rhythm wolf teardown: any questions?

From circuitbending to homebrew stompboxes & synths, keep the DIY spirit alive!

Moderators: Joe., lisa, luketeaford, Kent

Post Reply
User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

rhythm wolf teardown: any questions?

Post by guest » Tue Feb 18, 2020 8:44 pm

so, curiosity fiinally got the better of me, and i dropped 100$ on a used rhythym wolf. i had never seen a synth get panned so hard as this thing, and i was curious if the rumors were true, and if so, whats going on inside to make that the case. so im going to be using this thread to document that exploration.

first impressions:

1. the LEDs are blinding. i cant see anything. it would have been nice if the pads lit up instead.
2. all the voices are pretty flat, and have limited useful ranges (the majority of the knob travel isnt great).
3. the bass synth has some decent sounds in it, but only on the square wave. oh my god is the sawtooth horrible. unfortunately those decent sounds are in a limited window. as you turn the cutoff up it gets better, and then all of a sudden gets worse.
4. the case is sturdy and a nice size/weight.
5. the "howl" is ok on the bass synth in small doses, and pretty good on the bass drum in heavy doses, but there is no control over individual voices. its all howl all the time!

i havent tested out the sequencer/workflow yet, but so far i give it a "meh". its not as bad as i was expecting it to be, but is by no means great.

if you have any questions, or things youd like for me to test/fix, please let me know.

[EDIT] 3.14.20 - changed the title so rhythm is spelled correctly
Last edited by guest on Sat Mar 14, 2020 7:25 am, edited 1 time in total.
openmusiclabs.com

User avatar
SlightlyNasty
Common Wiggler
Posts: 202
Joined: Wed Nov 25, 2015 9:18 pm
Location: Adelaide, Australia

Re: rhythym wolf teardown: any questions?

Post by SlightlyNasty » Tue Feb 18, 2020 11:19 pm

I like the Rhythm Wolf, despite its many, MANY flaws. It's a fun box of quirky primitive drum sounds that respond really well to processing and distortion. The synth voice is fun to play with when you're mashing it all through effects, just to see how it reacts with everything.

It's not a great machine by any stretch, but the intensity of vitriol from boring techno dads the world over is a little undeserved. I used to have a 606 and I'd take the Rhythm Wolf over that any day of the week.

(Fight me.)
www.slightlynasty.com - 4U DIY Modules for the Loudest Warning format

User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Tue Feb 18, 2020 11:46 pm

yeah, i dont want to come off as hating on it. the thing is built like a tank on the inside. when i opened it up my first thought was "oh god, this cost them a lot to assemble". there are so many screws, its crazy!

i just finished the filter. its a 2-pole SVF, which explains the lack of bite. seriously, one more OTA and it would have been a lot better. i think i have a quick fix for the lack of "high end" in the filter. the very first thing i noticed about it was that the filter wouldnt open up fully when cutoff was turned all the way up.

have you pinned out all the individual voices? they labelled it internally for that, and i think that would make post processing a lot easier.
openmusiclabs.com

User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Wed Feb 19, 2020 12:17 am

alright, so there is no exponential control over the cutoff frequency, its entirely linear, and with a limited range. this would have cost an extra 10 cents or so for a pair of transistors.
openmusiclabs.com

User avatar
col
Wiggling with Experience
Posts: 447
Joined: Tue Feb 04, 2014 3:47 pm
Location: Melbourne

Re: rhythym wolf teardown: any questions?

Post by col » Wed Feb 19, 2020 1:17 am

Also, there was the Geert Goltstein mods:

https://web.archive.org/web/20190505163 ... ifications

User avatar
SlightlyNasty
Common Wiggler
Posts: 202
Joined: Wed Nov 25, 2015 9:18 pm
Location: Adelaide, Australia

Re: rhythym wolf teardown: any questions?

Post by SlightlyNasty » Wed Feb 19, 2020 1:28 am

guest wrote:
Tue Feb 18, 2020 11:46 pm
have you pinned out all the individual voices? they labelled it internally for that, and i think that would make post processing a lot easier.
The previous owner did - as well as adding the open hat decay knob mod - but honestly I almost never use them (I really like the intermodulation effects and dynamics interplay that come with distorting and compressing bussed parts simultaneously).

I would love to hear some behind-the-scenes insight into the development of these and the Timbre Wolf etc. There's almost an element of Outsider Art to them - with all the analogue design knowledge floating freely around the internet they couldn't even make the VCO remotely temperature stable, or design a filter that sounds like... a filter. The entire synth section is a wonderful piece of "how on earth did this ever reach production?".

