Worrying noise....

Hey people,

Slight prob; when switching off my power amp (was a dynavox with EL34's now heavily re-worked with 5881) I get a loud 'thud' through the left speaker. Its only started in the last week or so and doesnt do it every time. Otherwise, all sounds fine, its just this noise which doesnt sound good.

Ive checked all earthing and it appears okay. I've tried various ways of turning things off (ie cdp first etc etc).

So, any help would be most appreciated, thanks.

 

Hey.....hows things !

My left channel on the a for mentioned amp is making a scratching noise on power up ...any ideas ?

I thought it would be one of the caps but the ones I've swapped so far have improved the sound but not eliminated the noise. It remains in the left channel even when all valves are swapped to the other side !

 

tbone : check the values of the resistors that go direct to ground, how long since the rebuild?


zanash : is it a rustling noise like leaves? try cleaning the valve pins or maybe buy new sockets, or it could be a dodgy resistor, check values between channels. but I'd say it was a dodgy socket have you tried wobbling the valves while it's on?


good luck

 

yes spot on.....exactly like rusting leaves

I'll give it a whorl today......

 

I used to get that with my Concordant Quad IIs. A flick of the valve with a fingertip cured it

 

Hey Pete,

That sounds like a valve socket problem, maybe a clean with isoprop might cure it or even changing the sockets for some ceramic ones.

Im still no closer to my 'thud' sound, though it has been suggested a problem with the output caps on my pre, any further suggestions?

 

I've had a number of amps that thumped on turn on or off normally equal on both channels ...never had much sucsess sorting it.. sorry.

 

Have you got a component bridge? just check all the components against the good side, I think the popping is probably something to do with a dodgy component direct to earth, try checking/comparing the resistors with a multimeter on both channels. and checking for caps with low resistance.
good luck.

 

good one !

 

did it work?

tbone : have you tried swapping the rca's round?

 

seems less but still there.......

I'll try again with some deoxite etc.

 

i'd change the sockets, if cleaning the valve pins doesn't work then they are probably dead, it should only cost about £30 and should improve the sound, try changing the resistors for 2w 1% tantalums or just 2w 1% standard types as the thicker leads seem to improve the sound, especially when used on the input, also helps with keeping the channels in balance. (the 1% bit)

or you could try bending the pins in the sockets a bit and get a length of wire and put some small pinches in it at the end with pliers and twist it in the holes, are the valves really easy to get in and out?

good luck

 

Okay, I checked everything on the power amp and all is okay. I can switch the amp on and off all day long and no noises come through the speakers, so it must be the pre. I checked everything on there as you suggested and it all seemed okay. It has been suggested that some DC maybe getting through so Ive changed the output caps on the pre (I already had them on order anyway). Ive not switched it off yet but it does sound much better (a hum I had has totally gone) so it will be a partial success even if the thud is still there!

I'll see what happens when I switch it off tonight.

 

Still no luck, well did at first.

found a very dodgy joint on a bypass cap I fitted (smacked hands for me). When I resoldered all was well with the world till last night when the 'bang' came back. I can only describe it as a similar sound to removing phono's when evrything is switched on. i know this points to an earthing problem but Ive checked everything over and over again and everything sounds great other than this. Any more ideas??

 

If the was amps ok for a while...then look for a similar joint or one thats opened due to heating ?

you could remobiles all the solder joints one at a time ...at least thats sorted in your own mind.

My amps stopped rustiling ...but has decided to be a little over the right just as if you had turned the balance......after a couple of hours it seems to settle back into the middle ??

Picked up some big 470uf 400v caps off flea bay ....very nice sounding/better than the cheap chinese ones fitted!

 

try changing the resistors around the pre valves for 1% types, or look for resistors that are close to other resistors, that aren't close in the other channel.

tbone I'd do what zanash suggests, then test again, it should only take a couple of hours, also do the connections to the output transformer, as this has caused me problems before.

good luck.

 

also have you changed ic's? and try connecting the ic with only the conductor and see if it pops then check the earth on the phono connector.

 

here's a pic under her skirts.......

 

can't see much from the photo, I'd get that heater wiring twisted up if I were you *blue dots*(does it hum ? or is the heater dc?, if it is then you don't need to worry as much) *won't affect the balance* the resistors look fine space wise. (I didn't realise it was all on a board) the problem could be a dodgy resistor, the red dots indicate where I think the cathode bias resistors are ? check because it looks like they might be in a different place on the right. normally pin 8, 2+7 are heaters, *On the output valves* count clockwise from the biggest gap in pins from underneath or key, the pre I cant see well enough but probably 8+3 and 4+5 for heaters. try these first, just make sure they are what they say they are, in circuit at first just to see if they measure similar each side,and you could check voltages on startup and when it's running well. *take precautions gloves etc* but before all of this, you have tried changing valves and ic's round ?




ps I must stress I'm no expert, but I have built and repaired amps before with only moderate smoke and fire

 

I've got the schematics .....
So thats not a problem

I sent you a pm ....

Thanks so far !