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: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Fri Feb 21, 2020 1:26 am

VCO: the VCO is a linear design, with an exponential control voltage (V/Hz), which is a reasonable design decision considering how small of a range the VCO needs to operate over (3.5 octaves - 1:10 ratio of low:high note). this means any drifts "should be" linear, and can be compensated for with a simple tune knob turn. they buffer the PWM signal with a reference voltage, and send it to a mixing stage, which then goes to the OTA for waveform generation. unfortunately there are a lot of nonlinear drifts in the remaining stages, which can not be accounted for with a simple tune knob. for example, the current into the OTA is not linearly related to the CV, as the OTA input is 2 Vbe off the negative rail, and those Vbes vary with current and temperature. replacing R65 with a transistor would fix this, but it would also invert the relationship of CV to frequency, and im not sure if the rhythym wolf tuning software could overcome this. the next major issue is the existence of R140. this causes a nonlinear drain on current, which is amplitude dependent, and the amplitude at this point varies with the output buffer Vbe drops. the only reason i can think of for this resistor, is to discharge the cap when the VCO is shut off. the only problem with this is, is that the Vbe drops in the output buffer make it so that the VCO output is not at zero when this happens (more on this later).

the PWM signal is at 731Hz (a sub-multiple of 48Hz) which is quite low. there is a lot of ripple after the filter. the filter has a 2ms transition time, with a few milliseconds of ripple afterwards. any portamento happens on the digital side (does it have portamento?), but a pot and capacitor could be added after the filter to have manual control. in order to get 1cent resolution over a 1:10 ratio of frequencies, 14b resolution is required, which means the main PWM clock is probably running at 12MHz, and the low output is just to achieve this level of accuracy. a dual PWM scheme could have gotten the same accuracy with a higher frequency (and the buffer they use is a dual unit already!).

the VCO is a pretty standard dual OTA setup, which could be easily converted to a triangle wave, if so desired (remove R63). this would drop all notes down one octave. for both this and the VCF, the 13600 is an odd choice over the 13700. the 13600 send a copy of the bias current to the output buffer, which helps match the buffer impedance to the OTA output impedance. unfortunately this makes the buffer input current higher than it could otherwise be. swapping them out for 13700s would help. im a bit confused as why they buffer was used at all in the VCO, when the signal goes straight to an opamp buffer in the very next stage. the amplitude is set by R64,36. this is ~+/-2V, which is a fair bit of overdrive for the transistors at the input of the OTAs. this isnt a big deal on the one used as a comparator, but the other one will have leakage currents associated with this, which will effect timing. putting a resistor between R64 and R36 would fix this, dropping the overdrive to 0.6V. R63 should be removed, as it provides a leakage path for the frequency setting current, and is not required fro the reset cycle. speaking of which, the reset cycle is quite long (100us), which adds to the nonlinearity. there really isnt much that can be done about this, other than going to a different reset format (JFET across the integraction capacitor).

one other oddity of the VCO, is that there is a comparator on the VCA envelope, to turn off the VCO when the envelope gets below a certain level. the problem here, is that the VCA is not shut off at the same time, and just continues to amplify the DC voltage the VCO spits out when it shut off. so the output of the VCA has this long DC tail on it, which adds to the noise of the synth. also, the LM393 could have been tied to -Vcc, and the internal transistor used for this purpose, and Q4,5 and all those resistors could go away.

the tune knob has its bottom leg tied to -Vcc through a resistor. this completely defeats the point of having the reference voltage on the other side of it. any powersupply drift now ends up in your VCO frequency. that should have been connected to ground.

[EDIT] one last thing, the square wave is made with the comparator U2A. PWM could be added here by removing R54 and adding a pot in its place.
openmusiclabs.com

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Fri Feb 21, 2020 1:40 am

VCF: this really needs exponential control, but other than that, its a pretty straightforward SVF design. resonance is taken from the final stage, rather than the usual middle stage, and this reduces the low frequency amplitude with resonance ([EDIT] but also shifts the cutoff frequency with resonance setting).
openmusiclabs.com

ihav2p
Common Wiggler
Posts: 68
Joined: Wed Apr 28, 2010 2:03 pm

Re: rhythym wolf teardown: any questions?

