wrote in message
oups.com...
Paul Stamler wrote:

Not really -- you want it to be a good deal higher than that.
Typically a
cathode follower circuit has an output impedance of some 300-600
ohms, but
hang 3k on its output and suddenly it sucks, because you've eaten up
most of
its open-loop gain.

Hi Paul! how much higher? and what 'loop' are you refering to?
there's no global feedback happening in this pre.

No, but there's local feedback. I was guilty of using shorthand; for
open-loop read "gain without feedback". A cathode follower basically has
100% feedback, bringing the gain down to unity.

How much higher? Basic rule is that you figure out what load the thing can
drive based on its current capacity, not its output impedance. Like I said,
10k for a 12AU7, 5k for a 6SN7 if you bias it good and hot, and if you want
to drive a 5k load with tubes you're best off setting the jumpers for -10dBV
nominal sensitivity rather than +4dBu.

and what
performance change are you refering to when you say 'it sucks'? a load
that's 10x greater Z than the follower will have a small effect on its
output level. but of course, it may turn into a fuzz box.

Exactly. Look, back to opamps for a second. A 5534 has a specified output
impedance of 0.3 ohms, but that doesn't mean you can hang 3 ohms of load on
it and expect anything but fuzz. Same goes for tubes.

soon i
will investigate the effects of loading on distortion behavior with
the power of FFTs, i just haven't gotten the unit to my practice space
yet where my computer is.

You may be horrified at what you see. Tubes really don't like low
impedances.

also, can you recommend a transparent sounding zener diode series, for
the purpose of output clamping? with the swing available from tubes,
they really could wipe out any subsequent solid state stage that wasn't
input-protected. go over to a friends house, "dude check out
my...fftfftttttttssszzzz". oops, toasted your AD. sorry!

Check with the manufacturer -- you may find the AD already has clamping
diodes installed for exactly such reasons.

Peace,
Paul