Reckless Abandon: a 2 channel guitar preamp pedal

[jonny.reckless]

This is the Reckless Abandon: a 2 channel guitar preamp pedal. There's nothing particularly original or special about the circuit, but I do think it's nicely voiced and tonally versatile.

IC2A is a pre-gain treble booster with variable turnover frequency controlled by the voice switch. The -3dB rolloff is selectable between approximately 1.5kHz (typical of the Rat and many other distortion pedals) and 500Hz (more typical of a Vox amp). The lower setting adds in more pre-gain midrange to the signal and changes how the amp feels to play. I found the 500Hz setting generally sounds better with single coil pickups, and the 1.5kHz setting sounds better with humbuckers.

There is a 3 band passive EQ with mid shift and treble shift controls, moving the frequencies from a roughly Marshall curve to a more Fender curve. The bass control is a passive James style, placed after the usual tone stack, to get more adjustment range and a bigger bass boost than is possible with the traditional FMV 3 band tone stack. There is a huge amount of low end available with the bass control turned up! IC2B recovers the large amount of gain lost in the tone stack and buffers the output.

IC1 is a pair of diode clipping stages around an inverting op amp - one for each channel, with channel 2 having 20dB more gain available and a slightly darker sound. I normally use cascaded JFETs for gain stages, so this sort of circuit topology is not something I am experienced with. I experimented with different op amps and clipping diodes, inverting vs non inverting, diodes in the feedback path vs diodes to ground, to get something I really liked the sound of compared with a JFET preamp. I tried TL072, NE5532, LM4562 and LM833 op amps. I tried red LEDs, 1N4148s, 1N4007 clipping diodes. The current combination of LM833 and 3mm red LEDs, inverting, with diodes in the feedback path, sounded warmest and most tube-like to my ears. One red and one blue LED also sounded nice. TL072 was quite cold and clinical sounding, and 1N4148s were just a bit too crunchy. You can experiment with different op amps and clipping diodes here just like you might with any distortion pedal, it's all a matter of personal taste. I also discovered that the feedback capacitor you put in parallel with the clipping diodes is critical to getting a nice distortion response. I needed a bigger feedback capacitor with a TL072 to bring some of the warmth back. Perhaps more surprisingly, the value of the decoupling cap on the op amp power supply pin affected the sound quite a lot. I ended up with 10uF multilayer ceramic caps there rather than the standard 100nF, which tamed the shrill harshness I was getting, and seemed to increase the amount of low end warmth.

The right footswitch turns the pedal on and off (bypass) while the left footswitch changes the channel. It's a true bypass design using 3 pole footswitches. Everything is mounted on the PCB, including the sockets, switches, LEDs and pots. You need to mount the pots on a SIL header to raise them up 10mm so that they are the same height as the top of the footswitch. The PCB is designed to perfectly fit into a 1590DD chassis. I could have made it quite a lot smaller, but I wanted enough space between the controls so it didn't feel super fiddly to adjust with my left hand while playing. I imagine it would be possible to cram the circuit into a 1590BB.





More details, including bill of materials, photos of the build, and PCB Gerbers here:
https://drive.google.com/drive/folders/1D_56xzxK7AiYIwTiOtLHrCclqK8NDKdA?usp=sharing

Listen to it here:

[11-90-an]

Another amazing build, @jonny!   

I suggest you put your all your builds, in somewhat like a gallery, perhaps, on your "signature" down there... It would be easier to navigate through all yer nice builds!

I do, also have a question... Would there be any problems if one were to remove SW4, that one would be able to freely mix in between both channels? (Of course with some equal-valued resistors in place of SW4B as to not make each channel interfere with one another...)

Hmmm... another question... can the output be plugged directly to headphones?

[jonny.reckless]

I do, also have a question... Would there be any problems if one were to remove SW4, that one would be able to freely mix in between both channels? (Of course with some equal-valued resistors in place of SW4B as to not make each channel interfere with one another...)
Hmmm... another question... can the output be plugged directly to headphones?

You could mix the channels if you like, maybe use a 4k7 resistor from the wiper of each volume pot and then onto the tone stack. If you're mixing the channels then I guess you want them to interfere with each other?
You can't plug this directly into headphones, it will sound like crap! It needs the frequency response and coloration of a guitar speaker cab to make it sound right. Normally I would plug a preamp like this into the effects return of my head and mic up my 4x12, but for this demo I used a Celestion 4x12 G12M impulse response instead.

[roseblood11]

Sounds amazing! I listened to an overdrive demo to the end - that happens once a year...

To mix both channels: Why not add a simple dpdt switch that connects them in parallel, overriding the footswitch?

How does this circuit sound in front of an amp with a typical Fender TMB tonestack?

[antonis]

Excellent (as usually) Jonny..!! 

P.S.
Is C1 bias decoupling cap only placed on ICB non-inverting input (and not on IC1A, also) just for layout convenience..??

[Ben N]

Wow, Jonny, really nice--again! And this time with non-fiddly parts!

[jonny.reckless]

How does this circuit sound in front of an amp with a typical Fender TMB tonestack?

It will sound a bit odd due to having the 2 tone stacks in series. If you set your amp EQ with the mid on 10, and treble and bass on 0, then that is closest to a flat sound. I found plugging in into the front of my amp, it sounded a bit too bright, and too scooped. It's already pretty bright as you can hear in the demo video, and was designed and voiced as a preamp, i.e. into an effects return or directly into a cab sim. I like the crunchiness of a bright distortion, but it's easy to go too far into fizz, especially on an amp with a big preamp treble boost like a Fender clean channel. I guess you could tame it by putting a 47n from the output to ground, along with the 1k series resistor R24 this will form a treble cut around 3.4kHz. You might need a bigger cap (or make R24 bigger) depending on your personal taste. Another option might be to make C14 bigger so the treble control has a more dramatic effect when turned right down. But keep the treble on your amp channel down at 0.

