Moog Prodigy VCF

[the fiddler]

Hi there,
I have a question about the VCF on the Moog Prodigy.
I have noticed when the VCF is self oscilating that when I lower the cutoff freq the signal starts to roll of at about 150hz to nothing at about 80hz.
I just wanted to know if this is normal or if I should get a tech to take a look?
Whilst using the oscilators everything seems fine and the output signal goes down to 40hz. I also have a moog opus and a sequential pro1 which both go subsonic when the cutoff freq is lowered whilst self oscilating. A function I find to be very useful for speaker destruction.
Any info would be much appreciated.
All the best.
The Fiddler.

[MC]

That's normal. The modulars and minimoog do it too.

It's a big reason why the minimoog is famous for its bass.

[till]

I think this has something to do with the capacitors size in the ladder.

I remember exchanging the capacitors in the moog ladder filter clone (with written OK by Bob Moog!) of the MonoWave during beta testing one of the three prototypes. And using bigger ones changed this behavior to very low self oscillation frequencies below my speakers and my ears low frequency range.

[the fiddler]

Many thanks guys, all I needed to know. I'll just have to destroy those speakers with a rusty screwdriver and a blow torch.
All the best,
the Fiddler.

[Voltor07]

Many thanks guys, all I needed to know. I'll just have to destroy those speakers with a rusty screwdriver and a blow torch.
All the best,
the Fiddler.

Why would you want to intentionally destroy good speakers? Seems wasteful to me, especially in this economy.

[Just Me]

I don't recall him saying they were "good" speakers.
I made the mistake of listening to my Voyager in 2 channel for the first time yesterday. I've GOT to get a dual channel amp and speakers now. Meanwhile, I've been playing it through a set of 5" near field monitors. Every now and then I see them take a pound to full excursion. These are good speakers and I don't want to damage them, but it would be very easy.

[Voltor07]

I don't recall him saying they were "good" speakers.

That's true...I just assumed they were good speakers, because there are few bad ones that can't be parted out, whether it's the cone or dustcap, or even rubber surround. Not the foam or cloth surround though.

I made the mistake of listening to my Voyager in 2 channel for the first time yesterday. I've GOT to get a dual channel amp and speakers now. Meanwhile, I've been playing it through a set of 5" near field monitors. Every now and then I see them take a pound to full excursion. These are good speakers and I don't want to damage them, but it would be very easy.

I'm looking into powered mixers with stereo inputs for various audio tricks. Even though the LP is mono, there are nifty stereo effects to be had with 'Foogers. I'll be getting some custom 12" woofers from Kinetic Audio to go with it...especially for when I get my Taurus III.

[MC]

The reason that self oscillation dropped out was due to increasing resistance between the top transistor pairs of the ladder filter coupled to the differential recovery circuit that is part of the resonance loop.

There are ways to "fix" this, but usually at the expense of resonant quality of the filter. The Micromoog could self oscillate all the way down, but that filter had wussy resonance compared to the Minimoog. As did the Liberation, Polymoog, Rogue...

[the fiddler]

Hi again,
If that's the way it was intended to sound then that's fine by me.
However I have been considering some other modifications and would be interested to see what you guys thought, or if you had any suggestions that may be more sympathetic to what I would like to achieve.
Let me start by saying that I love my Prod, and that I think its a fantastic sounding synth that oozes with moogness and has a quite distinctive sound that sets it apart from my other analogues. It does however have its limitations. Although it has the ability to create some gorgeous bass and lead sounds it can be a bit boring when trying to be a bit more experimental. I almost sold it for this reason, then when cleaning it for said sale, I started to fiddle and fell in love again (you may start crying at this point). I thought to myself I can't lose you baby, but if we are gonna make a go of it some thing has got to change. So I started looking around at various mods on the net and after careful consideration I decided on what I wanted to do. First I should say that I am very reluctant to cut any tracks on the board or to swap out any original parts so the mods I'm proposing should be seen purely as additions.
Here are the mods.

1. Adding a second LFO.
2. Adding a Noise generator for audio plus feeding a S/H on the LFO
(These mods above would be based on the circuits of the micromoog)
3. Taking the voltage from the VCF contour and sending it via amount pots and polarity switches to both oscillators individually they would also reiceive the new LFO voltage switchable between Contour or LFO modulation.
4. The Osc2 to VCF Mod that's already on the PCB.
5. Ext audio in to VCF (also with an amount pot).

I'm proposing to place the new controls on the strip just above the keyboard.
I am not an electronic engineer although I have an understanding of the theory. The guy that would be carrying out the mods is not a synth tech but has had a very long career designing and building profesional mixing desks.
I have told him my ideas and shown him the various schematics and he doesn't seem to think that it would be to difficult.
I was also thinking of getting the woodwork redone by a professional cabinet maker, maybe in walnut rather than the badly sanded driftwood that is in place at the moment.
What do you think is this a good idea or just Heresy (please don't kick me of your site).

All the best,
the fiddler.

PS, I don't really get off on destroying speakers.