Post by ihav2p » Fri Feb 21, 2020 2:03 am

This is so great ... I will have to unscrew all 5000 screws again. Thank you for the detailed analysis.

User avatar
latigid on
Ultra Wiggler
Posts: 908
Joined: Fri Oct 25, 2013 11:05 am

Re: rhythym wolf teardown: any questions?

Post by latigid on » Fri Feb 21, 2020 2:06 am

I love this analysis! :hail:

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 » Fri Feb 21, 2020 3:11 am

Seems to me that the developer had greater ideas but was hit by a deadline! :hihi:

User avatar
SphericalSound
Common Wiggler
Posts: 207
Joined: Thu Feb 01, 2018 9:23 am
Location: Spain

Re: rhythym wolf teardown: any questions?

Post by SphericalSound » Fri Feb 21, 2020 1:14 pm

Impressive. Did you guess all this just looking and poking with the multi at at bare 4 layers PCB?.

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Fri Feb 21, 2020 2:59 pm

SphericalSound wrote:
Fri Feb 21, 2020 1:14 pm
Impressive. Did you guess all this just looking and poking with the multi at at bare 4 layers PCB?.
yes, although i dont think there are any signals routed on the internal layers - just power and ground. there really arent too many signals, and it could have been done in a 2 layer board.
openmusiclabs.com

User avatar
SphericalSound
Common Wiggler
Posts: 207
Joined: Thu Feb 01, 2018 9:23 am
Location: Spain

Re: rhythym wolf teardown: any questions?

Post by SphericalSound » Fri Feb 21, 2020 4:33 pm

guest wrote:
Fri Feb 21, 2020 2:59 pm
SphericalSound wrote:
Fri Feb 21, 2020 1:14 pm
Impressive. Did you guess all this just looking and poking with the multi at at bare 4 layers PCB?.
yes, although i dont think there are any signals routed on the internal layers - just power and ground. there really arent too many signals, and it could have been done in a 2 layer board.
It it still impressive. I have a hard time guessing thru hole PCBs. I dont want to imagine 4 layer SMT´s

Image

:hmm: :yay:

What size, by the way?

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Fri Feb 21, 2020 8:27 pm

everything is 0603 on the inside, so not too bad. but they use the EIA resistor code, so reading the values is a bit tedious.
openmusiclabs.com

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Sat Feb 22, 2020 1:37 am

ihav2p wrote:
Wed Feb 19, 2020 4:11 pm
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.
ok, im going to assume the VCF envelope is fine for you, and focus on the VCA envelope here. i would probably swap out the C147 for a 2.2uF cap, which would drop the attack time to less than a millisecond. this will also reduce the decay time to 5s max. removing R311 should get rid of the gate on the envelope. if find that the gate is cutting off at the end, remove R37. this will disable the VCO mute after note ends. this might introduce random thumps when the VCA turns on when the VCO is at a high point. i think the reason they put the mute in, is to eliminate those random thumps (which are common to any freerunning VCO). increasing R10 might also work here, as that will lower the voltage at which the mute occurs. going up to 100k or 1M might work well.

as for the VCF, there are a few different things that can be done. the first is to put a pot in series with R174, and use that as resonance. the issue with this, is that the filter will get louder as you turn resonance up, and will start to distort. this could be interesting. by twisting the 2 resonance pots you might be able to get a mix you like. the ideal is to replace the resonance pot with a dual pot, and mix some of cut the input volume as you turn resonance up (assuming you put it in place of R174, otherwise if you leave it where it is, you increase the volume with resonance).

ive been playing around with the synth voice some more, and there are quite a few interesting places in it, and my main gripe at this point is that the knobs arent really set to the most usable ranges. in particular, that VCA envelope is just ridiculous. ive been looking at the noise at the output as well, and that seems to be a lot of pickup from the switching powersupply, and the gain on the distortion just amplifying the noise floor like crazy. i put a bit of shielding in, and that seemed to help a little bit.
openmusiclabs.com

ihav2p
Common Wiggler
Posts: 68
Joined: Wed Apr 28, 2010 2:03 pm

Re: rhythym wolf teardown: any questions?

