This Week on the Breadboard: Gator's OD - cb mod

[Chuck D. Bones]

We held a design contest over in The Breakroom earlier this year. There were a lot of great entries and Gator's OD was one of 'em. It's based on a Timmy, with a toggle switch that allows one to drive the 2nd stage to clipping. which gives it a whole new flavor. This is the design Gator submitted. It works great as-is. Other opamps sound good too.



I couldn't leave well enough alone so I tried using an NJM386 for the 2nd stage. Unfettered by the rules of the Design Contest, I moved the BASS trimmer to the front panel. At the moment, I have an LF351 in place of the TL071. They sound the same. The MODE switch does a few things. In the "NORMAL" position, the 386 does not saturate and this circuit behaves like a Timmy, except the 386 makes it capable of driving a mile of cable. With MODE in the "SWAMP" position, the 386 is driven hard enough to saturate, the low frequencies are rolled off a bit going into the 386 and there is some high-freq roll-off after the 386. R9 has to be selected to accommodate the variation in input impedance of different LM386's. The spec sheet says the input impedance is nominally 50K, but the one in my breadboard has 95K input impedance.


Knobs, L to R: VOLUME - TREBLE - BASS - DRIVE



Try it (either way), you'll like it!

 

[jwin615]

R9 has to be selected to accommodate the variation in input impedance of different LM386's. The spec sheet says the input impedance is nominally 50K, but the one in my breadboard has 95K input impedance.

Great, now I'll be binning 386s too. Thanks Chuck.
But sincerely, thanks Gator, Chuck and everyone else who participated and facilitated that contest.

 

[Chuck D. Bones]

R8 can be varied to adjust how nasty it gets in SWAMP mode.
D1-D4 can be replaced with two IR LEDs, as per Gator's original schematic.

 

[The Gator]

I didn't know you posted this...I am late to the party once again. I like your naming better than mine! I have yet to try the CB mod with the 386 in the circuit. Not sure I can wait now. Should be soon. I am building a batch of Nexus of Plexus(golden falk...aka plexibreed) right now. Gotta feed the addiction.
This is a circuit that I have been tweaking for months. I decided to enter the contest by modifying it to fit the parameters of the contest. I'm glad I did. The contest was certainly fun and educational.
The variation on my breadboard now still uses a TL072, but has a pot to dial in how much swamp you can handle.
Personally, I likes ta feel the mud betwixt my toes...lol

 

[Chuck D. Bones]

I should have dropped you a note. I like your idea about making the Swamp mode variable, so I added a trimpot. With the trimpot dimed, it runs hot like the schematic above. If I dial it down to about 3 (10:00), then the tone is more like the original.



I could see building this into a guitar.

 

[music6000]

Something like This:

 

[The Gator]

Music6000 - Oh man does that already exist!!!
I Love it!!!

All good Chuck. I really like you that you dig the idea and sound of the circuit.
I have since changed the purpose of the switch and just use a full potentiometer for swamp. Now with the switch I am trying to come up with a mid control I like. Basically go from mid forward to mid slightly scooped.
I love what you have done here. I am building this this week!!

 

[szukalski]

I was sold on the demo of the originals, so I whipped this up.

 

[Chuck D. Bones]

NICE!
Maybe add the trimpot?

Just a reminder for anyone who builds this: LM386 is 9V only!

 

[The Gator]

Oh damn. You guys are my heroes!
That is awesome!
Now if I was doing it, designing a board to utilize my connectorized set up with the pedalpcb I/O power boards, I would move the jack in, Gnd, 9V, and Jack out points all up in one spot. Either at the top above the toggle or off to one side just above the drive or volume pot connections.
Am I pushing it too far? Lol.

This is blowing my mind!

 

[szukalski]

Good point with the Swampiness trim pot!

 

[szukalski]

Now if I was doing it, designing a board to utilize my connectorized set up with the pedalpcb I/O power boards, I would move the jack in, Gnd, 9V, and Jack out points all up in one spot. Either at the top above the toggle or off to one side just above the drive or volume pot connections.

It's a compromise where I put the top IO. The Jack pads give reasonable routing on the PCB (the main reason they are where they are), and let you tuck away the cabling if you're using standalone jacks. Plus, I do the cabling on IO right at the end, so having the pads away from other components makes the build process easier (for me).

I do use IO boards similar to the PPCB ones, and the layout works well. I don't use ribbon cable though.

 

[The Gator]

That is understandable. The fact that you have jack in and out points is cool.
Do you have a link to your I/O boards ?
Yeah I do not like ribbon cable much.
I use JST XH connectors and silicone insulated wires.
It looks awesome!
Now I can't wait to breadboard it any longer!

 

[The Gator]

I just built this on the breadboard using my IR leds for clipping being the only real difference...
Oh man, hard to put the guitar down after ya start digging in...
Gonna do a demo vid tomorrow.
I am having a great time with this.
Thank you to all of you.

 

[szukalski]

That is understandable. The fact that you have jack in and out points is cool.
Do you have a link to your I/O boards ?

I haven't made them publicly available because they're no different than the PPCB ones and I don't want to pull a dick move (potentially) reducing revenue. They look like this though:

I am going to refactor them to have the jack IO on the outside edges, and include the ring pad for stereo jacks.

 

[The Gator]

here's some rough demo video. Some mid gain, high gain and some fuzz tones with a Les Paul.
sitting on the amp top board next to the bread board is the original circuit I changed to enter the contest. I will get a copy of it to you chuck for reference.

Here is some swamp tones with the Tele. I do a comparison with the Stratodrive deluxe with that tone. then some full on high gain tele bridge pickup.

 

[music6000]

Good point with the Swampiness trim pot!

View attachment 70933

Pedal Mockup above adjusted to suit PCB Layout!

 

[szukalski]

Built this up last night. Pretty fun one to play, not your usual OD. Swampy is an apt description!

Dave, is there a simple way to check the input impedance of the 386?

 

[Chuck D. Bones]

1. Try measuring it with a DMM on the Ohms setting. No power to the LM386. Meter + lead to LM386 pin 3. Meter - lead to pin 4. You may need to set the meter range to manual (if it has auto-range). Nominal resistance is 50K, but it varies quite a bit depending on mfgr.

Method 2 requires a scope & sig gen.

 

[Chuck D. Bones]

I still couldn't leave well enough alone. Gator expressed an interest in being able to control the mids. I tried a few different tone stacks and landed on a modified BMP tone network. My experimentation with the tone stack on the modded Hoof lead me to try it on the Gator OD and it seems to work pretty damned well.

On the breadboard I'm using a pair of IR LEDs for the soft clipping diodes, just like Gator did in the original design.

Knobs (L-R): VOL - PRES - TREB - MID - DRIVE - BASS - GAIN


The DRIVE knob replaces the MODE switch. It controls whether we overdrive the LM386 and by how much. The PRESENCE knob at the end can be used to shave off a little or a lot of the harmonics. We can set the GAIN to zero, keep the DRIVE below noon and it's a clean EQ pedal. We can dial in any amount of soft clipping, any amount of hard clipping, or any combination of the two.

One crazy trick I pulled was using the LM386's Vref (pin 7) to bias the TL071. It's clean, it's near 1/2 Vcc, so why not? Another was using R10 to bump up the LM386's gain and bias the output a little off-center for some asymmetric clipping.