RF interference puzzler

[sublro]

I have a Fishman Loudbox Artist , and am attempting to run a mic cable from the DI output on the back of the unit into a keyboard amp that has a mic in as one of its channels. When I attempt to do this I get very noticable reception of an FM radio station coming from the keyboard amp. I've tested carefully and can provide this additional detail.

Running the DI out from another one of my acoustic amps in the same location into the keyboard amp (in other words replacing the Loudbox with another amp) produces a completely clean signal, no RF interference. Have also tried multiple mic cables so it has nothing to do with a bad cable. Have also run from the Loudbox's DI out to a different receiving amp and gotten the same RF interference, so it is not related to the amp I am sending the DI output into. It seems to be related to the Loudbox DI out specifically.

Thoughts? anything I can try short of sending the amp for repair?

thanks!

[ljguitar]

Originally Posted by sublro

I have a Fishman Loudbox Artist , and am attempting to run a mic cable from the DI output on the back of the unit into a keyboard amp that has a mic in as one of its channels. When I attempt to do this I get very noticable reception of an FM radio station coming from the keyboard amp. I've tested carefully and can provide this additional detail.

Running the DI out from another one of my acoustic amps in the same location into the keyboard amp (in other words replacing the Loudbox with another amp) produces a completely clean signal, no RF interference. Have also tried multiple mic cables so it has nothing to do with a bad cable. Have also run from the Loudbox's DI out to a different receiving amp and gotten the same RF interference, so it is not related to the amp I am sending the DI output into. It seems to be related to the Loudbox DI out specifically.

Thoughts? anything I can try short of sending the amp for repair?

thanks!

Hi subiro...

I have stopped using the DI output capability of acoustic amps because I find they are very 'dirty' compared to the line out and XLR of my preamps/blenders or even a simple passive direct box.

I get everything from 60 cycle hum (ground issues), to RF, cellphone interference, fluorescent buzz, and sometimes distortion. Pulling a direct feed from the preamp has cleared it up every time.

This has been over a range of several brands of amps. Sometimes different amps (same model, different amp) will be clean or dirty. It's just easier to use a direct feed (and one step cleaner). It's like the amp is not shielded properly in the output section.

I prefer the preamp/DI solution, because it sends the same basic signal to every source, where the tone and volumes can be set for that particular amp/PA/Mixer.

That way, if I adjust my stage amp, it doesn't affect all the others.

All my guitars use a preamp/blender with full line and balanced XLR output capability, and those signals are the cleanest. The only thing more direct would be a feed directly from the guitar.

[sublro]

Thanks Larry - good info as always. my only problem is that it's not just my guitar I'm sending. I have a mic and some drum sounds also going into the amp and am looking to basically send all of it to the keyboard amp as an extenstion to make a small PA. hoping that it's a gain structure issue, which it may be as I've now tried the DI to a mixer's mic input with gain control and it doesn't seem to have the same RF issue...

[sublro]

Interestingly, taking the DI output to the input of a mixer with gain control gives me a dead silent signal... either it means that more flexibility on input gain does the trick, or the fact that the mixer is plugged into the same power strip and side of the room as the Loudbox has exposed some less than ideal electrical wiring in my basement. ...

[dberkowitz]

What Larry said. I don't work with a single engineer who would even think of using the direct out of an onstage amp. We simply don't trust them. Safer to loop through the DI and into your amp.

[ljguitar]

Originally Posted by sublro

Thanks Larry - good info as always. my only problem is that it's not just my guitar I'm sending. I have a mic and some drum sounds also going into the amp and am looking to basically send all of it to the keyboard amp as an extenstion to make a small PA. hoping that it's a gain structure issue, which it may be as I've now tried the DI to a mixer's mic input with gain control and it doesn't seem to have the same RF issue...

Hi Sublro...

Sounds like you need a mixer to 'sum' your sources and send the mixed signal to the Keyboard amp/mini-PA.

[sublro]

Originally Posted by ljguitar

Hi Sublro...

Sounds like you need a mixer to 'sum' your sources and send the mixed signal to the Keyboard amp/mini-PA.

Yeah, I guess. Just disappointed if that's really necessary, as i've always thought of the di outs as a great feature. You love the ultrasounds, same deal with them in your experience?

[Bob Womack]

I've always just dropped in a direct box 'twixt instrument and amp and let the performer have his amp for his stage use and me my clean feed for recording, FOH, and monitors. It just never seems to work out the other way around.

Bob

[brad4d8]

Try putting one of these RF chokes on both ends of your cable, might do the trick.
http://www.radioshack.com/search/ind...=RF+Choke&sr=1
Brad