freestompboxes.org

NickH wrote: ↑30 May 2022, 12:40 Hello,
I found this schematic online and had a go at making it. It works...

thanks for trying this. A is log, B is linear, C is rev log. there are other conventions but this is by far the most common.
If you remove the baxandall you end up with something similar to the JHS maleficent mids but with nicer values. you also end up with 3 op amps which is unfortunate. you could use the 4th as output buffer or terminate it, or you can go with a bjt/jfet input stage and 2 op amps.
About the howling, the proper term is oscillation. I don't have any, even at extreme settings, so check your build. the gain isn't high enough to create problems.

dylan159 wrote: ↑30 May 2022, 13:24

NickH wrote: ↑30 May 2022, 12:40 Hello,
I found this schematic online and had a go at making it. It works...

thanks for trying this. A is log, B is linear, C is rev log. there are other conventions but this is by far the most common.
If you remove the baxandall you end up with something similar to the JHS maleficent mids but with nicer values. you also end up with 3 op amps which is unfortunate. you could use the 4th as output buffer or terminate it, or you can go with a bjt/jfet input stage and 2 op amps.
About the howling, the proper term is oscillation. I don't have any, even at extreme settings, so check your build. the gain isn't high enough to create problems.

Thanks for the reply. Just to be sure....
Mid Pot is Linear
Mid Freq is rev log
Volume is log
Bass and Treble are linear ??

I will recheck my circuit and hopefully get rid of the oscillation.
B
Nick

NickH wrote: ↑02 Jun 2022, 06:47 Thanks for the reply. Just to be sure....
Mid Pot is Linear
Mid Freq is rev log
Volume is log
Bass and Treble are linear ??

I will recheck my circuit and hopefully get rid of the oscillation.
B
Nick

yes. forgot to write it for bass and treble. midfreq can be linear in a pinch, or log wired in reverse

Hello, me again.
I have put this on a vero board and don't seem to have the oscillation i had on a breadboarded prototype. Not sure where the issue was but it has gone away so I won't worry too much.
Can I ask if the 100 uF capacitor on the lower part of the vero board schematic is the correct way round? Isn't pin 5 negative compared to ground and so connected to the negative side of the capacitor?
Tried it both ways and it didn't go bang!!!

Good news!
The capacitor is correct, pin 5 outputs the negative rail.

But doesnt the vero diagram show the positive side of the 100 uF electrolytic connected to pin 5? Pin 5 is negative output and therefore below ground. Sorry if I am being stupid but the schematic makes sense to me but not this part of the vero board diagram.
Thanks
Nick

NickH wrote: ↑06 Jun 2022, 07:27 But doesnt the vero diagram show the positive side of the 100 uF electrolytic connected to pin 5? Pin 5 is negative output and therefore below ground. Sorry if I am being stupid but the schematic makes sense to me but not this part of the vero board diagram.
Thanks
Nick

The schematic is correct, the vero isn't. wow, thanks.

Thank you for this little gem Dylan.
It does excellent job and then some

Considering I'm borderline ignorant when it comes to electronics ( just above soldering skills ) i have a question.

Can it be dirtied up a little?
I was thinking inserting TS like clipping and of course it would be switchable to stock design.

Does this make any sense?



Again , thanks for the design.

McOmio wrote: ↑25 Jun 2022, 23:01 Thank you for this little gem Dylan.
It does excellent job and then some

Considering I'm borderline ignorant when it comes to electronics ( just above soldering skills ) i have a question.

Can it be dirtied up a little?
I was thinking inserting TS like clipping and of course it would be switchable to stock design.

Does this make any sense?



Again , thanks for the design.

the stage you've added diodes and a gain pot to is an active version of something called a Wein Bridge network. here the gain is carefully tuned to boost or cut equally around a specific frequency. if the gain of this stage isnt balanced than there will be more boost than cut or v/v. if the overall gain, set by U2B, is too high (more than 3, in fact) the whole thing becomes an oscillator, which in some cases is another handy use for the wein bridge circuit but not one we want here.

if i were to suggest any place to put gain adjustment and clipping diodes in this circuit it would be the very first stage, keeping the present resistor values for the minimum setting and adding a maybe 100k or more pot to up the gain.

also, putting the diodes across just lugs 2 and 3 of the pot, while there is a series resistor from the output of the opamp, will short the diodes out at minimum gain for a clean sound and soften the clipping slightly at higher gain. a neat trick, but i haven't seen it used that much.

Thanks!
Will do & will try it.

Can it be dirtied up a little?
I was thinking inserting TS like clipping and of course it would be switchable to stock design.

