New Hum From 18v In My 4700 System
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Author:  valis [ Thu Oct 18, 2012 2:55 pm ]
Post subject:  New Hum From 18v In My 4700 System


I've been putting quite a bit of work into my 4700/2720 system as of lately. I've recapped the entire machine, as well as bought a new transformer and attached a fuse to it. I'm using the 4771 power source module.

Everything was working like a dream when I tested the modules together so I screwed the panels on and declared the project completed. Shortly thereafter, I started getting a noticeable hum from any audio module that was on the 18v line (i.e. the 4710 Balanced Modulator and the 2720 BPF). The hum would go away when I disconnected the module from the 18v supply and was not there on the +/-9v supply.

So I went back and looked at the 4771 power module as well as the transformer. All the voltage measurements are correct. I touched up some solder connections just in case there was a problem there. I even re-replaced C1 and C2 on the 4771 even though I had done that recently.

Even after all of that, I still get the hum. I did read the previous posts where people had hums in their 2720 VCA or other PSUs but I can't find the source of the problem. It was working cleanly and now it's not.

Any thoughts would be greatly appreciated. Thanks as always.


Author:  Kyhotay [ Fri Oct 19, 2012 12:04 am ]
Post subject:  Re: New Hum From 18v In My 4700 System


I've been working through this exact same problem. As soon as I have my issues worked through I'll post my results. The basic solution is to replace the 4771 with the 9771 and a
regulated 18V supply.

Brian Folkes

Author:  valis [ Fri Oct 19, 2012 1:29 pm ]
Post subject:  Re: New Hum From 18v In My 4700 System

Thanks for the reply. I was, for quite some time, running the machine off of a regulated Power One +15v supply which worked great, but I had the 4771 module so it seemed reasonable to utilize it.

The part that baffles me is that the synth was working just fine utilizing the 18v from the 4771 and my new transformer, and then it developed a hum. That leads me to believe that something went wrong within the power chain which is correctable.

I tried examining the hum with an oscilloscope to understand it but it really didn't answer the question. Maybe it's a ripple accentuated by the higher current demand? It seems like the caps should take care of that, right?

It should also be noted that I am using a three prong power cable (instead of the original two prong) and everything still appears to be properly grounded.

Author:  PAiA-Scott [ Fri Oct 26, 2012 3:47 pm ]
Post subject:  Re: New Hum From 18v In My 4700 System

When replacing capacitors, I wonder if you went with the preferred 1000uF at C6, the 18v filter capacitor on the 4710?

Double-check to be sure the polarity is correct on all the capacitors you replaced.

If there are lots of modules connected, it might be the heavy load increasing the ripple appearing on the unregulated 18v supply. The +9v and +18v supplies both tap off the positive rectifier and filtering. More capacitance on the 18V supply, and, the modules using 18v should help with reducing the ripple and hum. On a 2720-5/4750 also increasing the value of the resistor (1k) into the 18v filter capacitor helps too.

Maybe there is a ground loop in the connection between devices in your set-up with the earth/wall-outlet ground circuit introduced with the three prong power connector you added. Try using a three to two prong adapter between the line cord and wall-outlet to determine if this helps reduce/eliminate the hum.

Author:  valis [ Tue Oct 30, 2012 12:46 pm ]
Post subject:  Re: New Hum From 18v In My 4700 System


As always, thank you so much for providing support for thirty year old DIY products.

Anyway, I haven't had the time to follow up with your suggestions but I will in the next couple of days and report back.

Thanks again for your time,

Author:  valis [ Fri Nov 02, 2012 5:29 pm ]
Post subject:  Re: New Hum From 18v In My 4700 System

Again, thanks for the response.

So, to update I still have the hum.

To summarize a few things that I've done:

- I have double checked the orientation of C1 and C2 on the 4771 power supply and they are correct.

- I increased C1 and C2 on the 4771 from 1000uf to 4000uf.

- I tried using a three to two prong adapter between the line cord and wall-outlet to see if that helped reduce the hum.

- I entirely disconnected one wing of the cabinet.

That's definitely a good point regarding the 9v modules adding to the ripple as well as the 18v modules. I'm kind of at a loss right now as to the problem. I'd love to get it to work but at this point I'm thinking I might as well just build a regulated 18v (or 15v) supply.

I haven't changed the caps in the 4710 or the 2720 BPF. I'm being stubborn there and wanting to fix the problem at its source.

Thanks again for your time. I'll report back when I get things figured out.

Author:  Kyhotay [ Fri Nov 02, 2012 7:08 pm ]
Post subject:  Re: New Hum From 18v In My 4700 System

Hi David,

More of my $0.02 worth of wisdom, painfully gleaned on working through this very issue. You need to get the K60 kit and 18 or maybe a higher powered VR ( and the 9771-15, with a beefed up transformer. You can downgrade the -15 to "-9" easily enough. You can then make power distribution strips or, better but more bucks, the PS-CONN from

I am finishing up a dual wing cabinet and using just one supply and plan on selling my 2 4771 PS. I converted a Wing Patch Panel to house the on/off switch, power LEDS and O/I LED. You could probably mount a din plug to route power to the other cabinet. I drilled holes in the back of the cabinet.

I am finishing up this project and as soon as I am satisfied I'll be posting it here on PAiA Talk & Muff Wiggler with more details and pictures.

Good luck!

Brian Folkes

Author:  valis [ Sun Nov 11, 2012 2:54 pm ]
Post subject:  Re: New Hum From 18v In My 4700 System

Thanks for chiming in.

I don't think the problem is with my transformer. It's a brand new transformer and of good quality. It's rated at an 1amp which I know I'm not pushing, especially when only one wing is plugged in. I'm not discounting some sort of ground loop, like Scott said or just a ripple in the voltage.

I'm going to add a 15v or 18v regulation circuit. I don't have a problem when powering the machine on my bench supply so I really have to figure that regulating the 18v-15v will solve the problem. I agree though, utilizing a regulated power source seems like the best option.

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