Post by ihav2p » Sat Feb 22, 2020 5:01 am

I havent had enough free time to digest your schematic yet, but this thread has been great.
ok, im going to assume the VCF envelope is fine for you, and focus on the VCA envelope here. i would probably swap out the C147 for a 2.2uF cap, which would drop the attack time to less than a millisecond. this will also reduce the decay time to 5s max. removing R311 should get rid of the gate on the envelope. if find that the gate is cutting off at the end, remove R37. this will disable the VCO mute after note ends. this might introduce random thumps when the VCA turns on when the VCO is at a high point. i think the reason they put the mute in, is to eliminate those random thumps (which are common to any freerunning VCO). increasing R10 might also work here, as that will lower the voltage at which the mute occurs. going up to 100k or 1M might work well.
For the VCA, I'm actually fine with having only a gate a la 303. I want keep the tied notes. To achieve this I just bridged the part of the dual pot that goes to the VCA (P26A), leaving the envelope signal to the filter.

The filter envelope should decay immediately though. Can I just cut the trace between R312 and R313 to remove the gate signal?
as for the VCF, there are a few different things that can be done. the first is to put a pot in series with R174, and use that as resonance. the issue with this, is that the filter will get louder as you turn resonance up, and will start to distort. this could be interesting. by twisting the 2 resonance pots you might be able to get a mix you like. the ideal is to replace the resonance pot with a dual pot, and mix some of cut the input volume as you turn resonance up (assuming you put it in place of R174, otherwise if you leave it where it is, you increase the volume with resonance).
I will poke around with this. If you bridge R175 the max resonance is increased just shy of self-resonance, but you lose almost all the volume, so you have to turn up the gain at the same time. But you are saying to *increase* the resistance at R174? Or replace it?
In particular, that VCA envelope is just ridiculous.
Totally agree! Also, the weird linear filter decay is one of the best things about the synth, but of course it would be nice to have the option to have a more traditional decay.

User avatar
rurs
Learning to Wiggle
Posts: 47
Joined: Thu Dec 18, 2014 9:33 pm

Re: rhythym wolf teardown: any questions?

Post by rurs » Sat Feb 22, 2020 11:38 am

So much good info here, a feast of food for thought.

One mod I'd like to perform, which should be straightforward forward (I hope!), is to add footswitch control to fill and A/B buttons. Any pointers?

ihav2p
Common Wiggler
Posts: 68
Joined: Wed Apr 28, 2010 2:03 pm

Re: rhythym wolf teardown: any questions?

Post by ihav2p » Sat Feb 22, 2020 11:50 am

rurs wrote:
Sat Feb 22, 2020 11:38 am
So much good info here, a feast of food for thought.

One mod I'd like to perform, which should be straightforward forward (I hope!), is to add footswitch control to fill and A/B buttons. Any pointers?
I suppose you would just need to wire a momentary switch in parallel to the switches you want to control

User avatar
rurs
Learning to Wiggle
Posts: 47
Joined: Thu Dec 18, 2014 9:33 pm

Re: rhythym wolf teardown: any questions?

Post by rurs » Sat Feb 22, 2020 12:29 pm

I should just open it up and give it a go alright.

User avatar
joncharliefeathers
Common Wiggler
Posts: 51
Joined: Thu Dec 15, 2016 3:40 pm
Location: mole valley, england, uk

Re: rhythym wolf teardown: any questions?

