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Behringer Model D - DIY Mods Modular Synthesizers
Old 1 week ago
  #151
Gear Maniac
Quote:
Originally Posted by AndyHornBlower View Post
I now sort of see how the Release On/Off works, and why the Decay pot affects the length of that too... which suggests a fairly simple mod to add another knob for the Release time - I expect that was a fairly common mod for the original Moog Model D.

- see the attachment in my last post. The Release switch is towards the bottom right hand corner - labelled DECAY, for some reason.

With the Release switch in the instant Release position, C143 discharges quickly through R354 (1k5). With it in the timed Release position, that switch is just open, so we have the option of making an adjustable non-instant Release that's quicker than the Decay portion of the envelope (but still influenced by the Decay knob... I think) - or even quicker than "instant Release", to add a click on release (to make Korg fans more at home).

So, we could add a switch to engage the mod, a pot wired as a variable resistor (two connections) to set a faster timed Release (relative to Decay) and a 1k5 series resistor, to set the minimum "instant" Release time, with the mod engaged - or less, for a nasty Release click, but probably best not to go too low, or the capacitor could be damaged... and the pot, and the switch.

It wouldn't require any desoldering or track cutting - it would just need wires tacked onto the Release switch.

The Attack and Decay pots are 1MA (audio taper, aka log pots), so anything up to that value seems appropriate.
Might be worth having a look at this - came across a muffwiggler thread where sonmeone tried this on the original: MUFF WIGGLER :: View topic - Minimoog EG decay/release mod and offset issues

in the schematic in the first post we can also see that pin 3 of attack and decay pots are not really disconnected as seen in the behringer schematic. so i assume at the minute that my previous mod really just drained that cap after attack phase is passed. i don't get the workings of attack to decay transition enough to see an easy remedy for reducing that click spike without having c143 emptied fo sustain.

fun fact on the side - in the muff's thread you can slo see that the oberheim SEM has very similar envelopes.
Old 1 week ago
  #152
Gear Addict
Quote:
Originally Posted by Maffez View Post
Might be worth having a look at this - came across a muffwiggler thread where sonmeone tried this on the original: MUFF WIGGLER :: View topic - Minimoog EG decay/release mod and offset issues

in the schematic in the first post we can also see that pin 3 of attack and decay pots are not really disconnected as seen in the behringer schematic.
I can only see resistors, where the pots should be... am I missing something?

I'm guessing he put them as fixed values because a simulator is mentioned... and I'm not clear if the traces he shows are actual scope traces, or produced by the circuit simulator.

It looks promising though - and he mentions the DECAY (Release) switch produces the offset he was worried about too... whether that's real or simulated, I'm still not clear.

I don't get how the transition works, well enough, either. That's the clever part, really. I've seen envelope designs involving logic chips, which I understand better, but this transistors-only design is more interesting, to me.

Does autoy read this thread? It might be an idea to mention fitting a switch for that envelope click mod, in the other thread.

Which reminds me - I liked your schematic pictures showing that the shark tooth waveform is a passive mix of triangle and saw, from the other thread. That probably ought to be here too... back shortly.
Old 1 week ago
  #153
Gear Addict
.. a link to the post by @Maffez with the pictures showing how the shark tooth waveform option is made:

Behringer Model D - post 8327



- and the related bit, a couple of posts later (post #8330 ):

Quote:
Originally Posted by Maffez (other thread)
and hm, more like passive mixer (the 10k and 47k resistors) in the schematic between tri and saw; so nothing like a waveshaper or so; yet tilting the values more towards tri (say 5k and 52k or so) would give a bit of a rougher tri that is not as saw-y and maybe closer to the og....
- which, with the mod to add PWM on the pulse waveforms (and the fact that all three, including square are variations of the same pulse), means that the waveforms offered by the switch can be reduced to just triangle, saw and pulse with variable pulse width (aka mark/space ratio), like the Doepfer A-111-3.

