west wrote:
I built a typical phono amp with 3 12AX7s... CF... 100k step resistor volume control
...My problem is that when I use any of my tube amps, the sound has limited
bandwidth. A SS amp does not exhibit this problem
[re-editing my previous post that contained errers]
My understanding of "step resistor" is the resistance between volume
control positions* - but does "100k step resistor volume control" mean
a volume control made from 100k step resistors (perhaps around 2M
total?!) or "a 100k volume control that contains step resistors"? I
jumped to the conclusion it was the first, which isn't a sensible
conclusion, thinking about it.
You're using a 100k 'pot' effectively to drive a cable...
With as little as 1 nF of cable capacitance that forms a low pass filter at 6kHz !
Your answer is what I suspected but not having much experience, I asked this
NG. I was looking for a Tube Basic 101 generic answer that I did not find in
my reference books. I don't understand where Patrick is coming from....
(1) the limited bandwidth problem being only there _when driving a
valve amp_ is presumably due to the high output impedance (not the same
as a 100k output impedance though - depends on volume setting). The
test is to measure (on a scope or whatever, with a low capacitance
probe) directly at the output socket, with no cable connected, what is
going on with the frequency response. The trouble with this test is that
you have to rig up a reverse RIAA circuit for your signal generator's
output. Another, easier, test would be to shove a 22k resistor or so
across the output and then see if the valve amps are then happier
(albeit with a low volume). If the test shows a high frequency cut is
still going on, maybe the RIAA values are wrong and maybe the SS amp has
some slight high frequency lift (some nfb circuits do this) that is
compensating - two wrongs making a right?? (pretty unlikely).
How much "limited bandwidth" are we talking about, by the way?
*defined at
http://ww1.microchip.com/downloads/e...es/01080a.pdf:
The Step resistance. This is the change in resistance that occurs
between two adjacent wiper register values. It is also the RAB
resistance divided by the number of RS resistors (resolution) in the
Resistor Ladder.
Mark A