PWM...Minus the Modulation

[liquids]

Yeah, I don't care for that modulated sound, which although synthy is not what I'm looking for. The clips on Tim's site both use the LFO for pulse modulation.  So I wasn't too amped about building it, but last night said, what the heck.  While interesting and all, I build it without the LFO and just the pot. 

For those who haven't built it--Minus the LFO it's just a pulse wave shaper.  It tracks beautifully. It's got a sound that is unbelievably synthy, as a basic building block (minus filters, etc), and even useful on it's own as a lead tone, especially combined with some delay or reverb for dramatic effect. 

I like starting with a pure synth tone, a wave.  I like the simple square wave shaper made simpler's sawtooth shape synthy sound, for one, but still have that on the back burner for finalizing it, thinking I can improve it as I gain more experience with effects. 

I love my modified Shocktave/Shocky that I just built for that low-octave square wave synthy kind of tone, and will be using that a lot as well.

I have been wanting to try a Moog Freqbox for ages now and it's hard to find a place with one in stock to demo...but this scratched my itch...but also made it worse, out of curiosity for what else could be done.  But now I've got stuff akin to a square wave, a sawtooth wave, and a pulse wave on tap!  All DIY! This is crazy!

Anyhow, I am digging this thing, but wanted to mention some finding, and some Mods for the PWM circuit for other peoples experimenting:

I found that changing the .005uF (really, .0047uF) cap between pins 2 and 3 of the trigger as per the schematic made interesting differences.  Making it bigger more so removed usable sounds along the pot range, and added nothing to the other side. But making it smaller, such as my changing it to .001uF cap added usable sounds - the "thinner" pulse wave range (the side of the pot approaching <1K to ground) was much nicer and more real sounding thiner pulse waves, but you loose the warm 'bleepy' Nintendo lead sound of the full pot resistance, which is a wider pulse wave.  I'm going to experiment more with various caps and possibly hook two or more to a toggle on any final build, for different tones depending on musical situation . I didn't try anything lower than .001uF, but intend to soon, as it may have even better range for the thin pulse wave settings?

As you approach no resistance to ground on the pot, the pulse wave does "zero out" into silence.  This was something I found odd about the sound clips, and has been mentioned here elsewhere.  I think with some finagling, a low value resistor in series with the pot to ground would keep the pot (and especially the LFO) from doing that if need be, if that frustrates you.

Questions: The sustain on this thing could be better.  I can live with it, and stock it's better than the SWSMS is stock, as low notes, and notes below the 12 fret are good enough that I don't worry too much about sustain when playing around with horn-type melodies. But I'd like more sustain anyway, especially as notes on the high E and B strings approaching the 12th fret and beyond pretty much just "bleep" for a milisecond or two, and then gate.  Running my Ross-style compressor into it helped a bit, but I'd obviously when I veroboard my final build I'd like to combined it with a simpler circuit running into the input; when I build it is all as "one unit," but without changing the tone and response too much. 

So, of course, I went with the simplest dirt circuit I could think of and breadboard to see if it would improve anything---a bazz fuss---running into the 386D, but I pretty much had to power down for the night after I plugged that in, so I'm not sure of the result and have to wait all day today to find out.  It's going to kill me until I get home late and have a total of 3.5 minutes to try it.     I may try a simple elektra type circuit, too, after I mess with the bazz, for comparison.

Anyone else have ideas or success at getting more sustain out of it?  With more sustain, it's nearly perfect, if you ask me! Surprisingly convincing.   Does the input of the 386D need a lot of 'gain' at the input to drive it correctly in this circuit?  My pickups straight in did fine, but I don't understand this thing at all, so it's hard to tweak it with any intelligence, it's more stabbing, er, breadboarding in the dark...

[earthtonesaudio]

As shown here: http://www.geocities.com/tpe123/folkurban/fuzz/pwm.gif
The gain of the 386 is set at 200 (which should be PLENTY) but the input impedance is 50k (set by internal resistors).  This low of an input Z can load your pickups down a bit and possibly reduce the total gain of the system.  Your boost-in-front might be the cure.  If not, check the supply voltages.  386 chips come in different flavors, and some like to work better at slightly higher (12-ish) voltages.

