rhythm wolf teardown: any questions?

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ihav2p
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Re: rhythym wolf teardown: any questions?

Post by ihav2p » Mon Mar 09, 2020 3:50 am

I FINALLY got the filter to work in a way I am happy with:

Instead of altering the resonance feedback in the VCF itself, I decided to make changes after the filter. Basically I mixed the resonance knob and filter output at R58. I think this is in line w/ the basic idea of what the 303 does to compensate volume.

(The actual mod is: remove R58, replace with 200k. Connect the wiper of the resonance pot to the northern pad via 10k resistor). These values give a good mix and even at "laser" resonance, the volume drop is about what you would expect from a normal synth. If you want a little boost as the cutoff goes down, go for 20k instead of 10k. It sounds really good! So happy! :love:

The rest is like the OG schematic before any alterations, except the following: I reduced R124 to 1k. R175 is bridged. R174 is 6.8k, but only because I didn't have 7.5k. These make the resonance more sharp at the knob extreme.

The funny thing is I researched OTAs to death, and even learned LTSpice before discarding all of it and just doing the above. I also can't believe I never blew up the chip, knowing what I do now about the bias inputs. Be careful if you are experimenting!

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Re: rhythym wolf teardown: any questions?

Post by guest » Mon Mar 09, 2020 4:58 am

yeah, thats a good way to go. both positions of the resonance pot have their downsides. in the other position, the volume increases with resonance, so this allows for a good compromise.

im a bit suprised by the component changes in the filter, though. i would expect a fair bit of distortion and noise from those low values. but if it works, it works! congrats on getting it the way you like it.

i just got a camera, and am working on getting the sound right for the A of my A/B video series.
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Re: rhythym wolf teardown: any questions?

Post by ihav2p » Tue Mar 10, 2020 1:21 am

You're actually right, there is some distortion at settings with low resonance and low cutoff. So keep that in mind if you like the smoother bass tones.

Looking forward to the video. Will definitely be watching.

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Re: rhythym wolf teardown: any questions?

Post by guest » Sat Mar 14, 2020 5:59 am

just did the power supply section.

http://www.openmusiclabs.com/files/rw_power_lg.png

i think i already described it, but it basically takes the 12V switching supply wallwart, and runs it through an LC filter for the +12V analog supply line. it also takes the input and inverts it with a switching regulator to make the -12V analog supply. and, a second switching regulator is used to make the 3.3V digital supply. there is a signal to mute the audio output for a bit on powerup, and another signal back from the microcontroller to delay the start of the analog rails until the digital line is set.

i should have the first video up soon. it took a long time to get the audio somewhat ok.
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Re: rhythym wolf teardown: any questions?

Post by ihav2p » Sat Mar 14, 2020 6:04 am

I was wondering if it really draws 2a. I can run it on a usb step up converter and a battery for about a half hour, but it overheats and shut off. The converter is rated up to 800ma...

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Re: rhythym wolf teardown: any questions?

Post by guest » Sat Mar 14, 2020 6:18 am

it draws 0.5A @12V, so a 5V source would need to supply 1.2A if it was 100% efficient. a 12V cordless drill battery might be a good choice.
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Re: rhythym wolf teardown: any questions?

Post by ihav2p » Sat Mar 14, 2020 6:25 am

good to know, thanks!!

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Re: rhythm wolf teardown: any questions?

Post by resynthesize » Mon Mar 16, 2020 12:18 am

so i got my rhythm wolf. i actually like it a lot as is - but one thing i'm missing is an analog reset trigger input. @guest, while you're hacking around, if you come up with a way to add this I'd be keen to try it out.

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Re: rhythm wolf teardown: any questions?

Post by guest » Mon Mar 16, 2020 2:37 am

by reset, you mean reset the sequencer to step one? is the only way to do that now by pushing start/stop twice? you could put a transistor across the start/stop button, and have it get pulsed twice with an incoming pulse. a logic ic should do the trick here.

there are definitely sounds i like in the rhythm wolf, but for me its like only 20% of the knob settings are good, which seems unreasonable.
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Re: rhythm wolf teardown: any questions?

