How to eliminate LFO bleed?

zonkout · Joined: Oct 12, 2006 Posts: 33 Location: ozztin, teggsitz

I recently completed my modular:
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Everything works great, but I have a fair amount of LFO bleed, which is quite audible in the oscillators but likely affecting the filters and VCAs as well.

All my modules are running into two Cynthia combo/distro power distribution boards. I am guessing this is the issue, since the LFO bleeds to the other modules without being connected in any other way.

At first I thought the problem was crosstalk between the VCAs in my Modcan 4VCA, but even running a single oscillator straight into a single VCA, I still get the LFO bleed.

How can I solve this problem?

Scott Stites · Posted: Mon Mar 12, 2007 11:03 am Post subject:

What's the current rating of your power supply?

mosc · Posted: Mon Mar 12, 2007 11:18 am Post subject:

Try to narrow down the problem. Unplug from the power supply virtually all modules except the one you are listening to (VCA or Filter). Then connect a single LFO. Any feed through?

If not, disconnect that one and try different LFOs.

This kind of trouble shooting takes a lot of time and patience. You will teach yourself as you go along. It's something that is difficult for someone else to identify remotely.

A problem with LFOs is that since the frequencies are so low, filter caps don't do as much as they do for higher frequency noise.

zonkout · Joined: Oct 12, 2006 Posts: 33 Location: ozztin, teggsitz

My psu current rating is 3 amps.

I will unplug the LFO's and plug them in one at a time and check that way--but I can hear the bleed from each of my 2 Blacet LFOs and my Blacet Binary Zone (LFO/clock) independently.

I can defeat the LFOs by plugging a loose cable into the clock inputs. This stops the bleed, but I want to be able to use pure oscillator tones alongside modulated and clocked events, so I need a more permanent fix.

zipzap · Posted: Mon Mar 12, 2007 1:54 pm Post subject:

afraid i can´t help any more, but that synth looks great!!!

Mikmo · Posted: Mon Mar 12, 2007 2:15 pm Post subject:

Try to read this thread, if you haven't already, it might get you one step further.

http://electro-music.com/forum/topic-16482.html

zonkout · Joined: Oct 12, 2006 Posts: 33 Location: ozztin, teggsitz

My combo/distro boards are connected in series. Each is feeding 10 modules (4 modcan and 6 blacet modules per distro board). If I connect my ENV, LFO, and clock modules to one board (with VCO, VCF and VCA on the other), should it be the first or last in the series?
My power supply is in a separate case, so I can't connect the combo/distro boards to it independently.

Scott Stites · Posted: Mon Mar 12, 2007 3:02 pm Post subject:

3A would definitely be more than enough for your module complement.

I'd try the noise producers last in the series - the closer the VCO's are to the trough, the better.

If possible, it might not be a bad idea to try the two distros in parallel, though I realize things may be pretty much hardwired at this point.

Good luck,
Scott

Uncle Krunkus · Posted: Mon Mar 12, 2007 6:01 pm Post subject:

Does your power supply have analogue and digital(dirty) ground connections? It doesn't matter so much about individual feeds to each module, as long as dirty grounds supply digital boards only.
You'll know if your PSU has dirty ground connections, because the connectors will have 4 connections each. (+ve, -ve, AGND, DGND)
Some boards won't discriminate between AGND and DGND. If they contain digital chips, and/or noise generators, they will need to be on a separate feed.
If any of this doesn't make sense, just let me know. It took me a while to get my head around the whole AGND/DGND idea.

zonkout · Joined: Oct 12, 2006 Posts: 33 Location: ozztin, teggsitz

I am not sure if the supply has analog and digital grounds.
I tried putting the LFOs at the end of the chain but still get the bleed.
What is the best way to run my two distro boards in parallel? Can I solder two sets of wires to my power jack and then to the boards?

I believe I am using the combo/distro boards correctly. They are meant to be chained together (as described on Cynthia's site: http://www.cyndustries.com/modules_powerdist.cfm). Wouldn't this be a problem for everyone using these boards?

zipzap · Posted: Thu Mar 22, 2007 3:51 am Post subject:

ehh - try to place the lfos at the beguinning of the chain, the vcos and filters to the end. with what you tried you get this: lfo makes noise, sits at the end of the chain, noise has to go through all modules on the way to finally get to the psu-cabs.
when i started i made the same mistake. thought that if a module is farther away it can´t do as much harm to the supply. it´s the other way around Wink

doctorvague · Posted: Thu Mar 22, 2007 6:11 am Post subject:

Cool modular! Love those colors!!
I'm no expert, but could this be a bad bypass cap on an LFO board? Or an electrolytic wired backward or something like that?
Phil

zonkout · Joined: Oct 12, 2006 Posts: 33 Location: ozztin, teggsitz

Scott Stites · Posted: Thu Mar 22, 2007 10:26 am Post subject:

If it's any help at all......a Synth DIY list posting from the inestimable Harry Bissell:

zonkout · Joined: Oct 12, 2006 Posts: 33 Location: ozztin, teggsitz

What is the best way to run my two distro boards in parallel?
Is it a bad idea to run two sets of leads (one for each board) from the jack that brings the power into the case? Or do I need to split off power on a strip board? Sorry, I'm new at this.

