From: paul@gaitlab1.uwaterloo.ca (paul j guy)
Subject: Re: Passive preamps revisited
Organization: University of Waterloo, Waterloo, Ontario, Canada
Date: Fri, 21 Aug 1992 03:59:52 GMT

	If you plan to use a cd player ONLY, you don't always need
an active preamp, unless you want to alter the frequency response (and
CD's need it, IMHO), or want to change the balance. Most CD players
have around 2vac outputs, and that's more than enough to drive an
amp to full output. Typical output impedances are less than 1000 ohms,
my NAD5100 will drive 500-1000 ohms no problem, and the output
impedance looking back into the outputs, is about 200 ohms. If you have
a short run to the preamp (less than 4-5 ft) and have reasonably low
capacitance cable (less than 30pf/ft) you won't have any problem
with THIS END (the input to preamp) until you get up to the
megahertz area.
	At the OUTPUT END of the passive preamp, it's another story.
You need to know the maximum impedance, looking into the preamp
outputs. You need to know the input resistance of your power amp,
and also its input capacitance. You need to know the capacitance of 
the cable going between the preamp and amp.
	I built my passive preamp, so I know its maximum output
impedance is about 5000 ohms. My Linn LK100 has 10,000 ohms input
impedance, so I will get a few db straight loss (no sweat - I can
still put out enough to overload it). I have 3 ft of cable, say
30 pf/ft, so it's around 100pf, and let's add 100 pf(this I think
is unreasonably high for the Linn), so total capacitance - worst case, is
200pf. Looking at my chart, the -3db point works out to be between
100-200 kHz. No problem !
	Now, I will have a problem if I run 30 feet of cable, and
your passive preamp may have a much higher output impedance, which
will proportionally cut back the highs. If you have a noisy (electrically)
enviroment, forget about passives, go for an active with differential
outputs (XLR).
	I guess the long and short of what I'm getting at is, if you
know enough about your systems, and also want to customize your
sound and controls, then passive may do the job, for cd players,
or other comparitively high level outputs. If you don't know, and
you're at the mercy of the manufacturers, go active, especially
if you want phono inputs.
	As far as a passive preamp limiting the dynamics, I don't
buy it, unless the frequency response calculations show the roll-off
creeping in toward the audio top end (20 kHz). The noise floor
for a passive preamp is limited by the thermal noise of the resistors,
and that's a long way down from the point of view of a power amp.
The passive is not limited to maximum levels, until it starts to
smoke, or fry the internal parts (assuming them to be of reasonable
quality). Poorer quality power amps may give somewhat poorer
behaviour driven from high impedances, when it comes to slew-rate
limiting or overload recovery. Assuming that the power amp input
stages never get close to overload when the output stages clip,
and they have good isolation from the feedback signals, I don't
see input impedance being a significant issue as far as dynamics
go. I have seen crummy power amps putting out a lot of noise
(wide band) on open circuit inputs, and getting tolerable quiet
on a low impedance input, but I think that this stuff is not
representative of the quality most of you would buy.
	.... just my two cents' worth, give me hell if you think
I'm full of it......

	-Paul

-- 
-----------------------------------------------------------------------------
Paul J Guy			work phone:519-885-1211 ext 6371  
paul@gaitlab1.uwaterloo.ca	home/FAX/message:519-576-3090
pguy@healthy.uwaterloo.ca	..remember...bullshit baffles brains...