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I'm using a 250W Fender Passport PA system (model P-250) with a vocal mic (MP-75) and an electric bass guitar hooked up to separate channels (e.g. mic to MIC-1 and bass to LINE-2). When both are active, the vocals sound fine but the bass fades in and out. This only happens during vocals -- the bass comes through fine with the vocal channel set to an active level as long as nobody's singing.

What could possibly be causing this, and how could I go about troubleshooting it?

EDIT: It's been figured out. It turns out this was not a hardware failure. It resulted from an inadvertent setting which was functioning as the manufacturer intended. Turning off "VIP" mode on the vocal channel solved the problem.

  • Is this a new amp? Or did it usually work ok and it started acting up recently? – Schizomorph Aug 26 '17 at 16:38
  • I suspect this could be a crosstalk issue. I had a look at the schematics but my understanding of electronics is limited and I don't want to give you any false positives. Have you tried asking at electronics.stackexchange.com ? – Schizomorph Aug 26 '17 at 16:44
  • Thanks, @Schizomorph. It turns out this was not a hardware failure. It resulted from an inadvertent setting which was functioning as the manufacturer intended. Turning off "VIP" mode on the vocal channel solved the problem. – David Tresner-Kirsch Aug 26 '17 at 17:44
  • Oh cool. A built in ducker. I thought the description sounded like ducking but I had no idea they put them in amps. I suppose this can be handy in many cases. – Schizomorph Aug 26 '17 at 18:50
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The "Fender Passport" model name seems to pertain to a number of different amps. Assuming it is the portable one, we'd be talking about 7W. This is a PA rather than an instrument amp, so its failure mode for drawing more power than it can deliver will not necessarily be musically useful.

However, what you describe does not sound like a typical overdrive reaction: most particularly it sounds as if vocals are reasonably fine.

So my guess would be that you do a solid bout of dynamic compression on the end result and the initial gain for voice is higher than that for bass. So whenever the voice comes in, the bass is disproportionally turned down.

Where does that dynamic compression come from? My guess would be some inadvertant setting: it would be unusual if the PA dealt with getting overdriven by turning itself down on its own initiative, in particular by mostly turning down the bass channel.

What would the symptoms be? One indication of overall gain getting turned down is also the level of white noise getting turned down: does the general noise level go down with the bass?

Another would be that moving the mic closer and further from the singer will not change the volume of the singer as much as it will change the volume of the bass.

So check out those compression settings if your amp has them: if you need compression on your voice, you need to do it with an external stomp box rather than generally on the PA (at least if the PA's routing options do not permit doing compression on the voice alone).

  • @user22825 -- thanks, this answer was close and got me pointed in the right direction. It turns out the MIC-1 channel has a "VIP" setting that suppresses other channels when it's receiving active signal -- the idea being that it will automatically turn down music volume when someone is making an announcement. That mode is useful for trivia jockeys, bingo callers, etc, but not useful for band practice. Turning it off solved the problem! – David Tresner-Kirsch Aug 26 '17 at 17:41
  • PS -- Great point that "Fender Passport" wasn't specific enough -- I've updated the original question to include the model # and wattage (250). – David Tresner-Kirsch Aug 26 '17 at 17:43

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