Post by guest » Wed Mar 18, 2020 12:56 pm

so im finally getting started on the mods. right now im working on the "replace components only" mods. i think i have the VCF envelope working as it should, and i now understand what R209 is all about. when it works properly, the attack time is less than 1ms. you then have 4 more milliseconds of accent signal going through R209. this discharges the cap, and puts a peak on the signal, followed by a slow decay. as accent is higher, the decay time goes down becuase more charge is taken off the cap.

the 750ohm value is really low, so right now im tweaking it to see what a better value would be.

i also found a few mistakes in my schematics, and will be updating them soon.
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Re: rhythm wolf teardown: any questions?

Post by guest » Fri Mar 20, 2020 6:15 am

ok, i think ive got the VCA envelope to do what i want:

1. replace C147 with 4.7uF
2. remove R265 (or cut trace to P26A, or cut P26A pot leg)
a. connect collector of Q47 to C147/P26A juncture with ~5k resistor (if you leave R265 in place you can reuse that resistor)
b. connect cut side of P26A to ground with a 22k

this sounds really good. basically, its both gated and decaying, just like the 303. so while a note is held high (or tied) it will slowly decay as set by the decay knob. but, once the note goes off, it shuts off the voice. the 5k (50ms) resistor sets the decay when a note goes off, and the 22k (200ms) sets the min decay while note is on. the max decay time is set by the capacitor, which for this case is around 6s for max accent, which i think is a nice time for those really long slides.

the rewiring of R265/P26A can be done a number of different ways.
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Re: rhythm wolf teardown: any questions?

Post by ihav2p » Fri Mar 20, 2020 7:47 am

sounds good! as i recall one problem i ran into with the sequencer was that the gate time per step is not really optimal. it doesnt really relate to the bpm. have you seen that too?

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Re: rhythm wolf teardown: any questions?

Post by guest » Fri Mar 20, 2020 8:29 am

it looks like the gate time is 1/2 the note time. on the 303 i think its 2/3 note time. but, it does that stupid PWM thing on the gate line during ties, which C155 helps with.
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Re: rhythm wolf teardown: any questions?

Post by ihav2p » Fri Mar 20, 2020 8:32 am

Really? Maybe mine is messed up. It's about 1/2 step time at 40bpm but at 140 its more like 4/5ths. I'll play around with it.

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Re: rhythm wolf teardown: any questions?

Post by guest » Fri Mar 20, 2020 8:42 am

maybe whats happening, is that the decay time is stretching the notes. thats the nice thing with this mode, it cuts off the note with the gate. honestly, the VCA and VCA envelope were my main gripe with this thing, and doing this fix has really made a difference for me.
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Re: rhythm wolf teardown: any questions?

Post by ihav2p » Fri Mar 20, 2020 8:45 am

Ah yes that would make sense; if the minimum decay time was fixed, the faster bpm would crowd the notes together. Thanks for the mod, will definitely give this a try.

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Re: rhythm wolf teardown: any questions?

Post by guest » Fri Mar 20, 2020 8:59 am

there might be an easier way to do the mod:

1. cut the trace from P26A that goes to U27A/C147. be sure to cut it before the branch, so that C147 is still connected to U27A.
2. on the bottom of the board, run a 22k resistor from the P26A leg closest to the line you just cut, to ground.
3. on the bottome of the board, run a wire from the opposite leg of P26A to the +pin of C147.

this reverses the pot, so maybe that jumper between the pot legs also needs to be cut (which would be a bit of a pain), and soldered to the other leg. ill think on that a bit. at any rate, this makes all the work THMT work, rather than trying to solder to small 0603 pads.
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Re: rhythm wolf teardown: any questions?

Post by ihav2p » Fri Mar 20, 2020 11:50 am

Don't have mine open but it seems like using the first configuration you could cut and bend up the leg of P26, then wire the pad it was soldered to to the P26 wiper. Then just use a 22k resistor to ground from the leg.

Then you replace the cap, which you have to do anyway

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Re: rhythm wolf teardown: any questions?

