Preamplifier power supply

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.

Kirkus, I'm not saying that an oversized power supply is all you need. I was trying to find out why some experts keep telling that a big power supply is essential, even with preamplifiers. Why do you think manufacturers are making two (or four) chassis preamplifiers with the entire power section occupying an entire chassis? Spectral preamplifiers for example have a power section that is bigger than some power amplifiers! You can't deny that this is way overkill. So what are the benefits of such a design?

Chris

I see what you're asking.

Power-supplies are commonly separate from the main chassis usually either for UL approval (when pre-fabricated wall-warts are used) or to reduce magnetic coupling from the power transformer into the audio circuitry. In the latter case it may be actually cheaper to produce a separate chassis than to apply the correct parts and engineering time to eliminate the interference when these parts are in close proximity.

Truly low-noise circuit designs usually require low signal impedances, which means more signal current, which means more power-supply current, which can easily lead to a bigger supply . . . and everything runs warmer, so it has to be spread out more for heat dissapation. Also frequently, internal grounding mistakes can cause supply ripple currents to be injected as hum into signal grounds . . . and if the cause of the hum isn't well understood, then it may be solved simply by heaping on more power-supply capacitance and reducing the ripple current.

There are also standard core sizes that transformers are made in . . . and other electrical characteristics (like load regulation, EM field output, and leakage reactances) that are related to the size of the core and the winding methodology. Again, it simply may occur that to acheive other engineering goals . . . bigger ends up being the cheaper/simpler way to get better.

You might also run into issues where these products are sold in countries that have different power tolerances (50Hz runs hotter and requires more capacitance), or tropical climates where a lower temperature rise is requires.

So, in the end, it's these and a thousand other factors. In the end, just like underwear . . . it all Depends.