That simplifies making a waveform mixer because it only needs three wires from each switch, plus the mod to allow PWM. That's a lot of knobs, but even just doing it for one VCO would be good, or just the output sockets could be added, to allow it to be mixed externally, then fed back into the synth via Ext In.

I use a passive mixer, with my Doepfer A-111-3, which is basically three attenuators, with a 10k resistor in series with each output, all wired together to make the mixed output.

There's then another attenuator after that, for total level, but that part isn't essential.
Old 1 week ago
  #154
Gear Maniac
Quote:
Originally Posted by AndyHornBlower View Post
.. a link to the post by @Maffez with the pictures showing how the shark tooth waveform option is made:

Behringer Model D - post 8327



- and the related bit, a couple of posts later (post #8330 ):



- which, with the mod to add PWM on the pulse waveforms (and the fact that all three, including square are variations of the same pulse), means that the waveforms offered by the switch can be reduced to just sine, saw and pulse with variable pulse width (aka mark/space ratio), like the Doepfer A-111-3.

That simplifies making a waveform mixer because it only needs three wires from each switch, plus the mod to allow PWM. That's a lot of knobs, but even just doing it for one VCO would be good, or just the output sockets could be added, to allow it to be mixed externally, then fed back into the synth via Ext In.

I use a passive mixer, with my Doepfer A-111-3, which is basically three attenuators, with a 10k resistor in series with each output, all wired together to make the mixed output.

There's then another attenuator after that, for total level, but that part isn't essential.
thx man!

about the muff thread, yeah, that's a simulator and the pots are in there as fixed values(just right to q6 in bold)

well, the general principle if i'm not mistaken is gates and sorta logic; if charged then open, if discharged, then closed kind of material; variable resistors (potis) set the amount of recharge current reentering the circuit (lesser value, then it takes longer for threshold to be reached and next stage to be entered); roland envelopes and the respective frequency central clones work the same way

think autoy will find his way here since we linked it a couple of times in the main thread
Old 1 week ago
  #155
Here for the gear
 

Quote:
Originally Posted by AndyHornBlower View Post
Yep. It still seems like a useful option to have, anyway.

I hadn't seen that site before. Thanks. I found a categorised index of the schematics, here:

electro-music.com wiki | Schematics / Listbytype

- looks useful.

I now sort of see how the Release On/Off works, and why the Decay pot affects the length of that too... which suggests a fairly simple mod to add another knob for the Release time - I expect that was a fairly common mod for the original Moog Model D.

- see the attachment in my last post. The Release switch is towards the bottom right hand corner - labelled DECAY, for some reason.

With the Release switch in the instant Release position, C143 discharges quickly through R354 (1k5). With it in the timed Release position, that switch is just open, so we have the option of making an adjustable non-instant Release that's quicker than the Decay portion of the envelope (but still influenced by the Decay knob... I think) - or even quicker than "instant Release", to add a click on release (to make Korg fans more at home).

So, we could add a switch to engage the mod, a pot wired as a variable resistor (two connections) to set a faster timed Release (relative to Decay) and a 1k5 series resistor, to set the minimum "instant" Release time, with the mod engaged - or less, for a nasty Release click, but probably best not to go too low, or the capacitor could be damaged... and the pot, and the switch.

It wouldn't require any desoldering or track cutting - it would just need wires tacked onto the Release switch.

The Attack and Decay pots are 1MA (audio taper, aka log pots), so anything up to that value seems appropriate.
A lot like my own experience. I have a Boog but haven't felt the need to modify it (yet). I have my own miniMoog contour generator clone design that I've used in several synths now. It's as exact a clone as I can manage without actually copying track layouts, uses OG design germanium diodes and TIS93s for example. My only deviation is to use a 1M log pot in series with the 1k5 in place of a 'Decay' switch - this does indeed give a very useful pseudo-Release control where R can be shorter than D but not longer. Highly recommended.
Old 1 week ago
  #156
Gear Addict
Quote:
Originally Posted by Maffez View Post
thx man!

