Sawtooth LFO

[s16016wb]

I'm looking for a way to generate sawtooth modulation that can ramp in either direction.

So far I've tried:

OSC 3 in low freq mode: works but only in one direction, and eats up an oscillator

using a combination of a square wave+lag on the CP251: this doesn't work very well because the a true ramp should run in one direction for the entire duty cycle (execept the moment that the voltage resets). using this technique the voltage is contstant for half the duty cycle. also, the the lag has to be adjusted if the LFO speed is change - not very useful.

my old MS-20 (had to sell it to buy the vger) had a continuous control for the LFO waveshape that ran from up-ramp to triangle to down-ramp... i really miss this feature.

does anyone have any suggestions or do i need to employ a non-moog piece of kit to get my ramps?

thanks

[MC]

You obviously have the VX351.

You may need an external waveshaping kit that converts the Voyager LFO's triangle to up or down ramp. Not only has it been done on the MS20, but also the Crumar Spirit; so finding a tech who could build that specific circuit from schematics would be the answer.

I can't think of any available synth module that does what you need other than Serge, and they sell only whole panels - not separate modules.

[moogmusic]

Hey - What about a CP-251?
You can use the Lag Processor to turn a square wave into either a falling or rising ramp wave - it slows down the rise or fall of any CV signal. You can also use the CV Mixer's inverting output to invert any CV and it has an offset control. Plus you get another LFO and another sample and hold...
Just a thought...

[s16016wb]

Yeah, I already have a CP251. I mentioned in my first post why the lag processor doesn't really get the job done. Too bad the CV expander doen't carry OSC 3's output.

[moogmusic]

That's funny, I skimmed through your post but missed the bit about the lag processor. Sorry...

You can obtain Osc. 3 as an output of the VX-351, but, you have to assign it as a source in a mod buss. The outputs labeled Busses on the VX-351 carry the Mod buss CVs as they appear at the output of the busses (affected by the amount and Mod wheel, or Mod 1 input depending on the buss).

If you want to get the Osc. 3 waveform CV this way, but don't want it modulating anything else, set it up as the source on the Mod wheel mod buss and set up a "dummy" destination in that mod bus- something you won't hear, like Noise level (if you're not using noise), or if you're not using the LFO, LFO rate... Then when you turn up the Mod Wheel, Osc. 3 appears at the VX-351's Buss "Wheel" Output.

You can also get Osc. 1 and Osc. 2 signals this way, setting them up as a PGM Source.

Hope this helps,
MM

[FreqOut]

s16016wb,
Have you figured anything out yet?

I'm also really interested in this, because I sold my old Minimoog Model D to a friend after I bought the Voyager. I had thought that I could pretty much do everything that the Minimoog could do, but now I realize that I'm missing that 'up' ramp. The Minimoog had it as a waveform on OSC3, and it was perfect for creating "bubbling" type sounds with OSC3 in LFO mode.
In fact, on the Minimoog, ALL the sawtooth waveforms were "up" ramps, but OSC3 also had a "down" ramp.
On the Voyager, all the sawtooth waveforms are "down" ramps. There is no "up" ramp anywhere that I can find.

At the risk of embarrasing myself (I don't know much about electronics), I'll put forward a few ideas and maybe we can figure this out. I would really like to have this function, even if it requires more external gear.

1. Would inverting the voltage on a 'down' ramp cause it to be an 'up' ramp? If so, then I think a simple voltage inverter circuit could invert the wave coming out of the VX351's OSC3 and you could use it to modulate whatever you want. But I don't know anything about electronics, so I have no idea if this would work. Anyone??

2. I've looked at some MOTM modules, but can't tell if any of them produce the "up" ramp sawtooth. You can see them here.
Of course to add one of the MOTM oscillators, you'd also need a power supply. Is anyone familiar enough with these know if that will do the trick?

FreqOut!!

[s16016wb]

Nothing yet. I'm starting think I'm gonna have to buy another MS-20 just to get that damn LFO back But I think you are correct about inverting OSC 3 - if I had the 351 I would try this with the inverter in 251's CV mixer.

I have a couple of electronics-savvy friends looking into designing a custom circuit for me.

One idea is this: have a square osc with pulse width variable from almost no T at maximum voltage to a perfect square to almost no T at minimum voltage. Then have a lag that scales to the duty cycle. As the pulse width is swept it should go smoothly from a down-ramp to a triangle to an up-ramp. Anybody know if this would work?

