Limited Bandwidth
west wrote
I built a typical phono amp with 3 12AX7s, the last tube a cathode
follower
(low impedance). It has plenty of gain (70db) and can easily drive
any amp
directly. I like using a 100k step resistor volume control instead
of a line
amp.
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, if fact the SS
sound is
rather impressive to me (for silicone).
With my limited know-how, I'm swagging that an impedance mismatch is
the
culprit.
If my contention is correct, is there a simple cure besides using a
line
amp? Perhaps some kind of buffer is called for.
You don't say which part of the bandwidth goes missing.
A resistance ratio of at least 1:10 generally does the job, as a rule
of thumb.
The max output resistance of a 100k attenuator is 25k + source
resistance/4. Its minimum input resistance is its load resistance in
parallel with 100k.
What are the input resistances of the SS and valve power amps?
Bandwidth isn't hard to measure. Input resistance not so easy, but in
the case of the valve amps you can look at the circuits.
Incidentally, don't be conned by the AB test thing, West. A difference
cannot be heard, only inferred. For a meaningful inference, the
listener must be trained in AB testing. Training is a self-fulfilling
prophecy.
All comments welcomed and
thank you.
Hostage to fortune...you have failed to keep this promise.
cheers, Ian
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