Running a CD player directly into power amps,

A rule of thumb is to have the input impedance 10 times the value of the output impedance of the driving equipment. If the input impedance of the amp is 10K ohms (most are at least that) then the output impedance could be as high as 1000 ohms.

An analogy to a battery might help to understand the situation. A battery can be modeled as a perfect voltage source (maintains its voltage even if the current is infinite) in series with a small value resistor. As you pull more current out of the battery, the voltage drops a little because of the low value resistor that is "inside" the battery. As the resistance of the load gets smaller it will try to draw more current from the battery, but the "resistor" limits that current somewhat. One can see that the "resistor" inside of 12 volt lantern battery is much larger than the one inside a automotive battery. If a low resistance load (like a automotive headlight) is connected to an automotive battery it is going to be brighter than when hooked up to the little flash light battery.

The idea is the same in a passive volume control. (several CDP's use this on the output while others put active stages following the volume control and then there are the ones that use digital attenuation in the output stage) A passive's maxium output impedance is TYPICALLY at mid position (electrically) and is approximately half the value of the variable resistor. This means that a 10K pot will have about a 5K output impedance and might be suitable for use with a tube input amp as long as the IC cables were low in capacitance. If the volume adjust is followed by a electronic stage then the design of the stage determines the output impedance. (typically a 200 ohms or below)

I do agree that a well designed CDP could be better than a preamp just because it will have less connections in the signal path. The big question is what entails a "well designed" CDP. I believe that a good output will sound good and will look forward to comments from others regarding the sound of CDP with variable outputs. Chris

One comment on Croese's dislike of digital volume controls: Accuphase and Wadia have taken steps to overcome their USUAL defects, and in my experience with an Accuphase DP-75, they have succeeded. I used my DP-75 with excellent cables and an excellent passive preamp, and still lost detail versus the DP-75 "straight into" my power amp(s). An internal stepped attenuator might be better yet, saving the cables and their connectors, but I can't hear any degradation if I keep the digital volume attenuation below 12 decibels, which in my system is always enough. Accuphase claims you can go up to 20 decibels of attenuation and I have no reason to disbelieve this, but if you can arrange to use as little as possible, that couldn't hurt.

Croese, i'm assuming that your commentary pertaining to "bullshit" and "charlatans" was aimed at myself or possibly one other post previous to yours. While i can't respond for the other participants, I was simply trying to cover all the bases in a very "general" statement.

Since i am not familiar with ALL of the equipment out there, nor do i think that anyone else is, i used an example for those not electronically inclined to follow along with. I was trying to make sure that the analogue output of the CD player or DAC would not be "loaded down" under ANY circumstances. As such, i chose to post "worst case scenario" using generic figures and figured that ANYTHING above that would perform acceptably. While i'm sorry if my use of 50 ohms threw you for a loop or led to a misunderstanding, re-reading of my post should clarify things. I specifically stated that it was preferred that the input impedance of the amp be higher than the output impedance of the source. If i would have used non-RF based impedance figures or figures in the thousands or tens of thousands of ohms range, would that have made you happier ???

As Chris stated, it is common practice in the audio field to shoot for a "X10" difference in impedance from the output of one piece to the next gain stage. As such, i was not suggesting that one need worry about VSWR in audio gear although i do not doubt / deny that this factor MIGHT come into play in different ways. This is a subject that still has a LOT of "grey areas" to it and subject to much debate by folks FAR more knowledgable and experienced than i will ever be.

As far as that goes, i know my limitations and try to work within those confines. If i am making suggestions or comments based on anything but verifiable data or first hand experience, i typically state such. There is nothing wrong with presenting ones' opinion, but it should not be presented as "fact" unless one has done all of the necessary testing under various conditions to verify such. Sean
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croese@creatoaudio.com:

**do not listen to self-educated charlatans with distorted technical
orientations, but check with retailers and listen to the equipment
yourself**

since you choose to be so obnoxiously pedantic, perhaps you'd like to reveal your qualifications. while you're at it, why don't you register so we can obtain more of your "knowledge." BTW, you’ve obviously not heard or heard of the accuphase cdp’s. the dp-75v, for example, has a digital volume device that drops NO bits. -kelly