Is zipzap's suggestion an alternate solution? It seems to illustrate Harry's point (The idea is to minimize the area of the loop made as the current flows in the circuit. The smaller the loop, the lower the parasitic inductance. ) because I'd be making a smaller loop for the noisy modules. Right?

I appreciate your help in this guys--you really know your stuff.

Scott Stites · Posted: Thu Mar 22, 2007 11:41 am Post subject:

I'd try ZipZap's idea before doing anything else; if that doesn't work, then pull out the guns Very Happy

Cheers,
Scott

Scott Stites · Posted: Thu Mar 22, 2007 5:17 pm Post subject:

FWIW, here's the method I use.

I've uploaded a pic of my homemade distribution board. It's driving the KS01 plus a couple of VCOs (which explains the alligator clip connection). That's four LFOs,two four channel mixers, two MS-20 filters, a 2040 filter clone, four VCAs, a Blacet Dark Star 2000, three envelope generators, a CGS wave multiplier, two VCOs, a ring modulator and a heavily armored modified CGS Analogic circuit acting as a peak/trough function plus various submixers and powered buffers added to these modules all powered from an 800 mA per rail Midwest Audio power supply. On the far lower right is the connection from the power supply. Closest to the power supply are the VCOs, MS20 filters and VCA. Further down the line is the DSC2000 (read major current player), the LFOs (four LEDs) and the EGs. This arrangement is merely by chance. I've never had to juggle modules - this system has never displayed any interaction from one module to the next. This includes the two VCOs currently patched into it - they're Rene Schmitz VCO's, which require a bit of attention to how they're grounded (an awesome VCO, BTW).

The idea is you don't want rails and ground passing 'through' , but to get as close as possible to a common connection through a star arrangement. A serially connected system is asking for trouble, that's why there are distribution boards.

With a distribution board, the 'through' applies to the power and ground starting at one end and branching off to the various modules til it gets to the other end. For big 'pipes' I used a fairly heavy gauge bus wire and crossconnected the rails and grounds so that any point is no longer than three or four inches from the power supply spigot - each rail and ground is more or less a web that more closely resembles a single point than just starting at one end and moving to the other. You'll see that nothing relies solely on serial connection. Compared to the length of the wires running parallel, or 'starring out' from this point, this length is insignificant. You'll notice no capacitors on this board - I rely on the decoupling electrolytics of each module for that, and, in addition, each IC gets a 0.1 uF decoupling cap whether it needs it or not (the Bissell Patchell mode of circuit design). As many caps as I use, the power supply has enough of a capacitive load as it is.

Cheers,
Scott

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zonkout · Joined: Oct 12, 2006 Posts: 33 Location: ozztin, teggsitz

Looks like I'll be pulling out the big guns. Sad

Scott Stites · Posted: Thu Mar 22, 2007 8:52 pm Post subject:

Well, like Mosc says, these things are hard to troubleshoot from a distance. Wish I lived closer to TX. Anyway, curious - how long is the power connection between your distribution board and the power supply?

Scott Stites · Posted: Thu Mar 22, 2007 8:53 pm Post subject:

Forgot to ask - did you try putting the VCO's closer to the power in of the distribution board also?

Edit: whoops, guess you did.

zonkout · Joined: Oct 12, 2006 Posts: 33 Location: ozztin, teggsitz

Scott Stites · Posted: Fri Mar 23, 2007 12:56 pm Post subject:

Is there any way to hook a shorter connection between the PS and the synth (short of major surgery anyway)? It'd be interesting to see if that alleviated the symptom any at all.

mosc · Posted: Fri Mar 23, 2007 4:36 pm Post subject:

XLR mic cables can be a little bit thin. There are XLR speaker cables that might be a better choice.

blue hell · Posted: Fri Mar 23, 2007 5:24 pm Post subject:

Still 6 feet is sort of asking for trouble.

Scott Stites · Posted: Fri Mar 23, 2007 7:36 pm Post subject:

That's kinda what I'm thinking, too.

For a couple of years I've had this superfluous module tacked on the end of my synth - don't know if anybody's noticed it, no one's ever commented on it, but it's a module with two pots on the top and then two columns of banana jacks. It's from a panel for a module I was going to build for a guy, but things didn't work out, and I've scratched my head as to what to do with it (I have a limited amount of blank panels before I have to start searching again). This week it finally dawned on me that I could use it for a utilities panel. It could supply power and ground from the modular to my keyboard and a couple of other controllers I have in the works (joystick and touch sensitive thing).

Anyway, banana jacks would be a great way to go in this situation - get three banana jacks, one for each rail and ground, and build some *short* (I wouldn't want to go much over a foot or two) thick power supply cables with them. Extra banana grounds could be installed on the power supply to ground equipment you're connecting to.

Always the surgeon after the patient's closed, eh?

Cheers,
Scott