Need a +/- 15V PSU - MS20 Filter Clone...

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Milkmansound

Well-known member
Joined
Jan 29, 2005
Messages
449
Location
San Francisco, CA
and believe it or not I still need guidance. Can I get +/- 15V from a 12VAC transformer? I looked at PRR's simple circuit and that appears to put out +/- 18VDC - I intend to make this circuit:

rs20.png


I feel like I can get the whole thing onto one breadboard and mount it into a radio shack project box - I will want a regulated supply though, cause there are opamps involved.

Thanks!
 
15VDC regulated from a full-wave rectified 12VAC is marginal. But if the transformer has very poor voltage regulation by itself and you are using it at a fraction of the rated load you might just get there.

A perfect transformer fed from a perfect a.c. mains source will output a sine wave whose peak value magnitudes are square root 2 times the rms output voltage. So for 12VAC you would get 16.97V peaks. Then your rectifier diodes will reduce this by about 0.6 to 0.8V per diode. If it is a center-tapped bridge config. each of the + and - rectified outputs will be diminished by one diode drop, so you will be down to around 16.2V. But that's only at the exact peak, and you have to have some time conducting to charge your filter caps, so your rectified d.c. with a load will be less. Then, in between charging at peaks your load is discharging the cap and you will droop accordingly until the next half-cycle.

Then your voltage regulator will require some input to output differential to function. In the popular newer LDO (low drop out) regs this can be only a few tenths of a volt, but the more conventional ones like 7815's and LM317's you want more like about 1.5V. So, you wouldn't get there.

But....most trafos are designed with a lot of sag when loaded with the rated resistive load current. You will be making life hard by instead pulling current on peaks, but typically if the average current is a fair amount less than the rated trafo current the a.c. voltage peaks will still be a fair amount higher than the earlier calculation, possibly enough to get there with normal line voltage.

Another gotcha is when you have line power loaded down by a bunch of equipment that pulls power on the peaks just like you are attempting to do. This results in a flattening of the wavform tops and thwarts your attempts to extract power there. And then you may have a brownout and things will drop out of regulation due to that as well.

The moral is: try it, but don't count on it for anything critical. Better to invest in a higher voltage transformer.
 
well, the good news is that I do not even have a transformer in mind for this project! Should I look for an 18VAC transformer? Is there a schematic for this type of dual supply handy?
 
yeah, thats the one I looked at. The answer has to be in there somewhere - and thats where I saw PRR's little circuit with the 12VAC transformer into +/-18VDC


I just need a little hand holding is all - this is going to be my first build that did not come in a kit :p
 
Ahh---looking back at your app now, which I only glanced at to begin with, I would urge using regulated supplies. PRR's was specifically an op amp supply, and op amps have usually pretty good power supply rejection, at least at low frequencies.

But the other parts of the filter circuit have varying amounts of power supply sensitivity so would be risky to unusable with an unregulated supply.

An 18V trafo is slightly overkill but with your modest current requirements is not extravagant. Followed with 7815/7915 regs PRR's schematic should be fine, and no heat sinks should be required on TO-220 regulator packages. Use 35V rated caps on the unreg side.

If you wanted to get fancier you could get a trafo with two separate secondaries and do two bridge-rectified LM317-regulated 15V supplies, then tie them in series and make the center point ground. The ripple freq is doubled and the regulators work a bit better than 78/79 types, but this is probably overkill.
 
ok - so this:

simple-PS.gif


followed by a 15 and -15 volt reg should do it then.

I tie the ground points of the circuit to the 0V? Including the audio grounds?

Anyone have any idea what the 47k lin pot in the top left corner does? Near the CV inputs? (heh, are those inputs even? I need to do some more staring at this schematic!)

thanks - I feel like i am on the right path already :sam:
 
Yep---I would use an 18V trafo if you have the option, but the circuit is fine. Yes, the reg. grounds should go to the "0", which you can use as your star ground for everything. If your layout resembles the schematic, the main charging current spikes will be flowing in loops away from your star ground so it should be low hum/buzz.
 
> I will want a regulated supply though, cause there are opamps involved.

Does not follow. Most op-amps have very good supply rejection; within wide limits, they don't care what you feed them. Because the ear is sensitive to buzz, you want low ripple: couple big supply caps will do the job, and is more perf-board friendly than regulators.

