PSU questions for huge multieffects box please help!

[cathexis]

Hi!
I've been away from this forum for way too long because of family business, but the love for building is very much intact. Only this time I think I went in a bit over my head!

I'm making a very large multieffects box for a friend. He wants a machine that can simulate old reel to reel tape recorders, the consumer-grade variety, because he loves how they hiss and crackle and distort and warble. He's not happy with whatever microcontroller varieties the market offers (i. e Instant Lo-Fi Junky). So I figured I'd dust off the soldering iron and give it a try.

Here's the front panel sketch-up:



The separate modules, in signal chain order:

- 5$ microphone preamp

- Overdrive with germanium diode clipping in the feedback loop

- BYOC 3-band parametric EQ

- modded Little Angel Chorus (PT2399)

- Ray Wilson's Noise Cornucopia from MFOS

And this is basically how they are hooked up:



A right mess, as you can see:



Phew!

The thing is, it sounds great! It actually accomplishes what I set out for. But I can't figure out the best way to supply the power.

The mic preamp and the noise board wants +/-15VDC. The rest of the effects want +9V.

Initially I built a bipolar supply according to the schematic on GGG

http://www.generalguitargadgets.com/wp-content/uploads/ggg_bipolar_ps.pdf

I fed that with an 18VAC wall-wart transformer. That gave me a perfect +/-15VDC supply, then I added a 7809 regulator to get the 9VDC from the same board. That worked perfectly as well, but there are a lot of circuits sucking current here, and my 7815 regulator got very hot (I had no heat sink, so that is a problem of course).
I then figured I should just feed the +/-15VDC circuits from the bipolar supply, and feed the 9v circuits from a separate wall wart. Hello Big Ground Hum!

Does anyone have some tips for me with this? How would you do it? I haven't built anything like this before, mostly single 9v circuits actually.

Also: I suppose I could use some buffers in there. Where would you put them, and why? The noise circuit is one example - I would like to be able to switch it from either pre-EQ or post-modulation. I suppose I need to buffer it before adding it to the signal. I tried that with one of Jack Ormans op-amp buffer designs, but it still seems to load down the signal in weird ways.

Any help on the way would be so greatly appreciated!

Regards,

LARS

[antonis]

If you got +/-15VDC from 18VAC with 2 parallel bridge rectifiers, you have about 9VDC voltage drop on each one of +/-15V regulators..

-15V current is probably small enough to raise power heat on negative regulator but current drawn from 9VDC is added to +15VDC regulator resulting in 9V X total positive current power heat..

You may try lowering that 9V drop on positive only side by placing a series resistor between filter cap and +15V regulator input..
(its value and wattage according to total current passed and desired voltage drop - keeping in mind the minimum IN-OUT voltage difference required for regulator..)

Same as above could be obtained with series power diode(s) - you'll need more devices than a single resistor but you'll have a voltage drop practically independent of current flowing..

Or use a small heat sink (or even enclosure mounting) and let it be..
(but DO NOT use regulator's metal cooling tab as GND in a mains grounded chassis..)

[diffeq]

Most 9V circuits can run fine on 15V. It may be feasable to:
1) check all the ICs for their power ratings, including $5 preamp (is it based on INA217?);
2) fit LM7x15s with sufficient (or over sufficient) heatsinks and run all the circuitry from a single supply rail. Little Angel Chorus already has it's own LM7805, don't forget to heatsink that one also.

Regarding buffers, regular (non-inverting) buffers aren't well-suited for mixing signals together, inverting configuration is used instead. Like this one:
https://www.electronics-tutorials.ws/opamp/opamp11.gif

The amount of wiring for a single project is impressive!

[PRR]

> I had no heat sink, so that is a problem of course

So.... get a heatsink? A chunk of aluminum bar or angle? Some finny thing pried out of an old PC or VCR?

With proper sinking, that regulator sure should power the whole thing.

It may also be worth knowing *which* +15V load is sucking the most. It probably runs warm.

[anotherjim]

If you ran the 7809 from the output of the 7815, that may explain why the 7815 gets hot? It may be better to run the 7809 from the same unregulated input as the 7815, but it would help to know the unregulated voltage (possibly its around 24vdc?) and it may then be the 7809 that needs the heatsink. Both 7809 and 7815 have 0v body/tabs so are dead easy to heatsink : simply screwing or clamping to an aluminium surface can be sufficient - plenty of commercial products only do that.

[cathexis]

Thank you all! I'll stick with my original plan for the PSU that sounded good and was straightforward, only I'll get some hefty heatsinks (I've had some on their way in the mail for weeks, but they seem to have gotten stuck somewhere).

Thanks to diffeq for the buffer tip! I hadn't thought of that.

I'll post some pics and soundclips as soon as this monster of a one-trick-pony is finished!

/LARS