> Does anybody know what the actual input impedances are of some of
> our familiar PIP-input recorders, like LS-10, H2? Maybe we've been
> matching all along in the PIP world.
> Mics have higher output levels and have flatter response into
> higher-impedance inputs. Preamps with lower input impedances have
> less noise. A design trade-off.
Dan,
When it comes to input noise figures, the key parameter is the
"equivalent noise impedance", not the input impedance. You won't find
this is any specs as it is a difficult concept to put over. For an
optimum preamp noise level, the noise impedance (which is the ratio
between the voltage noise over the current noise) should match the
input power impedance. The input voltage noise is the short circuited
noise figure and the current noise is the open source figure.
Think of this in another way. If you have a 200 ohm mic going into a
5000 ohm optimised input, using a 200 to 5000 matching transformer
(+14dB) will always reduce the input noise in propertion to the input
sound. If it's not optimised, you've got a higher noise level at
either high or low impedance by definition.
In practice if you can hear input noise with a powered mic, you are
away off the optimum input spec anyway. With my pro SQN mixer I can
use a 10db attenuator without increasing the noise over the incoming
MKH mic excess noise which is one of the lowest you will find. My
Tascam DR-100 XLR input noise on other hand is only just acceptable.
My other objection with regarding a 3.5mm jack as two additional mic
inputs is that they are not balanced line and would need two
transformers for use with long mic leads or high earth noise levels. I
get mains hum just with the AC ground gradient here between the house
and my electricity supply pole ground. I also had bad ground loop
trouble when I wired the XLR shells to screen on a stereo pair and put
them on damp earth.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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