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
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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
I have to admit that the common detractors' rant of diminished dynamics rings true for me. For what I put into my system, however, I consider it an acceptable trade-off (for now). Plus, my system isn't perfectly optimized for a passive preamp, like the op's. This brings me back to the, more or less, main point of the thread as expressed in the original post, which is to determine when one should use a passive preamp versus an active preamp given a certain set of non-negotiable pre-existing system components.

This thread seemed to point out an Option C, however: the buffered passive. It seems that if enough gain is provided by the source output and amp(s), then a passive or buffered passive should be sufficient to accurately reproduce the source material. The only issue is impedance and possibly voltage(?). (Based on my reading here, this is my impression). If this is so, then shouldn't optimal dynamics (e.g., "attack") be achievable with a buffered passive, if not a discreet passive? For the proponents of actives, I suppose a more direct question is, what specific attribute of a proper active preamp achieves that vivid "attack" that a passive does not? Is it more than an impedance issue? If so, then what?

In short, it's clear to me that not all systems are passive-friendly, but a buffer rectifies the situation. That is, a buffer seems to be *the* Band-Aid for passive-hostile systems, based on any thread I've found on this subject. Or is it? When is a buffer plus a passive still not enough, and how common might this situation be? If this question deserves another thread, please say so and I will post anew. But this seems like a natural progression from the current conversation following from the op's expressed concerns.
I wouldn't necessarily consider a buffered device a band aid for passive-hostile systems. If you look at the designs of a Pass B-1, Burson-160, and Horn Shoppe Truth as examples, these buffered devices are anything but band-aids. They are also technically active devices as the buffers require a power supply, but in general they do not add gain to the system. The main purpose of the buffered output is to provide a consistent and low output impedance, generally under 100 ohms, but this could vary depending on the buffer design. Those using opamps or other solid state devices will tend to have very low output impedance, while tube buffers will in most cases have greater than 500 ohms, perhaps even over 1000 ohms.

IMO optimal dynamics can be achieved with both passive and active buffered preamps, my Lightspeed attenuator, Truth, and Silicon Arts Design preamps being examples in my system. However, system matching with a passive is a bit trickier than an active buffer (which if the output impedance is very low and due to the active circuitry may also provide the benefit of using longer interconnects). To really determine if your system is passive friendly and more importantly if you like the resulting sound, go to Arthur Salvatore's web site and read up on the Bolero test. This should give you a good idea if your source has enough output voltage to drive your amp. If so you might be ready to experiment a bit with resistive, magnetic, or LDR passives, as well as active buffers (preferably those with built in attenuation).

I could go on about how component specs can influence the sound, as can cable length and capacitance, but there is nothing like hearing it for yourself (although I think your system would benefit more from an active buffer). Lots of options out there at very reasonable prices to try both passive and/or active buffers. I know Ed Schilling offers a money back option on The Truth, and at $800 or so it is well worth a test drive. There are others that may offer trial periods as well. Take advantage of those opportunities to listen for yourself.
Although I have not heard the buffered passives Clio09 mentioned, the one's I have heard did not sound as transparent as the pure passive preamps. But this was a long time ago, and I don't even remember the brand of the buffered passives I auditioned. On my Bent Audio Tap X, I was thinking about putting in a Burson active buffer on the output of the Tap X, but this is akin to putting a transformer on a OTL amplifier. Seems ass backwards, but I've been known to do ass backwards stuff in the past.