>|3. Adjust levels *before* the A/D to prevent overload while maximizing dynamic
>| range. Most modern MD recorders have fixed gain *berfore* the A/D; not a
>| good thing!
>Absolutely. So you use the LM1973's you mentioned in a programmable gain
>configuration, right? Are these parts quiet enough to work with
>microphone-level signals?
I was planning on using a topology that had the LM1973 in the *feedback* loop
of the microphone preamp stage. Therefore, it would get its drive from the output
of the mic preamp, after 40 dB or so of gain and attenuate it back to microphone
level before feeding it to the inverting input. They spec these chips at over 100 dB
S/N, and this configuration exploits the available S/N to the max, so my guess is
that it would be quite acceptable. This topology is what I was refering to when I
was talking about channel trim.
The LM1973 contains three pots; I'd use the first one for level trim as described\
above, and the other two as conventional attenuators, one feeding the left
channel and one feeding the right. That would allow the controlling microprocessor
to emulate channel gain and pan, all the while maintaing an optimum gain
structure to maximize S/N while minimizing overload.
>you're well over
>1/2 watt--roughly 20 hrs. with 3-4 AA cells-- is this what you have in mind
>for a portable MD system?
That would be great... I'd planned to use NiHM AA cells...
>Also, I would think that some MD titling interface built into the mixer
>might be a nice feature that would distinguish your mixer from anything else
>that's commercially available.
A great idea! I'd probably use an 8051 derivative (we've had great success
here at work with the Philips 87c552 in battery powered equiopment; it has
some nifty power-save and sleep modes). This CPU would probably have
enough resources left over to run a titling program.
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