I think the drums have a cool primitive industrial grit to them though - like an organ rhythm section with a safety pin through its nose.
www.slightlynasty.com - 4U DIY Modules for the Loudest Warning format

User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Wed Feb 19, 2020 1:36 am

yeah, there is no temperature compensation in the VCO. im having a fun time reversing this thing, at every turn im like "i suppose you could do that??????"

and col, thanks for the link, ill check that out.
openmusiclabs.com

User avatar
lisa
Super Deluxe Wiggler
Posts: 4395
Joined: Fri Jun 02, 2017 7:00 am
Location: Sweden
Contact:

Re: rhythym wolf teardown: any questions?

Post by lisa » Wed Feb 19, 2020 2:23 am

Following this thread. Very interesting to read what you find.

Maybe I should get a Rhythm Wolf just to try it out. It just can’t be as bad as most people say. The Tom Cat is nice. I’ve used it for quite some tracks.
New modular track! My first in six months. :party: Messy and unglued.


User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Wed Feb 19, 2020 4:22 am

i think i have the whole VCO done now. it doesnt use an exponential converter, and instead uses a PWM DAC to set the frequency. this in itself isnt a bad thing, but the implementation has a lot of error sources, so unless calibration is essentially constantly run, it will always drift. since the tune knob is analog, there is no way the digital section could adequately perform autotune. there is a JTAG connector on the board. i might see if the lock bits are set.
openmusiclabs.com

User avatar
Noodle Twister
Common Wiggler
Posts: 197
Joined: Wed Jan 02, 2019 10:22 pm
Location: UK

Re: rhythym wolf teardown: any questions?

Post by Noodle Twister » Wed Feb 19, 2020 6:12 am

There's some Hi res pics, notes about it here too. May be useful :)

https://carillonaudio.wordpress.com/201 ... t-bending/

User avatar
Kattefjaes
Common Wiggler
Posts: 194
Joined: Thu Dec 12, 2019 9:47 am
Location: UK

Re: rhythym wolf teardown: any questions?

Post by Kattefjaes » Wed Feb 19, 2020 6:20 am

SlightlyNasty wrote:
Wed Feb 19, 2020 1:28 am
I would love to hear some behind-the-scenes insight into the development of these and the Timbre Wolf etc. There's almost an element of Outsider Art to them - with all the analogue design knowledge floating freely around the internet they couldn't even make the VCO remotely temperature stable, or design a filter that sounds like... a filter. The entire synth section is a wonderful piece of "how on earth did this ever reach production?".
Yeah, this is why I have been watching on ebay on and off for ages, looking for a dirty cheap one. I sort of like terrible little drum machines, and the notoriety of this one has piqued my interest. Cranked up and crunchy, there's mileage in getting something you like out of something famously crappy- pouring enough love into it and getting used to its restrictions etc..

Edit:

This looks totally legit:

https://www.ebay.co.uk/itm/Akai-Profess ... 4071021351

:mrgreen:

User avatar
Paradigm X
Super Deluxe Wiggler
Posts: 2019
Joined: Tue Nov 02, 2010 5:43 am

Re: rhythym wolf teardown: any questions?

Post by Paradigm X » Wed Feb 19, 2020 6:34 am

Lol, this is infamous for being out of tune all the time and youve figured it out in minutes :hihi:

id be curious if itd be possible to get the triggers out easily. if this could also be a useful trigger sequencer, it might be worth the money? maybe.

thanks for sharing.

User avatar
astrosound
Common Wiggler
Posts: 241
Joined: Wed Aug 06, 2014 11:14 am
Location: New England

Re: rhythym wolf teardown: any questions?

Post by astrosound » Wed Feb 19, 2020 12:55 pm

Hooray for another Rhythm Wolf thread! The almost universal disdain makes it all the more fun to discuss. Gotta wonder if they'll be sought after in a few decades for the hate alone.

Excited to see you digging into this, guest! I'm eager to see what you find regardless of any potential for modifications.
Paradigm X wrote:
Wed Feb 19, 2020 6:34 am
id be curious if itd be possible to get the triggers out easily. if this could also be a useful trigger sequencer, it might be worth the money? maybe.
Yes, the triggers and/or accent can be extracted. The trigger accent amplitude corresponds to the velocity of the drum hit(Low, med, high). Max velocity gives about 10V trigger and lowest velocity is 2V or so. That could be inaccurate though, last I had my RW was over a year ago and I didn't write anything down.

