Paia 4700 with 2700 power supply (how do I kill the hum?)

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bloodyhell
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Paia 4700 with 2700 power supply (how do I kill the hum?)

Post by bloodyhell » Mon Jul 15, 2019 12:13 am

I recently got a Paia 4700 and I love it, but what strikes me odd is that it has a lone 2720 power module, which is inherently dodgy and seems to produce plenty of hum. I do not like that one bit. Did the early 4700's use to come with the old supply instead of the regulated one or the watt bliocks? I'm planning to put in a 4781 sequencer I've got sitting around but I'm worried that it will drain too much current away and make everything more unstable than it already is. Is there any way to beef up the power supply and maybe evern regulate it to kill those buzzy ripples?
Last edited by bloodyhell on Mon Jul 15, 2019 1:02 am, edited 1 time in total.

bloodyhell
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Post by bloodyhell » Mon Jul 15, 2019 12:22 am

this was supposed to be a separate thread but I accidentally posted it as a reply to this post. Please remove this reply, thanks.

Rob_C
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Post by Rob_C » Mon Jul 15, 2019 8:55 pm

See Dave's website for a PAIA 2720 restoration. Link ->PAiA 2720 Synthesizer Restoration

Dave says: There is a lot of ripple in the unregulated power supply so I increased the filter capacitors from 1000 µf to 2x 3300 µF (two stacked in parallel).

When bought as a set, the 4700 modules came in a road case. There were a few different configurations. You could also bought the road cases separately. So it is possible your system was not a pre-configured kit but rather put together module by module by the original owner. It's possible the original watt blocks were never there or were swapped out. I don't think the 4700 systems ever came with the 2700 power supply.

In the 4700 systems, the oddball was the control-oscillator/noise source, not the power supply. This control osc/noise module had the brighter panel finish of the 2700 series. Later PAIA made a panel that matched the rest of the 4700 series, but few people had them, they are pretty rare.

Paia's spec for the 4780 sequencer: 50 ma on the 18 volt line and 1 ma for the plus and minus 9 volts.

You could benefit from a beefier and regulated power supply. At the very least I would try adding the additional electrolytics as suggested above. Wouldn't hurt to use new capacitors rather than junkbox supply.

Or you could try this for a plus and minus 9 volt power supply: Link-> Dual-Output Adjustable Linear Regulated Power Supply Kit The specs are probably a bit high on what this supply can provide. You could mount this anywhere in the case and still use your original power supply panel.

KSS
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Post by KSS » Thu Aug 01, 2019 4:32 am

You'll want to put larger filter caps on every module which uses the ++ (18V) line. Each module. In addition to any improvements you make the to the power supply itself.

If you're using watt blocks or the abysmal 2720 PSU, try to build a 4771 at least for a significant improvement. Still need the per module caps increase ont he ++ power entry.

Ditch the 2720-3L for a moog ladder. The minimoog version will easily work on the -/+9v rails.

As Kevin Lightner pointed out, the PAIA VCO's being linear have more in common with the moog 901;'s than nearly any other synth.

4730 well-adjusted is one of the better PAIA designs.

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Post by KSS » Thu Aug 01, 2019 4:41 am

Watt blocks were the first 4700 PSUs. They replaced the 2700's supply. The 4771 came later. 4780's were often run on watt blocks in a separate cabinet. Cheesy phillipine mahogany. Very 70's.

Alligator clips to the buss wires is not that uncommon. Early bus wires were actually piano wire, and if you have them, those need to be replaced with at least copper. Use some #14 or #12 romex -stripped- from home depot. Better still set up a non- bussed power distribution.

Have used old 4 pin PC disk drive connectors for this in the past. Molex types as used on 5-1/4" floppy drives, not the white .100 4 pin. But those are also okay and can be mad using vero stripboard. Be sure to heavily load the strips with solder if you choose this path. The other type doen more like a true star is better. See the Power supplies used by dotcom for an example of what this looks like.

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emmaker
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Post by emmaker » Thu Aug 01, 2019 11:04 am

First thing I did when I got mine running was make a power supply with 3 tab 78XX/79XX regulators. I'd do the same thing now with low noise Linear Technology regulators.

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Kyhotay
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Post by Kyhotay » Fri Aug 09, 2019 2:18 pm

When I rebuilt my 4700 I redesigned the look of the front panels. For power I took 9771-15 FRAC power supply, dropped the voltages to +/-9 and then added the K60 Regulated Power Supply and got a 7824(?) VR to get the +18V regulated. I used the Blacet power distribution boards in the cabinets to avoid the much despised OEM power distribution system. I added different colored LEDs for the 3 voltages as well as power on. I also added a +/-9V(?) bias control.

It's highly kludged together but it works great and the noise and ripple is eliminated. I replaced most of the op amps with Burr Brown and changed all the electrolytic caps to higher grade audio ones.

The result is plenty of power headroom and a cleaner sound. I suspect it would be cleaner if the wave forms were greater than .5V p-p but I am not an expert on that. Would anyone care to comment on that?

I have more pictures and notes somewhere if anyone is interested. The two I've uploaded give a pretty fair representation of what it looks like. ImageImage
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