..

well, the general principle if i'm not mistaken is gates and sorta logic; if charged then open, if discharged, then closed kind of material;
No problem

Yep - and TTL logic chips were just made out of transistors, of course.

I once designed a cash drawer burglar alarm for someone, with a long one shot timer and a slow LFO that tested the battery (with a pip noise, like in a smoke alarm, if it started to fade). I made it with all transistors, and tantalum bead capacitors, because that was the only way I could think of to get a few months out of a 9V PP3 battery, at the time. These days, it could all be done with a low power microcontroller, I guess.

The Model D envelope is a bit more complicated than that though


Quote:
Originally Posted by Channelizer View Post
My only deviation is to use a 1M log pot in series with the 1k5 in place of a 'Decay' switch - this does indeed give a very useful pseudo-Release control where R can be shorter than D but not longer. Highly recommended.
Cool. Thanks for the confirmation.

I rethought that a few times. I wasn't sure it couldn't make it longer, at first, but it's still discharging through the Decay pot as well - in parallel, so that makes sense.
Old 1 week ago
  #157
Gear Head
 

Is there an overall schematic that's updated to show the differences between the prototype and production units or will I need to roll my own?

(Apologies if I missed it earlier in the thread...I'm perpetually short on sleep--typically 3-4 hours per night--and don't always notice things even if I'm staring straight at them...unless they're pretty girls, those I tend to notice regardless.)

Grey
Old 1 week ago
  #158
Gear Addict
Quote:
Originally Posted by GRollins View Post
Is there an overall schematic that's updated to show the differences between the prototype and production units or will I need to roll my own?
AFAIK, there isn't - I've only seen the prototype one (P0CQJ_Schematic Diagram 01-MAIN RevA_2017-03-09_Rev.0.pdf).

I can empathise about the sleep problems. I've been getting more than that, but to no discernable pattern.

This reminds me of a quote, attributed to George Burns:

"I ran into Jack Benny once and he said, 'I didn't sleep well last night.' So I said, 'How did you sleep the night before, Jack?' He said, 'Great.' I said, 'Then sleep every other night.' "
Old 1 week ago
  #159
Gear Head
 

Circuit tracing...my specialty. Just not my favorite thing to do--tedious as all get out and I tend to want something to drink whilst I'm suffering. It's enough to make an alcoholic out of a man.

I'll drag out a Moog schematic and the prototype Behringer one and the unit and see what I can come up with. It just won't be in the next couple of days as I'm up to my [ahem] in alligators.

Yeah, I get a "normal" night's sleep about once or twice a month. The rest of the time I'm just running on pure, dogged determination.

Grey
Old 1 week ago
  #160
Gear Addict
That's intriguing, about the alligators, Grey. Have you tried counting them, to get to sleep?
Old 1 week ago
  #161
Gear Head
 

Too many bloody teeth, man. Sheep are nice, passive, safe creatures. Alligators? Half the size of a submarine, coupled with a bad attitude. More likely to cause nightmares than pleasant dreams.

I haven't got any immediate plans to hack my D. As detailed in the Model D thread, I had firmware update problems, then the so-called service center turned out to be a joke, then Uli & Co. stepped in and I got a D to replace my missing one. I'm not anxious to mess with my karma at this time, but...I'm making a mental list of things that would be nice to have, like some more CV options. If I run across a used D for a good price I might tinker with the second one, using it as a test bed for ideas.

Plus I've got this nagging idea about something I want to try with the oscillators...might ought to just build a stand-alone oscillator circuit as a guinea pig as that could get invasive and I'm not excited about messing with all those itty-bitty SMD parts. Gimme that old-fashioned, through-hole stuff that you don't need a magnifying glass to see. Worked okay for Bob Moog and I reckon it'll work okay for me.

