I think you may be a bit off here man.USB can
not pass an
analog signal so any analog device (microphone) to USB intreface has to do a the analog-to-digital conversion or you have an unusable signal.
You can not just add mic to a "USB" adapter and be up and running.that is why for simplicity i suggested the all-in-one chips.Far easier to use something that already has the circuit blocks figured out than it is to assemble multiple analog and digital chips onto a workable board unless you have done digital audio designs before.
The quick and dirty way to go and the one I have used many times in the past is to choose the part then hunt up the evaluation board pdf.They know their own product and so the layout will be optimised for the part.
The eval acrobat reader files normally include a 1:1 scale pc board layout which can be copied.Being an evaluation board that must be set out from the beginning for testing purposes there will be a lot of extras that you do not need so they can be left out of the final design.
Until you have done digital audio layout it is the best way really and it is how i learned to do it myself-by doing it from the already engineered layout.
BTW-when doing analog-to-digital input circuits accuracy and a stable clock are essential and the noise level tough to get right (I still can not get my ADC noise input down to 24 bits) but when doing digital-to-analog it is all about the
sound and that takes precedence.Special attention to the sonics is actually useful once going back to the analog world.Sound means absolutely nothing at the input so choosing "designer" caps and resistors would be silly and an uneccessary expense.More you need to shoot for electronic accuracy and tightly specced parts.
just a suggestion
BTW-if it was ME doing the USB mic i would add an analog mic buffered "pass through" so the mic signal could be tapped off before the inteface electronics allowing analog use s for the mic also.A simple dual jfet buffer or bipolar pair ( potentially lower noise level) should suffice for the connection to the outside world.
BTW ll-you also need to figure out your bandwidth requirements and then add filters for the analog input high and low cut off points.Trying to reproduce a bass down to Dc is dangerous and over the sampling frequency impossible if you want to avoid trouble.And if for voice only applications there is really no reason to go outside of 100hz-10khz or so.Will save you headaches later.Even for live music it is a real good idea to have filter turnovers in a range no sane person would accept if it were an amp or speaker but recording is different (as is PA work ) and you MUST have set cutoffs depending on application or you will be wishiung later you did.
BTW lll (
)-input levels : You will never hear worse sounding digital than if you exeed the input level overload point and that even on peaks.Analog can distort gracefully whendriven to hot but digital is either GO or NO GO.all or nothing.So even though the
potential is there for a huge dynamic range the reality is you never approach the upper voltage input limits out of fear.Where you get dynamic range is the low noise floor of digital which allows for signals that in analog would have been lost in the system noise floor and would have to be brought up or tossed out.
i would add and adjustable input pad and 3 LED mini meter (go-almost no go and approaching trouble-overloaded, SCREWED)to the mic input PLUS a volume control at the minimum but really a limiter is the way to go and some of the chips have this already built in to the design.Actually both
Or all the out solution if voice only is not what you are looking for : Analog Devices SSM series balanced mic preamp/compressor chips mated to a USB ADC.set the upper limits of allowable signal and
just some more options thrown your way and something to think about.
as you can see the all-in-one chip can really simplify things,especially if mated to a DSP chip to handle all the EQ/Compress/volume/etc
