Mixer Woes (FP42)
 

Hello Everyone!

I recently bought a Shure FP42 Field Mixer.

I am having trouble testing it because it runs way to hot. I can't turn the mic volume up past 3 or 4 and master past 3 or 4 without a few seconds of extreme peaks and feedback at random. Hot Mic – Hot Mic! It is like I have two volume controls, quite and screeching.

I am using a Hyper mic (AT4053) and a normal set of headphones (AKG K66). I am testing it on myself. Could I be getting feedback because the mic is pointed at me (1.5 feet away) and I have the head phones on as I talk while I am listening to the audio through the mixer? This could be a really stupid mistake to make, but I don't think that should be the cause of it.

I am running it on the Mic setting on the mixer. I tried the Line setting and didn't get any audio on the Line setting. I thought that I should be running it on the line setting but maybe the microphone I am using only uses the mic setting…? Turning the limiter on and off doesn't have a great effect on the feedback.

What is the best way to test and practice with mixer with only myself to be the test subject and the audio person. Is there some time saving method that works best? Recording the audio from TV maybe.

The contributors to this web forum have already helped me out a ton. Right now I am feeling a little stuck with the FP42. As you can tell, I am a newby and any words of wisdom are greatly appreciated.

Eric

I'm thinking the phantom power for the mic is inadequate, leading to instability in its preamp circuits. Audio Technica's specs for the AT4053a say it requires 48 volts +/- 4 volts. Shure's specs for the FP42 says its phantom supply only sends 30 volts to the mic.

Definitely using the Mic setting on the input the mic is connected to is correct, line requires a much stronger signal than any mic provides.

Thanks Steve! I didn't even think about checking the amount of phantom power volts the FP42 sends to the Mic. I think that explains the difficulties that I'm having with the FP42.

Thanks for your help.
Eric

It is certainly possible you're getting feedback from the headphone leakage back into the mic, in addition to the low phantom voltage. This mic has much more output than the mixer's mic inputs are designed for in regular usage. The "normal" mics that would be hooked to that mixer are Shure vocal dynamic and low-sensitivity vocal condenser mics that would have much lower output than the 4053.
In addition, you could have a bad cable which would create more problems with any phantom-powered mic.
Do you have other mics with lower phantom voltage requirements and different cables you can test with the mixer? To avoid feedback from the headphones, I'd place the mic at some distance from the mixer. Have someone else, or even a CD player with speakers, or the TV, be your test subject while you listen on the headphones.
Are you running the mixer from AC power or internal 9-volt batteries?
Let us know the results, then you can decide what to do next.

The AT 40 series mics can run on 9-52V phantom. ( at least they use to) so I don't think that's it.

Does it behave the same with the headphones disconnected?
(You can tell by observing the meters going nuts if there's feedback.)

Are the FP-42's outputs plugged into anything? .. for instance a camera... feeding a mic-level input from the FP-42's line-level output?

Could be a malfunctioning mixer.... Try exercising the volume potentiometers some, my old FP-32's pots would act-up occasionally. But not in the manner you describe.

Except if you go to the AT website and look up the formal specs on the 4053a it expicitly says 48 volts +/- 4 volts. When a mfgr states a tolerance like that they usually mean it and you're on your own if you go outside of the 44-52 volt range.

The only mics I know of in the 40-series that ran on 11-52 volts of phantom were the AT4071a and AT4073a shotguns. That was a very handy feature since I had an 18-volt belt-pack phantom supply and my DAT deck didn't supply phantom at all.
All the other 40-series, including the latest versions of the BP4071 and BP4073 specify 48 volts.
My new AT4021 should arrive any day and hopefully it will sound as good as its specifications indicate... I'll let you know.

So I have spoken to the person (David Riddle, ebay) that I bought the mixer from who is also an audio engineer. His recommendation is to get a 20db pad to run in the line with XLR and mic. The pad should reduce the the mic's output by 20db and hence reduce the signal low enough to something that the mixer can handle.

However, I think that my best option is get a different mixer that is designed for 48v Phantom Power. (I am kicking myself now for not buying the MixPre innitially.)

When I tested the mixer, I ran on AC Power from the wall outlet and I didn't run an output through to the camera. There was only one mic (4053a) and one set of Headphones connected to the mixer.

Thanks everyone for helping me out. I am currently weighing my options as far as selling the FP42 or buying additional 20db Pads and testing it. The MixPre has gain control and the FP42 doesn't so I think that it would immediately solve this problem.

Eric