Grey
Old 1 week ago
  #162
Gear Maniac
Quote:
Originally Posted by GRollins View Post
Plus I've got this nagging idea about something I want to try with the oscillators...might ought to just build a stand-alone oscillator circuit as a guinea pig as that could get invasive and I'm not excited about messing with all those itty-bitty SMD parts. Gimme that old-fashioned, through-hole stuff that you don't need a magnifying glass to see. Worked okay for Bob Moog and I reckon it'll work okay for me.

Grey
yeah, working with smd parts can be tedious on the eyes. i'd started going through the vco secion when working on the paraphonic mod by taking pics in higher resolution and trying to read values off the parts. for some that worked quite nicely but others not so - will attach the work in progress pics below

about the schematics - there is no comprehensive work on the final pcb layout; in terms of structure, there are only a few minor differences between the prototype schematic and the final one, yet the changed part numbering means you have to double check practically everything
Attached Thumbnails
Behringer Model D - DIY Mods-cvsumlabelled.jpg   Behringer Model D - DIY Mods-cvsumlabelled_b.jpg  
Old 1 week ago
  #163
Gear Head
 

How many layers is the board? Two, at least, right? Please tell me it's not three (or more...).

What I typically do in a case like this is start my own schematic, fresh. Having the Moog schematic and the prototype Behringer schematic will be a big help, plus it will be interesting to see what changes Behringer made (other than go SMD) and try to guess why. Now I just need to hunt down a used D to play with. Having just gotten back into business with the one I have now, I'm reluctant to take it back out of the rack for an undetermined period of time while I dissect it. I have a principle I call "deep oscillators" that is integral to the way I arrange sounds and I don't want to be without those three oscillators--I already lost a month due to the firmware fiasco and don't want to lose yet more playing time.

Grey
Old 1 week ago
  #164
Gear Maniac
Quote:
Originally Posted by GRollins View Post
How many layers is the board? Two, at least, right? Please tell me it's not three (or more...).


Grey
It's single layer. most on the front, some on the back. the rotary knobs obscure traces in some places tho
Old 1 week ago
  #165
Gear Maniac
some new sounddemos on paraphony, fm capabilities and internal overdrive modification

was playing around with vcv rack software and mixed tracked vco's with 1v/oct cv per vco, i.e. produced some crude analog 2op fm. the other one is just testing distortion modes

Paraboog FM by Uibkmedan | M M | Free Listening on SoundCloud

Paraboogdrive by Uibkmedan | M M | Free Listening on SoundCloud
Old 1 week ago
  #166
Gear Addict
Quote:
Originally Posted by Maffez View Post
some new sounddemos on paraphony, fm capabilities and internal overdrive modification

was playing around with vcv rack software and mixed tracked vco's with 1v/oct cv per vco, i.e. produced some crude analog 2op fm. the other one is just testing distortion modes

Paraboog FM by Uibkmedan | M M | Free Listening on SoundCloud

Paraboogdrive by Uibkmedan | M M | Free Listening on SoundCloud
Nice. I've never heard a Model D sound like that before, AFAIK

I'd only thought about getting different waveforms from the VCOs. I hadn't even considered getting them to modulate each other.
Old 1 week ago
  #167
Gear Maniac
Quote:
Originally Posted by AndyHornBlower View Post
Nice. I've never heard a Model D sound like that before, AFAIK

I'd only thought about getting different waveforms from the VCOs. I hadn't even considered getting them to modulate each other.
thanks!

