New CMOS design, help much appreciated

[earthtonesaudio]

Here's a sort of piled-together bunch of CMOS ideas.

Gain1 is from Frank Clarke's "Hot Harmonics" design.  Gain2 is from the Runoffgroove "UBE Screamer," and Gain 3 is an adaptation of Tim Escobedo's CMOS drive (he used switches not pots).  They are just there to showcase the different ways of using a gain control.  I would probably only use one in a "final" design.  I tried voltage divider between stages, but got a bad scratchy pot sound.  Tone is based on the Big Muff tonestack, but with the extra 10n cap on the wiper of the pot to have a mid-scooped sound at one end, and a mid-boosted sound at the other.  After the tone stack are 3 gain-of-two buffers to bring the level up after the tone control.  They probably add some distortion too, but I'm not sure.

One could probably tweak this for whatever purpose, but my goal was an alternative to the BSIAB/Hot Harmonics/Dr. Boogey/cascaded gain stage type of pedal.  I wanted to use a single active component (4069), and I don't like trimpots so I ruled out JFETs right away.

..Values are theoretical.  I haven't tested this out yet.  ...C6 is supposed to be .1u, forgot to put that in.



Any help would be much appreciated.  Oh yeah, and I accidentally bought a bunch of 10k pots, so I'd love to use those anywhere I can.

[nelson]

Why not lower the feedback resistors of the last three gain stages? You will introduce a ton of noise the way it is and there is no need for them to be so large.


I think if you want to go the route of cascading gain stages to mimic tube amps with CMOS you should include interstage filtering to mimic the tone shaping, mimic the input Z of a typical tube amp (1M), not to mention tailor the gain for each stage, that will get you as close as possible with a CD4049.

[earthtonesaudio]

Thanks for the reply Nelson.  I just arbitrarily made the last 3 stages identical.  I wanted to feed the tonestack into a high input Z stage, so that's the reason IC4 has a 470k input... and I wanted to bring the level up so I made it and the ones following each have a gain of 2. 

I was thinking about the tone shaping between stages, and I had the idea to use the capacitor as a lowpass, and the input impedance as a highpass... so small cap, small resistor in series would be a bandpass.  I am having trouble simplifying the JFET based amp-mimics into simple gain stages.  Not sure what the gain should be between stages... I guess more tweaking is in order!

I would like to keep the resistor values (and capacitor values) as low as possible throughout so as to keep noise down, but I'm not sure how to do that while keeping gain and impedances high.  Hm...

[dschwartz]

it looks pretty interesting..

but just looking at the schem my guts tells me that the output would be 50% noise and 50% splatty fuzz

there´s too many stages and almost no filtering/tone shaping..tubes are less noisier than cmos, and putting 6 tube gain stages is simply insane..imagine what would be with 6 cmos stages...Check ROG double D project and read the experiments they did..

if you don´t want to waste inverters and keep a low impedance output,.. you could try putting 2 inverters in parallel..

maybe 2 single inverter stages and 2 double inverter stages..(hmm or even a push-pull stage?)

or better yet.. 3 double inverter stages..probably could even drive a small speaker

[WGTP]

I was working on a similar design this weekend.  Sort of a compilation of various ROG CMOS stages including the EQ.  I guess you have seen the Twenty Two Sevenths (22/7ths)?  It has some similar ideas, with clipping diodes (that can be omitted), a BMP workalike.  For some reason, I was surprised that the little inverters will drive LED's very well, both in the feedback loop and to ground.   

[nelson]

I am working on a cmos fuzz that uses all 6 stages - but they are not all adding gain and I am trying to minimize noise and the "muddy farting" associated with CMOS fuzzes. As well as having lots of tweakability, oscilation, starving etc. So far the preliminary fuzz has 8 pots and a switch. Cmos starved of current creates interesting sounds and note decays - there is a project over at the GGG tech pages that uses an adjustable current regulator to starve the inverters, I am messing with something similar.

I also have an idea to design a cmos overdrive pedal that has built in compression and downward expansion stages for noise control and sustain purposes, utilising a couple of the inverters as VCR's a la the pearl phaser or Bad Stone.

Hope that gives you some more ideas for fooling with cmos.

[earthtonesaudio]

I gotta say I'm impressed with the amount of CMOS based distortions out there.  I hadn't seen the 22-7ths yet, thanks!  And I'd never thought of using a CMOS stage as a VCR.  I've only ever seen that done with a single MOSFET, never a complimentary pair.  I'll have to mull that over for a while.

I looked up the 22-7ths pedal, and this thread: http://www.diystompboxes.com/smfforum/index.php?topic=48815.msg363590#msg363590  ...has the info I was looking for regarding the tone shaping between stages.

I've found that a simple series resistance on the V+ pin makes a good starve control.  I have a 317-based adjustable supply, and a 10k series rheostat.  They both drop the voltage down, and they sound the same for every pedal I've tried so far (TS pedals sound great when starved, by the way ).  For a low-current draw pedal, a smaller pot would work better.   500 ohms to 1k is probably just about right for most circuits, but if your pedal is a current hog, 5k might work.

