Thursday, December 2, 2010

Modified Gakken SX-150

Here's the result of several hacks and mods for the Gakken SX-150:
Top View

Power, CV in, Gate in and envelope trigger switch

External LFO input jack, internal/external LFO switch, high pass filter kill switch, and pitch envelope kill switch

Guts,... rough job, ey?!?!

The LFO input and outputs, along with pitch envelope disable switch, CV in, and resonance feedback knob were found here:

This is one of the best blogs I've seen.

The LFO to filter switch and envelope trigger switch were found here:

I later chaged the LFO to filter switch tap points in order to install a separate LFO-VCF depth knob, since I wanted it to be independent of the LFO-VCO depth (see below). Instead of tapping right to the filter knob I went to the north side of R33. I kept the switch as a LFO-VCF disable switch. This works very well. If you want a more squibbly and unstable sound, you can go off of the south side of this resistor (or north of R32 - same thing).

The envelope trigger switch is an (on)-off-on type, as I found some wierd sounds with trigger held on. Well, what really happened was I soldered for too long with an (on)-off switch and melted something inside! Then I found it useful to install said type.

The Square wave with Pulse Width Modulation, LFO saw-up and saw-down wave shapes and selector switch, LFO depth (to VCO, or rather pitch), and decay kill switch were from this dude:

This schematic is pretty awesome. I did try the temperature compensation, but couldn't get it to work. And I tried the filter on the carbon panel section, and that also did not work. I think I was using the wrong Schottkey diodes (over-rated). Because of these trials, I had to replace a few of the resistors (as they're SMD), so that's why there's a few extra wires inside...

Anyway, I added a PWM master switch, as this circuit still affected the original sawtooth wave - which is cool, but the original wave is so fat that I wanted the option. The kill switch is DPDT, in order to kill the 5V feed and 2.5V reference connections, thus killing the LED too. I'd recommend this mod to anyone seeking a cheap and very fat square wave/PWM sound. This really does "smash it proper!"
The LFO depth pot was used to control all LFO effects - to VCF and VCO - and it also controls the input LFO (total fluke), which is nice. I later added a separate LFO-VCF knob (see above) which also controls the input LFO to VCF (again, a total fluke!). The decay kill is nice, but the synth will hold the pitch at the position determined by the pitch env depth - again this is pretty cool - and if you disable the pitch envelope, it just holds the filter envelope open.

Though my favourite mod from Hoshu is the square wave/PWM, the Saw Up/Saw Down LFO wave shapes is a close second. This is very easy to do, and only requires a few parts. You could even let them float, although I did install them on a tiny piece of PCB. What makes these really cool is actually a consequence of them being a bit rough. I'll explain: These wave shapes actually oscillate at a highr rate (faster) then the origunal waves, so you can actually tune in some serious ring modulation/FM effects. Furthermore, these shapes also get sent to the PWM section, so you can imagine the possibilities...
I will probably put a bigger knob in for the LFO rate, as I heard that you can really slow the rate down (one meg VR, apparantly), which will be nice for the extra wavesahpes, as they have higher rates, right?! I'll try this and post results.

The VCF tracking switch is nice too, can't remember where I got this??!! Anyway, I added a knob to adjust this, so you can tune it in a bit better. I just installed a 100k pot in parallel with a large resistor (and through a kill switch) between R31 west and R5 south. I also put a resistor in line so that a minimum VCF tracking cannot overcome a threshold - I did not want to get a dead short for obvious reasons. This actually ended up being really good for filter tweaks, since the original filter knob is a bit cheesy.

The high pass filter was an accidental outcome from mucking around, touching a resistor to different points - fun!  I installed a 10k pot to ground and the east side of R40. I could only get this to work with a logarithmic (audio tapered) pot, as the signal gets a bit 'quantized' (steppy) as you reduce this. Also, I had to install it reverse operation; that is, it works as a high pass filter counterclockwise, instead of clockwise - a bit counter intuitive. I added a kill switch here also, as there was a tiny bit of leakage with 10k pot. A bigger pot would do too. This is by no means a real HPF, but it does the trick - especially for tuning in some sounds with PWM on either the square wave or the saw wave. Not so great for real-time tweaks, especially since it doesn't have its "own" resonance feedback control (suggestions?).

This whole process started with DinSync's blog (above link), where he posted a great vid displaying how fat this machine can sound. I will be adding some sounds of the Gakken in the near future.

