Tube DIY Asylum: Here it is ........ "Figure Of Merit" of an Audio Amplifier's Power Supply: by drlowmu

Hi Henry,

So you ask for a "rudimentary technical analysis". It is given freely below. Quit the name calling. Try buiding and listening rather than just theorizing all the time.

The following is a letter on this subject ( of audio amplifier power supplies ), written by a audiophile / music lover / tenured Electronics Enginnering Professor, some 15 years ago, and it makes specific reference to a Dyna ST-35 series of supply mods I was executing back then. As I recall, I went from one tube rectifier to FOUR rectifier tubes in Parallel, to lower Z of the supply and really liked hearing the improvement in sonics.

The author is a dear and sadly departed friend, Dr. Chales E. Halijak, E.E., Ph.D. Please study this following letter carefully, as it has served me well over the years:

1989

Dear Jeffrey,

This letter is concerned with the power supply and the use of chokes.

A choke must have some internal resistance and its non-standard figure of merit is the ratio R over L. The best choke is the one with the smallest figure of merit and the best power supply has the least series resistance. It is not too difficult to obtain R over L = 10 but one can easily obtain R over L = 30 or more. The latter should be avoided but the ongoing shows that in a way it is unavoidable.

The choke input filter controls the diode by timing and not by ohmic losses. ( jm: current vs: voltage waveforms) The best place for a choke is at the input and the worst place is at the other end of a power supply filter chain because resistances tend to be higher there.

Suppose we start with your case and R over L = 10. When diode ( 5U4GB) and transformer resistances are taken into account we have :

15 ( choke ) + 70 ( rectifier ) + 70 ( power transformer resistances )
------------------------------------------------------------------- = 103
1.5 ( HY )

( the 70 ohm power transformer resistance is a guess )

If four 5U4GB are placed in parallel one has :

15 + (0.25 ) ( 70 ) + 70
__________________ = 68
1.5

Both figures of merit are way above the original 10 but your listening experiment told you that 68 was alright; this actual figure of merit is the best you can do because the largest power transformer was used and the smallest resistance available choke was used. You can see that stock power supplies are not anywhere near being the best cases *. Most tube and transistor electronic engineers are unconcious about power supplies, but good.

* Also the best cases need not have zero internal resistance - a utopian goal. One strives for minimal non-zero resistance !!

- - - - - - - - - -

Henry, on the subjective side, below is what Audio Consulting of Switzerland says about their four and a half ohm DCR @ 10 HY choke, from their literature, note their wording "one optimum" and "only way to make real progress":

"Get micro and macro dynamics out of your amplifier's power supply. Get the power into the music!

First we would like to explain our general philosophy� :

There always is one optimum for a given problem.
It costs tremendous amounts of time to find it, but this is the only way to make real progress.

Low DCR plays a key role in getting life-like dynamics out of an amplifier.

We have noticed that the lower the DCR gets, whether in a SE OPT of in a power supply choke, the more micro-information the system is able to provide.

We have gone as low as 4.5 Ohms for a 10 Hy choke that provides incredible sonic results when used in tube amplifiers.

The 16 Hy / 16 Ohms model is also very good, and maybe more reasonable when it comes to weight and size than the previous one.

More classical chokes of 30 to 50 Hy and 105 Ohms DCR may also be offered."

- - - - - - - -

I will try to address this power supply subject further in a day or so. Cheers.

Jeff Medwin