Re: Reducing hiss by changing op-amps ?

pcmangler_at_ntlworld.com
Date: 03/25/05 

Date: 24 Mar 2005 16:46:26 -0800

> > 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 ?

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.

> 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 ).

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 ! :-)

> 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.

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

> .....since the mic inputs are balanced, you may want
>something like this:
>
>
>tip--------- 120K ---+----XLR 2
> |
> 1000
> |
>sleeve---+-- 120K ---+----XLR 3
> |
> +----------------XLR 1
>
>
>The two 120k's are basically in series with your signal to add up to
>something near your 270 k unbalanced attenuator. "

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 ! :-)

> In short - it's not a reverb.

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 !

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