[jonny.reckless]

Is C1 bias decoupling cap only placed on ICB non-inverting input (and not on IC1A, also) just for layout convenience..??

Just for layout convenience. Both pins of IC1 are so close together that another cap would be superfluous, but there are a few inches of trace from Vmid to those pins.

[jonny.reckless]

I shared the Gerber and NC drill files, bill of materials for the project in my Google drive:
https://drive.google.com/drive/folders/1D_56xzxK7AiYIwTiOtLHrCclqK8NDKdA?usp=sharing

I've got 3 PCBs left. If you would like to build one you can have one of my PCBs for $10 plus postage, just send me an email with your name, postal address and telephone number (I need this for international shipments).

Jonny

[peterc]

Another amazing project, thanks Jonny.

[Vivek]

Hello Jonny,

Thank you for sharing this project.

Do you share the SPICE files ? I would like to analyse your circuit !!!

Do any of your opamps clip due to rail saturation ?

[LightSoundGeometry]

johnny, you have the gerber files here ..would it  be possible for me to download and have five or ten of them made for myself? I want to learn how to make and use files, then turn the files into usable pcbs; thought your stuff here would be a good way to  introduce me to a this method.


can anyone link a free program to use? or will I have to buy a program ?  I used to do etching but that kind of sucks , went back to strip board and perf board since then

i jut submitted your file to oshpark , they said it would 154.00 usd for 3 boards lol 

[jonny.reckless]

Hello Jonny,
Thank you for sharing this project.
Do you share the SPICE files ? I would like to analyse your circuit !!!
Do any of your opamps clip due to rail saturation ?

Hi Vivek, I don't have a SPICE simulation for this, I just built a prototype and tweaked it until I liked the sound. As it is, with the opamp inverting with diodes in the feedback path, the opamp never hits the rail. All the distortion comes from the feedback diodes.

[jonny.reckless]

johnny, you have the gerber files here ..would it  be possible for me to download and have five or ten of them made for myself? I want to learn how to make and use files, then turn the files into usable pcbs; thought your stuff here would be a good way to  introduce me to a this method.
can anyone link a free program to use? or will I have to buy a program ?  I used to do etching but that kind of sucks , went back to strip board and perf board since then
i jut submitted your file to oshpark , they said it would 154.00 usd for 3 boards lol 

That's a crazy price. I got 5 boards for $46 plus $20 shipping in 48 hours direct from the PCB supplier in China. You're welcome to make some boards from the Gerbers for personal use. If you want to sell them commercially please let me know first. I've got 2 PCBs left, you can have them for $10 each plus postage if you want.

[LightSoundGeometry]

johnny, you have the gerber files here ..would it  be possible for me to download and have five or ten of them made for myself? I want to learn how to make and use files, then turn the files into usable pcbs; thought your stuff here would be a good way to  introduce me to a this method.
can anyone link a free program to use? or will I have to buy a program ?  I used to do etching but that kind of sucks , went back to strip board and perf board since then
i jut submitted your file to oshpark , they said it would 154.00 usd for 3 boards lol 

That's a crazy price. I got 5 boards for $46 plus $20 shipping in 48 hours direct from the PCB supplier in China. You're welcome to make some boards from the Gerbers for personal use. If you want to sell them commercially please let me know first. I've got 2 PCBs left, you can have them for $10 each plus postage if you want.

i want one of your boards for my collection, but I wanted to lean the layout process in cad. I am using your zip file as an experiment. for example, I was submitting your file to get price quotes, and to familiarize myself with the process ..i wouldnt use your design in a commercial setting lol ..if I would have ordered your stuff it would of been like the pack of five, one for myself and the other four to give away or sell a few pedals to make mine free! 

check out jlc, im pretty sure they quoted me 5 boards for around 10 dollars + postage

[Vivek]

Hi Vivek, I don't have a SPICE simulation for this

Dear Jonny

It is kind of you to share all your project files from your Google Drive

The schematic is a PDF.

Could I have your electronic files and I can try to import into LT Spice ?

or could you inform me about the different export formats available on your software

Thank you

[Vivek]

You have two channels set with different filters and gains

and a switch that can choose between one of these channels

Is there any sonic advantage of offering various series and parallel options of these gain stages ?

Like for example, low gain stage feeding the high gain stage ?

I read somewhere that multiple gain stages leads to more amp like feel and response. Is that true ?

[jonny.reckless]

I've got 1 PCB left if anyone is interested in building this pedal.

[R.G.]

Nice work. Well thought out.
It reminds me of the guy in the UK who's making a preamp pedal and an onboard preamp, same circuit, which replicates the circuit of the old UK Vox Special guitar preamp. He's after the British Invasion guitar tone, and seems to be very happy with the results. I don't know if he sells them or not, though. They may be only custom builds.

[iainpunk]

reading back your OP, i think the LM358 is rather nicely suited for this kind of distortion, since it is a bit slower than the others, the 833 slew rate is 7v/us, the 072 has 20v/us (harsher) and the lm358 has only 0.3v/us, which is way slower, which makes it smoother. (this is a bit of a simplification, but it works well in most circumstances.)

cheers, Iain