Re: Reducing hiss by changing op-amps ?
From: Pooh Bear (rabbitsfriendsandrelations_at_hotmail.com)
Date: 03/25/05
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Date: Fri, 25 Mar 2005 18:13:41 +0000
pcmangler@ntlworld.com wrote:
> > > 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).
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.
> 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.
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.
> > 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.
That's good practice.
> 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 ! :-)
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. ;-)
> > 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. "
I can understand why someone might suggest that arrangement but I don't
happen to agree - lol !
> 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 ! :-)
I suggest sticking with the XLR input in view of the info you gave me.
> > 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 !
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
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