difference between an active and a passive preamp?


hi,
I have a nad c272 amp and am looking for a good preamp to go with it, but I am on a very tight budget. I see lots of preamps that are acive and some passive - I have no idea of the difference? I have quad 22L speakers and listen to cd only. Any help understanding these differences would be great. I just want simple 2 channel preamp, with as tube like sound as possible. Please help, and many thanks,
jason
128x128audioflyer67
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
Ts0711, you have a very sensitive amp so it won't take much to get your amp to full power. However, I would think you would lose some dynamics and high frequency response with interconnects that long. Have you tried to get your passive preamp and source close to your amp so you only have 3 feet from preamp to amp and about same from your source to preamp? I would think you would hear a noticeable difference. Your present setup may sound good to you now, but using shorter cables may significantly improve the sound.
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
Al, thanks. My understanding of TVC or AVC vs resistor/LDR volume control is that as you apply more attenuation, TVC will decrease its output impedance whereas resistor volume control will increase its impedance. That is one argument for TVC because your suppose to get better sound as you attenuate more with TVC, while you get worsening sound as you attenuate more with a resistor based volume control. At least that's the argument I heard, but I don't know if in practice this is true.
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