Power Supply Musings

[Paul Schreiber]

>What are your thoughts on increasing the 7805 (1Amp) with a 78H05 (5Amp)?
>Would this help or would it cause more problems?
>

I bet not!

Here are my observations and opinions based on 28 years of messing with this
stuff:

1) Designers have a habit of breadboarding and testing their prototypes with
a $3800 HP lab-grade bench
supply (0.005% line/load regulation, <1mv ripple). "Works great, boss! Ship
it!"

2) The synth power supply is the *LAST* thing designed. "Hey Fred! Let's use
all those transformers
from that last synth!" They should be OK!" Ha Ha HA!

3) No one wants to do it: "boring", "low tech". But worse, no one knows HOW
to do it!

4) There are very few analog designers that REALLY understand power
supplies. Things like a Schade curve
(in order to pick the proper rectifier/capacitor combination, which was
published in 1943 and *long* forgotten
by most people), proper heat sinks (I got a news flash: bolting a regulator
to the painted chassis is NOT a good idea!!),
and understanding basic things like in the US the AC line voltage varies
from 95VAC to 132VAC and you'd
better take care of BOTH cases properly.

5) News flash #2: the 78xxx and 79xxx are NOT GOOD ENOUGH without *proper
and careful componen selection*
to get you "studio quality" SNRs. You can get OK SNR, but not -90dB SNRs.
They are too noisy. The ripple is too high.
The line/load regulation is crappy. IF you have $$$ caps, a star ground, and
protection diodes you can get a "decent"
supply. Has this ever occured in a synth? Not to my knowledge. The WORST
supply in a synth I've seen is in the OB-8.

6) Another factor: capacitor aging. Capacitors age over time. If your synth
is over 15 years old, I'd STRONGLY SUGGEST
replacing the big caps on your supply. Chances are, they are cheap 85C-rated
caps. Get 105C, low ESR (effective
series resistance) caps. Mouser and Digikey in the US are wonderful sources.
The cost will be $10 or less. Get a value
equal to OR LARGER for your cap package (the case sizes  are standard,
thankfully!!).

7) Lastly: it's amazing (but not *surprising*) that National Semiconductor
has published page after page of 78xxx regulator
app notes and the synth designers ignored them TO SAVE 25 CENTS! You can get
PDF files of these, and then
add the diodes and change the caps and beef up your rectifiers like it
SHOULD have been done. You can unbolt the
regulators from the black painted chassis (the black paint is a WONDERFUL
heat insulator! This just bakes the regulators into toast)
and spend a whopping 75 cents and get a PROPER heatsink that will drop the
temperature by half (note that the HEAT
is the same. As we say in EE101 "power is power" meaning the wattage
dissapated is the same if you stuck it in
liquid nitrogen, but the TEMPERATURE can drop!)

Paul Schreiber
Synthesis Technology
www.synthtech.com  <<synth supplies done right