Re: Reducing hiss by changing op-amps ?

From: Pooh Bear (rabbitsfriendsandrelations_at_hotmail.com)
Date: 03/24/05 

Date: Thu, 24 Mar 2005 23:06:17 +0000

pcmangler@ntlworld.com wrote:

> > You are attenuating the mixer output by nearly 50 dB and then
> > amplifying up again. This is not good. I suppose you are
> > feeding into the low-impedance balanced input of the Carlsbro.
> > You would do better to feed into the high impedance unbalanced input
>
> I opted for the low impedance mic input on the basis of advice in
> rec.audio.pro where it was suggested to me to be wary of using the high
> impedance instrument input as "sometimes these also contain fixed EQ to
> sort-of simulate the tone shaping of a guitar amp; ie, a midrange
> notch."

That sounds unlikely to me. Why not try it and see ?

Also your current pad is presenting too high an impedance to the mic input
which will worsen noise anyway. I'd suggest no more than 100 ohms at that
end and no more than 40dB of attenuation ( maybe less ).

Try 100 ohms and 10k in place of your current values. You shouldn't have a
resistor in series with the shield in case you used one btw.

Wire as follows.
Mixer jack shield to pin 3 of mic XLR.
Mixer jack tip to 10k series resistor mounted in the XLR - then to pin 2.
100 ohm resistor between pins 2 and 3 of the XLR.

You don't need to make any connection to pin 1 of the XLR. This will
prevent hum loops.

I doubt it'll overload the PA head's mic amp if you substitute 3k3 for the
10k above.

> I therefore figured it was easier to just go with the mic inputs to
> avoid this potential problem.
>
> > Is it electronic or electromechanical? If the latter, it will
> > certainly have been damaged. Sometimes they can be repaired.
>
> Electronic. I'm guessing it's based around IC706 /707 on the power
> board ( http://homepage.ntlworld.com/anengineer/1410_Schematic.pdf ).
> Does this look like a very basic delay circuit, or do you think it
> should sound like a proper reverb ?

IC 708 is a 'bucket brigade' analogue delay line and IC709 is a compander
chip ( no doubt to attempt to improve the s/n ratio ) IIRC.

In short - it's not a reverb.

Graham

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