It's pretty mazing what you can get out of that little one, once more cv options are added. with stuff like vcv rack (or any hardware eurorack system, actually) you can easily have a modulator vco per oscillator in the boog with quite musical results

poor person's equivalent would be to have the lfo tracked to v/oct and rout it to the entire vco section per modbus, which works super with monphonic sounds - yet ading envelope control over modulator pitch and fm depth is where the fun really begins

maybe more examples soon
Old 1 week ago
  #168
Gear Maniac
so, in the meantime I had a go at the (for me last) holy cow - filter pole modification accomplished! i.e. you can tap the filter at 6db, 12db, 18db and the usual 24db

Behringer Model D Filter Pole Modification - Test Drive by Uibkmedan | M M | Free Listening on SoundCloud

not a beginner's mod but with patience and a steady hand absolutely doable. what's described here is for implementing this within the model d by means of switches, but you can easily adapt this and have all four individual filter pole outputs simultaneously on a board --> with an inverter and mixer you can create bandpass filter types etc. in this case you'd need to build a little opamp circuit like described at this cool modder page here: www.haraldswerk.de Voltage controlled current source
that guy also has a very interesting booster circuit that compensates for level loss at high resonance

for easiest mod:
you need to lift out two caps partially for breaking the internal signal and then tap two signals respectively at the silver caps shown in the pic below.

after the core of the ladder, the array that looks pretty much like a spine, is a little amp section you need in order to have the signal at the right level before it enters the vca; mind the right alignment of tapping point and point of feeding the signal back into the circuit - if wrong, you won't blow up your boog but the sound will be all wrong/distorted until the wiring is right.

the rest should be pretty much visible in the pics I hope

on the first pic I also labelled audio mix in (input side of the filter) and vca in (where output of filter goes into the vca) - you don't need them for this modification but the info might be useful for other purposes
Attached Thumbnails
Behringer Model D - DIY Mods-polemod1.jpg   Behringer Model D - DIY Mods-polemod2.jpg   Behringer Model D - DIY Mods-polemod3.jpg  
Old 1 week ago
  #169
Gear Maniac
for now latest version of the modding guide, including parapony, filter mod, and audio examples
Attached Files
Old 1 week ago
  #170
Here for the gear
 

I have had little bjsy January but the chassis is mkre less ready, I have to screw it together. The answer for earlier question is that I made the drawings and a local company cutted it by laser than bended it on press brake.
It the moment I have to decide wether I go with modification of behringer or building the analog part (I found the Bergfotron modules based on Model D, everithing is modified for voltage control). The lroblem with Behringer is that as I use it longer time it has a litch drift and it seems really unstable. The feedback for bergfotron oscillators is that they are stable after 15mins.
Attached Thumbnails
Behringer Model D - DIY Mods-img_20190213_181653.jpg  
Old 1 week ago
  #171
Gear Addict
Quote:
Originally Posted by Maffez View Post
so, in the meantime I had a go at the (for me last) holy cow - filter pole modification accomplished! i.e. you can tap the filter at 6db, 12db, 18db and the usual 24db

Behringer Model D Filter Pole Modification - Test Drive by Uibkmedan | M M | Free Listening on SoundCloud
Excellent! Well done. I thought that would probably work, but actually taking a soldering iron to it and doing it is a very different thing

I'll read through the guide properly, later. Thanks for the mention about the VCA In suggestions.

A 6dB low pass filter can be surprisingly effective. I've used that output of my Doepfer A-106-6 multimode filter, quite a bit. I mostly prefer using the lower dB options (6dB, 12dB etc).

I made a passive 6dB one for guitar use, a while ago, based on the usual guitar tone control circuit (low pass by bleeding off higher frequencies, with a high pass RC to ground - a pot wired as a variable resistor, in series with a capacitor)... except I made mine with an added rotary switch to select one of six different value capacitors.

I added a switch to that to put a 10k (IIRC) resistor in series with it, which lets it also work with low impedance sources, like the line out or one side of the headphones on a synth. I found that useful with the Bass Station II - more so, before they finally added adjustable key tracking.

I might make a vactrol based voltage controlled version... easy enough, conceptually, to replace the pot with a vactrol, but actually getting a useful response curve could get more complicated.
Old 1 week ago
  #172
Gear Addict
Quote:
Originally Posted by GRollins View Post
Having the Moog schematic and the prototype Behringer schematic will be a big help, plus it will be interesting to see what changes Behringer made (other than go SMD) and try to guess why...

