difference between an active and a passive preamp?

04-12-11: Almarg
TVC's are a completely different story, that I'm not particularly familiar with. But since a transformer provides an impedance transformation corresponding to the square of the turns ratio (with attenuation or gain being directly proportional to the turns ratio), I would expect that as long as the volume control position that is used corresponds to a significant amount of attenuation (i.e., it is not near the top of its range), cable length and component impedance matching would be much less critical than for a resistive passive preamp.

I should have added to this statement that it assumes the transformer itself does not introduce significant resistance or impedance into the signal path. I have no particular knowledge of how true that assumption may be for typical TVC's.

Regards,
-- Al

04-12-11: Dracule1
I'm not too certain on about how the length of the interconnects affects this. But I think longer interconnects will decrease the amount of effective volts the amp sees from the source. So I try to keep my interconnects as short as possible which also helps to prevent significantly losing high frequency response. If anyone more technical can find any error in my explaination, please correct me. I'm not that technical.

04-12-11: Ts0711
@dracule1
What am I missing? My amp has a sensitivity of 775mV (VTL compact 100), and I've been running 20 foot interconnects between the amps and various passive pre's with great results.

With resistive and LDR-based passive preamps, the need for short interconnects derives from their high output impedance, and applies to the cable between preamp and power amp. I can't envision a reason why the length of the cable between source and preamp would be any more critical than with an active preamp.

The reason cable length on the output side matters is that its capacitance forms a low-pass filter in conjunction with the output impedance of the preamp. Cable capacitance, besides being dependent on the cable type, is proportional to cable length. If it is too high, the bandwidth of the low-pass filter will be low enough to fall within the audible spectrum, resulting in some amount of roll-off of the upper treble.

The degree to which that is significant will also depend on where the volume control is set, since the volume control setting will have a major effect on the preamp's output impedance. For a passive preamp consisting of a potentiometer or a resistive stepped attenuator, the worst case output impedance will occur at a setting corresponding to 6db of attenuation, which will typically be very loud if the source is a cdp or dac.

As was noted, it is also important for the output impedance of the source to be low (in relation to preamp input impedance), and for the input impedance of the amp to be high (in relation to preamp output impedance). Otherwise frequency response irregularities will result, to the extent that the impedances that are involved are not constant as a function of frequency.

TVC's are a completely different story, that I'm not particularly familiar with. But since a transformer provides an impedance transformation corresponding to the square of the turns ratio (with attenuation or gain being directly proportional to the turns ratio), I would expect that as long as the volume control position that is used corresponds to a significant amount of attenuation (i.e., it is not near the top of its range), cable length and component impedance matching would be much less critical than for a resistive passive preamp.

Regards,
-- Al

04-12-11: Dracule1
My understanding of TVC or AVC vs resistor/LDR volume control is that as you apply more attenuation, TVC will decrease its output impedance ...

True, which is why I referred to TVC sensitivity to cable length being less "as long as the volume control position that is used corresponds to a significant amount of attenuation."

As you apply more attenuation ... resistor volume control will increase its impedance.

That's not quite true. Assuming the resistive volume control is configured as a conventional voltage divider, it is true that the lowest output impedance will occur at the maximum volume position (where it will to a very close approximation be equal to the output impedance of the source). However, the worst case (maximum) output impedance will occur when the volume control is set to provide approximately 6db of attenuation, which in terms of subjectively perceived volume is not greatly different than the maximum volume position. As the volume setting is reduced from that point, output impedance will then gradually decline.

Best regards,
-- Al

04-12-11: Almarg
I can't envision a reason why the length of the cable between source and preamp would be any more critical than with an active preamp.

05-18-11: Atmasphere
With passive controls, the cable length is quite critical as is the construction of the cable. The interaction is between the source impedance, the installed variable of the passive control, the characteristics of the cable that goes from the passive to the amplifier, and the input impedance of the amp. Its a bit complicated, but with passives what you will universally hear is that as you turn down the control, the bass and impact in the system will be diminished. It does not matter the quality of the control itself- that part is really not a variable, although the *value* of the control is.

Ralph, thanks for your comprehensive and characteristically informative response. Could you explain a little further, though, what parameters of the cable connecting the source component to a passive preamp would interact with the things you mentioned, beyond interaction with the output impedance of the source component that would also occur with an active preamp?

Best regards,
-- Al