Active preamps have the ability to add gain (amplify) to the incoming signal. The vast majority of preamps are active. The tip off is whether it has a power cord.
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In general usage, a passive preamp is one that does not have an amplification circuit. I.e., its volume control acts only to reduce the volume, starting from the full input signal from a source. Most sources output at high enough a level that they can drive a system pretty loudly without any amplification at the preamp stage. As a result, all the preamp really needs to do is reduce the level to a loudness that the listener wants to hear. This can be done in a number of ways without requiring a power source inside the preamp, including using a variable resistor in line with the signal or using a transformer.
Of course, there will always be instances when the listener may want to hear something louder (perhaps because his speakers are very inefficient or because of a signal level-diluting impedance mismatch at his amplifier), and this would require that the preamp increases the level of the signal using an amplification stage. Such a preamp would generally be called an "active preamp" because it would have an active circuit that would require a power source, usually wall current or in some instances battery power.
Of course, a phono preamp would have to be active since phono cartridge output levels are usually quite low and their signals are not recorded with a flat frequency distribution and thus require RIAA equalization, but that's a different subject.
Reading further in to your question, I agree that all preamps are passive but only in the sense that they are not intended to drive speakers directly. "Amplifiers" or basic amplifiers are required to drive speakers. (When the preamp is built into an amplifier, such a unit is called an "integrated amplifier.") Your use of the word passive is in a broader sense than the audiofile use of "active" and "passive" when referring to preamps.
The general definition of 'active' and passive' is similar to the audiophile definition.
Passive components are resistors, capacitors inductors and transformers, none of which requires a power supply in order to function. Passive preamps are made of these devices.
Active devices include transistors, integrated circuits and vacuum tubes (among other things) require a source of AC or DC power in order to function and their inclusion in active preamps is what is indicated by the AC cord.
Actually, you can make the volume attenuation even simpler - avoiding a "box" and having to add another set of interconnects. I have a pair of "shotgun" attenuators which sit between my interconnects and my amp.
Attenuator in my System
As with most things audio, there are many opinions regarding the sound quality provided by active and passive preamps, including the in-line attenuators discussed above. In general terms, a preamp accomplishes three things, a. switching between sources, b. attenuation (or gain-if active) of the source output signal, and c. impedance matching (or in some cases, mismatching). Item c (in addition to output levels and sensitivity), is a reason that passive preamps do not work well in all systems. If you are really interested, or really bored, there is a ton of reading on those subjects in these threads.
Thanks for the responses.
The main reason I asked this question is because I was wondering about not using preamp at all. That question led to this question.
I actully posted another question on that subject, but no response yet.
I'll kind of repeat what I said on that posting.
I'm looking at some of the newest high performance DACS such as Benchmark DAC1, Bel Canto DAC3, Slim Devices Transporter, etc, and I noticed most of those DACs have built in volume control with remote.
So, I was thinking that I can connect these DACs directly to my Power Amp eliminating preamp entirely.
I was wondering what's the implication on that approach?
What'll be the pros and cons?
Is it better because I'm skipping additional device that I don't need? or A good preamp somehow does some good thing to improve the sound quality?
"Is it better because I'm skipping additional device that I don't need? or A good preamp somehow does some good thing to improve the sound quality?"
I'm of the opinion that the simpler the signal path, the better the sound MUST be, in theory. HOWEVER, practically, that doesn't always result. There are techical reasons having to do with characteristics measured by input sensitivity, impedance, etc. I can't fully understand that stuff so, I resort to trying it out and listening. It's pretty inexpensive to try...Give it a shot!
Eandylee, most volume controls that are in DACs are not of the best quality- particularly if they are digital. Digital volume controls essentially subtract bits to lower the volume; the result is audible loss of resolution as you turn the volume down. If you want to do it right, an active preamp (especially one *without* remote) will have a better chance of doing the job right- you would run it and have your DAC turned all the way up for maximum resolution.
One other task of active preamps is to control the interconnect cable. If set up properly, the line section of the preamp will minimize or eliminate the effects of the interconnect cable. Be careful though- many preamps may do their other jobs well but fall flat on their face in this department, introducing coloration due to the cable. BTW, this is a serious weakness in 'passive preamps' which in fact are not preamps at all.