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Korg Monotribe modifications thread
MUFF WIGGLER Forum Index -> General Gear Goto page Previous  1, 2, 3 ... 36, 37, 38, 39  Next [all]
Author Korg Monotribe modifications thread
Starspawn
Hmmm, is a decay stage after the gated mode possible?
When using it as an extra osc for other things thats the main issue.
Would a diode and a pot do the job perhaps?
zaphod betamax
[img][/img]super simple attenuator to tame the 8v cv hz/v from ms-20
to allow you to run monitribe in hz/v mode after adjustment

this should also work to allow you to control the Yamaha CS-x
family of synths. However, I do not have one to test.
Scott Willingham
Starspawn wrote:
Hmmm, is a decay stage after the gated mode possible?
When using it as an extra osc for other things thats the main issue.
Would a diode and a pot do the job perhaps?


Sorry, it's not very feasible. As you speculate, it would probably be possible to add a decay to the analog CV that controls the VCA. The problem is that once the microcontroller in the the MT has finished the generation of its envelope, it will assume the signal path is muted and auto-run its VCO tuning routines.
rotallicso
Quick query about the monotribe and MIDI i/o (& thru)

it's evident that there is no practical need for the pair of hex inverters on the MIDI org DIN spec guidelines for a monotribe
https://www.midi.org/specifications/item/midi-din-electrical-specifica tion

Now, I want to add small 2.5mm TRS jacks for In Out *&* Thru

Since 2014 there's a new guideline which caters for interference and I'm wondering, well, why not do it right, what's the harm
Image copy here http://hinton-instruments.co.uk/images/reference/promidi/midihw2.gif

This new spec shows a single non inverting buffer, now I'm a noob here, I believe this is essential on the Thru side
Assuming so, then why don't I just hook it up on the Out too ?

Why are other designs not needing the hex inverters on the Out, I'm asking for curiosity and to learn a bit

I'm also wondering if the lessons here will apply to some Arduino circuits
If I'm skipping the thru on those, should I forget the 7417 (or 7407 which I presume is compatible/same) ?

I have the 6n137 optos for my projects, one thing that's written up online somewhat confusingly is the value of the pull-up resistor after that opto (assuming that's the right lingo, why?) -if anyone can guide me in layman's terms to understand the best way to understand selection and to dismiss some of the schematics doing the rounds that'd be nice too

edit:not much footfall here, gonna ask this on the DIY sub forum as it's actually just a general query ...
.
see here >
https://www.muffwiggler.com/forum/viewtopic.php?t=160437
systmcrsh
finally got around to installing darenager's vco mod... fun stuff :))



got a buddy to drill the holes which came out pretty awesome... my soldering was a little crap, but it sounds good :)[/img]
Kennymester
Not really a mod per-say but I'm thoroughly impressed by the quality overlay that I got from styleflip.com. Makes it really easy to use the version 2 update.

Pase

+1 for Styleflip overlay, not cheap but useful and look good thumbs up
Majonymus
did some bending repairs today, found a hat to clap easy mod, first hat decay to sd noise decay, cause my hats were not sounding at all then a switch for claps



and midi in for mine seems to listen a little, but still not working fine u_u
rotallicso
There's a pointer earlier in the thread to the fuse type on the battery board being 0603, that may help experienced folks, but here's a screen grab of all the necessary details if you've blown one, like moi ..
TF16AT1.25 M = anti-surge 1.25A 32V
.
very easy to bridge the smd at more convenient spots

[/img]
rotallicso
If anyone's looking to source nice sockets to fit the gap between the output board and the voice board then these fit just neat if you get your sums right. They're 2.5mm TRS as i had some spare MIDI to 2.5mm TRS cables from a Line6 MIDI Mobilizer and there'll be no accidental cabling mishaps either. Pro Signal PSG08264.
.

.
One other contribution fwiw would be that sitting your optocoupler directly on the board would help space-wise (as opposed to sitting in a dip socket) i'll do this next time
.
Works perfectly, but what case scenarios have people observed that may push the midi i/o to fall over ? - i went with the 137 opto and 140R on the output side to account for the 3.3v VCC
420noscope
Nice stuff!
jimmie
Does anyone have a broken siwtch?

My monotribe's switch 4 (seq #4) had started having bad contract and it's finally dead. I opened the case try to fix it - hoping brewing dust or touch of contact cleaner would fix it - but it didn't seem to work. I've never seen that round switch and doesn't look like I can open it to clean the inside.. looking very fragile.

If somebody has some advice to fix it I'd appreciate.
marmad
Hey everyone!!

Sorry for my long absence... the last 2 years became engulfed by a long-overdue renovation of my 2000 sq.ft. (200 m²) studio and the death of my father last year.