January 25th 2011: Pic's for Anonymous hacker:

LFO to Filter attached to North side of R33
(See pointer - East of R39)

LFO Mod - See two diodes on mini PCB
Hope this helps, Mick


  1. Hi. Great blog. I also love the Gakken. I tried to make some mods to my Gakken also recently. The LFO depht mod is working great. But the LFO to FILTER seems not to work. I am not sure about the right LFO to FILTER connection. I tried to connect the right pin of R16 trough 150K ohm resistor to the left pin of R31 (according to Mod7 pic) and also tryed to connect the right pin of R16 directly to the right pin of CutOff pot. In both cases I only get lowered freq OSC sound, but can not hear any LFO influence - any CutOff changes. Whats the right, working LFO to Filter schematics, can you give me some advice please? Thanks very much for any info:-)
    I probably also have some mistake in the LFO sawtooth wave to ramp UP and DOWN mod. I can not hear any difference when I switch LFO wave from sawtooth to rampUP or rampDown. I did the mod only trough 22K resistor and 2 diodes, some idea please, what can be wrong? Should there be some audiable difference between the sawtooth and rampUP/DOWN LFO waves (I hope so)?
    Thanks a lot again.

  2. @Anonymous
    Hello - glad you like the blog.
    Even happier you like the Gakken!
    Sounds like one of your diodes might be the wrong way round in your LFO mod - which would explain why they don't sound different. They should sound very different at low LFO rates, but as you speed up the LFO the difference becomes quite subtle, but still different. If you check out the squre wave "basic-basic" demo, I show the difference between these waveforms - a comment will pop up on the sound cloud player declaring the modifications being tweaked. Check it out to see how it sounds:
    And double check your solder points with this:
    As for the LFO->Filter, you could always go straight to the filter knob - which is what I did originally. See here:
    Theres a good pic to show you. I found that this affected the pitch a little bit, but only after I put a second depth knob in for the LFO to filter, and that's why I did it differently.
    I'm not home right now, but I'll post another comment after I double check what I did for the LFO to filter. Good luck, Mick

  3. Hi Mick! Thanks very much for the quick reply! ;-)
    a) LFO to filter problem. I know both the pictures - links you post. Maybe the main problem is, that I made a mixture of this 2 mod schematics :-)
    I did the LFO DEPTH according the MOD7 pic. It is working great. Next I wanted to do LFO sawtooth wave mod and LFO DEPTH mod (all according MOD7 pic).
    So I first tried the LFO2FILTER connection according to SX150_Mod7.PNG.
    I have broken the connection between R16-R17 and between R17-R18. Then the LFO2FILTER connection should go from switch trough the added 150K ohm to the link between R31 and R32. But maybe now I see the problem - there is a link to the added 330K resistor I dont have - from CONTROLER IC1A (because I didnt want to make CONTROLER mod - only LFO ones). Maybe this can cause the problem?
    I am looking for the most simple LFO2FILTER mod. Maybe the connection is simplier. I watched this pic already before, but there is no comment, there are just 2 yellow cables - one from right leg of CUTOFF pot and one from the right side of R16. Are this 2 points just shorted by switch in the front?
    I also tried this, but the result was the same. Maybe this is because the broken link between R16, R17 and R18 (because of already done MOD7 LFOwave and LFOdepth)? Maybe.
    b) LFO wave mod
    Of course I was trying to hear difference at low freqencies - but as I wrote did not notices difference in sawtooth.
    How should be the diodes oriented? I just put one strait and one "reversed". What can be wrong on their orientation? Maybe there is some missing connect. Hm. Will check. Because the output of the diodes comes together and then is connected to the right of VR4 and next only to LFO DEPTH pot.
    Will check with multimeter if its connected to IC1C- but it should be.

    Well, all for now. Thanks very much again. Greetings

  4. @Anonymous
    Matt-the-modulator's mod is exactly that - a short is created by a switch, so his design does not allow for depth control. His resonance is also just a switch, so again no depth control. Matt is very helpful, and as a side note, I just bought a Dub Step Arcade board from him - not yet built, but soon.
    See here:
    Anyway, I'm going to add a couple of pic's to the post for you - showing the LFO to Filter connection (North side of R33). I did play around, and even if you went to the north side of R32 it'd work, a bit funny, but it does work - I was going to add a switch to flip to this connection as it did sound cool, but ran out of room to add anything else!!
    I also added a pic of the LFO mod circuit, which is a bit pointless since you can't see how its wired?! Hope this helps. Good luck, and have fun with it.

