DynaMaxx
11
SPL’s Double VCA Drive Technology
TM
Double VCA Drive Technology
TM
utilizes two VCAs per audio
channel, where one VCA handles positive current and the other
negative. This way, the control voltage can effectively be halved,
but the amount of gain change can be doubled. The benefit of
this configuration is that the transistors within the VCAs don’t run
into saturation problems, which in turn avoids offset noise, gene-
rally audible as clicks and pops.
Compared with the acclaimed DBX 2150 VCA, the new THAT
2181 VCA is less sensitive to offset noise, and the Double VCA
Drive configuration produces extremely good audio perfor-
mance, both subjective and measured. The diagram illustrates
the principle of this circuit:
The audio signal is split in two paths, one of which is phase
inverted, and after having passed through the VCAs, the two
signals are recombined in a differential amplifier. This topo-
graphy causes a cancellation of negative side effects, mainly
distortion and sound coloration, by the mechanism of common
mode rejection (CMRR > 50 dB) within the differential amp. At the
same time the original audio signal is increased in level by 6 dB.
What are the practical advantages?
Clearly one cannot expect the noise floor of the VCAs to be
reduced by 50 dB, because noise is a non-correlated signal, but
taking into account the 6dB signal boost, the noise floor is
improved by 3 dB.
However, the real benefit is the reduction in THD (Total
Harmonic Distortion). Although the THD spectrums of positive
and negative half cycles are not identical, a significant degree of
cancellation nevertheless takes place, and the residual THD is
right at the limits of available measuring equipment. The same is
true of intermodulation distortion and control voltage crosstalk,
both of which are cancelled in the differential amplifier stage.
Diagram:
SPL’s Double VCA Drive
Technology
TM
SPL’s Double VCA Drive
Technology
TM
and the new
„2181 Super-VCA“ from THAT
Corporation double the opera-
ting range, realize extremely low
noise and distortion figures as
well as a unique sound quality.
