How to fight a bad hum in a multi-power amp system?

[robertbartsch]

One of my 5-channel JBL systems that is located in my garage is powered by 3 large Crown power amps and these are driven by an Adcom 5-channel home analog pre-amp with RCA input/output jacks. The Crown amps (XLS 802s) have 3-prong input (balanced??) terminals.

None of the amps are running in bridged mode.

I was doing some work with hook up, speaker changes yesterday and one Crown amp now has a significant hum but the other two amps do not.

How would you chase down the hum and in which sequence?

1. Switch input wires from amp-to-amp?

2. Switch the speaker outputs from amp-to-amp?

3. ground all amps to a cold water pipe?

4. disconnect all ground wires?

[robertbartsch]

...the only modification I made to the Crown 802 XLS amps was that I added a 100 ohm resister to the electric fan circuit since I run them at low power and the fan noise is too much. None of the amps has ever been driven into "hot" shut down mode...

The amp that has the hum is connected using two commercially purchased 3-prong male-to RCA jack metal input jacks. The amp without the hum is connected using self soldered input jacks for which I used the Crown user manual to wire.

[allen mueller]

The amp that has the hum is connected using two commercially purchased 3-prong male-to RCA jack metal input jacks. The amp without the hum is connected using self soldered input jacks for which I used the Crown user manual to wire.

That sounds like the best place to start looking with your option #1. If the hum goes away when you swap over to your cables that you made, i'd dig deeper into the off the shelf cables see how they're done vs. yours.

AL

[quindecima]

99% of hums like that are due to a ground loop. I would first do an extensive ground check before I moved on to other things.

[svollmer]

...the only modification I made to the Crown 802 XLS amps was that I added a 100 ohm resister to the electric fan circuit since I run them at low power and the fan noise is too much. None of the amps has ever been driven into "hot" shut down mode...

Can you explain further how one does this? I have a PowerBase 3 running my subs. The amp is in the next room and I can still hear the fan. When I disconnect the fan, the amp won't power up. Is there a way around this, or should I do the resister mod you describe? If it's the latter, do I just solder a resister in series with the fan connection? Does it have to be a high-power resistor?

Thanks!

[quindecima]

See how much power the fan is drawing and select a high wattage resistor from that information.

[allen mueller]

Can you explain further how one does this? I have a PowerBase 3 running my subs. The amp is in the next room and I can still hear the fan. When I disconnect the fan, the amp won't power up. Is there a way around this, or should I do the resister mod you describe? If it's the latter, do I just solder a resister in series with the fan connection? Does it have to be a high-power resistor?

Thanks!

I think the Power Base models are like the Macro Techs which means you can't do the mod because the fan also contains the transformer for the lv side of the amp.

AL

[boputnam]

... with RCA input/output jacks.

That's your problem area.

That sounds like the best place to start looking with your option #1. If the hum goes away when you swap over to your cables that you made, i'd dig deeper into the off the shelf cables see how they're done vs. yours.

Or, go to the tried-and-proved foolproof route, using and isolation transformer/RCA connector patch cable. Radio Shack Audio System Ground Loop Isolator. Back-in-the-day, when I tried mating consumer to pro audio, there were quite of a few of these in the signal path...

99% of hums like that are due to a ground loop. I would first do an extensive ground check before I moved on to other things.

Yes, but start with the Radio Shack interconnect, first - it will save a lot of time finding things you cannot remedy...

When I disconnect the fan, the amp won't power up.

I would never, ever do that. The fan is not a "cosmetic option" - it is preserving the operating environment of the amp internals.

[grumpy]

Back to post #3 if the OP is still listening... I bet they're wired differently.

Those powerbase amps are very dense and designed for a constant blow through.
The heatsinks are not designed to even partly work without active airflow.
I have the 3.1 version and only turn it on for loud action movies where it isn't noticed
(fan-noise-wise).

[svollmer]

I would never, ever do that. The fan is not a "cosmetic option" - it is preserving the operating environment of the amp internals.

I agree and wouldn't have done so long-term either. I just wanted to see if it would work without the fan. If it did, I would have been able to keep it cool with the top off (since it's 8 feet off the ground on a shelf) and a much quieter fan bringing much more CFM over the amp.

It's only operating from 40Hz down, so it's ice cold after several hours of use and it's even surrounded on all sides but one by rigid fiberglass to keep the noise down.

What I really need to do is find a nice (not beat-up) K2!