  5. Hi Mick!
    Thanks again for the help!
    Here I uploaded the picture of the schematics how I did it:
    Please, if you will have some time, check the above link I posted.
    Maybe now I see the problem in LFO2FILTER - I connected it from right pin of R16 to the CUTOFF pot wired with right pin of R34(because it seemed connected so on the Matts picture). Maybe this is the problem. I will try as you said connect if to north pin of R33. I will see.
    BTW. I am doing quite complex (visual :-) ) mod of the Gakken. I unsoldered all the POTS and switches and put new ones (brand new metal pot cups and switches) on wires. The main reason is, that I am going to put it in completely new good looking panel and wooden box and make it look like a "big" oldschool analog synth :-)Also I have more place for modifications.
    Best regards

  6. And also, please, check out the LFO WAVE connection in the scheme link I post above. IMHO is maybe some wire missing... I have the diodes also on the separate PCB like you.

  7. This comment has been removed by the author.

  8. Hello Anonymous.
    I connected the output from LFO mod-circuit directly to the west pin of LFO rate knob, and then cut the trace at R18 as you did. However, I did not do the destination switch, as I put depth knobs in for both LFO to pitch and LFO to filter - the way Hoshu did it allows only one or the other to be modulated, whereas my mod can mix LFO levels to both destinations. I'd try without that destination switch first, get the LFO working how it should (for you) and then add switch after (you could temporarily short out the switch). This way you'll systematically find the error.
    It looks like your doing awesome. Can't wait to see your upgraded enclosure (the Gakken enclosure is a bit tight!).
    Happy Hacking, Mick

  9. Hi Mick! Thanks for the answer!
    So you just connected the output of the 2 diodes (from the LFO wave mod) with one wire to the LFO (pich) Depth POT and the second wire trought the LFO2FILTER POT to the north of R33, right?
    The LFO2FILTER connection was just before the LFO DEPTH POT, right?
    Thanks very much. I am really enjoying the mods :-) Will let you know.

  10. Hi Mick!
    Well, I tryed to remove all the mods and connected only the wire from right pin of R16 to the north pin of R33. Now I can hear something. But its a bit other than I expected :-) the LFO is too fast for the typical analog filter sweeeps :-), is it is making just a very fast LFO "helicopter" like FX. And it is only auditible at certain (lower) CUTOFF POT levels - with the CUTOFF freq at MAX I can not hear the LFO. Is it OK?

    I also find out, the VCO frequency (pitch) extremely LOW in this (LFO2FILTER) case :-(. When I use LFO2PITCH, the pitch of VCO is several times higher. Should not be there some connection to the CONTROLLER block to rise the pitch in this case? Is it the same in your case?

    BK :-)

  11. Hey there BK.
    If you started again, but already cut the trace at R16, then you'll need to put this back. Then you should hear effects of LFO.
    The pitch will sway a little with the LFO depth as it is a cheap synth with a cheap LFO - which I like alot!!! But if it goes drastically out of tune, then something else is wrong.
    As for the diodes, from there you go straight to the LFO RATE knob OR the other side of R18 which is other side to the trace you cut. These connections will be the same.
    I will get back to you with some details tonight as I have to take my Gakken apart anyway (my carbon panel stopped working, and I want to change the knobs like you have).
    The LFO mod waveforms will run at roughly double the rate of the triangle waveform (it has to do with the way the diodes "rectify" the incoming signal), but you should still be able to slow it down alot, but never as slow as the triangle or square waveforms. You can put a bigger resistance pot in, apparantly you can get slower rates when you do this - I'm going to try it out myself soon.
    Talk again soon.
    One more thing: at full LFOtoFilter you are shorting out that connection, so maximum LFO signal goes to the filter - this is too much and goes beyond what the filter can handle. I have not done this myself, but putting a resistor in (try to estimate based on position of pot at the point where you lose the LFO on the filter) you will effectively constrain the amount of LFO signal going to filter so you can turn the pot to max. the filter is a pretty bare design (only 2 or 3 transistors) so its pretty limited.
    I'll give you more details about how I did it later. Good luckk.

