Ever perform a preamp bypass comparision test?


I'm still in the throes of trying to evaluate my latest preamp acquisition. Without going into what exactly that is right now, part of my testing lately has been to try bypassing its gain and attenuation sections by taking the output from the processor bypass loop straight into the amps. This sends the unpreamplified signal from the source directly from the preamp's input jacks to the output jacks at unity (zero) gain, preserving the same jack connections and cable runs as are present when using the preamp in the normal way. By setting the volume control to attenuate the regular output so it equals unity gain as well, and swapping the output leads between normal out and processor loop out, I can make volume-matched comparsions of what effects the gain and attenuation stages of the preamp are having on the signal.

Obviously, there is going to be some degradation or changes to the signal revealed in this test, and I am hearing them. At this point in my post, I could go off on a rant about how I fail to understand many print and webzine reviews of preamps that indulge in rhapsodizing over the wonderous benefits conferred upon the music by XYZ preamp - as if a preamp can somehow not only give control over volume and source selection, but also somehow 'improve' the signal coming from the DAC or phonostage - but I will attempt to refrain from this for the time being.

What I am wondering now is how many of you have tried this in your systems, and what were your opinions of what you heard if you did? Has anybody done this and failed to detect a difference? Anybody feel there was actually an improvement of some kind with the preamplification engaged? My own feeling is that if you answered in the affirmative to either of the last two questions, you either have yourself one hell of a magical preamp, or your sources' outputs are not very hardy.

[If you have never tried this test and want to give it a shot, just take care beforehand to judge the resulting volume you will be subjected to when running your source unattenuated straight into your power amplification, because you won't be able to control the resulting volume (unless your DAC has a variable-level output feature) - the level will be determined by what's on the disk and source's own output level. Just move the output leads going from the preamp to the amp from the regular preamp output jacks over to either the processor outs or the tape outs (if unbuffered), and then set the preamp volume control to match that level when listening from the normal attenuator-controlled outputs (unity gain). From there it's just audition and swap, audition and swap, audition and cry...]
zaikesman
I believe your post, Z, can be narrowed down to this: garbage in, garbage out. Or: if one prepares a piece of dung for dinner, pouring the finest wine reduction sauce over it will not compare with serving a fine cut of prime tenderloin. Of course, tenderloin costs more than a bit of sauce. In audio, as in cars, the best does cost more. Some may argue that their "fast and furious" Hondas match a Ferrari. But that is mere product envy. Life, alas, is unfair. Audio imitates life.
Jb0194, I am clear that you prefer the sound of your system with your new preamp in place, rather than your previous one or the passive device you allude to. It is not surprising that your new custom preamp would sound better than one you describe as "modest". As for the passive attenuator, it would interesting to find out more about what type of device this is, and the context of its usage in terms of sources, amps, and cable runs. Two questions: Can you be more expansive and specific about what the "more compelling" differences are? And have you actually tried a bypass comparision such as I propose with any of the three units?

Jayme, although I don't believe that either of the preamps I used in my test was 'garbage', and also have never felt during normal use that what I hear from those preamps is 'garbage' (I've owned worse before), I don't disagree that it may take a larger preamp expenditure than I have made (can make?) to greatly ameliorate the situation I describe using my units, and I basically stipulate that much in my thread-head post.
Z-man: The "garbage" phrase is a popular metaphor, meaning in this context that the quality of the original source cannot be much improved downstream by a generalized application.
I guess I would describe it as "Something in/Something different out", whether that something is garbage or gold. A person is allowed to prefer the differences conferred upon the output by the preamp. And I acknowledge that whatever signal degradation is caused by the preamp, it might still be less than the degradation caused by an electrical mis-match between the source, cables, and amp input when running direct, or by some type of passive attenuation device, depending on the system context. My main point is that the only way for an audiophile to know any of this for sure, in their system, is to go ahead and perform the bypass test.