Biamp SR240 spring rever greater hum in one channel and general hum reduction

[rjb5191]

Greetings grounding experts!

I've been doing a lot of reading on the forum regarding grounding and it's starting to sink in (maybe)!  JR had a lot good posts on this that got me thinking that the grounding layout in my Biamp SR240 is probably not ideal.  I'm working on restoring/improving this old spring reverb and reducing the noise in the unit is a big goal.  I noticed that channel A had audibly greater 120 HZ hum than channel B.  I traced this with my scope to the output of the reverb recovery opamp (actually a LF351 but shown as a 5534 in the Biamp drawing ).  The 120 HZ was about 10-15 mV greater in channel A and than channel B.  This stumps me a bit but I guess it could be a number of things.  I haven't finished recapping the unit but have finished the power supply.  The one thing that I noticed is that  on the audio boards there is a "ground area" (large trace) that shares decoupling cap grounding, audio grounding, and shield connections!  One end of the ground area is then connected via a cable shield on the psu wire to the ground area on the separate psu board.  On this board the ground area shares the regulator ground references, filter cap grounds, and what I presume  the transformer center tap (it's shown as an earth reference in the drawing but it doesn't look like it actually is because it looks like a wire that is connected to the transformer).  I'm not sure if this presumed center tap is also an earth reference.  It's not clear as the this wire goes to the transformer and one of the transformer case connection is bolted to the earth reference.  Anyways, JR mentioned that ac currents from the filter cap grounds can induced noise in the regulator ground references.  How can I tell if this is going on here.  They are all connected to the same large ground area on the psi board about equipment-distant from the transformer center tap.  would it help to lift the filter cap ground connections off the board and connect them directly at the transformer center tap lead which is the presumed ground reference?  I'm sure this is difficult to imagine so I will work on taking some pictures and will add them soon!  Schematic linked below!

http://cdn.biamp.com/software/docs/default-source/downloads/sr-240_schematic_manual.pdf?sfvrsn=0

 

[Bo Deadly]

My guess would be that ground layout is not going to be nearly as important as other things. Specifically, the most hum and noise will probably come from electromagnetic interference picked up by the recovery transducer and from simple ambient vibration. To really see what is important, put the unit in the middle of a quiet room away from any power cables, power strips, lights, pumps, AC units, etc. If you can, remove the power supply transformer / rect / filter / reg board and make a 3 foot extension cable to deliver the +-18VDC+gnd to the main board. Then rest it on a couch cushion, put the metal cover back on (as best you can with wires hanging out) and drape some heavy blankets over it. Then take your noise measurements. That will be your baseline. I would be willing to bet a roll of toilet paper that your hum and general noise level will drop 10dB. Probably more. If you look at many of the pro units like from EMT, they're very well shielded and insulated with enclosures mounted on springs within enclosures mounted on rubber shock absorber, etc.

 

[rjb5191]

Thank you.  I agree with you and I think I will remove the power supply and build it into an outboard box.  More ground issues to consider then.  Although this doesn't answer why the hum in one of the channels is much greater.  If it is not some bad component, my guess is that it may have something to so with the orientation of the magnetic flux lines from the power transformer.  The power transformer and transducers are shielded but maybe only marginally.  I will post some pics!

 

[RuudNL]

Is it me, or is it a bit strange that they have put a 2.5:1 transformer after the reverb tank?
IMHO this wouldn't have been needed if they had changed the configuration of the OpAmp after the reverb springs.
(Higher impedance, 8 dB less gain.)