[Voltor07]

...Here are the mods.

1. Adding a second LFO.
2. Adding a Noise generator for audio plus feeding a S/H on the LFO
(These mods above would be based on the circuits of the micromoog)
3. Taking the voltage from the VCF contour and sending it via amount pots and polarity switches to both oscillators individually they would also reiceive the new LFO voltage switchable between Contour or LFO modulation.
4. The Osc2 to VCF Mod that's already on the PCB.
5. Ext audio in to VCF (also with an amount pot).

I'm proposing to place the new controls on the strip just above the keyboard.
/snip/
I was also thinking of getting the woodwork redone by a professional cabinet maker, maybe in walnut rather than the badly sanded driftwood that is in place at the moment.
What do you think is this a good idea or just Heresy (please don't kick me of your site).

All the best,
the fiddler.

PS, I don't really get off on destroying speakers.

First, these mods sound awesome. I would personally find a competent synth tech to do them, but if this guy knows his stuff well enough it might turn out okay. I myself am a fan of one-of-a-kind modifications.

Second, regarding the speakers, that's cool. Have you seen the YouTube videos of people intentionally blowing up speakers?

[CZ Rider]

I did just about all of your mentioned mods, and only had to drill two holes in the back to do it. My Prodigy was a late one with the extra jacks installed from the factory. I was never a big fan of seeing extra knobs on the front panel, so I tried to make mine look stock.
The wood is simple design, and a more exotic wood could be cut. I just sanded and refinished the stock pieces.

While I had the wood removed, the first mod was to make a cutout for an IEC power socket. To replace the hard wired original.

The two extra holes drilled in the back were for audio input and a modulation input.

I used white rub on lettering to label the jacks, and masked then applied a little clear coat over top. Here is the back of an Opus I did with 3 extra inputs. Looks almost stock?

The only really cosmetic mod was painting a replacement set of switch caps to match the color scheme of the Minimoog. (Kept the original set white.)

So with the extra jacks, adding a noise source and sample and hold could come from a Moog CP-251 or other source of modular gear. On the inside there were many mods. I changed the kbd. out to a buffered out from the summing amp to osc 1. This way the CV out included pitch bends, osc. mods, and osc 1 range, along with the keyboard with glide circuit. Great for controlling another slave osc/synth. The modulation mod input involved changing the pot on the wheel to a center taped EML type. The pot was wired so down is the onboard LFO, and up comes from the newly installed modulation jack, and center neutral. A center detent was added to the mod wheel just like the pitch wheel . The jack is wired with an isolated switch, so with nothing pluged into the Mod jack, there is a regulated 5 volt source routed to the wheel. The sync switch was rewired so that the pitch bend is not disabled, but rather, routed the mod wheel now to osc 2 when sync engaged. This way I get pitch bend and can simply mod osc 2 through the mod wheel with anything from an outboard EG, LFO, or use the mod wheel up position just like the pitch wheel did stock. Nice mod!
So anyhow you could do most of the mods you want with extra jacks and the CP-251. The CP-251 has the noise source, VCLFO, sample and hold, and 4 input cv/audio mixer to attunuate the audio input or extra LFO/S&H. I'm sure you could add a jack with the one of the Prodigy EG outputs to run through the CP-251 back into the Prodigy.
Or you could expand the Prodigy with another mono synth for extra oscillators and noise plus new mod sources like this one.

A Moog Pierogi?
Works for me!

[Voltor07]

CZ with more customized gear. Awesome job, CZ!

[the fiddler]

Great stuff, deffinatley considering a separate box so as not to have to drill holes on the front panel.
Nice one guys,
Keep them coming.

[Mooger5]

Hi again,
1. Adding a second LFO.
2. Adding a Noise generator for audio plus feeding a S/H on the LFO
(These mods above would be based on the circuits of the micromoog)

I´m no expert but the Prodigy runs on 12 Volts whilst the Micro runs on 15. You may need to use some resistors of lesser value in order to send the adequate control voltages. The Rogue shares a lot more with the Prodigy in this respect.
And I personally would choose a true noise generator circuit instead of the 5837-based. That chip doesn´t generate noise. Instead it sends a digital prerecorded loop. One might think 'noise is noise so what' but I always notice a pattern. Even the S/H sounds predictable.

[Peik]

Hello Moog-People!

I read about the subject and i must say i personally really miss the ability to resonate in the lower frequencies. I cannot confirm, what has been said about to Minimoog, that he reacts in a simillar way. As i remember the one i have had before did go down all the way, which is good for bassdrums e.g.
I personally think that for leads and stuff the prodigy is doing really, really fine, but for making sounddesign and for using it as a real, deep lowendy basssynth i totally miss the resonance on lower frequencies. SO please is there anyone who has actually done the mod and how are your results? Is there a detailed info on which parts must be changed? I just did install an external audio input, mainly to add noise, because i like it a lot. It´s a real simple mod to do. So i´ll be happy, to hear from you soon. Best,

Peik Rosenbohm