most of these devices su
ff
ered from substantial impedance mismatches with ei-
ther sources or loads.
If for example a 5kOhm resistive volume control where to be employed in a pas-
sive control unit the source would be required to drive all the time a quite severe
load of 5kOhm. If combined with a 1kOhm 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!!! If,
to provide the source with an easier load were we to choose to use a 50 kOhm
resistive volume control we must either accept drastically higher levels of roll o
ff
at 20kHz or we must limit the load
capacitance to less than 100pF.
Such a level of capacitance (100pF)
can be found with only 1m of high
quality interconnect cable and is of-
ten exceeded by the input capaci-
tance of many amplifiers!
Thus the resistive volume control em-
ployed in passive control units must
navigate a course between the
Scylla of excessively loading the
source, leading to increased distor-
tion and the Charybdis of excessively high output impedance. The introduction
of the magnetic volume control TVC neatly cuts through this tangled Gordian
knot of contradicting requirements, making possible passive control units that
actually work more e
ff
ectively and in a much wider range of environments.
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icOn Smart TVC preamplifier
