My 4B NRB hums when turned on

[Ghost rider]

It may have been there for a while but I noticed that my 4B when I first turn it on it physically hums when it's cold. Makes like a transformer sound as it warms up, goes away after 20-30 seconds.

My guess is I will be sending it in for a recap soon.

Interesting my even older 3B that I did have recapped had always made a surging noise when it was turned on. I found that by switching off the power at a power conditioner solved that issue. I always thought that the amp never discharged completely when switched off but switching it off at the outlet or unplugging it allowed it to completely discharge preventing this noise when turned on.

[BlueSky]

I don't think filter caps going bad would cause the noise that you describe. I've got both a 4BSST and a 6BSST in my system and they have always "protested" when being shaken out of a sound "sleep". The noise my amps make last for no more than a few seconds however. I think its just the sound these toroidal transformers make when initially energized.

Filter caps going bad would result in audible 60hz (or 50hz) noise coming from the speakers.

[Ghost rider]

Thanks for the explication. I think this is what mine is doing. When it is dead cold room temp 65f and is turned on it makes this noise it settles down to a constant soft hum that always had. As far as I can tell it sound great as always.

[Formerly YB-2]

The NRB amps are old enough that they would benefit from a recap. Recapped both my 20+ yr-old 2B-LPs and a nice improvement in clarity and speed. Cheap up-grade.

[Ghost rider]

I don't know if this is related to the hum impulses at the startup or the caps being old. Last night I noticed the amp does not sound good when cold. It sounded raspy no bass but once it got good and hot 20 min of playing it sounded wonderful.

[AudioGremlin]

Many power amps are like this, an initial power surge when switching on. Rapid reservoir caps charging may make the mains transformer buzz for a short period, nothing to be concerned about.
Its a good idea to recap after 20 years especially electrolytics.
Most power amps don't sound at their sonic best from cold, I usually play some music and leave the room for 15 mins.

[ajdo]

Purchased pre-owned 14SST as my first major step into hi-end audio, a few years ago. First thing I noticed was unpleasant humming noise! Called Bryston who responded it was "normal" to hear humming, up to roughy 3 feet away. Unfortunately, humming was noticeable up to 8 feet away and had to play music not to hear it. Since I also had issues turning on and off unit with front panel switch, Bryston asked me to send it back to them, it was still under warranty and based in its age and serial number needed servicing.
After several weeks, unit was sent back to me, Bryston explained they did not find any issue with front panel switch and humming noise was normal !!!! Had heard lots of good things about Bryston service but this was the most unpleasant warranty service I ever experienced. Sold my 14SST as soon as I got it back sealed from Bryston.
I'm now very pleased wth McIntosh... May not have 20 years warranty but never heard any humming from any McInstosh products I have ever owned.
Just my short experience with Bryston.
AJ