DIYbanter - View Single Post - Reducing hiss by changing op-amps ?

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