BBC modded Quad 405s - mains hum

[spwyatt]

I've just been lucky enough to have purchased a pair of the stunning Rogers/BBC LS5/8 speakers, complete the with a pair of the BBC modified Quad 405 frequency splitting amps (actually, one 405 and one 405-2...no idea why!)

The main problem is that both amps exhibit a low level but perfectly audiable mains hum - even with their gains right down and nothing plugged into the (XLR) inputs. Am I right in thinking it's reasonably easy to fix by changing some capacitors? I reckon these amps are 10-15 years old at least....

Any help greatly appreciated!!

Many thanks.....

[Puggie]

open them up and there is a pair of fat electrolytics in there, of they are original they are past their sell-by date, if they have been changed then they may be past it too. change those and see if that helps.

those amps are basically just a regular 405 with the input going to a seperate board to split the frequencies before it gets to the normal 405 inputs.

hum is usually the main PSU smoothing caps, thundery rumble and popping is usually transisitors or caps on the frequency splitting board.

[dcd]

You will most likely find that it's C5 (100uF 3V tantalum ?) that's to blame - the 405 has fairly good power supply rejection apart from the current source Tr1 that gets modulated by the +ve supply ripple when C5 gives up.

It's likely that the amplifiers and speakers have differing serial numbers and one being marked 405-2 is no problem. You should find that inside they are very similar as they were modified to be so.

The 405s could do with modifying, and the balanced input can be dispensed with in a domestic situation, which will let you hear them at their full potential

Regards

Keith

[AndrewT]

Hi,
how have you terminated the XLR input?

The XLR may need a temporary link to ground the inverting and non inverting inputs while you assess the hum problem.

Are you using a balanced source?

If not, then consider using a psuedo balanced source that achieves some of the interference rejection of fully balanced without any of the extra hardware that can degrade the sound quality.

Jensen AN003, fig 2.4 with the Panasonic upgrade achieves this at very low cost. Have a read of their other info as well.
The Rane site also has lots of good balanced info.

[Puggie]

Thread hi-jack, my one is now playing up

I had this problem, (hissy rumble before), and it was sorted by replacing the transistors and capascitors on the frequency splitting board, the problem has appeared again and ideally I would like to try and do something a little more specific than just replace all of the components that I have to hand.

so I'm getting a hiss and a poppy rumble, the problem appears to be on the frequency splitting board, any suggestions as to what it could be?

thanks guys