Your intuition about shrinking the capacitor seems good to me.  The second Schmitt trigger stage is acting like something between a differentiator and high-pass filter.  Really small caps will make it act more like a differentiator.  I'd start with 10pF and work up!

When the "pulse width" control is set to zero resistance, the signal is indeed shorted to ground. 


One other thing that might be interesting to try:
Add a 10k pot in series with the power pin to the 40106 (leave the 386 as-is).  This might be fun.  If my hunch is correct, it should function both to decrease the input trigger range (for more sustain) and change the oscillation frequency.  A voltage divider instead of a 10k series resistance should also work, provided the total series resistance to the power pin remains reasonable.

[liquids]

Still working on this...which has been both encouraging and discouraging.  I always forget that tweaking something I intend to set in solder always takes me a good while anyhow, even when I end up back at the stock schematic.   

I had already put a buffer into the stock schematic, and hadn't heard much difference either way previously, so I'm not thinking input impedance is contributing too much to sustain.

I tried a bunch of things yesterday that were all "hmm, interesting," but nothing I tweaked was a "there it is!" revelation in regards to sustai, yet.  That is, except for the need to really hit the high strings with more than a light touch like I'm used to, to get a bit more of a full note out of them.  Taht increased sustain a fair amount   

The 4.7uF cap on pin 1 to boost gain, that I believe I saw Dano recommend definitely was...interesting...but it created a lot of oscillation. It definitely improved sustain, but the oscillations made it hard to work with because it eliminated a bit of the 'gated' silence which makes the synth tones more convincing.   It may affect the tone a fair amount too, all other things being equal.  I'll keep working with this.  The oscillations may be due to the breadboard and the open, sloppy wiring I have on my breadboard interface, though.  Maybe another inverting preamp stage would remedy it...but I have no idea if the output is in our out of phase with the input as per my question: how many, and which stages of the 386 and 40106 invert the signal here?

The Bazz Fuss to add sustain on the input works to some degree so far, but not so much that it ended my search, as if I recall, at levels where it inncreated sustain, it likewise messed with the gated decay.  Of course I just realized I was sending a buffered signal to the Bazz when messing around....so more experiments are due here too.

Adding another stage from the 40106 as I saw on Danos page didn't do much to the sound that I liked, so I'll put that aside for now.

the Variable pot to the schmitt triggers power pin didn't do much, other than make a "ghost note"-like sound of the clean guitar signal in the background (?).  It gated better stock/with the pot at no resistance, without any dramatic or noticeable improvement in apparent sustain otherwise.

Messing with a low pass filter for the tone didn't do anything I noticed or thought was useful as an option, let alone an improvement--which is ironic, because filtering on the SWSMS and Shocky was a 'necessary' thing if not for tracking (the later) but for tone quality (both).  Not here, to my ears, at this point.

Going smaller on caps between pin 2 and 3 on the 40106--I tried a 470pf, for instance---didn't do much for me.  the .0047uF and .001uF seem to offer the best range with two notably different range of tones and to my ears were usable almost throughout the pot range with each cap, rather than just a narrow range.  If anything I may examine the subtle differences between a .0022uF vs. the .001uF, and a .0068uF instead of the .0047uF but that just down to nitpicking for when I solder it all up.  I'll want on of each range both, and toggle to switch between two will be very useful and all I need...

Again, anyone looking for synthy tones--ignore the sound samples, and build this!  You'll enjoy it if you like the sound of pulse waves.  Or, go here and listen to the sound samples: http://www.martoneaudio.com/PulseSynth.htm   The tones I'm getting are a lot like the first sample, but need not sound so reedy, depending on how you set the pot.   I wouldn't be surprised if that pedal is based on the PWM, and would also be interested in knowing if Mario, the builder of all the very cool pedals on that site (and more) surfs here.

