Help, Noisy amplifiers or just me being picky ?

Hi all,

I have been picky about this subject for a while, it regards noise evident through speakers when the amplifier is on but no music is playing.

I replaced an old Classe Audio CA-100 power amp as I thought it was producing too much hiss and hum when connected via RCA, it was evident at a few feet, though reduced to a slight hiss at a couple of feet via XLR. I thought, old amp, needs a service.

I replaced it with a Classe Audio CA-101 power amp, which is near identical but a later replacement model, but still there is a slight buzz at the tweeter and slight hum at the woofer via RCA which is reduced to a slight hiss via XLR at a foot or so, this is similar to the noise from the CA-100 and hardly changes when I disconnect either Classe power amp from the pre, or switch all other equipment off but the power.

Is this sort of noise normal from high current amps like these, and I am just being picky ? Or could it be noisy mains ? Or faulty equipment ?

Another problem I have, when I connect my Teac DAC I get an increase in hum, quite noticeable, and it is reduced when I disconnect the earth connection in the plug.
I use Tacima mains blocks with filtering, but the problem changes when I swap the Tacima to a cheap white plastic affair, their is a slight increase in buzz when the pre only is connected which is slightly reduced when I plug the DAC in.

Any ideas ?

Thankyou.

 

Right...

1) Hiss

Most amps exhibit this. If it gets worse when you turn the volume control right up, it's the preamp (usually the culprit). It's not necessarily to do with power... I've heard low powered amps that hiss noticeably and I've heard 450W amps that are silent with your ear pressed to the tweeter.

2) Hum through speakers

This is typically to do with ground loops in the system, in your case further evidenced by reducing the problem by lifting the ground pin.

3) Buzz through speakers

If the sound is better characterised as a buzz than a hum, this is usually 100Hz sawtooth, which is an artefact of linear power supplies. It can be aggravated by ground loops though.

4) Mechanical Hum

This one is surprisingly prevalent. Some amplifiers can be heard the other side of a room with buzzing transformers. It can seem to come from the speakers.

Try putting an attenuator between DAC and preamp.