EMI from cooling fan being picked up by preamp

[jwasbury]

Hello TBers. I have an Aguilar DB680 going into an SWR Stereo 800 power amp. Noticed a lot of noise coming from my speakers during a break at a recent rehearsal. I’ve diagnosed the source of the noise as the cooling fan in the SWR Stereo 800, which runs intermittently. It seems that the Aggie pre is picking up some interference and sending it to the power amp. While fan is running in power amp, noise persists whether bass is plugged in or not. If I unplug preamp output to power amp, noise goes away. Some internet searching indicates that this noise is most likely RF, and I see indications that either a small capacitor across the power and ground leads to the fan could filter out this noise, or a ferrite ring “choke” on the fan power leads might work. I don’t know how to go about sizing either of these potential options.

any ideas from the collective here? Other things I should look into?

 

[s0c9]

I'm NOT an expert, but I've never heard of that happening.. sure.. physically noisy fans.. but a cooling fan inducing noise into the amp?
Have you tried different power sources.. that type of noise can either be gain-induced or ground-induced.
Do you have pedals ahead of the amp?

 

[jwasbury]

I first noticed this at a rehearsal. Replicated the issue at home, so it’s not a venue/house wiring anomaly. No pedals or anything in the signal chain. Bass—>preamp—>poweramp—>speaker. It’s definitely something coming into the power amp from the inputs, because the noise goes away when I unplug the preamp line into the power amp.

both power amp and pre amp have ground lift switches, neither has any effect on the noise.

 

[jwasbury]

here's a link to info re: fan EMI to sound outputs which I found. Electrical interference from case fan causes speaker hiss - Ars Technica OpenForum

 

[Wasnex]

AFAIK, the noise can be injected directly into the circuit ground, where it is picked up by sensitive circuits.

 

[JKos]

One test would be to move the preamp away from the power amp. Does the noise go away or change? If it changes, then the preamp could be picking up EMI from the fan. If it doesn’t, then there is another route the noise of taking.

Does turning down the output from the preamp change the noise?

Is this a new noise? If yes, can you think of anything you’ve changed?

 

[jwasbury]

Thanks for replies folks. I did observe the sounds changes when I move the preamp's position in relation to the power amp. i did not adjust the volume controls on the preamp to see how that effects the noise...yet. I don't remember having this problem in the past, but I can't say for certain. The Aggie DB-680 is a more recent acquisition and I've often paired it with my Crown K-2 power amp which has no cooling fan. I've had the SWR Stereo 800 power amp since new and I don't recall having noise issues with it in the past. I gigged it for years with a matching Grand Prix pre-amp.

a couple tests I want to run: use my hand to slow the fan rotation, to see if the frequency of the noise changes in relation to fan speed. I'm also going to try a different input source (other than the DB-680 tube pre amp) to see if there is still a noise problem. If the noise persists with a different input source, then I know something is up with the Stereo 800. If there is no noise in this test, then I will focus on the Aggie DB-680 as my problem child.

 

[byacey]

Rotate the preamp chassis around the power amp and see if a positive null point is found. This would indicate EMI. I would be more inclined to think it's a ground loop issue between the pre and power amp chassis.

 

[jwasbury]

Following up. I was able to reduce the noise substantially by grounding the DB-680 chassis to the Stereo 800 chassis. It’s not completely eliminated though. With the Stereo 800 opened up, I found an easy way to bridge the thermoswitch to make the cooling fan run. It’s absolutely the fan running which creates the noise. Dead silent without the fan running. Even with the 2 chassis grounded together the noise is still present when the fan is running, though 90% less. I also plugged my mixer into the amp (instead of the DB-680) and there is no noise (fan on or off). So for sure it’s an issue pf pairing between these 2 components. Happy to have a 90% solution, but would still love a 100% if anyone has further ideas.