  12. Hi Mick!

    Well, I finaly found out, that the LFO2FILTER IS working. I just did two very simple mods - Resonance POT and Decay disable and VOU A LA! Now I can clearly hear the LFO2FILTER. I was almost thinking of leaving it out... With disabled decay and high resonance (almost self oscillation) ist amazing. I was awaiting someting else first - you know that typical filter sweeps, but its still great.
    Thus, as I said before, its "working" (or audiable) only at certain (middle pot position) CUTOFF frequency levels.

    The CUTOFF and RESONANCE pots have generally quite limited usable range - maybe adding a resitor to pot will help.

    Have you also tryied the ENVELOPE to FILTER mod?

    I also disconnected the LFO waveform mod temporary and disassembled it. I am thinking of where can be the problem. There should be really only wire from the line between C1 and R19, going then trough new 22K resistor (same value as R19) then divided by switch to one diode direct or the second reversed. Then wire output of the diodes together and go next to the LFO rate pot (same place as DEPTH connection)?


  13. Hey BK.
    The envelope already goes to the filter - its hard to notice, but I did the envelope disable mod (from Din and you can really hear it when pitch env is off. Hoshu's mod actually switches between these sends (pitch OR filter env.) but I just did the disable as it gets rid of the slight pitch bend when you first trigger a note - it still goes to the filter a bit, bit I'm OK with that as its barely noticable (for a cheap synth).
    The lack of range on the res. pot is because a 10k pot is kind of big - but if you do the calculation for parallel resistors then 10k makes the gakken 1K resistor a 0.9k, and if you put a 5k (which would use more range) then your 1k becomes 0.83k, so at minimum res (which is really max on the pot) actually sends some feedback, albeit not vey much. I found another mod where the hacker simply got rid of the res switch and put a knob straight in, but I don't want to take Gakken apart too much.
    As for the LFO, go from R19 to 22k, then switch, then diodes, then from branch point of diodes to west (left) pin of LFO RATE knob. Then your LFO depth knob cuts into the line between LFO rate knob and R18 - rerouting to R16. Here is the point where you'd do extra work for the destination switch.
    Thats all there is to it.
    Your absolutely right about the res making the LFO more pronounced - I never use Gakken without Res switch on!
    As for your filter knob fix - I did add a resistor between west and north pins of filter knob, but I don't think it does very much. The reason is that the filter pot is actually operating like a variable voltage divider, and so shorting out the pins won't really work. A volt divider actually works by the ratio of two resistors - you'll notice that one side of the filter pot has a resistor to ground. This is to limit the ratio between the two sides of the resitance in the pot itself. To fix it would require resistors added into the circuit as with R34. You'd have to experiment with different resistors to balance out the right ratio. Installing a different size knob won't help much in this situation either, because you can get the same ratios for different size pots (for example, a half is still a half, and a quarter is still the other chunk to three quarters,...).
    Still, better quality knob won't hurt, and since your doing this anyway, you can put the resistors in-line with your wires that go back to the Gakken PCB.
    Important note: if you do the env. disable switch (as dynsiync did) then you'll have to put a resistor to ground, or the tuning goes out of whack! If you look at Hoshu's env dest switch, he has put these in for both the pitch and the cutoff. You must put these in for dest switch on env.
    How's your square wave mod coming? This is the best mod for sure as its so much fatter than the original sawtooth VCO waveform!
    Keep at it, its well rewarding when its done!

  14. Hi Mick!

    I did the ENV disable switch just today :-) but curiously, it does not require a resistor to ground. The tuning seems to be absolutely OK even with disabled ENV.

    I also cut the trace from the VCO before C6. I am thinking of the VCO square mod, I also already bought all the part, but not yet started making it...

    The 2.5 voltage (BIAS) is already in the circuit, right? And there goes also the output of LFO to automaticly modify the pulsewidth?

    I am thinking of making a very simple noise generator circuit and mixing it with the VCO.
    Not sure how strong is the VCO output (or how much can the VCF input handle) - have to check.

    But mine biggest current problem is, why that LFO wave mod is not working.
    I think I did it OK - wire goes just from before the R19 trough the same size 22k RESISTOR, then trough the 3 positions switch - middle position not connected, right position goes to a diode directly, the left to another diode reversed, the output of the diodes goes together and than to the LFO RATE pot (the same please where LFO DEPTH pot is also connected).
    Maybe used I the "wrong" type of diodes? There was no specification so I used standard regulating diode (which should work IMHO).