So my Monotribe modification upgrades were shelved during that time (even though the design of the circuitry was finished and tested), but I'll be able to get back to it a little later this month - and the boards will finally be sent off to be produced.

In the meantime, I thought I'd start posting the schematics for it as I find the time to touch them up a bit and scan them, which might allow others to incorporate any pieces of it into their own designs they're interested in.

To begin with, here is the circuitry to generate CV (1V/Oct) and GATE OUTPUTS from the Monotribe, as well as the circuit to feed the EG into the Rhythm section, and the Click/Pop Suppression circuitry mentioned in my previous YouTube video.



Feel free to ask questions, modify, or condense as desired. For example, the circuit as shown uses all 4 sections of a TLC274 quad op-amp (with one op-amp being used to generate an alternate source for the VCA from my VCO2). But if you just wanted to generate the CV OUT, you could use a TLC272 dual op-amp instead. and eliminate the use of any switches or externally-mounted pots.

You'll need a decent voltmeter or DSO to adjust the 1V/Oct output - first setting the CV Offset trim (floor) with the lowest note, and then adjusting the 1V/Oct output using the highest octave multiple of that note.

(P.S. this is one sheet of a set of schematics - so some of my package choices or in/outs to the sheet are related to other sheets in the set).
Scott Willingham
Good to see you back, Marmad, and to see this thread bumped again!

The Monotribe is a special box. I love the Volcas and the Minilogue looks cool, but the tech in them is too tight to reverse-engineer and mod. The Monotribe is just right: portable, playable, yet enough room to work with and accessible circuits. Plus, the groove-box interface just works for me. I'm not generally attracted to sequencers, but the MT invites me to dive right in.
williambeucler
Not sure if this is the place to post my issue, but I've had the Monotribe a few years now, and I love it. I bought the Miditribe and installed it with no issues. All of sudden a month ago, the thing started clicking non-stop, and hasn't quit. I've pushed and pulled all the buttons, knobs, switches, and it just won't go away. I was thinking that maybe if I did the declick/capacitor solder modification, it could get rid of it. I've read that it's a common issue, but I just can't seem to get rid of it. I even took the batteries out for a day to see if it would reset itself, but no luck. The drum sounds seem okay, but once I add in the synth, there is that clicking again. If I add in the noise, the noise has no effect. If I modify the gates to full, and turn the VCA level to full, I can eliminate the clicks and hear some of the synth, but it's faint. Thanks.
marmad
williambeucler wrote:
Not sure if this is the place to post my issue, but I've had the Monotribe a few years now, and I love it. I bought the Miditribe and installed it with no issues. All of sudden a month ago, the thing started clicking non-stop, and hasn't quit. I've pushed and pulled all the buttons, knobs, switches, and it just won't go away. I was thinking that maybe if I did the declick/capacitor solder modification, it could get rid of it. I've read that it's a common issue, but I just can't seem to get rid of it. I even took the batteries out for a day to see if it would reset itself, but no luck. The drum sounds seem okay, but once I add in the synth, there is that clicking again. If I add in the noise, the noise has no effect. If I modify the gates to full, and turn the VCA level to full, I can eliminate the clicks and hear some of the synth, but it's faint. Thanks.

It sounds as if your trimpot adjustment (VR9) of the bias offset current to the VCA (the source of the normal clicks and pops that happen on the Monotribe) might have gotten out of whack, so that's the first thing you should check.

If you haven't watched my YouTube video on the subject, that might be a good place to start. As I mention in the video, I suggest replacing the stock Korg trimpot with a 10-turn trimpot for a more precise adjustment, but for your purposes, you can at least check if the bias offset is causing your problems without doing any desoldering/soldering. You'll need a decent voltmeter of some sort that has reasonable precision.

You want to check the adjustment of the bias current at the zero level of the Envelope generator, so make sure that no EG is happening when you do it. First read the voltage at resistor R95 (the side connected to transistor Q17), then, while checking the voltage at resistor R91 (also the side connected to Q17), adjust trimpot VR9 until the voltage is as close as possible to the one at R95.

If you are still having clicks happen constantly after this adjustment, then your problem is originating elsewhere. But the other modification mentioned in the video (the cap) will likely not solve your problem (or at least, it won't remove the source of it), since the stock Monotribe does not normally have a "non-stop" clicking problem; it only happens under certain conditions.

marmad
Scott Willingham wrote:
Good to see you back, Marmad, and to see this thread bumped again!

Thanks, Scott.

