CD4049 Revisited - i.e. Mark Hammer

[Fancy Lime]

I did some experiments with various unbuffered CMOS inverters a few years back, but never was satisfied with the results I could obtain. They are very noisy (hiss) and sounded quite grainy to my ears. I could alleviate the noise somewhat by using 4 in parallel for each gain stage, but I couldn't ever get rid of the grainy sound. This is true to a certain extent for all MOSFETs. They work well for super high gain, saturated metal tones, but I was never able to get a sweet mild overdrive sounding right. I even tried removing the feedback resistor and directly biasing the input to VDD/2 to smooth out the transfer curve, to no avail.

What's the secret to getting good tone out of these?

Really? I guess tastes differ. I absolutely love CMOS inverters as well as discrete MOSFETS for slight brake up. Even a simple Tube Sound Fuzz sounds great to me in that role. I find the transition from clean to overdriven quite natural. I don't hear any of that brittle fizzing out on decaying notes that annoys me to no end in many diode clipping circuits. I am quite fascinated that our perceptions seem differ so diametrically on this point.

That being said, pre-gain treble boost plus post-clipping treble cut does wonders for smoothing MOSFETS and CMOS designs, in my experience.

Cheers,
Andy

[jonny.reckless]

Thanks Andy. Can you give me a suggestion for the freqs and amount of pre gain boost and post gain cut? Pretty much all overdrive / distortion designs do that to some extent. The choice of turnover frequency, boost amount, 1st vs 2nd order shelf etc makes a big subjective difference to the end result. I used MOSFETs fairly successfully with the Little Jim pedal, but that doesn't do low gain very well.

[moosapotamus]

What difference in sound did the parallel inverters make?
Thanks!

Unfortunately, I don't have one to compare side-by-side. But, the parallel inverters do sound very nice to my ears.

Are there any sources that discuss using the inverters in a filter, or how to create filters between two inverter stages?

Runoffgroove took a swing at that, too.
http://www.runoffgroove.com/mreq.html

[Fancy Lime]

Thanks Andy. Can you give me a suggestion for the freqs and amount of pre gain boost and post gain cut? Pretty much all overdrive / distortion designs do that to some extent. The choice of turnover frequency, boost amount, 1st vs 2nd order shelf etc makes a big subjective difference to the end result. I used MOSFETs fairly successfully with the Little Jim pedal, but that doesn't do low gain very well.

Hi Jonny,

My go-to starting values for the pre gain boost are 12db with a corner frequency of 700-1500Hz for guitar or 3kHz for bass. In terms of high cut, I have developed a habit of quite strictly restricting the Bandwidth every chance I get. Depending on the design, that usually means I have as many poles of low pass filter in total as I have stages. Sometimes more. Anything over four poles makes fairly little difference to my ears. I like to have them around 5kHz for guitar overdrives, a little higher for low-gain types, a little lower for high-gain stuff. Around 7kHz for bass. I usually spread the corner frequencies a little so the whole thing is less than Butterworth.

These are just the starting values, though. The ones I use for the initial breadboard set-up. Everything may change from there. I will often end up with one pole significantly lower, like 1 kHz or so, or with using one mobile pole as a tone control (aka Rat filter). Of late, I have developed a taste for really dark overdrives, so I might reduce the pre-gain emphasis or reduce it to 6db or so.

Hope that helps a little,
Andy

[aron]

Working on a Vulcan front end with parallel inverter stage. Will see.

[Scruffie]

I did some experiments with various unbuffered CMOS inverters a few years back, but never was satisfied with the results I could obtain. They are very noisy (hiss) and sounded quite grainy to my ears. I could alleviate the noise somewhat by using 4 in parallel for each gain stage, but I couldn't ever get rid of the grainy sound. This is true to a certain extent for all MOSFETs. They work well for super high gain, saturated metal tones, but I was never able to get a sweet mild overdrive sounding right. I even tried removing the feedback resistor and directly biasing the input to VDD/2 to smooth out the transfer curve, to no avail.

What's the secret to getting good tone out of these?

I've had the same issues, taking care of the noise wasn't too big a deal (the frequency shaping at the start of most distortions can be considered pre-emphasis  ) but under scrutiny at lower gains, there was always too much fizz and by the time I'd restricted the highs enough to remove it, the character was more at the fuzzy end.

I tried phase shifting, parallel inverters, voltage starving, notch filters, alternative CMOS chips... offsetting probably had the most promise but only worked nicely in a limited gain range.

I think it's just a taste thing, definitely make good fuzzy & high gain drives though.

[pinkjimiphoton]

No wonder we don't see it in reality. Did you try parallel inverters?

i used parallel inverters on my boogie snake project to make a weird tone control. worked pretty good. but that was a 4069, same basic idea but different pinout.
been YEARS.. sold long ago.




dunno if this will be helpful. i seem to remember the snubber on the parallel stage being necessary so it didn't whine.

[aron]

Very cool. I haven't figured a simple way to wire the parallel inverters without a jumper wire. Still working on layout.
Thanks