Grey
Someone mentioned the Bergfotron Minimoog VCO (DIY), in the other Boog thread. I hadn't heard of that before. That might be worth a look too:

Minimoog style VCO
Old 1 week ago
  #173
Gear Maniac
the berfotron stuff is a++
Old 6 days ago
  #174
Gear Head
 

Quote:
Originally Posted by AndyHornBlower View Post
Someone mentioned the Bergfotron Minimoog VCO (DIY), in the other Boog thread. I hadn't heard of that before. That might be worth a look too:

Minimoog style VCO
Thanks, dude...I've got that loaded up and will look at it after I have some supper.

Grey
Old 6 days ago
  #175
Gear Maniac
Quote:
Originally Posted by macsaif View Post
The lroblem with Behringer is that as I use it longer time it has a litch drift and it seems really unstable. The feedback for bergfotron oscillators is that they are stable after 15mins.
awesome case!

about that pitch stability; bergfotron is based on later osc board models of the mini and the behringer on the very first (transistor only), which was said to drift a bit. in my unit not so much of a problem, but if you want to go full vc control, then diy from the start might be the way to go

his/her supersyncussion seems also an awesome build
Old 6 days ago
  #176
Here for the gear
 

I have had a problem with the drifting during my lead solo. I have started the solo, everithing was right than during the solo it went out of tune. I have tried to correct it by the tune button, but it should not be the normal way. This was the breaking point where I started to think about a design based on the more stable oscillators. Anyway I want to tweek them a little bit and want to include the pole switch and the HP switch in the Bergfotron VCF. The HF combined with the Feedback produces very strange voices and I would like to include them in the new synth.
Other nice option should be to add a filter overdrive similar to used in Little Phatty. I like it much more than the miltidrive in Sub37 (I have owned it and sold after receiving of the Behringer).
Old 6 days ago
  #177
Gear Head
 

My Little Phatty's auto-tune works pretty well. Stabilizes quickly and is rock solid thereafter. I used to be afraid of it, but I let the LP calibrate itself and that made a big difference.

Grey
Old 5 days ago
  #178
Here for the gear
 

My comment about LP was concerning the VCF overdrive wether is jt possible to achieve with some modification of Behringer D. I think it is assymeteical distirtion.
Old 5 days ago
  #179
Gear Addict
Quote:
Originally Posted by macsaif View Post
My comment about LP was concerning the VCF overdrive wether is jt possible to achieve with some modification of Behringer D. I think it is assymeteical distirtion.
It's certainly possible.

The usual way to do clipping, either for an overdrive or a distortion in guitar pedals, is to use back to back diodes - e.g. one or more pairs of diodes, with the anode of one connected to the cathode of the other, so it conducts both ways, but only above the forward voltage threshold to start each diode conducting, multiplied by the number of diodes - e.g. about 0.6V per silicon signal diode.

In a distortion pedal, they're often between two amp stages to ground, usually with a resistor in series with the chain of diodes, so it's not a low impedance path to ground when they're conducting.

In overdrive pedals, they tend to be in the feedback path of an op amp, or other amp stage, but the distinction isn't all that clear cut. Overdrive tends to mean soft clipping, rather than less (though that's also sort of implied). The resistor + diode pairs to ground still allows for that.

For asymmetrical clipping, just use more diodes going one way than the other - and you only really need to connect the ends of two chains of diodes, e.g.

-|>-|>-|>-

in parallel with

-<|-<|-

- gives asymmetrical clipping.

For a synth, we'd normally want the clipping to happen at higher signal levels than in a guitar pedal (i.e. let a higher signal level through unmolested, before it clips), in which case, just add more diodes in each direction. For more asymmetry, use more one way than the other, not just one more.

