cambridge audio a500 amp problem

[tim0850]

sometimes at switch on the left channel makes a loud static/discharge noise and the woofer cone moves fully out for about three seconds annd then in for the same time this is all accompanied by a loud hum then the music starts this sometimes also happens after switch on thanks for any sugggestions for a problem fix

 

[Extreme_Boky]

Your amp probably does not have a turn-on delay protection relay. If this is a case, your amp might have problems with dry capacitors, non-symmetrical power supply rails, and maybe faulty input differential transistor pair. It's very hard to tell more without checking some voltages. Can you measure power supply rails, and base voltages at input differential pair?

Has this misbehaviour been happening from day 1 to some extent, and did it get worse since?

Cheers

 

[cunningham]

You are getting a dynamic responce from the power supply. The coils (speaker, transformer) and filter caps is acting like a tank circuit and causing the very slow freq. occilation you described. I designed a sub-amp that had this very same problem.

On ways to fix it? Not to sure. If this is a new problem, then some componant may not be working so good. maybe a bad filter cap? If it has always done this, you may try adding some small resistors in series with the supply rails. (design problem)

also, do you have any passive filters on the speaker? chokes? coils?

 

[Biggles]

I had (still have in my attic) a Cambridge P35 amp.

There was no protection at switch on and the volume had to be down to switch it on - otherwise there was a horrible screech and then a big whump!

Sadly after many years (17) of faithful service it sarted making a fairly ghastly sound through one channel (left - I think) which quickly became continious. I tried to get it fixed but no one was sure what to do as it was not a particuarly common bit of kit least that is what they said as they gleefully sold me another amp

Hope you have better luck.