I've just been reading the previous 4 pages of posts that were made since my last visit here, and I had two responses to the posts you made:

I think you might have missed the series of posts I made on page 31 describing why the drum mods proposed by various posters earlier were not very good - or, in the case of BD "DECAY" & SD "DECAY" (actual gain adjust points of their respective circuits, and thus rather mislabeled by Korg), electronically "incorrect". For example, there's no reason for ON/OFF switches for the different drum outputs; simple add-on modifications to the associated pads provided by Korg can produce an OFF setting for the drum outputs automatically at one end of their respective POT settings (i.e. instant decay or zero gain).

Also, I noticed that you seem to prefer cutting traces on the Monotribe board as opposed to drilling holes in the case. That's interesting, because for me, the challenge is the exact opposite: I prefer to add mods that don't alter the original circuit in any way, but instead just insert extra circuitry for added features (usually with the added burden of drilling holes for pots/switches). I guess those are the two "opposing" directions of modding (in general).

With that in mind, I thought I'd re-post the mod I first mentioned back on page 31, but with the additional circuitry added that's included in the schematic I just posted above.

This mod feeds the EG into the unused rhythm section amp input, with each EG setting (saw, square, etc.) generating different accents on the rhythm (some of them producing very nice results). It requires:

1x 3.3 nF ceramic cap
1x 100k resistor
1x 100k log pot (for intensity)

At one end of the intensity pot adjust, the effect will be zero; i.e. stock Monotribe.

marmad
Here's a schematic for adding a non-destructive, completely separate, Pulse Width Modulated source to the Monotribe. In other words, this circuit doesn't modulate any signal on the Monotribe itself, but rather generates it's own square wave (synced to the VCO) that is pulse-width modulated by the envelope generator, and then fed to the summing amp.

To use it, you must tap off 3 points from the Monotribe (IC13 Pin 10, IC14 Pin 2, and IC17 Pin 8), as well as 3 power-supply points (+5V, Vbias, and Ground).

It uses 1 dual op-amp IC, 9 fixed resistors, and 3 pots (although you can replace any pot with fixed resistors if you don't feel you need the corresponding adjustment):

1x LM358 IC
1x 330k resistor
1x 270k resistor
1x 220k resistor
2x 100k resistor
1x 68k resistor
1x 47k resistor
1x 15k resistor
1x 4.7k resistor
1x 1M linear pot (initial pulse width adjust)
1x 100k linear pot (intensity adjust - one end will be zero modulation)
1x 10k logarithmic pot (volume adjust - one end will be zero volume; i.e. stock Monotribe)

BTW, this circuit, as well as many others (as mentioned back in my post on page 31 of this thread), will be included on boards which I'm currently (finally!) producing - although I've added circuitry for additional choices for the pulse-width output and modulation input.

Pase
Great marmad!
If you'll ever start moddin' other people's units I will buy my third monotribe and I'll send it to you.
Keep up good job! we're not worthy thumbs up
CeeJay
Thank you marmad for posting your mods! I will try them one day.
Scott Willingham
Love the new mods, Marmad. The accent mod is clever and looks like fun. Great bang for the buck! The PWM mod looks very well designed too. I think I'd want to have a cross-fader between the normal waveforms and the new pulse. Not sure I'm up for adding too many potentiometers sprouting out from my MT, however.

I look forward to seeing your PCB. Very tempting.
Scott Willingham
marmad wrote:
I had two responses to the posts you made:

I think you might have missed the series of posts I made on page 31 describing why the drum mods proposed by various posters earlier were not very good - or, in the case of BD "DECAY" & SD "DECAY" (actual gain adjust points of their respective circuits, and thus rather mislabeled by Korg), electronically "incorrect".


I did see those posts last year, but failed to comment on them at the time. I don't think extensive drum mods appeal to me much. I mostly feel like analog drum circuits have a fairly small sweet spot and I don't care much for long booming kicks or toms. In the meantime, I installed Daren's drum mod (very clean kit) and it hits a good compromise for me of variation and simple operation.

Some motion sequencing of the drum tones would be interesting... screaming goo yo

Regarding your drum circuits, I agree with the way you handled the decay mods for the hat and snare noise. Seems straightforward and clean. I'm not sure if I agree with your criticism of the common kick/snare-frame decay mods. My (mis-?)understanding of the tone generators is that they are simple resonant circuits and the amplifier gain not only changes the volume a bit, it alters the damping factor of the oscillation. So I think Korg's "decay" points are correctly used by effectively altering the emitter degeneration resistance. Unfortunately, while that works, it is a very tweaky adjustment.