[liquids]

Is anyone else working with the PWM currently?   I guess hopefully this info is useful to anyone else who stumbles across this thread and builds a PWM in the future...

Did more expirimenting. I did audio probe the sound coming out of whatever gain stage I had feeding the 386D as well as the output of the 386D for reference (which goes into the 40106 pin 1), to zero in on what each chip was hearing at input and output.

The 386D distorts a bit on its own with a hot signal feeding it, especially with a cap to ground on pin one.  I realized when audio probling that what I wanted to do was overdrive the input of the 386D hard to get more 'compression.'   The 'one stage boost' of the 386 or even the bazz fuss was mostly just a volume boost, but what I needed was some controlled compression and or distortion so that notes sustain more.

The bazz fuss based signal into the 386D didn't do it for me, it turns out, because of this.  Neither did feeding the 386D input directly, with a cap (1uF, 4.7uF, 22uF etc) from pin 1 of the 386D to ground for gain increase.  I needed a smallish cap to ground off the input signal when I did this as I got squealing as mentioned before.  The sound was grittier, but had a fast recovery from clipping, rather than sustaining.  I  realized I needed something clipping a fair amount by itself going into the 386D, to create a fuzzy distortion with a bit more sustain than just one overdriven stage would have.

The simple electra distortion (again, keeping it simple first and foremost) was an improvement, especially with some mods. I put a 2n5089 in there, put a cap off the emitter to ground to give it max clip, with something like a 4.7k resistor to ground off the same emitter, and fed that into the 386 input.  I'm still messing with the diodes to ground off the collector and if I need them at all, and likewise if smaller (germanium) voltage diodes would make this better or worse.

Audio probling at the 386D output revealed a compressed, harmonic-laden fuzzy distortion.  This arrangement created a fair amount of grit but so much gain going into the 386D, though, that it kept the whole thing "on," so the silence or 'gating' effect went away.  I then put a 4.7K/1K range resistor to ground (like a volume control) off the output of the electra feeding into the stock schematic, and the gating returned.  Now, compared to the stock schematic with no pre-gain stage, the harmonics had increased, as did the sustain.  It's not infinite--though it nearly is with low notes--there's now a fair amount of sustain on higher strings and frets--but it's much better. 

Tradeoff: it's much more sensitive and, as wanted, compressed, so little 'scratchy' noises across strings or mis-pick release the gating too, while present. This means you have to be much more careful with your playing, but overall I think it's worth it for the improved sound and playability--in a live mix, the effect will be worth it, and some glitches will probably add to it's coolness and oddity, rather than trying to create 'pure tones.'   I'm really digging it!   But will invariably continue to experiment.     Any other ideas? 

As mentioned above, the 386 has a gain of 200.  What is the gain of a typical op-amp, or CMOS stages like the tube sound fuzz, for example, for comparison...maybe that is another way to go, and one that is more familiar to me than the 386D.  If there is no magic to the 386D other than creating a lot of gain, maybe other manners of creating large amounts of gain to feed the 40106 a good signal would be tonally advantageous, even if involving a few more components.

[smnm]

I built a stock PWM without LFO into a wah casing, and although I like it a lot, it doesn't get used. I was thinking of building another in standard pedal format, and might look into some of your ideas. Any chance of a schem to summarise them?  Thanks.

[Heemis]

That's quite a lot of info you've provided and lots of great insight into this circuit!  I built a pwm about 6 months ago, and basically used a combination of the original design and a some of dano's ideas, and also incorporated a switchable LFO.  I really like the sound, brutally fuzzy, and totally synth like.  I had luck using a booster into the pedal, but like you, I wanted more control.

I am currently working on a re-work of the design to include an op amp booster before the 40106, and a buffer after it using a dual op amp.  Also I will be incorporating a triangle lfo shape in addition to the square, and lastly I will include an expression pedal jack to enable control of the pulse width using an external expression pedal or control source.

I will let you know when I've made some more progress, too many other projects filling up the bench at the moment. 

