Preamplifier power supply

if the preamp has a phono section yes. if not, no.
The reason is that all the signals coming into a line stage are ALREADY at a high level, and the pre is really only attenuating them, and selecting the input. Thus some use only an attenuator as a pre, or no pre at all.
On the otherhand, a pre with a phono section actually IS amplifying the signal a lot from the tiny cart output.
The only other time a big power supply in a pre might help is when a big mismatch between the impedance matching of the source to pre, or pre to amp exists. (then the hefty power might cover the problem.

Hmm, so those manufacturers are only implementing overkill power supplies for marketing purposes and the people who buying their preamps should know that they are in fact buying power supplies?

Chris

Yes you do need a good quiet power (toriodal) supply that keeps noise away from the circuitry - and big capicitance like on a big power amp is one way to do that. Of enormous criticality is how power supply is kept away from ground and how balanced the signal lines are designed with respect to ground.However, it can't be simplified to just that.

It is just as important to have features other than just a good power supply - balanced, high output levels with great channel separation and low output impedance and high input imepdance and of course low S/N. High quality tone controls with a "defeat" option for refernce. Volume control needs to be well implemented like with a discrete resistor network steps (like a crystal CS3310 or Burr Brown PGA2310) rather than just a cheap pot and wiper with continuous volume control, IMHO.

for example the built in preamplifier in the Benchmark DAC.

To me it sure looks like a pretty big overkill toroidal transformer in that little box...

Hi Chris . . . it's a mistake to so superficially "analyze" both the engineering decisions and marketing motivations behind any consumer product . . . it's true that there are many questionable design and engineering practices behind many specialist audio products - but simply looking at the size of the "power supply" (what does that include?) is akin to starting a thread with a title of "What's the proper diameter of a woofer?" . . .

. . . and if it's a mains transformer and filter capacitors you're looking at -- let's just say that dramatically oversizing these components is a pretty ineffective way of ripping off the consumer.