I did mark the trigger points on the PCB but never utilized them as the limited free space in the rear was already used up for drum voice outputs. A small multi-pin connector (D-sub?) might be necessary if one wishes to extract both discrete drum voices and triggers.
Last edited by astrosound on Tue Feb 25, 2020 10:12 pm, edited 2 times in total.

User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Wed Feb 19, 2020 1:58 pm

Noodle Twister wrote:
Wed Feb 19, 2020 6:12 am
There's some Hi res pics, notes about it here too. May be useful :)
https://carillonaudio.wordpress.com/201 ... t-bending/
thanks for that, lots of good info there.

im pretty sure i can get the VCO to stay in tune, i just need to figure out how it does calibration (if it does calibration???). i should have full schematics posted later today for the bass synth section.
openmusiclabs.com

ihav2p
Common Wiggler
Posts: 69
Joined: Wed Apr 28, 2010 2:03 pm

Re: rhythym wolf teardown: any questions?

Post by ihav2p » Wed Feb 19, 2020 4:11 pm

Thanks for having a look guest! On mine, I lowered the minimum cutoff and decay time, and raised the maximum resonance. These were very easy mods. Then I cut the connection between the envelope and the VCA so the envelope only affects the filter, and the VCA is only gated. If someone wants, I can share the mods, but its pretty easy to figure out. Sound is really good if you have a taste for this particular genre of sounds. I guess its a simple Lm13600 filter, but it doesnt match either the svf or korg schematics.

Since you ask, I would like to permanently change the AR envelope to an AD envelope. (well, no A) It seems like a simple task but it is a bit outside my competence to do it in an elegant way. Until then I use the midi feedback method I described in the other thread.

Also, I tried to figure out if it was possible to compensate the volume of the filter at high resonance, but this was also too advanced for me, especially with no schematic.

Let us know what you find!

User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Wed Feb 19, 2020 4:48 pm

ill keep an eye out for those things. right now im working on the VCO, which drifts like crazy. i havent found the feedback for calibration yet, and im beginning to think it doesnt exist. im taking data on the CV, and hopefully its a just a straight V/Hz, in which case i can fix it. if its some internal lookup table, then there really isnt any hope.

if anyone is into writing STM32 code, it looks pretty hackable.
openmusiclabs.com

ihav2p
Common Wiggler
Posts: 69
Joined: Wed Apr 28, 2010 2:03 pm

Re: rhythym wolf teardown: any questions?

Post by ihav2p » Wed Feb 19, 2020 4:52 pm

re: lookup table, there was this on the akai forum,

https://getsatisfaction.com/akai_profes ... on-to-file

Not sure if that indicates anything

User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Wed Feb 19, 2020 5:38 pm

ok, thanks for that. it looks like there is a tuning utility that will reprogram the lookup table via USB. since this requires you to plug the audio out into your computers audio port, that confirms that there isnt any internal feedback or local calibration. but, this also means that i can most likely do a bunch of mods, and then recalibrate it, and it will then be stable from then on.
openmusiclabs.com

User avatar
Paradigm X
Super Deluxe Wiggler
Posts: 2019
Joined: Tue Nov 02, 2010 5:43 am

Re: rhythym wolf teardown: any questions?

Post by Paradigm X » Thu Feb 20, 2020 5:22 am

astrosound wrote:
Wed Feb 19, 2020 12:55 pm
Hooray for another Rhythm Wolf thread! The almost universal disdain makes it all the more fun to discuss. Gotta wonder if they'll be sought after in a few decades for the hate alone.

Excited to see you digging into this, guest! I'm eager to see what you find regardless of any potential for modifications.
Paradigm X wrote:
Wed Feb 19, 2020 6:34 am
id be curious if itd be possible to get the triggers out easily. if this could also be a useful trigger sequencer, it might be worth the money? maybe.
Yes, the triggers can be extracted. The trigger amplitude corresponds to the velocity of the drum hit(Low, med, high). Max velocity gives about 10V trigger and lowest velocity is 2V or so. That could be inaccurate though, last I had my RW was over a year ago and I didn't write anything down.

I did mark the trigger points on the PCB but never utilized them as the limited free space in the rear was already used up for drum voice outputs. A small multi-pin connector (D-sub?) might be necessary if one wishes to extract both discrete drum voices and triggers.
awesome, thanks!

User avatar
SphericalSound
Common Wiggler
Posts: 210
Joined: Thu Feb 01, 2018 9:23 am
Location: Spain

Re: rhythym wolf teardown: any questions?

Post by SphericalSound » Thu Feb 20, 2020 1:41 pm

I should get a R-Wolf before you do a serious hack that propel prices to the stratosphere.

