Ian Iveson wrote:
Eeyore wrote
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 !
Little? 1nF is over 10 metres of ordinary audio coax.
That depends what you call ordinary. What do you use and what's its
capacitance
?
Erm...it's pearlescent and thick, so it may be this:
http://www.maplin.co.uk/module.aspx?...2&doy=8m4#spec
but around 90pF/m seems common.
I am using a 100k pot too...but with only 1/2m of cable, until I have
finished my remote controlled pre. 10k will be a better load for my
sources.
cheers, Ian
With a 100k log pot set at -20dB, R source = 9k.
1.2M of cable at 90pF/M = 108pF.
The pole caused by pot and cable = 163kHz.
The worst HF losses are where pot is set for -6dB, R source = 25k, so
the pole is at 59kHz.
So a 100k pot would offer no audible losses over a 10k pot.
The 10k pot would load a previous CF with too low a load if tubed.
Just because we have a triode set up as a CF which gives low Rout 1k,
it doesn't mean we are permitted to use loads lower that the common
cathode gain stage.
Ideally a 47k load on a 1/2 6SN7 should be used for eithe ranode of
cathode load.
Where the THD may have been 0.1% at 1V out with anode loading, it
becomes 0.0066%
when the CF is used, but if the load = 10k, perhaps THD becomes 0.02% at
a volt output.
So thus the gains brought by the CF are eroded if the load is reduced.
The bandwidth won't change at the CF; its Rout = 1/gm in parallel with
RL, or about 500 ohms
and whether RL =10k or 100k does not make any great difference.
Patrick Turner.