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Preamplifier power supply

Hi folks, should a preamplifier have a BIG (that is: an overkill power supply) to sound dynamic and authoritative? I'm asking this because some experts would say "yes" while others would say "no". Recently a well known audio journalist (Anthony Cordesmann?) said that the preamplifier doesn't have to have a big power supply because it doesn't have to deliver lots of energy (in the form of current). A preamplifier can sound "dynamic" even with very modest power supply --> for example the built in preamplifier in the Benchmark DAC. But some manufacturers rely on a truly overkill power supply in their reference preamplifiers: MBL, First Sound Audio, BAT, VTL, LAMM, Mark Levinson. So who is right?

Chris
dazzdax
kijanki's avatar
kijanki
4,428 posts
 
Casouza - yes, transformer should be oversized since mentioned current spikes, while driving recifier/capacitors, heat up the windings (RMS much higher than Average) and their high frequency content heats up the core but 10x is perhaps overkill. Imagine 40% efficient class AB 200W amp with 5kW transformer. Large amount of capacitors is beneficial providing better filtering but when you think of it - linear power supply is really a bad case of SMPS working at 120Hz. SMPS working at 100kHz will do the same with 10x smaller toroidal transformer, is line and load regulated and is quiet since higher frequency is easier to filter out, non-audible and switching is done (in modern SMPS) in zero voltage/zero current. For that reason Jeff Rowland uses SMPS in Capri preamp (where efficiency is not important). SMPS got such bad rap from cheap ones used in computers that people don't want them even for class D amps (that are SMPS) and designers do what sells.

Preamps need some oversizing perhaps 2x-3x to cover losses related to operation with rectifier and losses in voltage regulators that have to work in 90-132V mains range. Filtering here is done mostly by the regulators and current is pretty much constant (class A) with transitional current supplied from local caps.

Benchmark DAC1 supply is not an overkill - it takes from mains "16W peak" of average power but because of mentioned much higher RMS value of current spikes it needs to be about 32W and it looks like 30W toroidal transformer to me. It is toroidal because outside field of evenly woven toroidal transformer is zero and Benchmark achieves measured 140dB S/N.

You are right about power supply caps being in series with the speaker. People don't realize how important supply caps are. Cheap caps not only have high ESR "eating out" dynamics that amp could provide but also have high inductance filtering out fast transitions. Placing film caps in parallel is a band-aid and often a bad one since pure capacitance in parallel to main caps inductance creates parallel resonant circuit that rings (cap is in series with the speaker). There are low inductance caps (like slit foil caps) but very expensive. I would rather go with fast responding SMPS and that's what Jeff Rowland did (AFAIK) in latest high power class AB amps.
tonywinsc
583 posts
 
There is another thread talking about a custom made tube preamp that sports an oversized power supply so much so that the unit weighs 75 lbs (34 kg). The owners of these particular devices have only accolades for the sound that this special preamp creates. Of course their descriptions oft include the size of the power supply as though any preamp worth of hifi should have a power supply capable of driving speakers. I have seen that there is some science to putting an oversized power supply into a preamp for improved noise performance and voltage stability. Beyond that, I do not see the point. Unless I am mistaken, the power output (audio power) of a preamp is, at most, about 50 milliwatts. That is assuming a 20k ohm impedance looking into the amplifier. I have seen some preamps boasting 250 VA power supplies. All that to deliver 50 milliwatts? Sure, there are the tube heater circuits to power and other ancillary circuits, but that is not the case with SS preamps. It this just a case where we as audiophiles expect our preamps to be big heavy boxes with gobs of reserve power and so the hifi builders deliver to those expectations? Preamps have had oversized power supplies for decades now. It just seems like a type of arms race that each generation of preamps must have even bigger power supplies to show some type of advancement in technology. I remember when very expensive SS preamps could be left on all of the time because they consumed little more power than a night light.
ausernamethathasntbeentaken
7 posts
 
Could someone please simplify this discussion for me back to the basics? To quote Jeremy Irons in Margin Call, "speak to me as though I were a young child...or a golden retriever."

Why is a separate power supply necessary at all? In the last hi-fi system I had (Mac C22, MC275, Marantz 10B), the wall plug in the NYC apartment I had at the time seemed to be a pretty good power supply. I'm now getting back into hi-fi and recently acquired a Levinson 25S phono preamp with a separate PLS 226 power supply. I haven't yet used it since I'm still in the process of acquiring the other necessary stuff (signal source, preamp etc.)

I gather that the 25S (or 26S preamp) wouldn't be of much use if it could be plugged directly into a wall socket. Why, in very simple terms, is that?

thanks! 
auxinput
2,920 posts
 

The whole idea of a power supply in audio electronics is to convert the alternating current (A/C) from the wall to direct current (DC).  DC current is required for anything regarding audio (it just cannot run from straight A/C like some motors can).

The reasoning behind creating a separate power supply will vary depending on manufacturer.  In a nutshell, a separate power supply will shield the core audio circuits from the electromagnetic noise caused by the transformer in the power supply.  Generally it is not really a problem, but it can cause a slight buzz in the audio if the transformer is too close.  The second reason could be that the manufacturer just did not have the room to build in the power supply they wanted within the size of the preamp chassis.  A good example of this is the Krell 202 preamp:

http://www.hifishock.org/gallery/electronics/krell/preamplifier/

You can see from the pictures that the power supply is enormous with two large transformers and a lot of transistor heatsinks for their shunt regulated DC power supply.

The alternating current AC from the wall comes in waves that run at 60 hz (about 60 waves per second) where half the time the voltage is "pushing" and the other half it is "pulling".  DC requires a 100% constant "push" in voltage that is one direction only.  The power supply tries to "leak" the current pressure from the AC and also store as much voltage in the power supply capacitors so that it can continue to provide a 100% DC current even when the AC is not pushing current at all (which is between the waves).

The size of the power supply can also dictate how good of a sound quality you get.  With regards to extremely large power supplies, it can help further smooth out the sound.  My research and experimentation has shown that conventional circuits and op amps do not really require a huge power supply.  However, when you start using Class A type audio circuits, then the power supply size becomes hugely important.  The Class A circuits require a large amount of constant current and you can actually reduce the sound quality by providing a "too small" power supply.  You are actually starving the audio circuit and the end result could be things such as a thin/bright/harsh type of sound.  A prime example of this is the older Krell KAV-280p preamp.  This is also similar to the Krell Phantom III preamp.  They both have such a small power supply with limited capacitance (4 x 3300uf in the 280p) that the sound is bright/thin.  I found that doubling that to 4 x 6800uf significantly changed the sound and it was no longer bright/thin - had a tremendous amount of bass and midbass strength.  When you get into the larger Krell preamps, such as Phanton II or Phanton I, you will get the larger power supplies for very similar audio circuits.

ptss
1,964 posts
 
Power supplies should be very robust. They should provide an unshakable foundation for absolute control of the signal to the amp, plus a high degree of resistance to perturbations created anywhere in the system, including from the amp(s).  No one can deny one has more control with an ‘oversize’ power supply. Just like driving a car with a standard transmission and more than adequate torque provides more refined control of the drive. 
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