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New guy - PSU questions
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Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Mon Jun 12, 2006 2:27 am    Post subject: New guy - PSU questions
Subject description: How good does it need to be?
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Hi, i'm relatively new here (posted a bit over at the Soundlab forum).

I'm planning to build a modular synth of (hopefully at some point) considerable size. For this I need a PSU of course, but I am confused.
For example: how much power do I need, and how important is separation between circuits?

There are a few different designs floating around out there, some using 78/79xx reg's - some using LM317/337's or other.

For instance, the +/- 12v supply on Ray Wilsons site looks simple and easy.
He uses 78/7912 regulators, that if heatsinked, should provide 1A per side.
He says his VCO's use 23ma per side, more if running 15 volts (which I plan to do), so realistically I can maybe run 3-4 of his various modules per 100ma. That's 30-40 modules, a pretty large synth! But is it realistic?

And in terms of separation, is it sensible to run everything off one PSU?
Taking LFO bleedthrough as an example (I have some of that on my Soundlab) Would it make sense to provide every module with it's own set of regulators and filtering? Would it help? It it even possible to run several regulators off one transformer? I seem to have read that Paia does it with their 9700 series.

Some people seem to like having a dedicated psu for "dirty" modules like LFO's and logic stuff. Maybe that it the way to go?

Basically, I'm asking for anyone's practical experience with these things.
Perhaps you found that it wasn't a problem, or that a bit extra filtering on each board was enough. Perhaps it turned out that you needed 5A for a modest system, instead of 1!

Any input? Smile

Best regards

Christian
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mosc
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PostPosted: Mon Jun 12, 2006 8:09 am    Post subject: Reply with quote  Mark this post and the followings unread

You are in for a learning experience. This is an area where it's is as much an art as a science.

Since the 200 series designs, Buchla has been using a clean ground and a dirty ground. You might think of that from the start.

I would design the wiring of my console to accept multiple power supplies, but I would not build them unless it was necessary. You may find that one supply is fine. You'll only know that after you start putting your system together.

Grounding is one area where you can get a lot of hum and noise if you don't get it right. You may have to use a lot of trial and error. There are all kinds of schemes that people use. Typical is a central ground with spokes. I've used this technique, but it sometimes doesn't work, because patch cords can change everything. Another reason I like banana jacks.

Be sure to use reasonably heavy gauge wires for your power busses and grounds.

Don't worry about it unless you experience problems. If you do, try to strip things down to the smallest possible configuration, get the noise out of that, and then start adding modules.

Anecdote: I used to build ring modulators using the AD530 chips (early version of the 630). Each ring modulator had a board, a front panel. The power supply was on the board. On the front panel was a little ground buss. On a run of ten units, I hand selected the place the wire from the board to the front panel was connected. On every unit, a different point on the board gave the lowest noise.

Another Anecdote: Back in the 70s, we would buy Teac 4 track tape recorders. There was a guy in Oakland, California who would take the new decks and work on them to get the noise down. He could improve the S/N ratio by as much as 12 dB. He did this by adding capacitors and moving the grounds around. Sometimes he would reroute the signal cables inside the machines. He said every one was different. (I wish I could remember his name. He was a wizard. I learned a lot from him. I think his first name was Don.)

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Coriolis



Joined: Apr 11, 2005
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PostPosted: Mon Jun 12, 2006 9:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi mosc, thanks for the answer!

Nosed around for Buchla psu schems, but couldn't find any, so I ask:
How are those separate grounds realised? Would two star-ground systems (one clean, one dirty), merging as late as possible qualify?

I plan to use banana-jacks, so I guess that helps.

C
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zipzap



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PostPosted: Mon Jun 12, 2006 11:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Someone (ouncle k?) told me that the dirty ground thing is basically to provide a direct connection to the psu for modules that produce a lot of dirt (eg cmos stuff). So if you imagine the little dirty electrons, you can see them going directly to the psu, not throug any modules that you want to actually listen to.
Must of the time you get no problems, once had a flashing light of a little sequencer that i could hear very loud, i had it going to the ground of my vcf (chaos setup that was)
Right now i use one psu with 2*1A and a 2*2,5A transformer. big enough to add another regulator if i need one. 1 A is a lot to start with. You can use paralel regulators off one transformer, but you have to keep them parallel, musn´t connect the outputs of those regulators, unless they are trimmed to exact the same voltage!
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Coriolis



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PostPosted: Mon Jun 12, 2006 12:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

I see. So I could have a set of regulators for "clean" ground, and another set for "dirty" running in parallel, if only my transformer is big enough?

It seems that daisychaining the power to modules is a bad idea, like in the case of your vcf. Star ground it is, then.

Will post my findings, but that may be a while, since I just ordered parts...

C
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zipzap



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PostPosted: Mon Jun 12, 2006 12:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

i think it´s not necessary to have two sets of regulator. Again, imagine those dirty elektrons, running from the sequencer back to the psu. As soon as they get there, at least thats how i imagine, they are sucked in by those big filter cabs. They will have no motivation to run against that force, up the ground connection to your audio modules, and make a mess. Would be against nature somehow.
If someone reading this knows that i´m wrong please tell me!
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Blue Hell
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PostPosted: Mon Jun 12, 2006 12:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

zipzap wrote:
If someone reading this knows that i´m wrong please tell me!


It's quite poetic the way you describe it and pretty accurate, alhough it is not just/only the capacitors calming down the electrons but the regulatory action of the regulator does so as well. And indeed the disturbance sort of gets absorbed.

