BAJA Splawn Quick Rod style Overdrive 160220

bajaman · Post by **bajaman** » 16 Feb 2020, 06:03

Hi fellow diyers

A couple of folks have requested an overdrive design based on the Splawn Quick Rod, so here at last is my take on it.
I modeled the 2nd gear channel and paid close attention to the frequency response, phase behaviour and clipping characteristics.
I used the TLC2264 quad opamp for this design and (like Tech 21) biased two of the stages so they would clip assymetrically.
The TLC2264 is a rail to rail all cmos opamp - Tech 21 use the TLC2262 dual version in their circuits - Sansamp, GT2, Character pedals etc.
Interestingly, I visited the Tech 21 website and noticed that most of their Character pedals are no longer listed

Anyway back to this topic

Here is the schematic - I have built it and tested it and it works well - plenty of bottom end and gain with minimal hiss

I have used a slightly modified Boss silent fet switching circuit and buffered both input and output - it is completely pop free and noiseless when switching

I have printed circuit boards available that will fit a 1590B style enclosure - pm me if you would like to purchase a board. The boards are double sided, plated through holes and silk screened.
More pictures of my build to follow.

cheers
bajaman

bajaman · Post by **bajaman** » 16 Feb 2020, 06:39

Here is the prototype build without the asymmetrical clipping and lower gain stages - (the pcb was easily modified to provide more gain and the asymmetrical clipping.) I decided to use preset trim pots for the treble middle and bass controls - these are easily accessible through small holes in the enclosure bottom plate - set then forget

I used the Boss pedal font to reference the fet switching circuit.
cheers
bajaman

bajaman · Post by **bajaman** » 16 Feb 2020, 20:17

Just a small clarification

- Tech 21 only bias one of their TLC2262 stages to clip asymmetrically in their character series pedals clipping module - my design biases two of the TLC2264 stages to clip asymmetrically. The unbalanced biasing is the only point of similarity with the Tech 21 designs - I have absolutely no affiliation with Tech 21 and use them only as an example of how to bias the cmos rail to rail op amp to clip asymmetrically.

Here are some pictures of the final build - note the additional black wire and 220uf electrolytic filter capacitor supplying the vbias 2 line to the two asymmetrical clipping stages.
I used a piece of double sided self adhesive 3M brand clear mounting tape to insulate the wiring from the enclosure bottom plate.
cheers
bajaman

rcustoms · Post by **rcustoms** » 18 Feb 2020, 00:39

bajaman wrote:Just a small clarification - Tech 21 only bias one of their TLC2262 stages to clip asymmetrically in their character series pedals clipping module - my design biases two of the TLC2264 stages to clip asymmetrically. The unbalanced biasing is the only point of similarity with the Tech 21 designs - I have absolutely no affiliation with Tech 21 and use them only as an example of how to bias the cmos rail to rail op amp to clip asymmetrically.
IMG_4051.JPG
IMG_4050.JPG
Here are some pictures of the final build - note the additional black wire and 220uf electrolytic filter capacitor supplying the vbias 2 line to the two asymmetrical clipping stages.
I used a piece of double sided self adhesive 3M brand clear mounting tape to insulate the wiring from the enclosure bottom plate.
cheers
bajaman

awesome like always,i made de pcb soon as posible.thanks baja

Ripthorn · Post by **Ripthorn** » 27 Feb 2020, 17:30

Looks super cool! Any sound clips?

bajaman · Post by **bajaman** » 28 Feb 2020, 01:00

Here is a minor update to the schematic - added extra low pass filter after the third op amp stage to more accurately model the clipping characteristics including the high frequency roll-off of the third stage. I also increased the gain of the final stage to maintain the correct onset of clipping in this stage with respect to the third gain stage

cheers
bajaman

Frank_NH · Post by **Frank_NH** » 28 Feb 2020, 15:22

Thanks again Bajaman, this looks great! In particular, I'm impressed with the "built-in" BOSS-style soft switching circuit. I've always wanted to try this out and was thinking to implement it on a separate veroboard as a generic unit. I would assume that one could then use this switching on any circuit, i.e. you could replace the Splawn overdrive with a Tube Screamer.

