The balanced output board includes two more output amps; the ones on the mainboard provide the non-inverting signal, while the missing board provides the inverting signal.

The four amp modules to the left are for the RIAA filter on phono equipped units (BP5).  It's theoretically possible to use two of these to provide your inverted output for the balanced outputs, but this would take some cutting of traces, and we've never tried it...

Thanks,

Mike

[Don Sutherland]

Thanks Mike, that makes a lot of sense. You have my vote for promotion to Senior Member! I noticed in the photo there is a checklist on the transformer that shows the balanced outputs working, so this unit had them originally.

[Mike Pickett]

Hi Don;

Thanks for the vote, but I'm too young to be a senior of any sort...

Our records confirm that the unit was indeed built with balanced output; I'm assuming someone stole the board to repair or upgrade another unit, weird as it may seem.

Mike

[rob80b]

I’m sure Mike has seen more than his share of weird things coming into Bryston for repair ...... but came across a few images that should help clarify the missing board.
http://www.ukaudiomart.com/details/649044998-bryston_bp5_preamp/images/541419/

[Don Sutherland]

Thanks Rob, that photo is very helpful. Hopefully, I can find artwork for the board or a photo of the tracks. It is a single-sided board, so it should be easy to duplicate. I would like to make this preamp as close to original as possible.

[rob80b]

Hi Don

You never know...but maybe ask Mike if Bryston may indeed have a board buried somewhere.