Do Passive Pre-Amps 'Limit' the Lower-Mids?

Whether the active's 'full' presentation is indeed a coloration, could be the topic of another discussion.

The fullness of an active is not a coloration. The lack of bass that is typical of all PVCs is. It has nothing to do with the quality of the control, and everything to to with the variables of source impedance, the output impedance of the source at 20Hz, the value of the control, the setting of the control, the capacitance of the interconnect cables and the input impedance of the amp.

That's a lot of variables! Here are some other points so you can get an idea of how they play out: The lower the value of the volume control overall, the less detrimental effect it will have as you turn it down. However, the lower the overall value of the control, the harder it will be for the source to drive it. The higher the overall value of the control, the easier the source can drive it, but as you *turn down* the control, the series resistance instantly turns against you and you loose bass and impact right away.

With PVCs there is no way around this dilemma- that is why active preamps sound more 'full'- they can play bass properly!

Now to TVCs- TVCs solve a lot of PVC problems I just mentioned. They bring on some of their own. The transformer in this kind of control behaves the same way it does in any other transformer situation: it has a certain bandwidth and it has an ideal impedance to drive wherein it will have the least distortion due to ringing and other inductive effects.

To make this work, the TVC has to be very carefully designed so that it is properly loaded at all volume control settings. This is a bit of a trick! The reason is that a transformer *transforms* impedance- that is why they are so named.

When you use a particular setting on the control, it will express a certain ratio of the windings from input to output and thus your volume control setting. Maybe at that setting it is also properly loaded and so there is no ringing. Now the source impedance does not change and neither does the input impedance of the amp, nor the impedance of the cables (mostly expressed by capacitance). Because these other aspects are constants, as soon as you move to another control setting the loading is no longer correct. This will result in distortion due to ringing, or a loss of high frequencies. It may also result in the turns ratio no longer being properly expressed- indeed, with no load the transformer will express the inter-winding capacitance rather than the turns ratio.

Finally we have the issue of overall bandwidth. Transformers always express a compromise of low frequencies vs high frequencies- how well you can play bass will have an effect on how well the highs are played, in a nutshell. The smaller the transformer, the better the bandwidth, as a general rule of thumb. **However** to play bass to 20Hz, the circuit has to have bandwidth to 2Hz, else 20 Hz will not play correctly. Transformers have trouble going that low- in essence, you will not be hearing the deepest bass out of a TVC whereas an active preamp *should* be able to do 2 Hz no worries (our preamps go to 1 Hz). BTW the test for this is '20Hz square wave tilt', IOW the measure of how level the top of the square wave is. To play 20Hz correctly it should be unmeasurable.

Now its my opinion that the only reason PVCs and TVCs have made their way in high end audio has to do with how abysmal some active line stages actually are. But just because some line stages are obviously colored does not mean they **all** are, and price has nothing to do with it!

What about those times when there can be said to be a synergistic match between source and amp to start with? Could this harbor at least the possibility that in that case the performance gap between those same passive and active pre’s within those systems could be said to narrow? Could passive pre’s in that instance then be thought of as being allowed (for once) to actually live up to the potential of their designs? That is, can the performance of a synergistic source-and-amp combo, when given an appropriately high quality passive pre, favorably compare to the performance of the source-and-amp combo, when given an appropriately high quality active pre? Can the synergy of the former pairing alone account for that difference?

The problem with synergies is that a weakness of one product is being balanced against a weakness in another product. A simple example is a dull preamp paired with a bright amplifier. Together they sound OK- in other situations those faults are revealed.

This scenario does not work. When a synergy is exploited in this manner, the result may have some good qualities but you will always have a system that is less transparent and less fulfilling than if the individual components did not require such manipulation to begin with!

Brownsfan, It is a common experience to hear things at dealers that might sound unnatural. Poor acoustics, other speakers in the room and other factors are easy contributors. Any decent dealer will allow you to bring in your front end to help out with an audition though.

Clio09, a preamp's line section has 4 functions:
select input
control volume
add any needed gain
control the interconnect cable

PVCs do the first two and most active line sections do three. Very few do all four, mostly from what I can tell due to the fact that the designers are unaware of the fact of the 4th function. The result is an otherwise neutral design's capabilities go untapped due to the coloration of the cable. Add to that the colorations that are common pitfalls with a lot of designs and you have a line stage that is a good attempt but nevertheless gets poor results.

See y'all at RMAF!