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The Bent Audio Model 102s Kit
Transformer-Based Passive Volume Control

(page 1)
Introduction
  
Bent 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:
 
  1. You need a good potentiometer. Better yet, a stepped attenuator. 
  2. There are different designs types to choose from: series, ladder, and shunt. Each of these has pluses and minuses that need to be considered. 
  3. 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. 
  4. The output impedance of the source component must be low and must have enough "juice" to drive the amplifier to full output. 
  5. 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. 
  6. 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.
 
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