2. amps-preamps
  3. 1034575658

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
basement
245 posts
 
I meant to say "sources that would seem good candidates for a passive, such as the theta's", but I should add "also from output voltage and what I have read". The theta seems to benifit less from going passive, while my phono, seems to benifit more from going passive. My findings so far are opposite from what I have read would be the case.
In the quicksilver line stage, the volume is before the line stage curcuitry. The path is fron inputs, to the rotary switch, then to the other swicthes (mono, ect.) then to the attenuater, then to the active curcuitry. I have added two leads going to output jacks from the attenuater, so the active and passive are in parrallel coming from the attenuater each with it's own set of output jacks. It is actually what seems to me a good quality passive, given the stepped attenuater and quality of jacks. The only inconvenience is that the active reverses polarity, so I have to reverse the speaker cables whenever I go from one to another, or else I would put in a switch.
You bring up interesting point about the bass frequencies, I have not payed that much attention to it insofar as a mismatch, perhaps I should. I have looked only at rolling of highs as a mismatch that just would'nt work well (sound quality is the main thing, but I am refering to situations where it is not possible due to the interfaces to evan consider).
I wish I knew a way to invert polarity on a single ended signal.
zaikesman OP
3,017 posts
 
JC, I appreciate your input, but I think you may have read some things into my post that I did not intend. I am not trying to make a case for running direct, and I do realize that my preamps (list prices about $1,800 to $3,000) are not SOA. Neither are most folks' preamps, and neither do most folks want to run direct or passive. I am also not trying to solicit advise concerning my own preamp situation in particular with this thread, which is why I'm leaving out the make and model info, since I don't want my comments taken out of context.

My post is just intended to explore the notion of employing a bypass test to look at preamp performance, and to see if other members have had experience with this kind of procedure and what their results may have been. In my own case, where I feel confindent that my DAC, at least, is just as capable of driving my amps' inputs as my preamps, I admit I am skeptical of the notion that a preamp could not only fail to cause *some* signal degradation, but could actually make the sound 'better'. I acknowledge that I don't have epxerience with the best preamps, but rest assured that I have no intention of removing the preamp function from my system. I just want to know exactly what it's doing.
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drubin
3,275 posts
 
But Zaikesman, if the preamp can't make the sound better, why would you not want to go direct or passive, at least for digital?
jetter
3,043 posts
 
Zaikesman,
Wasn't it you who yesterday took the time to belittle someone who posted a review on a Meitner MTR101 amp because it did not fit into the audiogon definition of a product review?
I was already thinking about what type of person has the need to do that to someone else and then post his own nonreview just like he took someone down the day before to do it.
zaikesman OP
3,017 posts
 
Jetter, need I reiterate for you that this is *not* a product review? This post, if I do say so myself, actually delivers on the promise implied by its title line, should you choose to click on it - something that other post you refer to did not (as of right now, mine is still the only response to that three-sentence 'review', which should be some indicator of how worthwhile it was). If you have some pertinent criticism of my post here, then please, by all means contribute it. (And if you care to see how I do write a product review, you know how to find that too.)

Drubin, I just want one control center in my system, without having to go around back and pull wires all the time, and with the gain I sometimes require to listen at higher levels. I realize there is always, at least in my own opinion, going to be some sonic tradeoff for this convenience. I know I wouldn't be the first audiophile to have ever concluded that a preamp is basically a necessary evil. :-)
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