Post by guest » Fri Mar 20, 2020 12:13 pm

that would be the easiest, except the leg to cut is on the inner pot. so its behind the first row of pot legs. i tried getting my snips in there, but they were too big.

i did the last version, and i forgot about the trace from Q44. there is a short link on the back of the pcb that makes cutting that trace easier, but then you have to run a link back from Q44 to the pot. it seems like theres no free lunch on this one.

im working on the VCF right now. although your fix for the volume is a good one, i just couldnt stand the shifting cutoff frequency as i turned the resonance up, so im experimenting with the other placement of the reso pot. my god does it get boomy and distorted. im going to have to back the gain down a lot.

also, i ran some tests on the VCF. the cutoff knob goes from 47Hz to 666Hz with reso at min, and from 250Hz to 3.3kHz with reso at max.
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Re: rhythm wolf teardown: any questions?

Post by ihav2p » Fri Mar 20, 2020 12:17 pm

guest wrote:
Fri Mar 20, 2020 12:13 pm
that would be the easiest, except the leg to cut is on the inner pot. so its behind the first row of pot legs. i tried getting my snips in there, but they were too big.
yikes what a pain. now i get it.

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Re: rhythm wolf teardown: any questions?

Post by guest » Wed Mar 25, 2020 2:24 pm

so, after many days of digging, i finally understand what the percussion section is supposed to do, and how its supposed to sound. i did a few days worth of work on it, and ended up right back at the original, with just a few mods. it actually sounds decent. ill have a more detailed post on this later, after i finalize the design, but so far:

1. D50/1 are there to clamp the inputs to the filter so that the accent doesnt have an effect on them. it has a small effect, but not as much as one might expect. why do this? wouldnt you want accent to make notes louder? well, that brings us to point 2.

2. U22B is meant to disengage the compressor for a short bit during the attack phase. but, since C126 does not have a discharge path as a result of D54, it has no effect. putting a 10k resistor to ground after D54 activates this circuit. the larger the accent, the more it disengages the compressor, letting a larger hit come through at the beginning of a note. so the accent is entirely handled by U22B and the compressor circuit, so the pinged filters are kept constant.

3. U22A is meant to silence the compressor amp when no notes are playing. this is a good idea, but its hard to say when a note is playing, so they just use the 150ms envelope for this. the problem here, is that this is too long, and deactivates the compressor for almost the entire note duration. reduce C149 to 100nF-330nF to make it 20ms - 50ms. i cant say for certain what this value should be, as ive gutted so much around that circuit at the moment.

4. the noise VCA is just a gate, and doesnt add much (in my opinion). you can swap the signals between the gate and drain, and get a useful VCA out of it, but it still just replicates sounds you can get elsewhere on the synth (hihats and snare). so i just removed R8 to keep the noise bleed from getting into the circuit.

5. if youre not going to use the noise source, then P24 can be used to adjust the mix between the 2 pinged filters. this actually sounds great, and i highly reccomend it. cut the trace from U21A to P24. replace R193 with a 10k (or lower if you feel the high note is louder than the low note). remove R107, C214, C120, and C131. run a wire from the output of U13B to the cut end of P24. replace R296 with 100ohms.

6. to make the compressor work as it should, replace R304 with a diode (cathode to U19A), and put a 100k resistor in parallel with C39. this gives it fast response for the attack, and a ~20ms decay time. longer decay times make is useless for the high note, and shorter decay times worsen the response for the low note (which is a little bad).

7. i would reccomend putting another pot in, in parallel with P4, and running the wiper to the compressor input. this will allow you to vary the compression ratio, which opens up the sound a fair bit. it changes how hollow sounding the hits are.

i have 2 more things i want to see if i can implement, and then ill call it quits on this voice. the first is getting the mute to work correctly. i think i can repurpose U22A as a comparator on the peak detector, and use it as a gate. this might make for some fun sounds by varying the gate amplitude. the second is getting a variable attack time. i got this sort of working, and it sounds pretty good, but it goes opposite to how the accents should work, which is a bummer.
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Re: rhythm wolf teardown: any questions?

