Clíodhna – Finalizing the Circuit Design

Let’s finish the circuit design today. The first and the second part went pretty well, I also covered tone control for the pedal. Let’s add couple more options and then finalize the design with actual switches I’ll be using.

You can jump to schematic which is just below, or to the finalized circuit schematic. I did not do breadboarding diagram, it would have been just unwieldy and probably pointless. The schematic is very slightly different from the previous post anyway.

As usual, if you can’t be bothered to read, just skip to the video. You can always come back for more detail.

Ancillary Options – Phase 4

In this part I’ll add two extra options. The last post left us with 18 different options for our circuit. By adding two extra options this shoots to 4*18=72 options 😮🤪. That’s way too much! There is no real and meaningful way to try all of them. But that’s where we are 🙂.

When I looked at Boss SD-1, one of the major differences was that it had double the gain than TS. So that’s what I did, I added an option to boost the gain.

On the other hand, when I had a look at MXR’s Zakk Wylde overdrive, one of the mods was to boost some bass (sort of) to get GT-OD style overdrive. So I did that as well: bass boost.

Let’s look at the schematic:

So let’s look at these two changes in a bit more detail.

Gain Boost

For gain boost, it is fairly simple (encircled in red on the image). I added R17 resistor and SW4 switch either shorts it so it does not do anything, or SW4 is disconnected and we get about 6dB extra boost.

This way, the gain range is from 20dB to 46dB in total. It’s a bit wonky when dB is used, but the gain range is 10-110 (ca. 20-41dB) when the switch is on, and it is 110-210 (ca. 41dB – 46.5 dB) when it’s off.

This options is really giving us control over the top half of the gain of SD-1.

Bass Boost

I called this Bass boost, but that’s a bit of a misleading. By connecting R18 in parallel with R15, that gives us 50% lower combined resistance and thus gives us 2x boost (6dB).

This lifts both lower and higher frequencies a bit, but lower frequencies are unaffected by the tone control, so when engaged, as turning tone knob adds more highs, bass boost becomes more prominent.

Breadboard

Things are getting even messier with breadboarding so I did not include the diagram here. It is pretty much the same as the diagram from the last time. The only thing I did was to add 2 resistors and move few wires around to make it easier to turn them on or off.

Bill of Material

Below is the BOM used for this demonstration. There’s just a couple of extra resistors in it from the last time (again, instead of the switch I used a wire, for now).

Designators	Component	Notes
	Breadboard	Any breadboard will do, I used full size breadboard. You probably don’t want anything smaller than that.
	Jumpers and wires	As many as you need. I got some online, but 24 AWG solid core wire will do just fine.
	9V battery	I used 9V battery snap with Dupont wire.
IN	Switchcraft 12B	In – stereo jack (mono will do too)
OUT	Switchcraft J111	Out mono Jack
RV1, RV2, RV3	Alpha 16mm 100K linear pot	Any 100K linear pot will do, I used 16mm with PCB pins
C1	10n (0.01uF)	Metal film PET cap
C2, C13	220pF	Ceramic C0G cap
C3, C4, C7	220n (0.22uF)	Metal film PET cap
C5	470n (0.47uF)	Metal film PET cap
C6	4.7uF	Electrolytic cap 25V
C8	47uF	Electrolytic cap 25V
C10	100uF	Electrolytic cap 25V
C9	100n (0.1uF)	Ceramic X7R
C11	330nF	Metal film PET cap
C12	2.2nF	Metal film PET cap
D1-D6	1N4148	Small signal diode for clipping
R1, R3, R16	2.2M	1% 250mW metal film
R2, R5, R8	1K
R4, R10, R11	10K
R6, R12	470 ohm
R7	5.6K
R9	100 ohm
R13	47K
R14, R15, R17, R18	100K
U1	TL072	op-amp

Final Circuit Version

Before I finish off, I was thinking about how I want to complete my pedal. The reason I’m adding the final schematic here is that I want to focus on pedal assembly next time.

Based on the schematic above, I’d need two SP3T switches and three SPST switches. I have those, but that would take a lot of space on the board. I also have dip switches lying around.

Have a look at the photo below, quite a big difference in size and having less components is a good thing for keeping pedal compact enough. Otherwise I’d need to use 2x SP3T and 3x SPST to achieve the same control.

Dip Switches are not as sexy as other switches but employing them strategically I can get 256! options for my circuit (8 dip switches, 2 positions each, that’s 2⁸ = 256). Have a look at the final schematic (I omitted power section just to keep the schematic simpler):

Let me zoom in real quick into clipping and tone sections, just for completeness.

Tone Options for Clipping

With three dip switches (SW2B,C,D) I control 8 possible versions of this clipping section:

As you can see, with some clever organization I can still get TS like tone with all 3 switches OFF. If switch SW2D is ON, I get transparent overdrive version and with only SW2B and SW2C ON I get RAT like tone shaping. The other 5 options I haven’t tried out in any of the videos, but differences are subtle.

Clipping & Gain Control

I rearranged clipping section, so instead of 3 way switch, symmetric and asymmetric clipping is controlled by SW3A and SW3B, and extra gain boost by SW3D:

If both clipping switches are ON, then we get symmetric clipping with one diode each way. With both switches OFF we get symmetric clipping but with two diodes each way.

When one is ON and the other switch is OFF, we get asymmetric clipping. Keen eye might’ve spotted that … except for an odd pedal stacking arrangement, either SW2A ON and SW2B OFF or SW2A OFF and SW2B ON should give us the same asymmetric clipping.

With the above in mind, instead of 4 clipping arrangements, we get 3. Thus, instead of having 256 options for the pedal in total – we get 2⁶*3 = 192.

Tone Section Control

Switches SW2A and SW3C control tone section:

The reason I assigned the switches this way is that, if SW1 has all dip switches in OFF position, and SW2 has them all ON, we get default Tube Screamer arrangement. I’ll just rotate SW2 so all dip switches are in the same position by default. Clever! 😉

The Video

Here’s the video with me trying out some of the options covered in this post. I employed a looper for this, and I even cheated since I saved my loops from the last video 😊. I promise to do some more original stuff once I complete the pedal.