So far I've found that 5 inverters in series is not too bad for noise, as long as they aren't all high-gain.  I've also toyed around with the idea of using one as a splitter, one as a mixer as in the UBE Screamer.   

Eventually I hope to play with diodes, as the inverters are suited to driving them (high Z outputs).  Thanks for the tips, I have a lot of things to try now...

[WGTP]

http://www.aronnelson.com/gallery/main.php/v/WGTP/cMOSilator.GIF.html?g2_imageViewsIndex=1

Here is a preliminary look.  It's reverse flowed because that is the way my breadboard is set up.   

[earthtonesaudio]

Cool, I'm home now so I'll try out some of these ideas.

[brett]

For a cool finish, try using an inverting stage for the output, using 100% feedback. ie. connect the output and input with zero resistance.

You might be thinking "You idiot! What will that achieve?  The inverter is parallel with a piece of wire - it'll do nothing because no self-respecting electron will go through the inverter rather than along a piece of plain copper".  Or you might not jump to a conclusion and give it a try.

[earthtonesaudio]

For a cool finish, try using an inverting stage for the output, using 100% feedback. ie. connect the output and input with zero resistance.

You might be thinking "You idiot! What will that achieve?  The inverter is parallel with a piece of wire - it'll do nothing because no self-respecting electron will go through the inverter rather than along a piece of plain copper".  Or you might not jump to a conclusion and give it a try.

I've seen that before and it always made me go "why...?"  Maybe a sly way to "tie down" the inverter inputs without using ground or +V.

[gez]

The inverter is parallel with a piece of wire - it'll do nothing because no self-respecting electron will go through the inverter rather than along a piece of plain copper". 

If the amp was non-inverting, ie a follower, then your statement would hold true.  However, inverters invert.  Yes, the electrons bypass the inverter and generate a voltage across the load at the output, but that voltage is still pulling the inverter's input up/down.  The inverter's output will, therefore, continue to function as a push-pull arrangement working against any changes seen at its input.  In practice, there won't be any swing in the opposite direction seen at its output, but the effective resistance/impedance (to an AC signal) of the inverter's output stage will reduce with input swings, in turn loading/compressing the signal as its amplitude increases.  So, doing what you outlined does have an impact on the circuit (Edward the Compressor).

[gez]

Cmos starved of current creates interesting sounds and note decays - there is a project over at the GGG tech pages that uses an adjustable current regulator to starve the inverters, I am messing with something similar.

This may (or may not) be of interest:

http://www.diystompboxes.com/smfforum/index.php?topic=50803.msg379819#msg379819

"Expansion" of the idea here:

http://www.diystompboxes.com/smfforum/index.php?topic=49860.0

[puretube]

You clicked the links right out of my mouse, Gez...

Just gimme some more... 

[WGTP]

Have you tried, or is there any reason that 2 of the compressorator stages in a row woundn't work?   Or maybe dispersed amoungst the clipping stages? 

[gez]

Have you tried, or is there any reason that 2 of the compressorator stages in a row woundn't work?   Or maybe dispersed amoungst the clipping stages? 

This may seem a little counterintuitive, but 2 inverters in a row (in series) with a wire from out to in actually results in the pair being wired in parallel.  I should think, therefore, that the compression effect would be more pronounced.  Probably too much: the loading on the previous stage - don't forget that CMOS is high output impedance - would likely kill much of its gain.

Inserting a stage (or two) in between the pair of 'MOSpressors" would work, though, and wouldn't introduce phase shifts despite the extra stages.  Whether it's of any benefit sonically, who knows?  Someone will have to try it.

[earthtonesaudio]

Okay... I've been breadboarding like mad lately.  Haven't tried doing anything with inputs shorted to outputs yet... I guess I'm still skeptical about its utility. 

But here's something I put together.  I wanted to get really basic, to hear individual stages.  I tried one, then two, then three... eventually I got to this:



I A/B'd it with my TS-7 (set to TS-9 mode) and it sounds a little like the TS with Gain 12:00, Tone 3:00, and Volume 12:00.  But this has WAY more output volume.  Sounds great driving the Noisy Cricket... which already sounds good on its own.

[WGTP]

My original design goal was to use the standard approach of boosting highs and reducing lows at the beginning of the distortion stages and boosting the lows and reducing highs at the end of the circuit.  The ROG EQ has booth a high and low boost section.  Then using sections of the DD, 3 Legged Dog,  and/or 22/7th's for distortion, I am hoping to improve on the basic designs by getting more distortion and sustain in the highs and less in the lows.  This seems to make it more dynamic in the lows for better rhythm work, at the same time the compression and sustain of the highs produces a sweet, silky tone for leads.   The additional compressorator stage(s) have added a new element to the possible designs.   All the recorded guitars we hear have compression after the distortions stages.  6 inverters may not be enough.   

[earthtonesaudio]

WGTP, what's this "compressorator" thing you mentioned? 

[WGTP]

The stage Brett was referencing.  Some where in the links, it was called that, IIRC.