|Audio Tweaks - Bent Audio 102s
The Bent Audio Model 102s Kit
Transformer-Based Passive Volume Control
Audio is a Canadian company based in Surrey, BC. Their
main focus is high quality passive pre-amplification. What got
Bent Audio on the map was their innovative TAP system, a
modular passive pre-amp for high-end audio applications. It
comes in both resistive and transformer-based flavors with
enough input/output options to meet anyone's requirements. The
transformer-based modules were then spun off into kits that
have now become very popular.
|I have always been enticed by passive volume
control. Its innate simplicity made a lot of sense to me
especially in the early days of my passive "preamp"
considerations. Going passive, however, is quite a balancing
act and not as simple as one would think. Several years ago
when I started looking into passive solutions I learned that
it took more than just a good pot to get good results. Getting
resistive type passive volume controls to work right involves
taking into account the following:
- You need a good potentiometer. Better yet, a stepped
- There are different designs types to choose from:
series, ladder, and shunt. Each of these has pluses
and minuses that need to be considered.
- Then there are the issues surrounding impedance
matching and mismatching. One concerning input
impedance and how it relates to output impedance of
the source component; another concerning output
impedance and how it relates to input impedance of the
power amplifier. Input impedance that is too low may
push the output stage of your source component too
hard and into distortion. Output impedance that is too
high may cause audible high frequency roll-off. To
complicate matters further, input and output impedance
may change with the change in volume settings.
- The output impedance of the source component must be
low and must have enough "juice" to drive
the amplifier to full output.
- Amplifier sensitivity has to be high enough and
within the source component's output capability. And
the amplifier has to have enough gain to amplify a low
level signal to sufficiently high levels capable of
driving your loudspeakers loud enough.
- Your interconnect cables, particularly the one from
the passive volume control to your amplifier, have to
exhibit low linear capacity so that they don't start
acting as high frequency filters. For this reason,
leads need to be kept as short as possible.
I'm sure there is more to keep in mind and get just right
before you can truly reap the rewards of resistive type
passive volume control. Poor or mismatched implementations
of passive "preamps" are often associated with a
sound that lacks dynamics and extension. However, used
properly, they can offer excellent results, both sonically
and economically. For the past year or so, I have been
using what I consider to be one of the best resistive type
implementations: a pair of EVS Ultimate Attenuators (1K
version). These regretfully are no longer in production;
however they still represent excellent value.
A transformer-based volume control, on the other hand, is
quite a different animal as compared to its resistive type
counterpart. The problems surrounding impedance matching
and mismatching are largely gone. The input and output
impedance of a transformer-based volume control, though
varying with different volume settings, can be kept within
optimum values for both the source component and the
amplifier. To some degree transformers perform more like
active preamps, some offering even + 6db gain or more. It
is believed that, contrary to resistive type controls,
they do not suffer from lethargic sound.
Well, being firmly rooted in the "passive way",
I had to investigate this promising transformer
alternative. Transformer-based volume controls are not
necessarily new; for some time I have been writing to
Santa for an Audio Consulting Silver Rocků but with no
luck. It being out of my budget range, I was pleased to
find a less expensive alternative in the form of a Bent
Audio Model 102s kit.