  15. Hey BK.
    I think I took Vbias from positive side of C8 - but measure to ground with a voltmeter to be sure. I think I grabbed Vcc (+5V) from positive side of C20. Both of these are the big caps on knob side of PCB. I would have to double check that for you, because I think I measured all the voltages. From Hoshu's schematic, he shows the source Vcc (as +5V) and source Vbias (as 2.5V) from the points I stated above. I had to double check these connections, as you're working with components on the reverse side of the board.
    Yes, the LFO does the PWModulation, but there is a switch for this in the mod so you can control with a knob or LFO, but you will need the knob control WITH the LFO to get good effect and sounds.
    As for your LFO, I'm pretty green with the electronics, so you could go to Muff Wigglers Forum or The Grid at Dogs On Acid, and someone will know about diodes.
    You can see the size/type from the pic I posted for you, they're pretty big. I don't think other types would work the same - like zener's or schottkey diodes, so regular type for sure. Now these are low voltages, but I don't know what kind of currents are running through the Gakken, so you could try big diodes and then recude size until you get one that works.
    I can't remember what I got, but know that it was rated at 30V max. Maybe 5A max??!!??
    Sure of voltage max rating though.
    Anyway, when I get more time I'm going to fix up some stuff on my Gakken and I'll let you know exactly what I did for LFO, but it sounds like your on track.
    The only thing you did differently was the destination switch.
    Anyway, if your still having problems in a week, I should have Gakken apart by then and will totally go through it with you.
    Good luck, and let me know how the noise generator goes!
    p.s. I'll put up another Squarewave tune soon, to demonstrate the Gakken mod some more, as I thnk the Gakken is a bit more fun to work on than monotron.
    p.p.s. Have you got a monotron yet? They're fun too, and sound pretty fat!

  16. Hi Mick!

    About the LFO wave mod - well I really dont see any other possible mistake yet, but the diodes characteristic. Mine are also kind of small "standard" diodes, but up to 1000 V.

    Yesterday I did the VCO square mod quickly. Yea, I got the right voltages (2,5V and 5V) according to the MOD7 schematics with no problems. :-)
    But probably I have a mistake in the circuit, because when I was making it, I checked it, and I have clearly heard difference between SAWTOOTH and SQUARE waveforms.

    But when I finished it, the old SAWTOOTH waveform seems to be modified in some way also :-(. I am not sure if is the bug or feature. I was making it quickly, so I have maybe shorten some wire, I have to check.

    So my questions are here:-) :
    1) The PWM POT is working - I can hear it very clearly - its really great. But the PWM POT affects the sound also when I have switched to the old SAWTOOTH waveform, is it correct? I tought it should work only when the waveform is switched to the SQUARE or?

    2) The PWM LED is still on. No matter if I have switched to SAWTOOTH or SQUARE the LED IS ON. Also the LED does not change anyway when I change the PWM with pot. What should that LED basicly indicate?

    3) There is a switch to choose between the Manual PWM by POT or automatic by the LFO. The manual works great. The automatic should come from the "LFO trigger" - I am not sure about it. I took the signal from the LFO OUT - where I divide it the LFO2PITCH or LFO2FILTER (after the 150Kohm R16 resistor). Is it right please? Well it seems to be working a little, thus its not very notable because the LFO also affects the same time either the PITCH OR FILTER (not only PWM modif. possible without adding next switch position). Is the LFO trig out to PWM comming from after R16 correct?

    As I wrote, I have probably make some mistake in the wire, have to check it this evening with the multimeter. Have a nice day.


    PS. I dont have the Monotron yet, I was thinking of it when I was buying the Gakken (the filter sounds great - MS-10 like architecture). I think I will probably buy one in the future.

