Re: Transformer attenuators

"Iain Churches" <taelNOSPAM@xxxxxxxxxxx> wrote in message
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"Arny Krueger" <arnyk@xxxxxxxxxx> wrote in message
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"Iain Churches" <taelNOSPAM@xxxxxxxxxxx> wrote in message
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If for example a 5k Ohm resistive volume control were to
be employed in a passive control unit the source would
be required to drive all the time a quite severe load
of 5k Ohm.

5 K ohm load is severe???

Hi Arny. That's a cut and paste from the maker's website,
not my personal statement.

You seem to lack the knowlege and candor it takes to intelligently comment
obvious on weirdness, Iain.

If combined with a 1k Ohm source impedance the worst
case output impedance of the combination would be 1500
Ohm at -6db attenuation, while at -20db attenuation the
output impedance would still be around 540 Ohm. If
combined with around 1nF load capacitance (easily found
in longer, high capacitance interconnects), this leads
to a 0.3db attenuation at 20kHz for a 20db attenuation
setting, practically showing the absolute permissible
limit for load capacitance. The worst-case attenuation
at 20kHz almost reaches 1db!!!

(a) This is a very atypical cable.

(b) Iain, you shouldn't get your panties in a bunch
about it.

See above. I have no opinion for or against at this
juncture.

You posted it without commenting on how strange it was.

I can however hear shortcomings in the passive controller
topology, which is usually a 1m screened pair to a box
with a stepped attenuator and a 1m screened pair leading out to
the power amplifier. In addition to the changes in volume
there> are changes in timbre at different settings of the
attenuator.

No doubt a badly designed passive controller.

If, to provide our source
with an easier load we choose a 50 kOhm resistive volume
control we must either accept drastically higher levels
of roll off at 20kHz or we must limit the load
capacitance to less than 100pF. Such a level of
capacitance (100pF) can easily be found with only 1m of
high quality interconnect cable and is often exceeded by
the input capacitance of many amplifiers!

Using a potentiometer that is over 10 K as a passive
controller is unwise.

Most people seem to pick 100k.

Nonsense. 100K is a good value to use in a preamp where the volume control
is followed with some kind of active buffer stage.

Counter-example:

http://www.mhsoft.nl/MySystem/HiFi.asp

"Some important points:

"Use as short interconnect cables as possible
"Use cables with low capacitance
"Use low attenuator resistance values (recommended total resistance:
10kOhms)



.

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