> Anyone have any idea what the 47k lin pot in the top left corner does? Near the CV inputs? (heh, are those inputs even? I need to do some more staring at this schematic!)

Are you building a voltage-controlled synthesizer? Or a box with a knob?

"Pitch" and the "CV" inputs allow you to set the center frequency with a voltage. In an original Moog, the oscillators were also voltage controlled. To pick-out a specific harmonic of a voice that melodically hops around in pitch, you feed the same pitch-control voltage to oscillator and filter. That will track the fundamental; to track a harmonic you introduce an offset voltage. If you don't want exact Volt/Octave tracking, you feed the CV pots and turn them to the desired mistracking.

If you have ever worked with such a machine, you are in awe of Carlos' Switched-On Bach. Getting one oscillator and one filter to track is maddening: microvolt offset and drift cause many-cent errors in pitch, sometimes drifting enough in a 60-second passage to sound like Bach's organ-pipes are shrinking. Even though Moog potted the loggers and used accurate regulation. Making multi-voice music on a Moog: awesome.

However, since you don't know the CV inputs, I bet you are wanting a knob-tuned filter. And you probably won't need the very high Q settings needed for harmonic-tracking. In that case, initial accuracy is meaningless, and drift is less problematic than Moog-tracking.

The frequency setting is a direct function of supply voltage plus a log function of supply voltage. 6% drift in supply voltage will move the frequency one musical semi-tone. If this clone were Moog-scaled, 80mV of drift at the CV summing resistors would shift pitch one semi-tone.

If you actually set the frequency to the nearest semitone (which implies a Q over 10; not musically useful outside a voltage-controlled synth), and your power company is wandering from 115V to 120V, you will notice. Of course, if it wanders from 115V at 5pm to 120V at 3am, you may not notice.

I think it would be musically-fine with 12VAC into two diodes, 2x220uFd, 2x470Ω, and a pair of 1,000uFd caps to give low ripple and very-slow pitch change for wall-power variation.

Alternatively: there is no reason it can't run on +/-9V. Demand is under 5mA. It is quite reasonable to do this with Zeners. Say 220uFd, 1K, 9V Zeners bypassed with 220uFd.

Demand is so low, that for many applications it would run happy on a couple 9V batteries.

OH! This is NOT a passable clone of what I thought was Bob Moog's "famous" filter. He stacked two piles of BJTs, with caps between, and drifted the bias current. Clever enough to patent. Also "inaccurate": it uses the BJT's emitter impedances as the R in an R-C network, but emitters are pretty bent Rs. The unintended result was a mellow funky distortion. This contraption with 3080s will do the VCF thing fine, but without any extra flavor.
 
Heck: if you are not using the Voltage Control feature, and don't need to cover the entire audio band in one turn, then A4, T1 T2 and all that stuff top-left is wasted.

The two 10K resistors that feed the "OO" end of the 3080 chips: change to 47K. Wire a 100K Audio-taper pot across the supply rails. Reading the terminals as "standard volume control", the "In" to V+, the "Ground" to V-, and "Out" to the 47K resistors feeding the 3080 "OO" node.

If you do need voltage-control: this scheme will not be near as stable as a real Moog (which wasn't as stable as a fine violin in the Arizona sun). T1 T2 must be potted, and the 1K must have a specific temperature coefficient. The "plain" resistors really should be high-stability (this matter on my ARP clone of the Moog). And while the 3080 is mighty linear for any ordinary purpose, it may a bit slippery for precision tracking.
 
Having attended on a Moog in my later school days PRR's comments about Walter/Wendy C. are well-taken. The difficulties may well have accelerated Carlos's decision to have The Operation.

The antilog converters drifted, the basic oscillators were based on current-source-fed unijunction transistors with their own temperature coefficients and intrinsic deadtime nonlinearity, etc. To make matters worse the temperature control in the studio was terrible.

All of these obstacles and irritants, however, paled in comparison with the challenge of working with the prima donnas in the Music Dept. :razz:
 
haha - yes, I am looking for a box with a few knobs scenario - resonance and cutoff - audio input attenuation would be nice, but is not totally essential. This is to be more along the lines of a line level stompbox - if thats even possible. Thats why I posted here - cause I don't know a thing about this and you all do :oops:

I can probably get the circuit up and running with batteries and worry about a PSU when I fit it into a case if the thing does not sound like doody.

