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Hum in left channel - Parasound Z

I'm using a parasound Z amp and a z tuner at work. I do not have a preamplifier, so I have been using the gains on the back of the z amp. It works fine, but it's a pain to have to reach to the back side of the amp to turn volume up or down. Plus, matching levels by channel is difficult.

I obtained a 10k stepped attenuator that I wanted to use as a temporary volume control. It's stereo as I didn't want to adjust each channel independently.

I tried the attenuator at home last night with a different system. It worked perfectly. I brought it to work this morning, connected everything, and it worked great. However, about 30 seconds in, I developed a hum in the left channel. Now, I can't seem to make it go away. It's a 60 Hz hum. I've swapped speakers, and determined it is the amplifier. I've also ruled out the source.

My ultimate question is, is it likely that I damaged the left channel of the amplifier with the 10k attenuator? If so, is there something obvious that can be replaced in the amplifier to make the hum go away? The left channel operates correctly, just has a hum - which is annoying.

Thanks for any input you can provide.
s7horton
sidssp
1,077 posts
 
There are a couple of possibilities I can think of.

One possibility is the hum has always been there but you didn't notice it
because the volume control on the amp was turned down. Now you have
turned the volume control to the max to use an attenuator and the hum
becomes noticeable.

Another possibility is that the hum is coming from an appliance you didn't
have before, perhaps a new light with dimmer, a heater, a fan, etc...

I have run into both situations before. It is worthwhile to check out.
marakanetz's avatar
marakanetz
5,473 posts
 
strange. for some reason i thought that zamp is digital, but you may be right. what speakers you're using?
s7horton OP
1,077 posts
 
Thank you for all the feedback.

I'm using a custom set of speakers design by Clearwarve. The speakers have performed without issue, and I swapped speaker cables to cause the hum to move.

It is only in one channel of the amplifier. Not both. I plugged the amplifier into a different power circuit and the issue is still there. I have played this setup with decent volume before and have never had issue.

It was only after I ran the setup with the attenuator in place that the hum developed and now won't go away.

Any reason to believe that there was some DC offset being carried on the positive side that may have done some damage?
marakanetz's avatar
marakanetz
5,473 posts
 
DC offset over tolerance is usually when DC caps getting old and drain reverse voltage. Zamp shouldn't be that old though.
s7horton OP
1,077 posts
 
A couple other notes. I used a cheater plug to lift the ground, no change. The volume of the hum is not impacted to the gain on the amplifier. In other words, I turn it up, the music gets louder, the hum stays the same.

Thinking of dropping by an electronics repair shop on the way home tonight.
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