Light guy needs "sound" advice

[JohnA]

Greetings to all,

I am primarily a lighting tech, but do sound occasionally. I have come across a situation in a local school & require some advice. The sound is controlled via a rack containing an AKG mixer, Marantz CD player, and JBL amplifiers (not 100% certain on brand).

The mixer has 7 channels (could be 8) of "gain only" control, i.e., 1 pot per channel. The vocal mics sound fine. However, I am having some issues with the frequency response on CDs I am playing through the CD player, a Marantz model PMD325. I am getting a lot of high frequency hiss on both commercial & custom burned CDs.

I have a Rane ME15 graphic equalizer available for "tweaking". Since the mics sound fine, I don't want to change their response characteristics. So, I was wondering if I could take the output of the CD player into the EQ, then run that to the CD input on the mixer. Presently, the CD output is R/L channel unbalanced direct into the mixer.

Ideally, I want to take the CD output from the <balanced out> into the Rane EQ <balanced in>, then go EQ <balanced out> to AKG mixer <balanced in> CD. Would there be any impedance mismatch issues or excessive voltage issues to worry about with this arrangement? In simple terms, I DON'T want to damage anything...

Thanks for your suggestions--John

[museav]

That's probably a discontinued AKG AS 4+3, 4 mic inputs with an automix function and 3 stereo, unbalanced line level inputs. There are no balanced line level inputs so although you might bee able to set two mic inputs low enough to accept the stereo line level signal, that seems very awkward.

Is the hiss there all the time or only when you play a CD? Have you verified that the hiss is there not on the output of the CD player?

If the CD and/or Master level controls on the mixer are at extremes, you also might want to see if adjusting those helps. If it does, then you might be able to compensate by adjusting the amplifier level and/or the mic input levels. The mic inputs have two gain controls; one on the rear panel (essentially the trim control) and one on the front panel (basically the fader) as well as a DIP switch on each mic input for a fixed +20dB gain, so there is quite a bit of adjustment possible.

[JohnA]

Brad,

Thanks for the advice. The hiss is ONLY on CD playback--not present on mic channels. I will have to go to the school & take a closer look at the mixer & the connections you mention. I will also try pushing each channel on mixer to max--one at a time--to verify hiss is only on CD input. I will advise exactly what I have over there (may take a day or two). --John