> DAN
> > >The way to listen to -just- preamp noise is to solder a 200-ohm
> >>resistor across pins 2 and 3 of an xlr, and plug that in as a "dummy
> >>mic." You can make valid comparisons of the noise of different
> > >preamps this way, assuming a method for setting them at the same gain.
>
> KLAS
> >Hmmmm. 2 * 200 Ohms in series with a condensor from pins 2 and 3 to
> ground!
>
> Assuming big enough capacitors, that network would simulate a 400-ohm
> source, rising to infinity at DC, center tapped, not floating.
DAN
> If the mic's output is transformer coupled, the simple 2-to-3
> resistor will be a closer simulation, I think, besides being simpler
> and an industry-standard way of testing. One might argue for a lower
> value like 150 ohms or even 50, or a parallel capacitor-resistor to
> lower the impedance at low frequencies. But I'd rather keep it
> simple. Big value caps that can take 48V aren't going to fit in an
> XLR shell, either.
>
> I only have schematics of a couple of mikes; I see the Sennheiser K-6
> preamp is transformer coupled. The Schoeps preamp is direct-coupled,
> no coupling caps. Explains why their low-low frequency response is so
> good!
>
> I see that your network might be a better simulation of
> capacitor-coupled condensor mikes, but isn't the 400-ohm impedance
> too high? As a mic manufacturer, you're probably more familiar with
> mic electronics than I am.
In phantom-powered setups, I would add two additional 22 kohm resistors
between pin 2 / pin 1 and pin 3 / pin 1. These additional resistors would
emulate
a phantom power current of about 3 mA. This may reveal addional noise caused
by the phantom power circuit (that circuit may behave differently under
load).
Regards,
Raimund Specht
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