Passive Preamp?

A passive preamp has no amplification stage. It is actually something of a misnomer. Typically they do not plug into an electrical source. I own two of them and they are both just switch boxes with a volume control. If you need more than this fundemental info, then you should follow Herman's advice above.

Check out the thread at: http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1159194844&read&keyw&zzno+regrets

It sounds better than most active units, it goes between your CD player or DAC and your amplifier.

It is mainly a volume control with no gain since the amplifier can handle the DAC perfectly well by itself without adding another gain stage and the problems that can come with it. A passsive volume control then will sound more transparent...you will have to do some reading in order to insert one in your system because they are system dependent:

What is your system?

Dynaco ST-70
PHILIPS 910-CD
Dynaco FM-1 tuner
Teac 330 reel to reel

Hello Groundhog,
If using more than one source, like you with the CD, Tuner, and reel to reel, it is best to use an active preamp or you will need to really do a balancing act to have separate gain from each source and diferent impedance each also. I am sure your ST-70 has enough gain, but impedance matching all the sources is a problem.
Its not impossible, but complicated.

You can also get a good active preamp, vintage amps (I have several) like newer preamps IMHO.

If you decide to go with one source only, the passive will work just fine.

All the Best

PS: Placette Audio has a passive preamp designed for various sources, you can check them out.