Reducing hiss by changing op-amps ?

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.

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

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

I'm somewhat confused here... if I use 10K and 100R then I am going to get 10mV (max) output for a 1V max input. If I use 3K3 then the output will be 29mV. In both cases, surely this will overdrive the PA mic input, which is expecting just 3.7mV max ???

I read in sci.electronics.design that snipped-for-privacy@ntlworld.com wrote (in ) about 'Reducing hiss by changing op-amps ?', on Thu, 24 Mar 2005:

I can't tell; it looks very complicated.

I don't know where you got the 3.7mV max info. Sounds wrong to me.

You may be interested to know that a low impedance mic can easily produce up to 1 *Volt* of signal when used close up and loud !

I recently designed a new unit with a fixed gain mic input and we allowed for an input signal of up to 200mV as a reasonable compromise between sensitivity and likely overload in its intended useage..

Graham

Ahhh - in that case the line input looks like an internally padded down' version of the mic input.

I wouldn't worry too much about which input you use in that case.

Let's stick with the mic input for now. It's balanced which is good for rejecting hum loop type problems.

I suspect the sensitivity figures are 'nominal' - probably actually the

That's good practice.

The lower the source impedance - the lower the thermal noise contribution will be. Low impedance mics typically vary in impedance from 150 - 200 ohms ( most 'pro' mics ) to around 600 ohms ( far eastern rubbish and cheap 'copies' ).

Typical professional mixers have a mic input impedance of 2 kohms. But I digress.

I recommend the use of 100 ohms for ease of doing the calculations and lower thermal noise. ;-)

I can understand why someone might suggest that arrangement but I don't happen to agree - lol !

I suggest sticking with the XLR input in view of the info you gave me.

Erk !

Any chance of holding fire on that ?

If you're simply going to junk it - I'll give you a few bob for it and arrange collection if that's good with you.

It would actually be quite handy to me for various reasons.

Graham

I read in sci.electronics.design that Pooh Bear wrote (in ) about 'Reducing hiss by changing op-amps ?', on Fri, 25 Mar 2005:

From the spec. And it presumably means the input required to get 0dBu or some other reference level out with **all gain controls at max**, which is not a realistic operating condition.

What matters in this case in the maximum source e.m.f of the mic input, and the spec doesn't say.

Graham, did you get my email regarding this ?

Cheers,

Kev.