Can anybody identify this circuit?

[Guybrush]

Hi all

Just come across this on Instructables:

https://www.google.co.uk/amp/www.instructables.com/id/NOT-Like-a-BOSS-Distortion/%3Famp_page%3Dtrue

Just wondering if anyone with a better understanding of schematics than me can identify if it's based on any existing circuit? I'd like to try and build it but I'm useless at working from schematic alone.

If it is based on an existing design I'm hoping I'll be able to find a layout of some kind for it.

Thanks in advance!

[somnif]

I've seen push-pull pairs in amp design, but not in pedals. What an odd choice.

[samhay]

The clipping is done with a variation of a 'rubber diode' circuit:

https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part2/page3.html
https://www.google.com/patents/US20090231011

This gives soft-ish clipping, and more interestingly allows the clipping threshold to be varied. As a result, it crops up a lot in solid state amps to prevent hard clipping of the power amp stage.

I have built some variants with the rubber diode driven from within an op-amp feedback loop (like in the patent ^), where it works nicely enough.
Is it better than just using diodes? I'm not sure.

[PaulL]

The clipping circuit looks similar to the one I designed in my Whiskey Cream pedal (Circuit posted here in Build Reports only last week) Originally I had pots to adjust the clipping shape of both the top and bottom waveforms, but not for bias as I wasn't looking for strong distortion or fuzz.

I came up with the arrangement based on an idea stemming from VI Limiters which are used to steal away base current from driver stages of solid state power amps. This is used as a form of speaker protection. My idea was to implement it as a way of controlling the clipping shape. In the end I done away with the two pots that control the clipping shape to keep things simple and to stop me twiddling with knobs rather than playing. I also played around with the topology and in the end the circuit evolved away from the VI Limiter.

The gain structure in the Not Like A Boss is different from my Whiskey Cream pedal, so they should sound very different.

[matmosphere]

No idea but I like how he put the schematic in there under the foot switch.

[stringsthings]

No idea but I like how he put the schematic in there under the foot switch.

Neatly folded!

[Kajnjaps]

Hi Everyone,

Silver Blues made a stripboard layout at guitar-fx-layouts:

http://guitar-fx-layouts.42897.x6.nabble.com/Instructables-Not-like-a-BOSS-distortion-td38120.html#a38131

[Guybrush]

Amazing, it was actually me that put the request in on their forum!

I've just ordered the parts so will verify it asap.

Thanks to everyone for their input. Much appreciated.

Cheers

[Kajnjaps]

Hi All,

Since there was some discussion on how it works, I drew up a quick schematic.

• Drawing A shows a non-monotinc 'bump' function: for low inputs, the output just follows. Once the transistor begins to conduct, the output will drop with increasing input voltage.
• Drawing B shows the classic 'rubber zener'. It more or less  clamps the output to that voltage which causes transistor to conduct (so it's base is at 0.6V). 
• Drawing C shows combines A and B, by splitting the collector resistor, so you get something in between A and B, depending on the position of the wiper.

The distortion circuit in the Instructables is just drawing C with an extra emitter resistor and mirrored, to accomodate the negative input voltages. The emittor resistors limit the 'depth' of the downgoing slope of the characteristic.

Have fun!

[PaulL]

Thanks for posting the diagram. Interesting to to how we had similar ideas but based on different circuits (VI limiters compared to rubber diodes). In the end I found that my circuit worked partly as the transistors acting like a regular diode shunting some signal via base to emitter and partly some signal via Collector to emitter.

Did you find similar with yours?? Neat idea on the variable biasing of the opamp, I will have to try that one day.

[Kajnjaps]

You're welcome. I needed the bump circuit for another project. It was only later that I wondered what it would sound like in a distortion. Haven't measured base current, but since the base resistors are considerably higher than the collector resistor, probably the collector current dominates. In other words: the transistors work as 'proper' transistors and not merely diodes.

[nzCdog]

Hi Kajnjaps!

Welcome to the forum and thanks for sharing more info on your design

[Guybrush]

Hi All,

Since there was some discussion on how it works, I drew up a quick schematic.

• Drawing A shows a non-monotinc 'bump' function: for low inputs, the output just follows. Once the transistor begins to conduct, the output will drop with increasing input voltage.
• Drawing B shows the classic 'rubber zener'. It more or less  clamps the output to that voltage which causes transistor to conduct (so it's base is at 0.6V). 
• Drawing C shows combines A and B, by splitting the collector resistor, so you get something in between A and B, depending on the position of the wiper.

The distortion circuit in the Instructables is just drawing C with an extra emitter resistor and mirrored, to accomodate the negative input voltages. The emittor resistors limit the 'depth' of the downgoing slope of the characteristic.

Have fun!

Thanks for this! I've been looking for a new distortion for ages. I put in the parts order yesterday and will get it built (as oper the Tagboard layout) asap.

Thanks again!

[Kajnjaps]

Thanks for this! I've been looking for a new distortion for ages. I put in the parts order yesterday and will get it built (as oper the Tagboard layout) asap.

Thanks again!

You're welcome, have fun!
Also, it does pay to experiment a little with the component values, especially the emitter resistors R18, R19 and the gain of the input amplifier U1a (lower R1 for higher gain).

[Guybrush]

Thanks for this! I've been looking for a new distortion for ages. I put in the parts order yesterday and will get it built (as oper the Tagboard layout) asap.

Thanks again!

You're welcome, have fun!
Also, it does pay to experiment a little with the component values, especially the emitter resistors R18, R19 and the gain of the input amplifier U1a (lower R1 for higher gain).

Noted!

I'll do some socketing. Any suggestions for R18 and 19?

Edit - On the tagboard layout the components are lablled. WOuld you be able to point out which resisors I should socket?

Cheers