Preamps... understanding volume...

Is my understanding that preamps don't "amplify" a variable line out, but only attenuate?I'm trying to understand how preamp selection is important with low sensitivity speakers, even if you have a powerful amplifier.

Well the "variable line out" is the out on a preamp. The key word to understand, what everyone above is sort of but not quite getting, is "line". Line level is around 2 volts. CD, DAC, tuner, tape decks, VHS, pretty muc everything you care to think of is line level. Even the output from a phono stage is line level. In fact the only things you are ever likely to encounter that are not line level are the output from a phono cartridge or microphone. That is WHY the thing you plug your cartridge into is called a phono stage!

Amplifiers also are designed for line level inputs. Pretty much all of them have enough gain to be able to put out full power connected directly to any line level component. In fact, almost all the time all you really need is attenuation. Like you said. So what do you need a pre-amp for anyway?

Well, to conveniently switch components. But also because the design and component selection in line stage components is pretty poor compared to a decent pre-amp. One of the more common problems is impedance matching. Your line component might work great with one amp that has a very high input impedance. This in effect is not asking the source to do much in the way of current. Another amp with lower input impedance might have your line stage sucking wind. Sucking the dynamics and life out of your music in the process.

So like everything else you just have to try and listen and compare. 

So where does an amplifier input impedance of 22K ohms land on the scale?

My plan is to go Mac Mini to a DAC/Preamp to a beefy amplifier to speakers with a sensitivity of 86 db at 2.8 Volts.
If I had an all digital DAC/Preamp combo that maxed out at "line level", is that enough to reach reasonably loud levels?

Since you are using a digital source (as opposed to vinyl, for example) more often than not the answer will be "yes," assuming the amp has sufficient power capability. But for a definitive answer to be provided you should state the make and model of the amp, or at least its sensitivity spec (which is usually, although certainly not always, defined based on the input voltage that is required to drive the amp to its maximum rated power capability).

Also, "line level" is a term that is loosely defined with respect to voltage. The voltage of an analog audio signal will of course vary with the volume of the music at any given instant, but even so the **maximum** voltage of a line-level signal can be anywhere from a fraction of a volt to perhaps 3 volts or more for single-ended outputs, depending on the component providing the signal.

Typically digital sources provide maximum voltages in the area of 2 to 3 volts, for single-ended outputs. Double those numbers for balanced outputs. In the case of phono cartridge/phono stage combinations, when a cartridge is providing its rated output (which is based on certain test conditions which represent relatively high volume but somewhat less volume than the dynamic peaks of certain recordings), the single-ended output of a phono stage providing typical amounts of gain (for example, 40 db for a 5 mv cartridge; 60 db for a 0.5 mv cartridge; 66 db for a 0.25 mv cartridge) will be about 0.5 volts. (40 db more than 5 mv is 0.5 volts; 60 db more than 0.5 mv is also 0.5 volts, 66 db more than 0.25 mv is also 0.5 volts). Again, double the 0.5 volt figure for balanced outputs if the phono stage provides them.

The input voltage required to drive a power amp to full power will often be somewhere between 1 and 2 volts for a single-ended input, but in some cases may be as little as 0.5 volts or as much as perhaps 2.5 volts. And double those numbers for balanced inputs.

As I said, the dynamic peaks of some recordings can cause a phono cartridge to output somewhat more than the cartridge’s rated output, which is based on certain standard test conditions. But even if it were putting out say three times its rating on dynamic musical peaks the phono stage would provide just 1.5 volts in typical situations, barely enough voltage to drive many power amps to full power, and not enough in some cases. So you can see why Clio09 indicated that he needs his preamp to provide considerable gain with his analog sources.

Finally, an important point to keep in mind: The gain specs of preamps almost always refer to the gain they provide when their volume control is turned up to max. And it is usually desirable to have enough gain to be able to drive a power amp to its maximum power capability without having to turn the control all the way up, assuming the power capability of the amp is not **greatly** more than would ever be used with the particular speakers. As you can see from the numbers I stated above, that is likely to be a concern only in some situations involving vinyl or other analog sources. If a digital source is being used the opposite concern is more likely to arise, especially if the source is used in conjunction with a high gain preamp, the concern in that case being having to use the volume control at undesirably low settings.

Again, though, what is the amp? And if it has been determined at this point, what is the DAC/preamp?

Regards,
-- Al

The amp is an Odyssey Audio Stratos Extreme.
The DAC/Preamp combos I am considering...
Pro-Ject Audio Pre Box S2 Digital
Pro-Ject Audio Pre Box DS2 Digital
Topping D70
RME ADI-2
MiniDSP SHD
NAD C658
... obviously quite a range. I’d like to spend the least amount possible balancing a great DAC with remote volume control. Sub out, MQA decoding, EQ, and streaming are pluses, but not required. Since my source is the Mac Mini, I have some flexibility in software taking on those features.

Unfortunately it appears that sensitivity is not specified at the Odyssey site for any of the Stratos amplifiers. Nor is gain, which in conjunction with the maximum power ratings could be used to calculate sensitivity.

And I couldn’t find relevant specs or measurements elsewhere after doing a quick Google search. So I suggest that you contact Odyssey and ask for that information.

Regards,
-- Al