Re: Mic pres - what's the story?

On Aug 4, 12:50 pm, poppy...@xxxxxxxxxxxxxxxxxxxxxxxx (Adrian
Tuddenham) wrote:
<jwillia...@xxxxxxxxxxxxxxxxx> wrote:
... Shoving a low noise discrete FET in front
allows very low current and voltage noise with very high input
impedance, this design I'm using in the Rhodes Mk. 7 pianos as I'm
measuring -115 dbu noise with a 2 meg ohm input impedance.

What measurement bandwidth were you using?

According to my calculations, the Johnson noise alone of a 2 megohm
resistor with 20 kc/s bandwidth is around 25 microvolts. That works out
around -90dBu.

How did you measure your input noise?

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)www.poppyrecords.co.uk

Audio Precision System One. Noise vs amplitude and S/N tests. 80 k
bandwidth. The 2 meg resistor is not the source resistance being
measured. I used the 50 and 600 ohm outputs on the AP. The application
is the preamp from a Rhodes piano pickup rack. The source resistance
of the 88 pickups is 1 k ohms. The 2 meg bias resistor is shunted by
the 1k ohm inductors so the 2 meg is not factored into the noise
measurments. If I use a capacitor capsule from a mic, this is of
course a much lower S/N, but still better than most mic body preamp/
impedance converters, even using 1 gig ohm bias resistors.

Jim Williams
Audio Upgrades

.

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