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
sean
6,231 posts
 
Zaikesman: I have a DAC with dual outputs i.e. one that is line level and one that is fed via a stepped attenuator with the capability of 4 volts of output. I have tried listening with the stepped attenuator feeding the amp directly and with the line level outputs feeding one of my preamps. Quite honestly, i could not hear a difference.

If i can make arrangements with a local individual, he has a "straight wire bypass" contraption that Quad built for a select few dealers. Supposedly, there are only about four of these in existence. This can be used to do what you are doing AND be done under DBT conditions as it can be set to do random switching with the unit in or out of the circuit. While it is surely not a "mandatory" listening tool, i think it would be kinda cool to have and experiment with. Sean
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zaikesman OP
3,017 posts
 
Sean, your result would seem to maybe indicate that the attenuators in both your preamp and your DAC are of roughly the same quality (I take it that you normally run the fixed outputs from the DAC and use your preamp for level control). Have you ever done the comparision I describe above, using the feed from either the DAC's fixed outputs, or from the variable outputs set to a constant comfortable level? That would tell you what the contribution is from the preamp alone.

(FWIW, it seems from the lack of response to this and previous posts I've made along these lines, that not too many other members are interested in trying to do this kind of isolation testing on their gear. I'm a little surprised, to tell the truth - I find it fascinating and informative, but maybe I'm just a freak! :-)
drubin
3,285 posts
 
You're not a freak. In some ways, what you propose is threatening to some of our core beliefs as audiophiles. There was a thread not too long ago about which component is most important, and many people weighed in with preamp. At times, I must say I agree with that. But it's also fundamentally irrational if you believe that the job of the preamp is to switch and attenuate (and maybe add gain). The mainstream audiophile press has steadfastly avoided seeking the truth on this matter and us sheep audiophiles have followed suit (not singling anyone out). It's scandalous, really.

On the other hand...What is frustrating is that a great preamp can make a system sound more like music even through it may well flunk the bypass test. It is adding to the sound--how else can we explain it?--yet doing so in a way that is musically compelling.
sean
6,231 posts
 
Zaikesman: I have done exactly as you stated. Sorry if i did not make that clear. I ran the fixed outputs from the DAC into the preamp and then fed that into the amp. I then switched over to the variable outputs of the DAC which directly drove the amp via the DAC's built in stepped attenuators. I could tell no difference between the variiable of the DAC fed into the amp in "direct drive" mode and the line level out of the DAC feeding the preamp which fed the power amp. This tells me that the preamp is either extremely neutral or that the stepped attenuators are "coloured" in a manner that is exactly the same as the preamp in question.

The "straight wire bypass" device that i was talking about allows you to actually put another preamp, power amp or both in SERIES with your existing "reference system". It comes with a device that allows you to gain match the system so there is no difference in volume. You can then manually switch between just your "reference components" or your references with the other components placed in the signal path to see if there is a difference. You can set up the switching device so that you can see if all of the components are active or you can do it in a blind and random fashion. If you can hear the difference with the "test components" switched in, they are either "coloured" or you are experiencing loading problems with the specific gear in question. Sean
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