One quick question - can you use other JFETs as substitutes for the 2SK118 for the soft switching? If not, I'll have to order some, though I may have a few in my parts bin.

bajaman · Post by **bajaman** » 28 Feb 2020, 21:25

One quick question - can you use other JFETs as substitutes for the 2SK118 for the soft switching?

yes - there are many jfets that will work in this switching circuit - 2SK117, 2SK30ATM etc are pin compatible - the source and drain pins are interchangeable in these jfets.
J201, 2N5457, 2N5458, 2N5459 may also work but have different pin outs and have not tried using them.
cheers
bajaman

bmxguitarsbmx · Post by **bmxguitarsbmx** » 28 Feb 2020, 23:54

Toshiba jfets are hard to come by. I horde them and can't imagine squandering any on switching. YMMV

bajaman · Post by **bajaman** » 29 Feb 2020, 04:23

Aliexpress has lots of jfets - 2SK117, 2SK118, 2SK30 etc
one example: https://www.aliexpress.com/item/3299562 ... web201603_
cheers
bajaman

bmxguitarsbmx · Post by **bmxguitarsbmx** » 01 Mar 2020, 00:16

Those are Fakes, not Toshiba transistors. I have a few piles of fake Toshiba's from china. They won't get close to the low noise or high Forward Transfer Admittance. At least for the 2sk117. I have noticed some nice offerings in surface mount form that are still available. The 2sk209 may be a 2sk117 equivalent even.

https://www.mouser.com/ProductDetail/To ... XQMlwpg%3D

bajaman · Post by **bajaman** » 01 Mar 2020, 05:26

Those are Fakes, not Toshiba transistors

ok - that maybe, BUT - they work well as switches for me

bmxguitarsbmx · Post by **bmxguitarsbmx** » 02 Mar 2020, 01:11

Perfect. It is still useful to know that they are not real, and if you have real Toshiba transistors, they are good to save for low noise projects (Or sell haha)

jhergonz · Post by **jhergonz** » 02 Mar 2020, 08:36

Mr. Bajaman, Goodday.

Just asking out of curiosity, are you planning to update all your ampbased circuits with tlc2262?

Marlon53 · Post by **Marlon53** » 06 Mar 2020, 15:13

how does it turn into true bypass version?

Ripthorn · Post by **Ripthorn** » 09 Mar 2020, 01:39

I whipped up a two layer PCB for SMD parts for this today. I think I'm going to order some PCB's before I publish the layout here for people to make their own, just to make sure I don't publish anything that won't work. My version does not have the boss switching (true bypass) and is a really easy fit in 125B with top mounted jacks. PCB is 2.2" (56mm) square, board mounted pots, board mounted LED. I hope to hear this guy roar!

Ripthorn · Post by **Ripthorn** » 31 May 2020, 02:52

I finally populated my board and it's working. Still finishing the enclosure, but it sounds good with these quiet run through I did. Only one small problem with the layout and that is the LED negative is tied to ground. Easily rectified. I'll share pictures and the layout once it's all boxed.

Ripthorn · Post by **Ripthorn** » 01 Jun 2020, 15:03

Here are a couple pictures. One of the guts, the other of the finished pedal, complete with my daughter's painting of a hot rod. I'll work on getting some sound samples together. Mine does have some background noise, not sure if it's due to how incredibly high gain this gets, or if it has more to do with something in my layout that I didn't take into account...

jhergonz · Post by **jhergonz** » 01 Jun 2020, 16:26

that is a beautiful build. we will wait for a demo or audio clip.

what is the size of the resistors and capacitors?

Ripthorn · Post by **Ripthorn** » 01 Jun 2020, 18:43

I'm using 0805 sized passives with SOIC and SOT semiconductors. I soldered them all in by hand, thus some of the components are a little wonky.

BAJA Splawn Quick Rod style Overdrive 160220 [documentation]