I graduated from High School in 1997 - but I tell ya, thats not that long ago!

Also - yes, I am fully aware of the "Switched On" series - the sex changes, etc. Interesting stuff.
 
Agree with PRR---if it's a manual adjustment and not a voltage-controlled scenario, you don't need to be so worried about regulation.

Based on that though, you could also do a design with a dual potentiometer and eliminate some other parts, and still get your manual bandpass adjustment. But that's another whole b of w.
 
Ok, the confusion is slowly dissapating. Mostly because I just got an email back for the circuit designer:


As shown it is just low-pass. The high-pass variant is archeived by lifting off the grounded end of the cap at the output of O2, and feeding your input signal there. (I have used a switching jack, which makes the contact to GND if there is nothing plugged, so I can have both options, even simultaneously. But one can also use a switch, and just one jack.)

Me: second - how forgiving is the input? Is it for line level - or should I plan to pad it down?

The sound is largely dependant on the level of the input, Line levels should be ok, but the sound will get more like the original if you drive it a little bit hotter, say 2Vpp or even more. There is no inherent danger if you drive it even hotter. With low levels, the self oscillation will be quite loud compared to your input levels, so you might need a voltage divider at the output.

Me: next - I am curious about the 3 pots on the top left corner of the schematic - are they trimmers for the CV inputs? Whats the pot with the +/- 15V on it for? And do I need to use CV?

With that you can tune the cuttoff frequency. The other inputs provide control voltages, two of which can be attenuated.

Me: Basically - I just want a simple filter for line level audio that has a similar sound to the MS20 high pass.

In that case you should consider adding a little amplifier stage. The basic text book inverting operations amplifier circuit will do nicely. Maybe a gain of 10 is a good starting point.

Me: Last question - there is a "pitch" input as well - what is that for? Is that yet another CV input?

Exactly, with that you can get keyboard tracking.

If you want a stand alone version, you don't need to connect anything there. The filter will work without that.

____________________________

I think the only thing left is to add that amplification stage on the input - should I just use any old opamp like a 4558 and look at the data sheet to make this little amp? Any advice as to which opamp I might want to use? Thanks again - I am almost there, and I think I will definitely use that simple unregulated supply for this :grin:
 
I finally ordered the parts for this - I am going to try the PRR PSU, and I have a 15 and -15 reg set leftover from something else if I end up needing them.

I got an 18VAC 1A transformer, which I think should be more than enough to handle this.

Also, I will be putting a non-inverting amplifier at the front of the circuit, to increase the gain a little more and send the filter a good amount of signal, since the designer recommended this.

Then a voltage divider on the output, so that the self-oscillation is tamed.

I'll be building this onto a breadboard, so that I can play around with the different HP and LP possibilities.

Thanks again to everyone who helped so far, I will post again in this thread when the filter is built!
 
If not too late, this is a very nice power supply: The Peter Green mic pre supply.

http://1176neve.tripod.com/id11.html

Simple to build, if you know how to etch a board. (If not, it's a good project to learn on.) I built this one with a 24V transformer and 7824/7924 and got a 24V power supply.

It's a standard design and not far off from the one PRR suggested. Oh, and if you're interested in a kit for a Korg filter, check out the MOTM kits:

http://www.synthtech.com/motm410.html

I built one of these synths and it NAILED that modular Moog sound (with the 490 filter). I had the big sounds from Abbey Road nailed in my bedroom. The oscillators were rock solid, too. I no longer have it, though. It turns out I was not a modular synth guy.
 
well, I drew this up - I think this is much more breadboard friendly, and I have most of this stuff lying around:

PSU15VDC.gif


this is based off of the simple PRR circuit - keep in mind, am using mostly opamps so I might even be able to skip those regs - but I have 2 on the bench... I'll expiriment.


I know about the MOTM stuff - its killer - but for just this little filter, the motm seems like overkill. I mean, I will be able to build 3 of these for $50! And I would like to make a stereo version as well.
 
Back
Top