mixer output into AMP mic/keyboard inputs

[writersf]

Can I ask a live performance/mic-related question here? Hope so, and apologies if it's the wrong spot. I use a Shure SM57 (vocal) and AKGC100s (on my acoustic guitar) into a Behringer MX602A mixer. Until I get around to buying something better, I'd like to feed my mixer output into my Peavey KB/A 30 amplifier -- as a mini-PA system. The Peavey KB/A 30 has a mic input as well as inputs for a keyboard or acoustic guitar. Feeding the output of my mixer "works" in any of the imputs, but I assume there's a mismatch, and the fact that it doesn't sound wonderful confirms it for me. Is there a line transformer or something I can use to better match the line level output of the mixer to either the mic or keyboard/guitar inputs on the Peavey? (The specs I have with each don't tell me enough to figure out the exact impedances, etc.) Thanks a lot for any help.
John

 

[crazydoc]

I'm just guessing here, because I'm not familiar with your equipment (and not with any stuff really that's less then 30 years old.)

You've probably tried this anyway.

First, your mismatch is probably not so much impedance as voltage. The voltage out of the preamp is probably on the order of a volt or so. Your inputs expect a few millivolts, unless you have a line input, which I guess your amp doesn't have or you wouldn't be asking the question.

The keyboard input is probably the most suitable. Plug the preamp output into this with the preamp and the amp volumes all the way down. Turn up the amp volume almost all the way, then slowly turn up the preamp master volume with a signal (voice or guitar) going thru it until the amp starts to distort (this will be the maximum volume), then back it off a little. Then you can turn down and control the volume with the amp's volume control. There will probably be a fair amount of noise, but this should give you the hottest signal with the least noise.

Alternatively, you could connect a 1k potentiometer from the preamp's output to ground, then connect the wiper of the pot to the amp's input and control the volume/voltage this way.

Or, if you had a schematic for the amp's circuits, you could break into the circuit at the input to the power amplifier section with a capacitor to block any DC voltage and hook in your preamp there.

I don't know if any readily available transformer is available that would do the trick, though it would probably color the sound to some degree.


BTW, any equipment damage or electrocution from trying the above is entirely the responsibility of Murphy (of Law fame.)

Hope this helps.

 

[writersf]

cranking the pre-amp output

Thanks, crazydoc. I haven't tried that. My frame of reference is 30 years ago, too (Dynaco amps and preamps. and 4-channel Shure mixers with both mic and line outputs). I'll try cranking the mixer output and therefore limiting the power amp's participation. Worth trying for sure.

 

[TexRoadkill]

I would try the headphone outs into the keyboard input.

 

[crazydoc]

writersf-

What I meant was the opposite of what you apparently understood - limit the mixer's output to a very _low_ level signal (which is what normally comes in thru the amp's inputs from an instrument or a mic), and use the amplifier's volume knob to control the volume from the speaker. I think a high voltage (line level) signal to the amp's inputs will overdrive and distort the amp's own internal preamplifiers.

Sorry if I wasn't clear.

 

[writersf]

Aha

Thanks, crazydoc. You were perfectly clear. My comprehension wasn't. Got it now.

 

[TexRoadkill]

You guys are mixing each other up. A keyboard puts out a line level signal. Using the keyboard input should work fine because you will be feeding it a line level signal.

If you find that your incoming signal is too low than use the headphone outs because they usually have more gain on them.

Most keyboard amps do not have a gain control. Just a volume. So WritersF's advice would apply more to using a mixer with a guitar amp.