Fun with CMOS - part 1 - The CMOS Raincoat

[Chuck D. Bones]

Good question. It's not straightforward to calculate. LTSpice is fairly accurate with transistor stages like the Raincoat, but the CMOS gain stages are difficult to calculate or simulate accurately because the behavior in the linear mode is not specified in the datasheet or controlled in manufacturing. I have LTSpice models of generic CMOS inverters, but because the actual devices vary so much, the simulations are not particularly accurate.

The previous stage has a non-zero output impedance, so that is in effect the input resistor of the next stage. By the same token, the input impedance of a CMOS inverter with feedback is low, so without an input resistor it loads the output of the previous stage. These impedances are non-linear and frequency-dependent, which further complicates matters. CMOS gain stages running in linear mode are capable of a max gain around 10x to 20x, so that influences the overall gain and freq response as well. The behavior is also influenced by the rail voltage. I've found that there is a sweet spot between 4V and 5V where the gain is highest. What I'm getting at is this: our best move is to breadboard and test the circuit.

 

[cdwillis]

I have a couple 4049ube chips I bought to play around with that I need to throw on a breadboard. Might finally tackle it tomorrow because I'm still fiddling with some distortion circuit on the "spare" breadboard.

 

[Chuck D. Bones]

My recommendations:
Keep it simple. Start with just two inverters. Adjust the rail voltage.
CD4049s manufactured by Texas Instruments are noisy. They're fine when used as digital parts, but not so good as analog parts. For reasons unknown to me, the CD4007s manufactured by Texas Instruments are quiet. That's why I prefer to design circuits with CD4007 over CD4049. I have some vintage CD4049s from Hitachi, National, Signetics and RCA that are quiet, but they are hard to find.

 

[aquataur]

I once aimed a shot at an inverter overdrive. I used N.O.S. Motorola MC14049UB, which are very quiet.
I used the output stage of the Fat Sandwich WH-301, which wires the unused inverters in parallel for even lower noise, with a front end like above. Indeed, there is no perceivable noise.
Note that this measure inflates the current consumption astronomically.

According to an internet demo of The Raincoat, its tone apprears similar to the aforementioned unit. It has the typical coloration a MOSFET imparts. Not bad, but it behaves far from any dynamics associated with an overdriven tube amp. If you push it hard, it sounds ugly to my ears.
I do not care about this pedal, although the optics grew very appealing.

 

[Robert]

According to an internet demo of The Raincoat, its tone apprears similar to the aforementioned unit. It has the typical coloration a MOSFET imparts. Not bad, but it behaves far from any dynamics associated with an overdriven tube amp.

It's a Big Muff, it's supposed to sound ugly.

(It also has no MOSFETs)

 

[aquataur]

It also has no MOSFETs

The Muff doesn't, but the inverters do. To my ears a discrete MOSFET stage like has the same "something" in its tone. Something dark.
I agree, Muffs can sound ugly - and they generally do to my ears. Unless you noodle along with some Gilmour-ish chords, then they work.

BTW, the inverter drive "barks", but it sound better if masked by a subsequent beautifier of sorts.

 

[Robert]

The Muff doesn't, but the inverters do.

Ahh, I was assuming you were referring to the original Raincoat circuit (since you mentioned a video), which is all BJTs.

Is there a video demo of this CMOS version somewhere? I kinda wanna hear it but I'm too lazy to breadboard it.

 

[Chuck D. Bones]

I had my best luck with CMOS inverter-based dirt boxes by carefully managing the gain and bandwidth of each stage. The best one I was able to make was this one.

 

[Chuck D. Bones]

Is there a video demo of this CMOS version somewhere? I kinda wanna hear it but I'm too lazy to breadboard it.

I'm too lazy to breadboard it again. My impression was the Raincoat sounded way better, which is why I abandoned the CMOS "Raincoat."

"Experiments that failed too many times..." BÖC

 

[aquataur]

... managing the gain and bandwidth of each stage.

Ah, the ole' interstage attenuation. The heart and soul of all tube amps.
Yes, this would probably need to be revisited. But truly, I don´t expect sonic miracles out of the technology.
Your self-knitted push-pull approach is remarkable though.

 

[aquataur]

Ahh, I was assuming you were referring to the original Raincoat circuit (since you mentioned a video), which is all BJTs.

... which I was indeed. I thought Chuck's version was just an improved version. But incidentally, the demo I heard sounded dark, and I thought I heard the ole MOSFET. So much for perceiption...