Moog Rogue LPF Cutoff Filter Modification - Kenton ProSolo II MIDI-CV Converter

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Moog Rogue LPF Cutoff Filter Modification - Kenton ProSolo II MIDI-CV Converter

Post by SteveECrane »

Anyone out there know how to perform the modification to the Moog Rogue (or the Moog-manufactured Realistic MG-1 Concertmate) monophonic analogue synth(s) in order to add external control of the LPF filter cutoff? lthough this is meant to be a cinch, I've subscribed to many a DIY forum but to no avail. Anyone know where there's a diagram, circuit schematic, a description or some other literature?

If anyone cares to read the following, this is what I've gleaned from the vast experiences of other experts on-line and what I'm likely to try this weekend :headbang: unless anyone cares to stop me, that is :crazy: :-

I've been searching the web for weeks looking for details of the Moog Rogue synth modification which provides external access to the LPF filter. I already have a Kenton Pro Solo II programmable MIDI-CV convertor (the gate and CV outs work absolutely fine) and wanted to use its third output - the Aux Out - to control the filter's cut-off. I have the schematics:

Moog Rogue Circuit Schematic

and just need to know where to make the requisite mods.

I'm presuming the 'filter input' is to the left of the circuit around the 100K resistor (R67 on this schematic) which connects to the taper of the 10K Filter Cut-Off slider (R68) forming a simple voltage divider between the +/-12V supply rails ... but I'm no electronics engineer, just an amateur meddler with a bit of common sense and a few ideas!

Anyways, the following is what I'm thinking of trying out in order to get external (voltage) control of the Moog's LPF to work (via a Kenton Pro Solo II MIDI-CV convertor, which outputs 10V4 max at its Aux Out depending on sensitivity settings, input controller position, blah, blah, blah):

1) The junction of R67 (Manual Cut-Off), R65 (Envelope), R69 (Key Track), R71 (Mod) and R64 is where various filter control voltages appear to be summed. These go into the base of transistor C in the CA3046 package, identified as U7 on the scheamitc. (I understand that transistor C forms some kind of exponential convertor(?) for the subsequent filter ladder constructed out of transuisters Q9 through Q14 and U14 pairs A & B, C & D).

2) It seems that the 1K resistor (R64) going to ground from the base of that transistor to ground means I'll need approximately 50K going into that junction to give around 1V/Oct control. (Or so someone tells me!)

3) Although the Kenton Pro Solo II unit has a wide degree of control over the maximum value of its Aux Out voltage (I've measured from 0V0 to around 10V4), I'm thinking of adding another pot to control the sensitivity here (10-100K linear - the precise value to be arrived at via a bit of experimentation, I'm afraid, but I'll start with 50K) and a resistor in a similar way as resistor R70 and potentiometer R69 respectively.

4) The way I'm thinking this should all be connected means that the Kenton Pro Solo II will feed its Aux Out (via a quarter inch "Filter Input" input jack I'll mount on the rear of the Rogue alongside the other 1/4" jacks) into the "top" of this new linear pot, through a resistor I'll possibly mount at the jack itself (so I don't have to mod the circuit board in any way or find a way to mount another daughter-board inside the Rogue's case).

5) For a wider range of control here I'm going to try to use a smaller resistor than the 45K R69 that's already employed in the existing circuit, probably ~1K-2K2 (again to be arrived at purely by experimentation!).

6) I also want to add a degree of input protection at this 'new' "Aux Filter Input" jack by using a couple of diodes that will connect to the centre tap of the pot in addition to the input 'sensitivity' resistor. One diode will have its cathode connected to the V+ supply rail and its anode towards the wiper of the pot, whilst the second diode will connect its anode to the V- rail with its cathode also connected to the wiper of the new pot end of the circuit. In other words, where these 2 diodes meet, they will connect to the wiper arm of the pot (see diag below to clarify!).

I believe that this will ensure that any input voltage above 12V or below -12V gets safely diverted to the power supply rails, rather than through the summing circuit itself, in a similar way to this:

Diode Input Protection Circuit

Obviously the resistor values are different, and the circuit shows a diode connection to ground rather than the V- power supply rail, but it's virtually identical in terms of illustrating just what I'm talking about).

As far as I can tell, input protection is not an imperative so long as the 'rules' governing voltage input(s) are observed, i.e. so long as the Kenton unit never outputs more than a +/-12V signal; however, as some modular synths runs on higher +/-15V rails - and who knows, I may connect one at some point in the future(!) - I agree with Dave Magnuson of HooHah Records / Resonant Frequency that a few pennies' worth of diodes is a cheap, no-brainer form of protection here ... so long as it works, that is!

8) So, ultimately, the input side of this little circuit feeds the centre tap/wiper arm of the 'sensitivity' pot, with the output from this little lot leaving the top of the pot via a 22K resistor to feed the summing point for the filter control voltages.

7) Finally, I may well attempt to use the switched connection available on the 1/4 jack socket to take the existing LPF filter cut-off control out of circuit ... if I figure how on earth to do that!

So, how's that all sound? I may appear to have a reasonable idea of what I'm talking about ... but I don't really! It's all simply common sense coupled to a little application of an even tinier element of emprical knowledge and speciulative theory gained from trawling site after site on-line and soaking up the knowledge of others.

Comments, criticisms and laughter more than welcomed.

PS Many, MANY thanks to Colin Fraser of Sequentix Music Systems Ltd and Dave Magnuson of Resonant Frequency / HooHahRecords for all their invaluable advice thus far! Greatly appreciated; and nice blokes to boot.

PPS I know there's an actual Kenton kit (Kenton Moog Rogue LPF Cutoff Filter Aux Input Cable/Socket Modification Set) available but, having already bought the Pro Solo II, Kenton now want a minimum order value for the mod cable itself ... and I simply won't do it out of principle, even though their ProSolo is a superbly engineered bit of kit. (Also, there appears to be no input protection with the Kenton mod which the below provides).

I simply don't need any additional (and expensive) cables to "make up the order value" and, together with the more-than-sufficient £GBP6.50 + VAT (!) shipping charges, there's really little justification for this additional (and quite unreasonable) criteria being introduced on an order-by-order basis. (Royal Mail's "Special Delivery" service is actually VAT free and would cost less than £5 for this weight/value combination for a next day before 13:00 service!). Will our British manufacturers ever learn it's a globally competitive marketplace and stop b!tchin' about cheap Far-Eastern imports? It's the way it is and they're getting better all the time. Jump on board the bandwagon guys and stop over-charging for stuff: then you won't lose our custom.

Anyways, rant over. :protest:
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Stephen E Crane
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Re: Moog Rogue LPF Cutoff Filter Modification - Kenton ProSolo II MIDI-CV Converter

Post by nathanscribe »

Wow. Old thread but, in case it's useful to someone, I have pretty much had the same thoughts lately and went and stuck a Filter CV input on my Rogue. I didn't bother with the input protection or scaling pot, but you're right, with a 45k3 resistor in-line with the input CV, you get the same scaling as the keyboard. Half that value should give twice the range, If my brain is right, and another pot of say 47k would allow a range of half to twice unity scale. Might well add these extra features later, but for now here's how I did it:

Moog Filter CV Input modification
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