Is the level of this noise variable or am I psycho-acustically challenged?

[ggedamed]

Good day ladies and gentlemen,

Please listen to this sample. The circuit is a take on the CMOS inverters distortion a la Emma ReezaFRATzitz, but with CD4007.

First, is the noise level increasing after the palm mutes or I am hearing things? I heard it on other circuits, but never actually thinked about it.

After some thought, I think that my ears don't lie to me and this phenomenon is caused by the 10k resistor being in series with the IC power rail and changing the inverters bias. Maybe there are Reeza clones buiders that experimented it?

I would only wish it would behave the other way around and the noise would decrease after the palm mutes  .

[Seljer]

It may be whatever it was recorded with has got auto-gain-control that is working as a compresser/limiter to avoid clipping and you're hearing the recovery as it brings the gain back up. Theres might be a psychoacoustic equivelent of the same principal too.

[anchovie]

Can't hear your sample as I'm at work, but I recently had a high-gainer based on a CD4007 on the breadboard. It was set up to use one inverter as a variable gain stage followed by two inverters near-enough maxed out. I was getting a flubby/crackling noise that would fade in after palm mutes on low notes and power chords with the gain set up pretty high, managed to cure this by reducing the value of the cap I had going from the first inverter to the second stage, so you might want to try reducing either the input cap or one of the early interstage ones to see if your CD4007 also has a dislike of too much low-end!

[duck_arse]

somewhere, someone (tim escobedo?) put a resistor in series w/ the supply on an inverters design, because when run in linear mode, they suck huge amounts of current (relatively), and get hot.

now I look at his ghost dance, he even uses a fet as a constant current source. some pedal designs use a pot instead.

it seems the inverter designs have a reputation for high end noise.

[jonasx26]

After some thought, I think that my ears don't lie to me and this phenomenon is caused by the 10k resistor being in series with the IC power rail and changing the inverters bias. Maybe there are Reeza clones buiders that experimented it?

You're probably right. CMOS inverter (linear) gain is inversely proportional to supply voltage.
So my guess is that the decoupling cap (assuming there is one) and 10k time constant is enough to make the 4007 VDD dip when you dig in..

[PRR]

> is the noise level

"Noise" covers too many entirely different types of garbage.

What I am hearing is mostly power-hum related. This can be a poorly filtered power supply, poor shielding around a high-gain circuit, and of course all the trash that a guitar pickup picks-up inside a room with power in the walls. This can, in principle, be totally eliminated by getting far away from all AC power. In practice, good power filtering and shielding goes a long way.

There is also hiss. This happens with *any* high-gain circuit. Also CMOS is notoriously more hissy than other amplifying devices. Also "starving" them with 10K in their power feed is liable to increase hiss.

You are slamming the signal so hard that it is difficult to tell if the noise increases in the rests. When most circuits are "slammed", their gain drops a bit. When you rest, the gain comes back. So it is very possible.

Put a voltmeter in there and watch DC voltages while slammed and while idle. They probably jump around a LOT. That's key to many distortion effects. Figuring out what voltage jumps are "good" or "bad" is much more difficult than just observing.

[ggedamed]

Wow! You guys are an impressive bunch.
Thank you for all the answers.

To add some details, the circuit is on the breadboard and it is hit by an op amp booster so the gain is really high. The noise it is mainly hum, yes. I don't care much about the noise level being high, that's to be expected when breadboarding a high gain circuit. It was its variation that made me curious.
Some very good ideas came from here: I'll monitor the supply voltage on CD4007 (thanks, Paul) and I'll try to reproduce the bias conditions met during the palm mutes. I'll try also the constant current source idea (thanks, duck_arse) and the bass reducing idea - probably an adjustable 2nd order op amp HPF (thanks, anchovie).

Again, wow.

[ashcat_lt]

It may be whatever it was recorded with has got auto-gain-control that is working as a compresser/limiter to avoid clipping and you're hearing the recovery as it brings the gain back up. Theres might be a psychoacoustic equivelent of the same principal too.

But remember that distortion is essentially the same thing as heavy handed compression.  Everything that goes in comes out the same volume.  When there's no guitar playing, the noise comes through.  When the guitar plays, it's a lot louder coming in than the noise, and the whole damn thing gets squashed down.  The noise does get quieter, and is probably masked some as well.  Haven't listened to the clip, but it is not unusual under these conditions for the noise floor to pump and breathe like that.

[ggedamed]

Well, I did measure the supply voltage and it's actually funny. It's obvious now that I didn't give enough details, so here goes.
The circuit is supplied by the same bipolar supply as the op amp booster. The power supply is based on a LT1054 in the inverter configuration. Of course, the CD4007 ignores the ground and just uses the + and - rails.
So, as the resistance in series with the supply goes bigger, the voltage supplied to CD4007 goes down (obviously). The headroom decreases, the noise and the gain (I think) go up.
But, when I palm mute, the voltage goes up. The inverters suck most current when idle. Palm-muting makes them suck less current, so more supply voltage and less noise. When the slamming effect wears off, voltage goes down again and the noise goes up again.
I'm wondering what would happen if I modulate the current to increase as the signal decreases. Kinda like a current compressor . It could be applied to any distortion: just starve it more as the signal goes higher.

[duck_arse]

do you have a diagram of the circuit as you built it? the 4007 is really only a bunch of mosfets, and the chip doesn't need a supply to work. you can wire the mosfets like an output stage or like a gain stage. or like an inverter.

[ggedamed]

I built it directly on breadboard, so no schematic yet. I formed the 3 inverters like in the datasheet. I know I should've said "MOSFET transistors push-pull stages supply" or "Vdd" or something like that, but I think the idea gets through. The chip is only a bunch of MOSFETs so when I'm talking about the voltage supplied to CD4007, I'm talking about the voltage supplied to the bunch of MOSFETs.