I see you coming

:foul:

User avatar
SyndieBot2000XL
Common Wiggler
Posts: 196
Joined: Sat Feb 16, 2019 2:45 pm
Location: Owe Hi-O

Re: rhythym wolf teardown: any questions?

Post by SyndieBot2000XL » Thu Feb 20, 2020 3:51 pm

SphericalSound wrote:
Thu Feb 20, 2020 1:41 pm
I should get a R-Wolf before you do a serious hack that propel prices to the stratosphere.

I see you coming

:foul:
You should get one anyway if you’re into funky drum machines. It has tons of character, punches way above its price if you want characterful, grimy drums.

I think most of the hate this machine gets is down to (delusional imho) people thinking it was going to be a kind of all in one drum-and-bass box for $149, and it falls *far* short of that - but honestly, people should have known better.

For what it is it’s fairly brilliant - a basic drum box with a bonus bass tone generator, with a toy-like (in both the sense of being very simple and very fun) ”TR lite” trigger sequencer interface. I’m following this thread with interest, but for me it’s mostly academic as I enjoy the stock functionality of the RW plenty as-is.
It began as a mistake.

User avatar
SphericalSound
Common Wiggler
Posts: 210
Joined: Thu Feb 01, 2018 9:23 am
Location: Spain

Re: rhythym wolf teardown: any questions?

Post by SphericalSound » Thu Feb 20, 2020 8:00 pm

SyndieBot2000XL wrote:
Thu Feb 20, 2020 3:51 pm
SphericalSound wrote:
Thu Feb 20, 2020 1:41 pm
I should get a R-Wolf before you do a serious hack that propel prices to the stratosphere.

I see you coming

:foul:
You should get one anyway if you’re into funky drum machines. It has tons of character, punches way above its price if you want characterful, grimy drums.

I think most of the hate this machine gets is down to (delusional imho) people thinking it was going to be a kind of all in one drum-and-bass box for $149, and it falls *far* short of that - but honestly, people should have known better.

For what it is it’s fairly brilliant - a basic drum box with a bonus bass tone generator, with a toy-like (in both the sense of being very simple and very fun) ”TR lite” trigger sequencer interface. I’m following this thread with interest, but for me it’s mostly academic as I enjoy the stock functionality of the RW plenty as-is.
Looking at prices, it is at 140€ in ebay right now.

There´s also one delusional japanese selling it at 1.450€, in case somebody is interested :yay:

The stock percusions are not that bad once they mix... but I can imagine some mods opening the machine to a lot of new synth terrain. CV control of the "percussion" block would be fenomenal, for example... Also the 4 pole mod suggested by guest seems like a winner

So.., if anybody spots a really cheap one, please share

User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Thu Feb 20, 2020 10:45 pm

ok, here are the schematics for the bass synth. there might be some other stuff going on with the envelopes, but i think this is the main part. there were some signals going off to the drum section that didnt make much sense to me. ill puruse that later. its a 4 layer board, so its a bit hard to trace things around, but im pretty sure this is accurate. im putting the files here as links, rather than as image files, as they are quite large. i drew them by hand, so they are a bit messy and the resolution is needed to read them.

http://www.openmusiclabs.com/files/rw_vcovcf_lg.png
http://www.openmusiclabs.com/files/rw_vca_lg.png
http://www.openmusiclabs.com/files/rw_env_lg.png

is there a way to have a reasonable sized image be a link to the higher res ones?

[EDIT] most capacitors are not labelled with a value, as its hard to measure in-circuit. electrolytics are labelled.
[EDIT] i meant to write on the VCO schematic: CV PWM frequency is 731Hz (/2^n of 12MHz). i dont know the bit depth.
[EDIT] updated (2.21.20) with some fixes to the envelope section.
Last edited by guest on Fri Feb 21, 2020 10:33 pm, edited 2 times in total.
openmusiclabs.com

User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Thu Feb 20, 2020 11:46 pm

ok, im going to do a discussion on this section while its fresh in my mind. but, id like to first say that this is in no way intended to be disparaging of the circuit or its designer. i have no idea what that persons intent was, or what their constraints were. a lot of the circuit leaves me baffled, but maybe thats a failing on my part. hopefully this discussion helps others who want to understand the circuit so they can mod it.

