[sci.electronics] Audio Preamp build-it-yourself

brian@ucsd.EDU (Brian Kantor) (09/06/88)

I've come to the conclusion that there isn't a preamp on the market
that will do what I want a preamp to do, so I'm going to build my
own.

I have only one low-level source, which is my turntable.  All the
rest of my inputs are at line level from tuner, CD player, various
tape decks, synth, etc.  So what I need is one phono input, and 
8 line-level inputs, with a separately-selected tape dubbing output.

So my design is: I'll use one nice large pushbutton switchbank to
select my input at line level, and one of those inputs will be the
output of a phono preamp.  The output of the selector switch goes
to the input of a stage which has level and (switchable bypass)
tone controls.  The output of a similar selector switch for selecting
tape dubbing sources drives a bus that goes to the inputs of all
the tape decks.

I can even put a separate input level adjust pot on each input so
that I don't have to wank the volume control whenever I switch
sources.  Sources which need some kind of frequency/gain compensation
will have that before the selectors so that the selected output
will be "flat" (as it were).  I think the only one of those is the
phono, and that's clearly best done in the preamp.

There's enough loss and some need for gain into the power amplifier
that I'm going to need some sort of simple gain stage.  And here's
the question:  when I was doing this sort of thing 15 years ago,
the right way to do it was with a simple two-transistor amplifier
with some degree of negative feedback (like in my old Dynaco PAT-4).
Is that still the right way to do it?  Are FETs, MOSFETs, or IC
op-amps a good idea, and if so, which ones?  What's the current
vogue?

What kind of tone controls to use?  Baxandall were popular a while
back; have they lost favour?

What to use for a phono preamp?

I know about grounding, building a quiet power supply, using
good capacitors and resistors, etc.

Ah yes: the appropriate degree of fidelity required for this is at
the low end of the audiophile spectrum: it will be feeding a Hafler
DH500 amplifier which pushes Vandersteen 2C speakers, with inputs
from 1/4" tape, VCR, CD, Shure MkIV on a Dual turntable, etc.  In
other words, not the top best, but respectable.

   M A I L   to me and I'll summarize if response warrants.

	Brian Kantor	UC San Diego
	brian@ucsd.edu	BRIAN@UCSD ucsd!brian