[sdiy] PSU ramblings

[Batz Goodfortune]

Y-ellow René 'n' all.

At 08:21 PM 6/28/02 +0200, René Schmitz wrote:
>Hi all,
>
>recently I bought some nice PSUs, that I plan to use to power my modular.
>
>The unit has +/- 18V @300mA and separate floating +25V @500mA output.
>The +18 and +25 are done with an LM317, the -18 with a 79G.

I'll leave aside the fact that they're only 300mA for the time being. 
(Can't cause serious welding with that kind of current.) And I don't know 
about the grounding thing. That could be more trouble than it's worth. 
Though on the face of it, it would seem to be something of a solution for 
de-synching VCOs. But! Often in mixers, I've used a pair of 78Lxx/79Lxx's 
on each channel. An "L" series will give you about 100mA local and if you 
had some LEDs or something that switched in with a thump, you can always 
run them from a separate "L" series reg.

I like the idea of a 723 with series pass as a pre-reg. Though there's an 
LM family of power regulator out there that is rather like a dual tracking 
723 but in a T03-a-like can. (You know the smaller ones. buggered if my 
brain will dredge up the screaming JDECs on that one right now.) (Nor the 
actual part number for that matter. Bugger it!) But often I've gone 
fencing-wire style and used a jacked up 78/79H reg as a pre-reg for a 
mixer. Sure you lose a bit of regulation by doing that but if everything 
else has local regulation then it's not going to be a big problem.

As for jacking the ground I'm not entirely sure you'd achieve anything. If 
you did that you'd have to derive everything from that source and that 
would be the same as not jacking it. It might provide some degree of 
isolation depending on topology but I doubt it would be worth the effort. 
Although it's harder to achieve in a modular or a modular mixer, a good 
STAR point is probably more beneficial.

Star-Ground distribution is very difficult to achieve in a great many 
designs that are modular and/or expandable. What I've found to be a better 
approach is to run a dirty great thick ground trace across the back. 2" 
thick copper rod if necessary. And in the case of mixers, I tend to try and 
cluster all the Is and Os together on the back and make that my Star Point. 
That's practical in a mixer but not terribly practical in a modular synth 
unless all your jacks are clustered away from the actual modules.

The only time I've seen good use of an artificial ground point was in a 
single supply PSU run from a battery. Where there was no centre. The design 
used a power op-amp to provide this reference.

And don't forget your Rs and Cs. You can get a lot of mileage out of 2 Rs 
and one or two Cs. I can't remember the last time I powered an op-amp 
direct from the PSU in an audio circus. It's always a matter of course for 
me to feed them through a pair of 100ohm resistors and whack a 1uF to 10uF 
cap across them. This may not be practical in every circuit in a synth but 
it does multiply the op-amp's ripple rejection out of sight. 
("out-of-sight" is a new quantifiable unit of measurement most notably used 
in the calculation of PMPO)

I'm told there's a new, more accurate  way of calculating PMPO these days. 
The engineers on a project are all given a dart. The team leader (in a 
large team) or the general manager as the case my be, is tied down to a 
bench with his/her ass facing the engineers. Each engineer throws one dart 
into the ass cheeks of the general manager/team leader and the louder the 
screams of pain the more PMPO the device has. But before you say. "But Batz 
m'man, that's hardly scientific." It actually is not only scientific but 
also quantifiable and can be graphed. The screams are measured with an SPL 
meter at 1.5 metres distance. each peak scream level is recorded and 
averaged to derive the number. You get the "Peak" in PMPO from this. And 
since it's generally music to the ears of the engineers to hear their boss 
screaming in agony you get the "Music" in PMPO from this fact. And since 
they all have a rare moment of power over their boss at this point you get 
the "Power" in PMPO.

But what about the "Output" in PMPO? I hear you ask. Well that's easy. 
Usually the boss either shits himself or pisses himself and thus the "Output"

Some less than scrupulous companies derive PMPO by simply counting the 
number of darts it takes before the "Output" occurs but you won't find that 
method used in the more trustworthy designs using the PMPO standard. Often 
these companies have a high staff turn-over in the engineering departments.

Jeez I just can't help myself can I.

Be absolutely Icebox.

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