Post by guest » Fri Mar 27, 2020 11:45 am

ok, here is the first of the finalized mod sheets: the percussion voice

http://www.openmusiclabs.com/files/rw_perc_mod_lg.png

i made the decision for my box to completely cut the noise tone. it didnt sound good, was a lot of work to get sounding ok, and duplicated the same sounds i got from other voices. instead, i used the mix pot to mix between the hi/lo tones. this gives a much wider range of sounds. this involved disabling the noise source so it wasnt splattering all over the circuit, and cutting the trace to the mix pot. for mine, i cut both ends of the pot, and wired directly from the outputs of U13. but, its easier to just cut one side and adjust the volume on U8B. C120 is removed to keep the opamps from having to drive a large capacitive load when the pot is at the extremes. R296 needed to be reduced as the levels were much hotter now. 100ohms was a good value to put it just at the edge of distortion across the whole range. you can go a bit smaller if you want it perfectly clean, but the SNR gets worse.

the next thing i did was mod the hi/lo tones so they are louder with more accent. i think it was originally done as a fixed volume because the mute circuit also did envelope control. but, once i got the mute function going, i decided i liked the variable volume effect anyways. this is done by removing the diodes at the inputs of U13, and replacing the capacitors with resistors. i picked the values to give a bit of distortion at the highest level. the resistors can be increased by 20% or so to eliminate this, or reduced by 20% to increase this distortion. due too these capacitors being replaced with resistors, the decay time was made shorter. its ok, but the notes lost a bit of that hollow sound, so i removed R279 and added another 1uF cap to bring the decay time back to where it was.

next up was the accent circuit, which just needed a 10k drain resistor for C216 to discharge through. this gives a click at the beginning of the notes, which is louder with more accent. im not sure i fully like it, but it can be easily switched out by disconnecting the drain resistor (or any other method of blocking the signal).

finally, the mute circuit, which probably worked on their bench, but not in production due to the wide spread of JFET parameters. so i measured the JFET active range for my VCA compressor, and it was -4V for full off, and -3V for 1/2 gain. full gain was up at -0.5V. so i modded R253 so the output was around -8.5V with no signal. this, mixed with the 0V output of the fullwave rectifier, gives -4.2V, which ensures that the VCA is off when there is no signal. this greatly reduces the background hiss of this voice. next, i decreased R6 to 36k to increase the envelope. this allowed the compressor to open up, giving longer notes that were louder, and acted more like a compressor. if you put a 1.2k from the inverting pin of U22A to ground, this increases the gain even more, and basically gets rid of the envelope effect entirely, just gating the VCA.

other things i tried, but didnt want to add switches/pots for at this time:

1. put a pot or switch on the signal going from the output to the compressor. a pot here is great. for testing, i just wired from the volume pot wiper, and was tempted to leave it like this. this allows a really wide range of sounds from the voice. without the compressor, they are very much just resonant ringdowns, very sharp and short. with the compressor, they get a nice hollow sound to them.

2. put a diode across R304. this makes the compressor turn on instantly, which sounds pretty good when there is no accent added. it also increases the compression effect, but does also distort the beginning of notes some.

3. i tried higher compression gain settings, and they were fun, but ultimately i decided it was too much for a fixed setting, and they distorted a lot.

4. fixing the noise source. this is possible, but complicated. basically, the JFET needs to be wired differently. the noise source should go to the gate, and the the envelope should go to the drain. since JFETs vary a lot, you will need to test values to get the response you like. the way to do this, is to take a 100k or so resistor, and tie one end to the noise source, and the other to the gate. with a variable 100k resistor with a 50k in series, connect it to -12V. adjust the pot until the noise source is sounding right. then check the value and solder that value in. the other thing you will need to do is to adjust the volume level. it will most likely be too high and distorting. to fix this, you can either use a smaller drain resistor, or attenuate the noise input. the latter is probably easier, as the envelope will need adjusting elsewise. with a 100k drain resistor to the envelope, the capacitance will need to increase at the envelope by another ~1uF. the attack time become a bit unreasonable above this point, and R295 might need to be shorted out. putting a few capacitors around the circuit would allow shaping of the noise a bit.
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Re: rhythm wolf teardown: any questions?

Post by ihav2p » Fri Mar 27, 2020 12:18 pm

This is amazing! Thank you!

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Re: rhythm wolf teardown: any questions?

Post by guest » Fri Mar 27, 2020 12:22 pm

thanks for following along. if you do any of the mods, id love to hear what you thought of them.

also, out of curiousity, how have you been on muffs for 9 years and only have 67 posts!
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Re: rhythm wolf teardown: any questions?

Post by ihav2p » Fri Mar 27, 2020 12:31 pm

more than 50 of those must be in this thread! At the time I joined I was doing Jurgen Haible projects, but then I kinda lost interest in modular and sdiy for a long time...Kinda fun to be back

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