The buzz from my Threshold 400A

[davstev]

I recently acquired a lovely Threshold 400A and I am buzzing with excitement. It is the best sounding amplifier ever to meet my ears. What bothers me some, however, is a slight buzz in one channel that I cannot get rid of. It's easily lost in most music but in quiet passages it's evident enough to bother me.

I know that these things are hard to diagnose without testing the amp and some real troubleshooting, but here is a clue. The buzz is very loud upon powering up. It subsides somewhat after 10 or so seconds. Also, it decreases considerably after I power up my preamp. It would seem that not having a signal going into the 400A is somehow a problem. Why? However, even with the preamp on and connected, it's still evident (although less so). Also, only in one channel. Cable changes made no difference.

Hmmm.

Bzzzz.

Any ideas are welcome.

Thanks.

 

[Zen Mod]

if problem persists with shorted inputs , there are two possible culprits - either cold solder joint or leaky electrolytic caps (on one pcb?!?)

pictures maybe could help ...if nothing else - to tell you that problem is not visible , but we must start from somewhere

 

[davstev]

if problem persists with shorted inputs , there are two possible culprits - either cold solder joint or leaky electrolytic caps (on one pcb?!?)

pictures maybe could help ...if nothing else - to tell you that problem is not visible , but we must start from somewhere

Thanks for the ideas, Zen Mod. I will open it up and take a look, but I didn't notice anything amiss when i looked in there before. What also makes me wonder is how the buzzing varies considerably, and decreases when you turn on an external component, one that feeds the signal into the 400A, like a Cd player or preamp. I mean, just turning either of these on while RCA connected to the 400A makes the buzzing decrease by about 75%.

Still wondering what that might tell us about the problem.

 

[rayma]

just turning either of these on while RCA connected to the 400A makes the buzzing decrease by about 75%.

The component presents a lower (active) source impedance to the amp input when powered.
Have you run the amp enough since getting it, so that the capacitors should be in nominal
condition by now? Have you tightened all the hardware, input sockets, cap screw terminals, etc?
Try cleaning the oxide off the ground plate on the two main caps.

 

[wayne]

If the power caps are original I would replace them.