8 input microphone mixer (AMZ mosFET boost-ed).

[cedric]

hi.

i'm looking to build my own "vintage-like" 8ch.mixer, before i go into my mono tube cassette deck.
i was thinking something along the lines of 8 AMZ mosfet booster, one for each channel.
but would this even work? (most mics are at 200ohm)
i suspect i'll need a transformer of somekind (200ohm-to-50Kohm), for the lo to hi Z thang.

any ideas?

- ced

[davebungo]

Professional mic pre's usually have transformer coupled inputs so I suppose this must be a consideration.  They also have impedance switching when the input gain reaches a certain threshold although I don't think this is really important.  Have you also considered phantom power?  Drum mics are quite often phantom powered and if you are using your mixer for band stuff then this may be something to think about.
Back to your original question, I don't see why a MOSFET boost wouldn't work provided that there is enough gain to be had (I seem to remember that the MOSFET boost has about 35dB of gain max).  I'm not sure about the distortion profile of this circuit i.e. It is meant really for guitar/bass use where a little distortion may actually sound nice - you may want a lower THD for your application esp at higher gain levels.  If the input impedance is a little high, then there is nothing to stop you putting in a shunt resistor on the input.  I guess some experimentation is required.

[cedric]

yeah, i'll try to experiment a little here.

i guess a transformer on the input wouldn't be a bad idea. although it might be better on the output of the mosfet.(?)

- ced

[ikiru-]

still a newbie...but something to consider would be increasing the voltage of the circuit (ie to +-15V) may increase the amount of clean headroom, and I'm quite sure it could take it...

let us know how you experiments turn out...

ps. you may want to look into the hamptone jfet pre...