[puretube]

Does it do the 98% duty-cycling? (1° - 359°)?

[mills]

Thanks for all the thoughts!  I breadboarded one not too long ago, and needed to tweak the LFO and controll options a bit before I boxed it up.  Move on to other things for a while, but I'm going back to it soon so I'll be watching ths thread.

[liquids]

I'm not that good with schems and stuff, but in a few weeks I may get something going.  For now, Ive got an electra distortion  http://www.geocities.com/CollegePark/Library/1355/electra.gif, and happen to be using a 2n5059 instead (or 2N5088, of the 2n3096 would be fine too I think), but with an added cap to ground off the emitter (to boost the gain), running into the stock schematic.  The volume pot out is good and will help you dial in the output into the stock PWM schematic.  I may just pick a value that works with my setup and leave it rather than have an added knob on my final build, but there are advantages both ways.

Otherwise I would have the PWM stock as well, with wired running from pins 2 and 3 (pin three connects to the pot) on the 40106, to a DPDT toggle.  One side of the toggle would have a .0047uF cap across it, the other a .001uF across it, to switch between difference ranges for "pulse width" that I like.   

I'm only in the breadboard stage so I haven't had to think too hard about all that yet. 

I think the next step for me is trying to understand the 40106.  I've not done much of anything on that end of the circuit because I understand 386s very little, and 40106's null.  So working with it is fun, but I keep thinking if I understood even a little bit more I'd be able to do a lot more...also, I have no idea what 98% duty cycling is. 

[bubbles]

hi,

i'm in the process of building a pwm (minus the modulation, too).  i breadboarded it, liked the sound, then put it on perfboard.  i plugged it in to make sure it worked, then boxed it up.

however, i'm having a hell of a time getting rid of the switch pop (dpdt true-bypass, no led).  i've tried pull-down resistors at the input and output.  i added a 0.1uf cap at the input (in series).  the resistors are on the outside of the caps.  i've tried 1m5 to 4k7 resistors, but the pop persists.  just now, i breadboarded another in case i messed up somewhere on the perfboard.  added a dpdt true-bypass switch to the breadboard and tried it with and without pull-down resistors, but the pop is still there. 

is anybody else having this problem?
is there anything else i can do?

thanks.

[auden100]

Digging up this old thread because I've been playing with it a bit along the same lines. I tried the Electra Distortion, but I was still getting a lot of unwanted harsh sounds from string rubbing. I tried another similar single transistor fuzz, the Orman Muffer and it was the same. Eventually, I decided it was because the PWM was getting too loud a signal. After sticking a RC lowpass in between the fuzz and the PWM, things really cleaned up a lot. Right now, I'm using a 220k resistor and a 22nF capacitor. But these might be tweaked a little bit more.

Also, someone may have mentioned this, but Beavis Audio incorporated an RC lowpass at the end of the PWM, and that can make this thing belch out some really fat bass. (1k pot + 1uF electrolytic).

Due to parts availability, I'm using a 250k pot for the Pulse Width control. I've got a 10nF capacitor instead of the 5nF in the schematic. Gives me more low end. Caps of larger values didn't make a significant change to the low-range sound. Making this switchable with another smaller cap still seems like a good idea, but I prefer the bass-ier sounds.

Orman Muffer -> Lowpass(220kR-22nC) -> PWM(10nC) -> Lowpass(1kPot-1uC) -> Volume.

All in all, I love this effect. Super synthy, ranging from fat bass to squelchy octave-up. Making a smooth sine, slow modulation for the PW control would be beautiful, but it definitely works on its own. When I get around to it, I'll probably make a new schematic and post it.

[liquids]

Nice!

[Strategy]

I built PWM without modulation, but added an expression pedal input for pulse width. You need a 500K pedal, which you must hunt down or build, but it's great. Very expressive

The only issue i have with this pedal is that the guitarist in my band has taken control of it. That is great, because he sounds great with the pedal and I'm glad he loves it - but now I have to build another one for myself 

- Strategy