On the other hand, your "gain mod" may also alter the resonance in a good way. I'll have to think about it more. confused

Edit: Ok, I see what you are doing. On one side of the pot's center setting, you are effectively doing the standard mod with a wrinkle: adding an AC coupling capacitor so you adjust the midband gain of the amp without disturbing its bias point. Good idea. On the other side of center, you are essentially just fading out the gain of the amp by loading down the collector side. At the extreme, this also mutes the path.
marmad
Quote:
I'm not sure if I agree with your criticism of the common kick/snare-frame decay mods. My (mis-?)understanding of the tone generators is that they are simple resonant circuits and the amplifier gain not only changes the volume a bit, it alters the damping factor of the oscillation. So I think Korg's "decay" points are correctly used by effectively altering the emitter degeneration resistance.


Q11 and Q15 (the NPN transistors for SD frame and BD) are simply unbypassed common-emitter amplifiers. Since the gain of the stage is approximately Rᴄ/Rᴇ (R56/R64 and R69/R78 respectively), varying Rᴇ (the common "decay" mod) varies the gain - but it also varies the DC collector current, thus changing the frequency response of the amp (which may lead to distortion).

Obviously, one can change the gain of a CE amp this way, but not only is it normally considered poor practice (for the reasons mentioned above), but just adding resistance in parallel with the respective Rᴇ of each transistor means you automatically alter the stock Korg output sound.

For example, the normal Rᴇ of the BD amp is 470Ω; a common "BD DECAY" mod (top waveform in the following DSO image) simply makes Rᴇ adjustable between 183 - 395Ω, thus never being able to achieve the original frequency response of the stock BD OUT (bottom waveform):



My alternate mod is just a well-known, improved method of gain adjustment for CE amps. Varying the pot bypasses more or less of the AC signal, increasing the gain, while also potentially reducing the volume at the following stage. The overall frequency response will remain essentially unchanged at various gain settings, with the middle position of the pot (top waveform) almost precisely equal to the stock Korg output (bottom waveform - BD OUT):

Scott Willingham
marmad wrote:
My alternate mod is just a well-known, improved method of gain adjustment for CE amps. Varying the pot applies more or less of the signal present at the emitter to the following stage. The overall frequency response will remain essentially unchanged at various gain settings, with the middle position of the pot (top waveform) almost precisely equal to the stock Korg output


I don't think your description of the circuit really matches the operation. The way you have connected the potentiometer is with the wiper grounded. This does not couple the emitter signal to the following stage. In effect you have two separate circuits: 1) shunting the emitter through a series R-C to ground. This adjusts the midband gain of the CE stage, yielding longer decays than the stock circuit. 2) Adding a parallel load to the collector (again AC-coupled). This reduces the amp's gain at the other end of the knob range, which shortens the decay from stock.

Your results demonstrate that this is an effective circuit and though I haven't tried it, I agree it is superior to the alternatives that you criticize. But IMO, your description of the circuit's operation is misleading.
marmad
Quote:
I don't think your description of the circuit really matches the operation.

True, I wrote the post quickly and my mind was elsewhere when writing. The description should read (now fixed): "Varying the pot bypasses more or less of the AC signal, increasing the gain, while also potentially reducing the volume at the following stage."

OTOH, your description of the circuit is also a bit misleading and not really accurate:

Quote:
1) shunting the emitter through a series R-C to ground. This adjusts the midband gain of the CE stage, yielding longer decays than the stock circuit.

Bypassing the CE amp alters more than just the midband gain, and this is not a specific adjustment to the "decay"; it's a gain adjustment of the entire envelope of the sound (see my previous statement about Korg's rather misleading labeling of those points).

Quote:
2) Adding a parallel load to the collector (again AC-coupled). This reduces the amp's gain at the other end of the knob range, which shortens the decay from stock.

No, the other end of the pot doesn't affect the amp's actual gain settings at all, and again, it is not specifically related to "decay" - it reduces the AC output volume of the entire envelope before the next stage.

So, one end of the pot bypasses all of the emitter resistor, leading to maximum AC gain, while having little affect on the volume before the next stage. The middle of the pot is equivalent to no bypassing of the emitter resistor, with just a small effect on the volume (i.e. almost stock Korg gain/volume). The other end of the pot just reduces the AC volume to zero before the next stage, with little affect on the actual gain setting of the CE amp.

This is not, to be sure, the most elegant way to do this - but it was designed with three strict rules in mind:

1) It should be non-destructive. In other words, there should be one position of the control which produces the stock Korg output.

2) It should provide zero volume at one end of the control. In other words, no additional switches, etc, should be needed to mute the sound.

3) It should only use the existing tap points (e.g. BD DECAY, BD OUT, etc) that Korg provides. In other words, any user, even those with little electronics knowledge, should be able to easily implement it.
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