Re: I/O Matching: Mic Pre and the Converter


Harry Houdini wrote:

> I'm trying to understand the relationship between mic pre's and A/D
> converters as far as the Mic Pre's Max Output and the Max A/D Input is
> concerned.

That's easy. There is none. There is no standard mic preamp sensitivity
(standard number of millivolts in for the nomminal -10 dBV or +4 dBu
output), nor is there an A/D converter standard for volts in to get to
full scale.

There are some conventions for converters, like -20 or -18 or -16 or
-14 dBu in for full scale (take your pick, but you won't find it on
most spec sheets) and of course if there's an input level control, you
can get any number you want. Mic preamps usually run from 40 to 65 dB
of gain (but it's always adjustable) and typically have a maximum
ouptut before clipping of 18 to 24 dB above the nominal output level.

> How important (in terms of sonic quality - of course!) is it for these
> to match?

It's not important that the number match, but what's important is that
there's something that you can adjust to get the digital level that you
want. In essense, it's up to you to make them match, to the extent that
you want them to match.

Ideally, the gain of the converter should be adjusted so that it
reaches full scale when the preamp is just below its maximum output.
That way, they both will clip at the same time. Avoid clipping in the
preamp and you won't clip at the converter. But sometimes you end up
with things (like multitrack recorders or computer audio interfaces)
where there's no input gain adjustment ahead of the A/D converter, so
you have to make adjustments somewhere else. You may find that you
can't get enough level (before clipping) out of your mic preamp to
reach full scale, or you may find that you have to attenuate the output
of the preamp to avoid clipping the converter when you still have
plenty of headroom in the preamp.

.

Loading