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

Showing 2 responses by basement

basement
245 posts
 
I have a quicksilver line stage that I have done a simple mod on- I have taken the leads from the tape in jacks and unsoldered them from the tape switch and soldered them in parallel with the output of the attenuator, so I have both an active on one output and a passive on the other, all using the same exact switches and attenuator.
I have never seen much sense to a preamp being an improvement, after all, all we really need one for is to switch between sources and volume control. Why pay big money for something that is suppossed to do as nothing as possible, why not do nothing in the first place?
There is another side to the argument that says a matching devise of some sort is nessessary for the best sound evan though a passive may work. while of corse we all know this is system dependant, and obvious because different components have different input and output qualities, there seems to me to be much more to this, and I find it intreguing.
Needless to say, I find the passive more transparent, (but realize that the quicksilver is not supposed to be the most absolutely transparent preamp possible). The passive does not seem to always go as deep in the bass as the active does, and while it may seem that the extra depth that the active does is due to either the effect of gain, or the fact it is tubes, I'm not convinced that it is artificial. The deepness that I am referring to is not just more bass that is always present, but rather deep bass that it seems is suppossed to be there when the program calls for it.
And also, I find that sources that are suppossed to be good for driving a passive and not good for for a passive (due to the output impedences) don't seem to be following the rules. My theta cd player seems to benifit more, or suffer less from a passive, while my micheal yee phono amp which has the lowest gain of all my sources, seems to benifit the most going passive.
Also, amps that I have tried this with that I would think would benifit from an active stage due to thier input impedences sound better with the passive, both a pse studio IV (50 ohms) and a hafler dh-200 (33 ohms). No loss of high frequencies at all. This seems to be true no matter how much I have to crank up the volume to drive them.
These have all been with only 1 meter interconnects, and I have yet to try long ones, and I plan on trying more amps. All of my comments have been thinking only of transparency, or what I am believing to be accuracy to the signal, but I am not really sure if while at the same time I am losing information and details going passive, that the active is not the most accurate to what the signal is intended to be. The quicksilver is neutral, I can say that, and right now I am enjoying the option of going either way. I'm on the fence.
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.
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