Harman Kardon 775 mono amps earthing

[metako]

I just bought some HK775 mono power amps and they have a low level hum when powered up and connected to the speakers but not the preamp. When I power up the preamp the hum gets louder. I moved the power lead to another wall outlet and the hum got softer. I cut off the earth prong on a double adapter and plugged the amps into this and the hum goes away.

Is it safe to use the amps without an earth (240V Australian power)? If not, what else can I do to eliminate the hum? Would this be caused by faulty power caps?

Also, when I switch the amps on the is a delay and then a static-like sound comes from the speakers-one amp is worse than the other. Is there anything I can do about this? Link to schematic below (PDF was too big a file to attach).

Harman Kardon HK775 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

 

[metako]

I've found some interesting stuff here

Audio Asylum Thread Printer

and here

http://www.diyaudio.com/forums/diya...udio-component-grounding-interconnection.html

I've started switching off computers and TVs and set-top boxes etc and results are promising for the hum at least.

 

[jaycee]

Do not use the equipment without an earth connection. instead find what is causing the earth loop. If you have Cable TV connected into the system in any way, this is usually a culprit.

The hum when powered up without the preamp connected may be simply the amp picking up stray interference. To test this you need to short the input connection. If you still have hum with the power amp inputs shorted, there's a problem.

 

[metako]

Thanks. To short the connection I just connect the positive with the negative on the phono input socket? This won't harm the amp?

 

[jaycee]

Yep exactly that. No it wont harm the amp

 

[digits]

You could also try a smple wire from one amp's ground to the other's, that's why I prefer my monoblocks, in one case, with two power inlets.

 

[metako]

When I short the inputs the hum goes away. I take it this indicates there is no internal problem with the amps-great news! Also when there is no RCA lead plugged into the amp there is no hum either. Thanks for your help Jaycee.

Digits: I connected the earths of the amps but the hum remained.

Any ideas re the noise on turn-on? The delay is obviously intended to prevent these noises so is there a particular component in the delay circuit which I could replace?

 

[metako]

I've scanned the schematic and the power protection board layout below.

Would noise in the protection circuit most likely be caps C17,18,19,20 or transistors Q1,2,3,4,436,437 ? Thanks