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TVC definition


There are several ways to attenuate an audio signal:

The classic one is a voltage divider through two resistors in series(or a potentiometer in place of the resistors).
Signal IN is connected to the top resistor, signal OUT is connected to the junction of the resistors. The bottom resistor is connected to signal ground and dissipates the unwanted electrical signal as heat. As we know, one can not fool mother nature: to get rid of energy one must transform it into some other kind of energy. In this case, a pot, stepped attenuator (ladder, shunt, series, does not matter) always converts the undesired signal into heat. Of course the quality of the TWO resistors and the wiper contacts affect the final sound quality.

The second one is a transformer attenuator. The turns ratio between primary and secondary determines the signal OUT level. TVCs usually have one primary and 24 different secondaries.
Transformer-based TVCs go through two energy transformation processes:
1-electric do magnetic on the primary, then through the core
2-magnetic to electric on the secondary.
This double processing is not the NEC PLUS ULTRA solution IMHO.
Transformers have one unique advantage: galvanic isolation between primary and secondary. Galvanic isolation helps avoid ground loops and is a natural filter for digital supersonic noise (above 50 KHz).

The third mode of attenuation is an autoformer. An autoformer has no secondary, it consists of a high-inductance primary with 24 taps wound in series around a transformer core. Voltage division is performed by varying the turns ratio, therefore changing the inductance ratio.
(two inductors in series configure a voltage divider).
There is always a direct connection between input and output, which IMO is a major advantage of autoformers.
Posts on the technical asylum indicate that autoformers perform the voltage division two ways: inductance ratio plus resistive ratio (resistance of the windings). So, IMO, autoformers do provide a tighter and deeper bass and a more "direct" sound, because of their direct signal path.
The downsides:
a- if your CD player puts out lots of ultrasonic noise, an autoformer will not filter it as effectively as a transformer attenuator. A typical high quality autoformer designed for audio responds to 100 KHz. OTOH, this transparency is what gives good autoformers their amazing ability to pass "air" and soundstaging details.

There is a fourth way to do a voltage divider: two capacitors in series. Unfortunately caps distort too much, AFAIK no one ever tried a cap-based audio attenuator.

I hope this is kosher according to AA rules and helps understand the differences, advantages and disadvantages of each approach.
(flame-proof suit on)...



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  • TVC definition - Carlos 01/14/0407:45:40 01/14/04 (0)


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