ENVELOPES: ok, so it seems like the intent here was to make both lin and exp envlopes, with the exp going to the VCA, and the lin for the VCF. since there is no exponential converter on the VCF, the lin envelope makes for a really weak filter sweep. it spends a relatively smaller amount of time in the lower range, near the cutoff frequency. there are 3 accent levels (the 4th accent level is "off"). these are evenly spaced between 0 and 10V. the accent and trigger signals arrive at the same time, with the trigger ending first. im not sure why there are 2 signals here, but its handy, if you wanted to add a manual accent knob. you would drive U27B with your accent voltage. the gate signal is even more bizarre, in that it has PWM on the first 5ms. im not sure if this is intentional, or just some artifact of the way the signals are multiplexed. im assuming the former, as it gets multiplexed the same way as the other signals. this pwm signal has no effect on the remainder of the circuit, as its over-ridden by the accent/trigger signal where they come together at R312. but, if it were pwm'd after key-up, then you could have digital control of the decay time. the existance and size of C155 suggests that pwm was intended here. if you dont want your envelopes to be gated, you can just remove R311, and instead put a 10k to ground there. Q44 follows the accent voltage to charge C147, while at the same time Q47 is turned off to disengage the decay pot. this sets the min time before decay to 4ms (the trigger time).

the decay time on the VCA envelope is crazy long: 45s! reducing C147 would fix this. another odd thing here is that its a linear pot. typically log pots are used for time and frequency control. there are only linear pots in the whole synth, but a number of different values, so getting a log wouldnt have been any extra cost. in this particular case, its a dual pot, so its an odd one out to begin with. again, an interesting choice to have 2 different styles of envelopes, as it required a dual pot.

the vcf envelope is less straightforward. first off, it has 2 comparators, each with startup pulses (Q38,43) on them to ensure a fixed state at startup. im not fully convinced these are necessary, and a quick pulse on the trigger would have done the same thing anyways. both of these comparators merely follow the trigger signal, so its not too relevant. what it looks like is supposed to happen, is that U25B detects the trigger, and starts charging the integration capacitor, and U25A checks the envelope voltage, and shuts off U25B when it hits 10V. but, this doesnt happen, as the signal is not strong enough. there are a few different way to fix this. but, the upshot is U18B saturates at 11V, and there is delay before decay begins as the integration capacitor needs to discharge quite a bit before U18B goes back into linear mode. R146 sets the attack time, and D48 ensures its only active during attack. D55 and D14 ensure that the gate signal does not discharge the capacitor. but, im not sure what this pathway is for anyways. it slightly adjusts the attack time, from 1ms to 1.5ms at higher accents. im not sure why you would want slower attacks for accented notes. personally, i would re-route this signal to the comparison voltage on U25A, giving higher peaks on accented notes.
openmusiclabs.com

User avatar
guest
Super Deluxe Wiggler
Posts: 4000
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Fri Feb 21, 2020 12:25 am

VCA: this is perhaps the most intriguing circuit so far. its a VCA with compression on it. perhaps it could be just as easily accomplished with a diode limiter on the CV, but i cant say for sure just yet. the main gain cell is a standard diffpair, but the tail current generator is unique. it has a current mirror that tracks the tail current, and then brings down the CV input as the gain goes up. my main issue with this, is that it completely defeats the purpose of having exp envelopes for the CV, as this compression completely undoes the exp behavior, and compresses the envelope. this compression starts when Q12 turns on, which happens when the current through R73,76 cause a voltage greater than a volt or so (Vbe + R56 drop). C16 puts a time constant on this compression, which is slightly backwards from what you might expect: fast moving increases in CV increase compression, and fast moving decreases in CV decrease compression.

the real oddities on this circuit are the inputs to U1B. U1B sets the lower rail that serves the tail current. U20B is a just a fixed voltage generator, but with its signal taken from the inverting pin, rather than the output. C97 is not populated, as this would make the opamp unstable by delaying the feedback signal. ultimately, this opamp isnt required at all, as a resistor from Vcc to the summing junction of U1B would have done the same thing. U9A is even more curious. this takes the peak of the VCF signal, and tries to adjust the lower rail accordingly. this could act as a sort of compression on the audio signal, as opposed to just the CV. this could be pretty useful for dealing with resonance related volume changes, or if there was a distortion circuit before this. but, since there is no resistor across C58, it just charges up and stays charged forever, so it cant respond to changing ampitudes. its just the peak value it has ever seen. im not sure what C55 is about, as it would only allow for a temporary compression. the oddest thing about this, is that the feedback is backwards, such that larger signals actually increase the gain of the VCA. reversing D8 would fix this. but, utlimately the CV compression circuit mostly over-rides any effect this has.
openmusiclabs.com

Post Reply

Return to “Music Tech DIY”