Post by joncharliefeathers » Sat Feb 22, 2020 12:41 pm

The inquiry of RW in this thread makes for a fascinating read. I bought the machine as a breakout box for Reason. I use the midi to play the drums on a Roland JV1080. Fair to say RW was the first significant move away from a computer I made, followed by the plunge into modular.

Would anybody be interested in making the mods to this machine? :hail:

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Sat Feb 22, 2020 1:18 pm

ihav2p wrote:
Sat Feb 22, 2020 5:01 am
For the VCA, I'm actually fine with having only a gate a la 303. I want keep the tied notes. To achieve this I just bridged the part of the dual pot that goes to the VCA (P26A), leaving the envelope signal to the filter.

The filter envelope should decay immediately though. Can I just cut the trace between R312 and R313 to remove the gate signal?

I will poke around with this. If you bridge R175 the max resonance is increased just shy of self-resonance, but you lose almost all the volume, so you have to turn up the gain at the same time. But you are saying to *increase* the resistance at R174? Or replace it?
the 303 VCA is actually an exponential decay, but its about 6s long, so you never hear the tail in most applications. it does come into play on long slides, which sounds great.

the envelope filter is not gated, but there is a 4ms delay as its held high during the trigger event. do you want to get rid of this 4ms?

you would have to increase R174 to get more resonance.
openmusiclabs.com

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Sat Feb 22, 2020 1:36 pm

i just double checked the signals, and the flat top on the VCF envelope is 100ms, not 4ms (it can be as long as 150ms). so the only way to fix this is to fix the actual circuit and get the comparators working like they were intended to. ill see if there is an easy fix for this.
openmusiclabs.com

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Sat Feb 22, 2020 2:03 pm

try removing D13, that should fix it. the lower level will drop by 400mV, but the comparator should engage. also, i think i have a good solution for making the VCF envelope track the accent.

[EDIT] that only fixes half of the problem. ill have to think on it some more. i think i see what the designer was trying to do here, but they got the polarities backwards. the accent was supposed to charge up the integration capacitor, but this made the output signal the wrong polarity, so they added a new pathway from U25B, but didnt remove the old pathway (D55 et al). but, U25B is what turns on the decay, so you are either decaying or attacking, there is no "hold". so D13 was put in to force the integration capacitor at a fixed value for a "hold". but this then made U25A not engage. but im sure at that point the synth needed to get out the door, so they said "good enough" and shipped it.
openmusiclabs.com

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 » Sat Feb 22, 2020 3:38 pm

This is fantastic, guest. Your efforts here are much appreciated!

Perhaps you could clarify something. I noticed two R63 on the schematic, and your reference to both separately:
guest wrote:
Fri Feb 21, 2020 1:26 am
the VCO is a pretty standard dual OTA setup, which could be easily converted to a triangle wave, if so desired (remove R63). this would drop all notes down one octave.
...
R63 should be removed, as it provides a leakage path for the frequency setting current, and is not required fro the reset cycle. speaking of which, the reset cycle is quite long (100us), which adds to the nonlinearity. there really isnt much that can be done about this, other than going to a different reset format (JFET across the integraction capacitor).
So from what I gather, removal of R63 (30.1k going from the comparator stage buffer to the amp bias input) would convert this to a triangle VCO since U3A's control input would no longer be assisted by current from the comparator output for half of the charge/discharge cycle. Leaving that 30.1k resistor in, but removing the (other R63) 10k resistor at U3B's buffer output would eliminate the leakage path for the control current, thus improving linearity, correct?

Regarding the VCA compression: Since the current mirror Q11/Q12 serves to pull down the envelope CV, it seems increasing/removing R77 would reduce/eliminate the compression.

