mohler@druxu.UUCP (MohlerDS) (07/23/85)
In the process of trying to find champagne sound at beer prices in preamps, I have found some design criteria that when utilized result in excellent sound. I decided to share this with the net since it may be of help to anyone trying to find some measures other than just listening to determine a preamps merit. Keep in mind during this discussion that poor quality could negate any of these good design criteria. In any preamp, better quality parts and tighter tollerances in phono equalization will improve the sound. A phono section implemented with two gain stages (one for the upper half of the RIAA equalization and one for the lower half) will result in better sound than either a one stage active or passive design. The input stage of a gain section of a preamp should be as low noise and as linear as possible. This is due to the fact that subsequent stages (even cascodes) are of only minor benefit in reducing the noise figure of the input stage. This is also due to the fact that with a typical input stage (diff amp, complementary diff amp etc.) feedback doesn't help to linearize the input stage. Experimentally I have proven to myself that a complementary diff amp input stage formed with either JFETS or low noise Bipolar transistors with or without a second stage cascode is the way to design an input. The output of a gain stage has a couple critical criteria in a preamp. One is that the output impedance be low enough to avoid frequency response problems, i.e. use an emitter or source follower output. The second is make the output complementary and class A. A power supply and any designed in regulation in the gain stages should have been designed to avoid the use of zener diodes (they are noise sources) and should also take into account good single point grounding, good regulation and the problems of producing frequencies lower than the AC line frequency. (how many times per cycle of a reproduced waveform do the capacitors in the supply get recharged? This is a real issue at sub AC line frequencies.) I am in no way saying these are the only issues in preamp design, I am merely saying that these are things that can be heard and measured and are therefore important. Many people have asked what is the best preamp that I have heard and measured. My reference is either the the Krell KRS or if you don't consider that a real preamp (you can't just walk into a dealer and buy one - special order only) the Krell PAM-3. They are the best designed, best implemented, best sounding preamps I have ever seen and heard!! If you are looking for a very expensive preamp like the Levinson, Spectral, Klyne, Threshold, etc. don't buy until you have heard a Krell PAM-3, it is unbelievable!! Those of you that love the big Conrad-Johnson or ARC tube gear, open your mind (and ears) and listen to this preamp! David S. Mohler AT&T - ISL @ Denver drune!mohler or druxu!mohler