A likely place to do it is the input of the VCA, with a switch to disable it, of course.

VCA In -> Switch-> fixed resistor -> variable resistor -> clipping diodes -> Ground

- the fixed resistor could be a trim pot, but it might be best not to allow for that input to be grounded... I'd expect it to be okay, but the design assumed it wouldn't be. So, if you do want to add a trimpot, to set the starting point of the main knob, still add the fixed resistor.

Experiment to find the values - run two wires to a small solderless breadboard; one for VCA In, and one to ground. Start with bigger resistors, and work your way down. I'd suggest starting with at least 10k, as the fixed resistor - which sets the maximum current to ground, when you turn the clipping knob for the variable resistor down to the minimum.

By "variable resistor", I mean connect just the centre lug ("wiper") and one end lug of a pot. You might prefer an audio taper pot, like for a volume knob - in which case you have to choose which end lug / pin, to use - wire up all three to short lengths of solid core wire, so you can push them into a breadboard, then decide later.

A bigger resistance in series with the clipping diodes will give softer clipping, as well as less clipping... The same applies to adding more diodes to the chains. Favouring more resistance, rather than more diodes, takes up a bit less space, and may let you set a gentler curve where it clips - more like a tube aka valve amp. On the other hand, using more diodes guarantees a higher clean signal level, before any clipping can happen - so experiment to find which combination you like best.
Old 4 days ago
  #180
Gear Addict
Quote:
Originally Posted by macsaif View Post
wether is jt possible to achieve with some modification of Behringer D. I think it is assymeteical distirtion.
... if you want the overdrive to be before the filter, you wouldn't put it at the VCA In, of course.

You may need three wires, to put it somewhere else, and you'd want to put this in series between two audio stages, ideally, but in the case of VCA In, there's already a resistor feeding signals to that point, so you can just tack it on, with the two wires.

You could tag this sort of circuit onto any point in the signal path, so long as you don't load it too much - the fixed resistor avoids that ever happening, and the switch disengages it.

Have a look at the attached schematic, for a classic simple "overdrive" - or light distortion, depending on your point of view. This one has asymmetrical distortion by having two diodes facing one way, in parallel with one diode facing the other way.

You can ignore the top section, which is about power smoothing and providing a bias voltage. On the left is an op amp, followed by a capacitor, but we don't need that part, only the bit to the right of that capacitor (C4).

You see a fixed resistor, R6 (10K), then the diodes, connected to ground. The clipping is done by the diodes, but the current through them is limited by R6. In this case, the voltage across the diodes is clipped at roughly +0.6V and -1.2V.

The output is then after R6 - so it has an in, an out, and the bit in between, which does the clipping.

If you want to be able to just have two wires, tacking this on to the circuit, so you don't have to desolder anything from the PCB, we can look for a point where there's already a fixed resistor feeding the signal to that point (like at VCA In) then tack this on after that resistor, at the input of the next stage - but still add a fixed resistor in series with the diodes, to make sure nothing gets damaged.

The output of our clipping circuit is then taken from the same point at its input, but that will still work.

In our case, we would probably want a few more diodes in each direction, but more facing one way than facing the other, to make the clipping asymmetrical, as here.

You could also add a switch or two to bypass one or more of the diodes, facing one way - e.g. to give the option of symmetrical clipping, or a different amount of asymmetry.

The circuit in the schematic also has a small capacitor from the input of the clipping stage to ground (C5). We don't need that part, but you could try adding a small value capacitor to see what happens, maybe with a switch to activate it. What happens is low pass filtering, with the amount depending on the value of that capacitor. If you put a pot in series with it, you can choose how much - like a filter cut off knob, but a lot less severe than the 24dB Moog one - more like a treble knob on a HiFi, or the tone knob on a guitar. Effectively, it would make a 6dB low pass filter.
Attached Thumbnails
Behringer Model D - DIY Mods-dod250-schem.jpg  
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