Preamps for dummies that would be me


This is one of those “why is the sky blue” questions that I feel like an idiot for asking, but here goes.....

Other than switching inputs and controlling volume, what exactly does an active preamp do? If that were all to it, we'd all be using passive preamps. I've tooled around the web looking for articles, but I'm not really hitting anything. I've seen some veiled references about 'conditioning the sound' for the amp. Whatever that means.

So what, other than a fixed and usually too high output level, is coming out of the CD player(or tuner or whatever) that the input of the amp does not want to see. Thanks in advance for not slamming my ignorance.
randalle

Showing 4 responses by marakanetz

In fact, it's all about the system's integrity.

Yes, basically the active preamp is designed not to only bring the signal to the line level but to restore drastically compressed and feedbacked output of the cheap CD-player with bi-polar output stage for example and bring it to the amp that also has quite large amount of feedback.
A good active preamp is usually built with small amount of feedback and can be a great interface between the components. The output impedance of preamp and input impedance of the poweramp have to match approximately 1:1000
Most-likely all active pre-s have a low output impedance that can match well with low input impedance of the SS amps. The low input impedance of the amplifiers is caused by deep negative feedback that highly limits the input sencitivity.

No, no active preamp shall be designed to 'condition' the sound but in fact they do despite being designed not to.

Yes, if you have invested into the high quality source component CD-player or analogue; if you have low neg. feedback on your poweramp, high input sencitivity all you need is just input selector and volume control i.e. passive preamplifier.
Just of the tech experience I can add the following:

Any preamp that is using variable electronic or op-amp feedback with bi-polar high-gain output stage will certainly deplete and compress the sound drastically. The sound change is very similar to turning-on Dolby NR system to the cassette or any tape as you plug in such pre- between your components v.s. no preamp.
The same amp with regular R/C feedback tend to colour the sound and make it extra-bright due to high instability of the bi-polar elements.

So choose either tubes MOSFET or JFET preamp that most-likely have low neg. feedback, linear and does only minimal 'conditionings' to the sound.
Z.., Forgive me my horrible arithmetics, even 1:10 is good enough.

The so-far called "conditioning" of the sound might(or might not) apply to "restoring" the sound after feedback on the source components. Certainly I should've placed it in quotes since whatever is lost on the first place can never be restored and that's why many prefer to call it sound conditioning. Active preamps do not neccesarily have a gain. They can have a unity maximum gain as well or very low gain that is not able to "condition" or "restore"...

Thus it's not neccesarily that 20dB-gain preamp will sound better than 2dB.

Volume control and pre-amplification in general is one of the most vast parts in audio and it certainly equally conserns passive preamps as well.

I prefere to invest more into source components rather than spending a fortune on the true active preamp that niether colour, condition or restore the signal. That's why I use passive and probably never will go active.
Gs..., I wouldn't realy think of any other than CAT if I had a budget for it:-)