I'm not sure what adding clipping diodes to the wien stage would do. If it was a virtual ground amplifier sure, it's the same as hard clipping, and it probably won't oscillate because it didn't before and the diodes can only limit the gain. It might have some "unintended" effect on the whole operation of the stage though.
That said, I agree in putting the clipping in the first stage and making it adjustable, it's free to do that and it allows you to shape the result afterwards. Scaling the whole baxandall up by 10, adding a series resistor and shunt clipping could be interesting too, but there's the problem of having enough gain for the clipping without clipping the mid band eq.

also, putting the diodes across just lugs 2 and 3 of the pot, while there is a series resistor from the output of the opamp, will short the diodes out at minimum gain for a clean sound and soften the clipping slightly at higher gain. a neat trick, but i haven't seen it used that much.

that works, I think I've seen it somewhere, but a tiny bit of series resistance goes a long way with diodes, and it's hard to combine that with the large resistances needed for clipping gain. At that point the op-amp is probably clipping more than the diodes.

McOmio wrote: ↑25 Jun 2022, 23:01
Can it be dirtied up a little?
I was thinking inserting TS like clipping and of course it would be switchable to stock design.

I think that the one of the bests ways to do that is to modify a bb preamp circuit, in other words, to implement a ts like overdrive before that circuit.

I just built a clone of the Systech Harmonic Energizer. It’s a parametric EQ. I preferred the sound of the overdrive/fuzz after the filter. To me the sound of distortion before the filter lost all character of the distortion.

Been messing with wein bridge stuff in simulation. If you make the U2B op-amp have adjustable gain then you will have the Q variable with a pot. Have to be careful though as you can achieve ocsilation with too much!

TWSpedals wrote: ↑31 Jul 2022, 09:34 Been messing with wein bridge stuff in simulation. If you make the U2B op-amp have adjustable gain then you will have the Q variable with a pot. Have to be careful though as you can achieve ocsilation with too much!

That's what the Q switch does, but changing two resistors at once so that you change the Q but keep the same boost/cut range, and in a more controlled way!

I made a PCB for this circuit. It works and sounds good. I made the PCB small to fit in a guitar and replaced all but the mid pots with trim pots. I am really pleased with it but I have a couple of issues I need to resolve.

I tried to use an on off switch on the power but this is really noisy so instead of turning the power on and off, I am using a switch to turn the Led on and at the same time switch my signal output from the pedal input to pedal output. This is quiet during switching but is there a more sensible way to bypass the circuit or switch it into my signal path.

The other issue which might be related but I am not sure, is that I can get a very high frequency hum. It is like a whistling sound and the dog hates it!

Any thoughts as to how this might be created and how I might eliminate it?

Thanks
Nick

I made a PCB for this circuit. It works and sounds good. I made the PCB small to fit in a guitar and replaced all but the mid pots with trim pots. I am really pleased with it but I have a couple of issues I need to resolve.

I tried to use an on off switch on the power but this is really noisy so instead of turning the power on and off, I am using a switch to turn the Led on and at the same time switch my signal output from the pedal input to pedal output. This is quiet during switching but is there a more sensible way to bypass the circuit or switch it into my signal path.

The other issue which might be related but I am not sure, is that I can get a very high frequency hum. It is like a whistling sound and the dog hates it!

Any thoughts as to how this might be created and how I might eliminate it?

Thanks
Nick

NickH wrote: ↑31 Aug 2022, 20:13 I can get a very high frequency hum. It is like a whistling sound and the dog hates it! Any thoughts as to how this might be created and how I might eliminate it?

Hi Nick, when does this happen? When the EQ is active, or bypassed? Any particular pot settings?

A whistle could be oscillation, or it could be switching noise from the charge pump.

Thanks for answering. A high pitched whistle is likely to be at about 10khz so I think this might be switching noise. It doesn’t seem to alter with the Eq, at least not in an obvious way. If this is switching noise, do you think that altering the capacitance on pin 5 might have an effect? I suppose it would alter the pitch of the whistle if the clock speed is lowered. Might try this as it is something I can do without having to redesign the PCB. However, this will just change the problem and not solve it.

Any other thoughts?
Thanks
Nick

It does sound like the charge pump. Even with pins 1 & 8 shorted to double the switching frequency, you can still get some pretty strong square wave radiation from C7 (and a bit from the body of the charge pump) with products in that 10 kHz range. If you have a scope, you can see it by bringing the tip of the probe within an inch or two of the top of C7 or the IC, the amplitude increases as the distance decreases. Possible solutions include (1) moving the charge pump and C7 farther away from the audio path, which might be difficult since it's on a PCB. Or (2) shielding C7 with a grounded metal cylinder. It's easier than it sounds, just get some aluminum or copper tape, lay the bare end of a wire on the conductive surface, cover the conductive surface with tape to secure the wire and insulate the surface, roll it up on a pencil slightly larger than the diameter of C7, secure the roll with another bit of tape, slide it over C7 and attach the other end of the wire to ground. You can even pinch the open top of the roll shut if you want. That will effectively block any radiation. I use both physical separation and a cap sleeve on all my charge pumps these days to keep the noise floor low, here's a pic with both strategies in play.