  17. For the affects on sawtooth:
    I put an overide switch in to kill the 5V and 2.5V feeds, and this fixes that. Also, whenyou put this switch in, it kills the LED.
    You can see this switch on my mod left of wave select switch. I used a DPDT switch, so one pole does 2.5V and the other does 5V.
    I think the problem arises due to feeding the extra signals from pure voltage sources back into the VCO out.
    The LED will flicker whenyou really modulate the square wave hard from the LFO.
    As for the LFO feed onto PWM circuit, it has to come straight from the triangle waveform of LFO section. I think I cam off the triangle side of the LFO selector switch; that is, dirctly to the triangle side of the LFO switch and took it directly to new PWM mod select switch.
    Good luck and have fun, Mick

  18. Hi Mick!

    So affecting the sawtooth is a "feature" of this PWM circuit (not problem in my cables :-)).
    I will really need to add a switch to turn it on/off, cause I liked also the old sawtooth wave very much.
    Now the PWM works in both cases - automatic by LFO and manual - its very coool :-). Its really great mod!
    I also tried add the envelope retrigger button according to the Matts flicker picture, it was working OK (with a little of strange sound when pushed), but I disconnected it, because I already have the frequency POT and a button to turn the trigger (I dont use the default "pen" and resistor string).
    Your "ENV" button on the side photo is envelope retrigger or?
    I am going now to buy and try another diodes to make the LFO wave mod working.

    What are your future modification plans?
    Looking forward to check your new tune.


  19. Hey BK.
    Yep, the switch labelled ENV. on the side is the envelope trigger - its an (on)-off-on type, which means that one way, (on), the switch is one-shot, or momentary, the other way, on, stays on - which you can get some nasty sounding effects from. In fact, for the Mighty Gakken clip, all the sound effects (with delay added) were created with the env trigger held on. Fun times!
    I agree, the original sawtooth sound is nice, which is why I put the switch in to disable PWM circuit. But you can also get some neat sounds applying PWM to the sawtooth - but certainly only a few setting generate "good" sounds, if you know what I mean... Still, with the LFO modulating the PWM circuit yu can get some interesting effects on the sawtooth.
    The square wave is fat, init?!?!?
    I love it!
    Future mods: will probably just replace pots, trying different sizes for the LFO Rate, Attack, and Decay controls. The existing pots are pretty cheap and cheesy. But I won't get to these changes until I've finished the monotron mods - its alot of work! Then I have to fix some stuff on monotron - the filter knob range, the carbon panel stopped working, the sawtooth seems to have lost some of its bottom end, and add a resistor on LFO2Filter so as not to over-drive the filter section, and I might try to do something a bit better for the audio out (starting with a (1/4)" jack!!
    Anyway, won't be around for about a week or so, as I have lots of non-gear-related things to catch up with - and my spare time is going to monotron mods and music at the moment.
    Keep up the good work, and I'll answer any questions if I have time.

  20. I Mick!
    Finaly I found the problem in the LFO wave mod - just the output of the MOD should be connected to another leg of the POT (not the same as the LFO DEPTH - like it seems to be in the schematic).
    I also replaced the diodes with the lower voltage "high speed switching" ones. The waveform mod works excellent. As you said - the frequency of Sawtooth up/down is doubled compared to the Triangle wave - because of the design of the circuit.
    Do you please now, how much Volts/Octave is the Gakken driven by default? (without the 1V/octave mod)
    BTW. the Envelope (attack, decay) is to PITCH and also to FILTER a little, right? Because it does not seem to work like the "typical" VOLUME ENVELOPE (VCA).
    What about the VCF tracking? Is it an interesting mod? Is it just a 100K VR in series with small resistor (maybe 1K or so?) between the R31 west and R5 south? Why is a big resitor (maybe 1meg?) needed in paraller to VR?
    Best regards,

  21. I forgot a question about that VCF tracking - is it basicly a "Keyboard tracking" filter function? It meens that the filter is depending on the pitch? The schematics of it is just as mentioned before? Not exacly values of that "small" and "big" resistor?

