[GrooveControl]

I have an older 2B-LP (non toroidal) with a fairly loud hum direcly from the unit, not the speakers. The hum is so bad that i can't use the amp in the house.  Even as a garage amp I find it annoying. 

Back in the eighties I recall bringing such a unit in for service once, but all Bryston could do was tighten the transformer screws. I tried that on this unit, but that didn't help. 

Forty years later...  has anyone discovered a way to solve the hum problem in these units without converting the transformers to the toroidal type?  I can't justify that cost, and am hoping for an inexpensive solution. 

I read once about dipping the transformers in some sort of shellac, but don't have enough details. 

Any thoughts?   

James,   any magic bullet here?

[GeorgeAb]

If the hum goes away when you float (remove from case and place on rubber mat, etc.) the transformers, I would look for non-flammable material to isolate transformer from case. Caulk, cork, rubber something like that you can put between case and transformer and then tighten screws enough to secure, but not tight enough to squash materal. If the hum is coming from transformer you are likely toast, but if it is the transformer vibrating the case something like this should work. I should state the obvious of do not handle transformers when unit is powered on and remember that those caps will hold a significant charge if there is no circuitry to bleed the current off.   

[John Tulett]

To supplement the suggestions kindly provided by GeorgeAb, if the cause of the hum is the power transformer vibrating the chassis, to share something I found effective.

I recently restored my 2B power amplifier, documented in the Vintage Circle forum, "Bryston 2B Power Amplifier repair - Hints and Lessons Learned".  This unit was manufactured in 1978 having standard (non-torroidal) power transformers.
https://www.audiocircle.com/index.php?topic=184949.0
Although my 2B is fully operational after restoration and sounds great, with it connected to our AC supply of 50 Hz you could hear some background hum noise that I don't recall when it was previously operated on a 60 Hz AC supply.

While shopping at a local hardware store I found this product called CELLDAMPER made by INOAC in Japan.
https://www.inoac.co.jp/en/solution/celldamper.html
From their website, "CELLDAMPER is a foamed polyurethane elastomer developed for anti-vibration material to reduce oscillation. It is used in various fields such as civil engineering related, machine tools, and automotive fields due to its superior anti-vibration performance which was not able to achieve with the traditional foam."
A single 5 mm sheet cost me 3500 Yen, attached is the package label (in Japanese) with important information; Product Code BF700-53350, Dimensions 5 x 330 x 500 mm, Load Capacity ~ 6.0 kg / sq.cm.
The standard sheet width 500 mm was a good match for my 2B (or any 19-inch rack mounted equipment), just placed my 2B amplifier on top of the sheet, no cutting or trimming needed.

I then experimented with some simple sound level measurements using a third-party SPL app on my cell phone.
With the 2B amplifier powered on and warmed up, no audio signal just the background hum, cell phone placed in front of the amplifier with SPL Meter set to C Weighting, observed a repeatable 3-dB to 5-dB reduction in background noise level when the amplifier was placed on the CELLDAMPER sheet, compared to the amplifier on the shelf with no sheet.
Of course, this was no precision laboratory measurement, but it did show that the CELLDAMPER sheet made an improvement in reducing vibration noise.

While not an endorsement of any component supplier, the background hum noise is barely audible now so hopefully this may give some ideas what you could try.

Thank you,
John

[Clive197]

To me it sounds like a classic case of DC on the mains. Try a DC Blocker between mains and unit. There not expensive and can have great results.

[GrooveControl]

Thanks for the input guys. The hum is there even when holding the amp in my hands.  There already is a rubber pad between the transformers and the top, but it doesn't help much.   I've had three encounters with the non toroidal version of this amp, and all exhibit the same problem.  I guess I'll hold out for a badly beaten non-working toroidal version and swap amp modules. 

thanks

[John Tulett]

To share one more suggestion to investigate the audible hum, after an hour or so of the amplifier powered on, is the chassis hot to the touch?
This is what happened to me when restoring my 2B, initially I didn't pay enough attention to setting the output stage bias current, with the result the bias current was running high creating excessive heat generation and a noticeable hum noise from the power transformers.
Taking the time to correctly set the bias current brought down both the chassis temperature and the hum noise.     

Here's a brief extract of what I posted in my report, "Bryston 2B Power Amplifier repair - Hints and Lessons Learned".  https://www.audiocircle.com/index.php?topic=184949.0
Measurements after repairs: "But if the surface is hot to the touch and/or the power supply is overloaded causing the chassis to hum then the bias current is likely running too high."

While I can't say if this is the issue you are seeing, with a decent DVM it's straightforward to check the bias value and the DC rail voltages - if the bias value is high or the DC rail voltage is sagging it can be related to excessive current in the power transformer.   When making these checks please be aware at all times of safety to equipment and yourself, when powered up there is both AC line voltage and DC rail voltages present.

Hope this give some ideas what to look for.

Thank you,
John

[GrooveControl]

just an update... I replaced the dense rubber pad on the inside top plate with a thicker 1/4" foam like material.  I don't remember where I go this material, but had it, so tried it.   

It helped a bit.  now it sounds like I'm hearing just the transformers and not the transform interacting with the case vibrations.   While still not good enough for in house use, I can live with this in the garage.