CD4007-Based Distortion Humming Issue

[Baran Ismen]

I was cleaning up some of my parts and it just so happens I had a CD4007.  Just to be clear I didn't build this circuit.   However, it occurred to me the reason this circuit is noisy and needs a lot of bypassing is because the first two stages use the P-channels of the CD4007 for amplification.   That means these two stages are effectively positive ground circuit.  What happens is the supply noise gets amplified with the full-gain of the amplifier!

A possible mod is to use the N-channels for gain like this,



Using the N-channels for gain makes the first two stages negative ground which will greatly reduce the amplification of the supply noise.

The gain look very similar for the P-channel and N-channel but since it's unverified I can't guarantee the mod sounds the *exactly* the same.  The bias points should be close and if they aren't close enough the bias point could be tweaked.

This project is still laying on the breadboard, I'll give this one a try too, looks legit and when it comes to P/N feeding difference, it makes much sense.

[Baran Ismen]

I was cleaning up some of my parts and it just so happens I had a CD4007.  Just to be clear I didn't build this circuit.   However, it occurred to me the reason this circuit is noisy and needs a lot of bypassing is because the first two stages use the P-channels of the CD4007 for amplification.   That means these two stages are effectively positive ground circuit.  What happens is the supply noise gets amplified with the full-gain of the amplifier!

A possible mod is to use the N-channels for gain like this,



Using the N-channels for gain makes the first two stages negative ground which will greatly reduce the amplification of the supply noise.

The gain look very similar for the P-channel and N-channel but since it's unverified I can't guarantee the mod sounds the *exactly* the same.  The bias points should be close and if they aren't close enough the bias point could be tweaked.

I made this modification last night and I can say it's verified. Tinkering with the values and tone section at the moment, and I am considering adding a clipping stage -or a variable one w/ multiple diode types-.

All goes right but with one thing; as mentioned before, the gain pot is acting a bit weird, I mean it's working indeed, but when at zero, it also removes some of the treble due to the cap connected to the gnd. I think it's sort of a pseudo-gain pot. I wonder if there'd be any workaround for it. Also during its travel, there's still a bit of ground hum. But when introduced into a clean channel, it's quite a high gain going on (muff-ish, rectifier-ish), and when boosted the dirty chain, it becomes sort of an HM-2/3. I'll carefully tinker the tone section in order to get a variable, yet mid-focused tone stack.

[Rob Strand]

All goes right but with one thing; as mentioned before, the gain pot is acting a bit weird, I mean it's working indeed, but when at zero, it also removes some of the treble due to the cap connected to the gnd. I think it's sort of a pseudo-gain pot. I wonder if there'd be any workaround for it.

The simplest mod is to put a 100k resistor in series with counter-clockwise terminal of the gain pot.  On the schematic that would be a 100k resistor between the pot and the cap.

That prevents the gain control acting weird - which I could see in the circuit simulations early on in the thread.

[Baran Ismen]

Quick question;

Would it make a difference if I placed the clipping stage before or after the tone stack? What's the general rule? Or is there any?

[Rob Strand]

Would it make a difference if I placed the clipping stage before or after the tone stack? What's the general rule? Or is there any?

I think it would be quite noticeable.   There's no rule really, people can do whatever they like.  It can be wired that way and it will produce sound.

It's more common to put the tone control at the end since it lets you shape the tone of the clipped sound.  If you put a stage that clips after a tone control the ability to shape the tone is more limited.   It can still work.   In some cases the stage after the tone control doesn't clip.  In these cases the extra stage doesn't change the pedal tone.  It just louder but that change the tone of a overdriven the amp.

In the case of your circuit hanging a tone control of stage 1 or stage 2 will load the circuit differently than hanging the tone control off stage 3, as stage 3 is wired differently.

So the short answer is you can do it but don't expect things to stay the same.

[Baran Ismen]

Would it make a difference if I placed the clipping stage before or after the tone stack? What's the general rule? Or is there any?

I think it would be quite noticeable.   There's no rule really, people can do whatever they like.  I can be wired that way an it will produce sound.

It's more common to put the tone control at the end since it lets you shape the tone of the clipped sound.  If you put a stage that clips after a tone control the ability to shape the tone is more limited.   It can still work.   In some cases the stage after the tone control doesn't clip.  In these cases the extra stage doesn't change the pedal tone.  It just louder but that change the tone of a overdriven the amp.

In the case of your circuit hanging a tone control of stage 1 or stage 2 will load the circuit differently than hanging the tone control off stage 3, as stage 3 is wired differently.

So the short answer is you can do it but don't expect things to stay the same.

Thanks for clarification Rob, some further trail and error is on the way for me, then