And indeed usually there is no need for two sets of regulators. Occasionaly I've seen a 2nd one, or 2nd set, being used but only when there were exteme differences, like for instance a motor control circuit combined with a microphone pre-amplifier. Oh and once in a high end modular tube oscilloscope where each module even had its own transformer - I bought it as junk though :D

Before going to double the regulators there are some cheaper ways of decoupling between clean and dirty circuits, like using inductors in the power lines or RC filtering.

But whatever you do, it's like mosc already said, very much like art and building up experience.

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piedwagtail



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PostPosted: Mon Jun 12, 2006 1:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

The more modules,the more space you'll need for a heatsink......
I have a dedicated psu for the vcos....and if i'd had more space pi choke configurations would have been nice.I've used capacitor-choke-capacitor(pi) filtering on valve circuits and the dc is lovely,so little ripple.
But the point is don't cramp the space,the rule is if the heatsink/reg is too hot to touch it needs to be bigger.A 7815 doing 1A will give you a blister!
So don't neglect the mechanical set-up.

Robert
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Coriolis



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PostPosted: Mon Jun 12, 2006 1:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you very much everybody, this is pure gold to me!
Schematics and general info is so easy to find, but someone's actual experience is another thing!
I'll start with the simple setup and just learn from there.

Cool Cool Cool

C
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Uncle Krunkus
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PostPosted: Tue Jun 13, 2006 2:29 am    Post subject: Reply with quote  Mark this post and the followings unread

I only just saw this thread. I agree with what everyone else has said. I'd add that Ray has gone out of his way to make sure the Soundlab uses very little current (so that it is viable on 2 9V batteries)
While it's possible that some modules could use as little as 30mA I'd assume an average of 100-150mA each. Experience is the best way to get it right though.
I'd start with a +/-15V PSU, with clean and dirty grounds, star connection to each module, at least 1A on the +ve rail. If you're building it yourself include a test link before the starpoints for the +&-ve. When you've put a few modules in check what you're drawing and then you'll be in a good position for prophesy.
Once you have less than 20% headroom it's time to start building (and making room for) the next PSU.

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Coriolis



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PostPosted: Tue Jun 13, 2006 4:15 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Uncle Kruckus! You mean something like this:

Edit: Btw, I had a sense I might in reality end up using more current than just adding up someone elses numbers would suggest... Wink


clean dirty ground.gif
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Clean and dirty ground for a +/- supply...I think
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clean dirty ground.gif


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Uncle Krunkus
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PostPosted: Tue Jun 13, 2006 8:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Yep,
same for the +ve & -ve.
Are you gonna build the PSU yourself?

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Coriolis



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PostPosted: Tue Jun 13, 2006 9:19 am    Post subject: Reply with quote  Mark this post and the followings unread

It's gonna be built by me, yes. I already did a bipolar 15v psu, but the transformer is only rated at 400ma per side at 15V, so I figured I would need something bigger for the actual system I'm planning, and just using the smaller one as a crude lab supply.

C
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mosc
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PostPosted: Tue Jun 13, 2006 12:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Coriolis wrote:

Nosed around for Buchla psu schems, but couldn't find any, so I ask:
How are those separate grounds realised? Would two star-ground systems (one clean, one dirty), merging as late as possible qualify?


You won't find anything no matter how long you look. I used to work for him, that's how I know.

Dirty circuits are stuff like digital logic, comparitors and square wave oscillators that have sharp transients. I've built many many digital/analog circuits where there was just one ground an never had any problems. Still, I always kept Buchla's scheme in mind in case I ran into a situation where I was hungry for a solution. He probably ran into a situation early on and discovered that trick. I never asked him about it, we just used it all the time.

A star configuration is sometimes not completely practical. Moog in his modulars used a power buss on every row of modules. The busses each went back to the power supply. So, that is sort of a hybrid star - probalby the most commonly used approach - similar to your drawing.

In general, I would try to use one supply - but it's not critical. I'd be affraid there might be the possibility of noise creeping in if there are multiple supplies. But it that happened, you'd identify it and fix it.

Figure out some kind of useful connector/wiring scheme so that you can plug in and remove your modules from whatever power distribution scheme you use. Even for purchased ready-built modules, you'll be doing that a lot. For example, Buchla used a 6 pin Cinch/Jones connectors. On each module there was a female in the chasis back, and a male on a 6 inch cable. The idea was that you plug power into one module's female and chain them along the row in the console. To remove a module, just take it out by unplugging the connectors.

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dnny



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PostPosted: Tue Jun 13, 2006 1:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

mosc wrote:
Coriolis wrote:

Nosed around for Buchla psu schems, but couldn't find any...


You won't find anything no matter how long you look. I used to work for him, that's how I know.


that reminds me that there is this wonderful story Grandpa Mosc, tell us a story? where Howard tels us about the project called the Electric Symphony Orchestra. (from the time he worked for Buchla)

sorry for the off topic

Howard, i know you are busy man and i appreciate all the input and time you give to this forum, is it too much to ask for the ending of that story? Confused

daniel

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mosc
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PostPosted: Tue Jun 13, 2006 4:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

Not too much to ask... I better do it before I completely forget. Very Happy
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Coriolis



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PostPosted: Wed Jun 14, 2006 2:07 am    Post subject: Reply with quote  Mark this post and the followings unread

This is great, the setup is starting to take shape in my head.

Great story by the way, mosc!

C
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Mr Swirling Vortex Man



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PostPosted: Fri Jun 30, 2006 2:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Just wanted to add -- I also am finding this info very valuable. Coincidentally, I'll be building a power supply over the weekend. So thanks folks! Hoping one day I may also be able to pass this kind of knowledge on to others.
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