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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.

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.

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

Reply to
Pooh Bear
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OK - but I'm going to have to change the values of the resistors - the low impedance input is ten times more sensitive than the high impedance one. (low = 6K8 ohms - 3.5mV, high = 68K ohms 35.0mV).

What would you recommend here ? The specs for the mixer output say it's 600 Ohm, 1.0 V and I'd be going into a 68K Ohm, 35.0mV input on the PA.

I arrived at the figures I'm using on the basis of reading somewhere that the resistor across the tip and shield should be ten times (or less) the input impedance of the unit you are driving. In my case, 6K8 Ohms. My 560 Ohm resistor would therefore be about 1/12 of that. I will gladly give your advice priority over this as if you design this stuff then you obviously know what you're talking about ! :-)

I am. This was suggested to me in rec.audio.pro as per :-

OK... so what I need to do now is to make up some more attenuators, a pair for the XLR low impedance input option using 10K & 100 Ohm resistors and another pair for the 1/4" high impedance input option using whatever values I'm hoping you can suggest ! :-)

Ah well, it was worth a shot, just in case !

Incidentally Graham, I don't know if you remember, but we exchanged a few posts a while back about a Studiomaster 400 Powerpack with a blown main transformer. Just fyi, I never did manage to locate any suitable replacement, so it's going in the bin on the weekend !

Reply to
pcmangler

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.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

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 ???

Reply to
pcmangler

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

Reply to
Pooh Bear

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

*minimum* voltage needed to produce max output.

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

Reply to
Pooh Bear

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.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

Graham, did you get my email regarding this ?

Cheers,

Kev.

Reply to
pcmangler

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