[FreqOut]

I have the 351, but not the 251, so I can't try out the inverter idea.
Does anyone out there have both, and would they be able to tell us if this idea works?

FreqOut

[spaceness]

I have both the CP and VX...but currently have my voyager at the moog shop awaiting the anniversary conversion so, unfortunately, I can't be of much help at the moment. Sorry. I actually wasn't aware that the Voyager didn't have the rising ramp capability. It's such a useful waveform and such a shame that the Voyager doesn't offer it as a stock feature. When I get it back I'll do some experimenting with the CP and VX and let you know what I come up with.

[GregAE]

<< 1. Would inverting the voltage on a 'down' ramp cause it to be an 'up' ramp? If so, then I think a simple voltage inverter circuit could invert the wave coming out of the VX351's OSC3 and you could use it to modulate whatever you want. But I don't know anything about electronics, so I have no idea if this would work. Anyone??
>>

Inverting a 'down' ramp will create an 'up' ramp. However, this may or may not get you what you're looking for. The reason is that an inverter will create the negative of the input signal. If the input signal is bi-polar (+5 to -5 volts), then the output will be the inverse (-5 to +5 volts), and you're good to go. But if the input signal is not bi-polar (+5 to 0 volts), then the inverter will produce the opposite (-5 to 0 volts). In this case, you would have to add +5 volts to the inverted signal (via a mixer) to get a control signal that goes from 0 to +5 volts.

Now for the bad news: while the CP-251 expander does have a CV mixer, it does not have an inverter (at least according to the specs on the Moog web site), and the 251's lag processor will not give you the result you're after. The good news is that a simple inverter is easily constructed and could probably be retrofited into a 251 if you are handy with a soldering iron and have a basic knowledge of electronics. This could be a low-cost alternative to any other approaches you're considering.

Hang in there. I hope to purchase a 251 soon and may be able to provide details on adding an inverter.

[GregAE]

Whoops, I spoke too soon!

With apologies to the Moog folks:

After downloading and reviewing the CP251 manual, I found out that there is an inverter: it's built into the control voltage mixer. So it does appear that you can get the 'up' ramp you're after. It IS unfortunate that there isn't a dedicated inverter in the box, i.e. a simple one-in, one-out jack for this kind of thing, but at least the capability is there.

Now I've got to go buy one to accompany my VX-351.

Cheers!

Greg

[FreqOut]

Thanks for the info Greg!
Looks like I'll be buying that 351 sooner than I thought!
I do wish there was an easier way to do this though, because I think it's really an essential part of an LFO.
Thanks again for the help!

FreqOut

[FreqOut]

^^^I meant the CP251...

Anyway, I just can't justify spending the $ on the CP251 since the only part I really want is the voltage inverter.
So I've decided to build my own voltage inverter. I'm not an electrical engineer, but I have experience building PAiA kits. I've found a couple of schematics online, and hope someone who knows more than I do can help me with a couple of questions.
1. Is it possible to build a "passive" voltage inverter?
2. If I need to build a powered inverter, does the power supply need to match Moog's +5V/-5V scheme?
Any help would be greatly appreciated.
Thanks!
FreqOut

[newname]

[quote="s16016wb"]I'm looking for a way to generate sawtooth modulation that can ramp in either direction.]

stock Vger can do some limited LFO waveshaping:
modulate LFO frequency with the square wave output, patch 'triangle' output to what you want to mod.

changes base freq of LFO, but also radically alters shape to approximate a saw. sounds ok but likely looks weird on a scope.
this is not a 'perfect' fix, but it provides lots of mod fun. i mostly use it from front panel, using the second mod buss, although it's fun to route through the 'wheel' buss...

haven't tried this with the little voltage mixer on the 251, perhaps the bias control and inverted outputs could be used to make it go the other way? i figure you should be able to bruteforce an 180degree 'phase shift' that way...

hope this helps.

lx
[forum choked on my new email addy. needed a new name anyway.]

[FreqOut]

^^^I meant the CP251...

Anyway, I just can't justify spending the $ on the CP251 since the only part I really want is the voltage inverter.
So I've decided to build my own voltage inverter. I'm not an electrical engineer, but I have experience building PAiA kits. I've found a couple of schematics online, and hope someone who knows more than I do can help me with a couple of questions.
1. Is it possible to build a "passive" voltage inverter?
2. If I need to build a powered inverter, does the power supply need to match Moog's +5V/-5V scheme?
Any help would be greatly appreciated.
Thanks!
FreqOut

Can anyone here help me out with this, or at least point me in the right direction?
Thanks...