  22. Hello BK.
    Yes, VCF points you mentoin are correct. I used a big resistor in order to let the pot reduce the keyboard tracking to zero - like switching it off. This is because the tracking can be quite effective at lower intensities - especially when you're driving the filter hard from the LFO and have the envelope and res up.
    You could put a 500k pot in the way you describe. I just did mine in parallel because I could not get a 500k pot at the time, but wanted more range. I definately think its a worthwhile mod. If you check out my Basic-demo, I label a part where I increase the VCF tracking - it gets pretty dirty, even a little crunchy. I like it!
    It also offers a little more versatility to the filter, as real-time knob tweaks are'nt really that great - especially when you consider the lack of range on the filter pot itself! Changing the pots (as you're doing) will make "playing" the filter better, but I find "tweaking" on the fly a bit more pleasing through the VCF tracking knob.
    Of course, you could just put a switch with a fixed resistor in line. But I think the pot is way more fun!
    Yes, the envelope does go to both the filter and the pitch. Thats why the disable switch is nice to kill the env to pitch. Then you can get some heavy env action on that filter!
    The Gakken is somewhere around (1/3)V/Oct, but I not certain this is an exact value. The voltage divider I added works for this, anI control the Gakken from a Kenton Pro Solo mk2. The only annoying thing is you have to tune it - like a piano or guitar. But this means I can control the machine from other gear (groove boxes and such). If you're going to use a computer, then I suggest you get Silent Way. With this software you won't have to worry about the V/Oct, as there is an auto calibration that "tunes" the output voltages from you computers interface to play proper notes on the Gakken. Of course, you'll need to get an interface that can use the outs as CV outputs. I think all the MOTU interfaces can do this, and a few others.
    I don't use computers to make music, so I cannot give any more advice about this. Din sync has a video on Silent Way if you're interested.
    I'm very happy you got the LFO working. Can you imagine Gakken without it now? I love that mod!
    Happy hacking, Mick

  23. Hi Mick!
    I am working on the build-in simple 8 step analog sequencer - its almost finished.

    What about the Gakken CV 1V octave mod?
    You have some short info in the next blog, but you are just saying about some "test" circuit and pin connection, but I am not sure about the circuit.
    Have a nice day.

  24. Hey BK. I added an update to the Voltage Divider post, with a schematic and an equation.
    Please be aware that the Gakken will still tune a couple of octaves highr than the standard. So that if C3 is middle C on the keyboard, then it'll be at appromimately C5 after tuning.
    Now, you can put another trimpot inline with the CV in to the volt-divider, which is what you have to do to monotron CV in to get 1V/Oct. The difference being that the monotron does not need a volt-divider - just some patience whilst tuning your Kenton Pro Solo!!!
    Good luck, and I hope this helps, Mick.
    Here's the post with extra info:

  25. Hi Mick!
    Thanks for info. Well, I tried a volatage divider - the POT before PAD3 (or lower pin of R7? if I remember), it works in the way that it changes the general pitch - the higher resistance, the lower pitch and vice versa.
    But I found out, that when connected, it modifies the same way (makes lower) also the PITCH OF STYLUS (when played directly on Gakken and not by CV IN (from analog sequencer)). Probably it will be needed to connect also a SWITCH not to modify the pitch when sequencer is dissabled.
    I agree with you, that the music is so much important. These dayes I also spend only a little time of making music - I have to change it.

  26. I forgot, there is no the Volume Envelope, right? Just the ENV to filter and pitch, but not to the volume?
    Have a nice day.
    Best regards

  27. Hey BK.
    I tied to R7, and it did not affect the stylus control. If you took CV to Pad 3, it might, but I don't know without trying it first.
    I also hooked up my trimmer incorrectly the first time I did the voltage divider, and it only reduces the pitch that gets played - the Volts per Octave stays the same. You should get semi-tones after you tune in the correct V/Oct on the trimmer.
    I did not need a switch for this mod.
    As for amp env - no, there is no amp env. It would be nice if there was though!!
    I think you could buld one yourself (see MFOS website, or any DIY synth blogs - most will be for modulars, but the same applies).
    It'd be interesting to see if it could also be controlled by the Gakkens' env section?!?!?
    Keep having fun with it, and show us a pic when its done.

  28. Hi Mick!
    The south of the R7 is (at least in my case) electricaly identical (conductive) to the PAD3, so I am not sure, but it should not matter at which concrete point I connect the CVin. But maybe, I have to check :-)
    Yes, it can be really cool to have ENVtoVolume(AMP) in gakken. The simplest way should be probably getting the Envelope ControlVoltage from existing VR3 pot. Have to check the values/amplitude. Thus there ist only attack and decay (no ADSR (or even delay - interesting ADSR can be done with simple 555 timer), it can be interesting enough at least for testing. So just a suitable VCA is needed. The VCA injection point should IMHO be after the VCO wave mod (before C6 filter input), but will have to check the VCA output properly (not to burn the VCF input)
    Best regards

  29. Hi Mick!
    Well, I found out that the ENV generator gives the peak output of 2,5 V. The more typical synth design should probably be the VCA just after the VCF.
    So I will try to look for some suitable simple VCA design.
    Have a nice day.