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Sat Feb 22, 2020 8:10 pm

astrosound wrote:
Sat Feb 22, 2020 3:38 pm
So from what I gather, removal of R63 (30.1k going from the comparator stage buffer to the amp bias input) would convert this to a triangle VCO since U3A's control input would no longer be assisted by current from the comparator output for half of the charge/discharge cycle. Leaving that 30.1k resistor in, but removing the (other R63) 10k resistor at U3B's buffer output would eliminate the leakage path for the control current, thus improving linearity, correct?
yes, this is correct
Regarding the VCA compression: Since the current mirror Q11/Q12 serves to pull down the envelope CV, it seems increasing/removing R77 would reduce/eliminate the compression.
removing R56 and R57 would eliminate it completely (although the bias level on U1B would have to be adjusted). leaving R56 in would change the compression to expansion. when the voltage across R73 and R76 is less than 0.6V, R56 is not active. but, above this level, it becomes active and draws more current than R73+R76, therefore increasing the gain. it might be an interesting effect. the DC bias on U1B would probably still need to be changed, though. those transistors could also be repurposed to make a perfectly linear control current by wrapping them around U1B, or just using it as a current mirror like a 3080 input.
openmusiclabs.com

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Sun Feb 23, 2020 1:19 am

im working on the hihats right now, and i must say i actually quite like how they sound - they almost sound digital, which is wierd. my main complaint is that the tune doesnt have much depth, and the decay doesnt do anything for the majority of its travel.
openmusiclabs.com

ihav2p
Common Wiggler
Posts: 68
Joined: Wed Apr 28, 2010 2:03 pm

Re: rhythym wolf teardown: any questions?

Post by ihav2p » Sun Feb 23, 2020 1:55 am

...
Last edited by ihav2p on Sun Feb 23, 2020 2:03 am, edited 1 time in total.

ihav2p
Common Wiggler
Posts: 68
Joined: Wed Apr 28, 2010 2:03 pm

Re: rhythym wolf teardown: any questions?

Post by ihav2p » Sun Feb 23, 2020 1:56 am

guest wrote:
Sun Feb 23, 2020 1:19 am
im working on the hihats right now, and i must say i actually quite like how they sound - they almost sound digital, which is wierd. my main complaint is that the tune doesnt have much depth, and the decay doesnt do anything for the majority of its travel.
a funny thing i did was to reverse the open hat part of the dual pot so you have long closed and short open or vice versa. the voices are the same so turning the knob eventually evens them out and then inverts the pattern.

ihav2p
Common Wiggler
Posts: 68
Joined: Wed Apr 28, 2010 2:03 pm

Re: rhythym wolf teardown: any questions?

Post by ihav2p » Sun Feb 23, 2020 4:41 am

some more data points: i tried removing d13, which didnt seem to do much, although it may indeed have taken the top off the envelope; it did add a kind of flam/double trigger at the end of the envelope.

[edit: put another diode in, and yes the flat top was removed when removing d13. but the flam is also gone w the diode. probably the env tail was going negative w/o the diode]

one thing that might be useful regarding the way accent was supposed to work. if i loopback the midi, the different accent levels shorten the decay envelope. high accent has the shortest envelope. whereas without the midi loop there is no influence on the decay time. overall the decay is also a lot shorter with the midi loop even comparing with d13 removed.

User avatar
guest
Super Deluxe Wiggler
Posts: 3954
Joined: Mon Aug 19, 2013 11:49 am

Re: rhythym wolf teardown: any questions?

Post by guest » Sun Feb 23, 2020 10:31 am

the comparison for "when to shut off" is 1 diode below ground, but that diode is a regular diode, and the one in the feedback loop was a shottkey. if youre going ot leave it like that, id reccomend replacing the comparison one (D19) with a shottkey, just to give it a bit more headroom and eliminate false triggers.

with the midi loopback, is it getting 2 triggers in quick succession? and is it the VCF and VCA envelopes that are shortened? i could definitely see the VCF envelope shortening under these conditions. the D55 et al pathway would make sense then.
openmusiclabs.com

Post Reply

Return to “Music Tech DIY”