Author Topic: Angry Grouse: An amp-ish overdrive for guitar and bass  (Read 564 times)

Fancy Lime

Angry Grouse: An amp-ish overdrive for guitar and bass
« on: January 04, 2021, 05:57:39 PM »
Hi all and a healthy new year!

I have been toying with some ideas on the breadboard again and found something worth sharing. I called it the Angry Grouse because I happened to be looking at old vacation pictures from Grand Canyon, which included some of an "angry grouse" that the rangers warned us about. It turned out to be a rather nice grouse, so I thought the name would be fitting. Anyway:



The schematic is not pretty but optimized for turning into a PCB layout in KiCad, which I intend to do one of these days. You can ignore the PWR_FLAG things, these are just to make KiCad's bug checker happy.

The inclined connoisseur will see some resemblance with the 1990's Marshall line of pedals. The idea of putting a Baxandall tone control inside the clipping section comes from old Orange amps (although they have a James before the clipping stage but same difference, in a way). Turns out that this makes for one hell of a flexible drive pedal, especially at low to medium gain. At high gain, the effectiveness of any boost on the EQ is naturally quite a bit diminished. So this thing really shines a medium gain and if you need high gain, put another drive pedal in front. That is why I consider it an amp-like overdrive, because I would use it and combine it with other drive pedals in much the same way as I would a one-channel dirty amp. But it is not supposed to sound like this tube amp or the other. However, the thing sounds very nice with guitar but almost better yet with bass. I found myself automatically playing a lot of Motörhead riffs while testing this. Go figure.

The gainpot can be lowered to 250k for a bit finer control on the most useful range. Just for good measure, I also included a primitive "cab sim", which is really just three orders of low pass. It is tuned rather darkly, which I like, but others may wish to move the corner frequencies up a smidgen. The way it is, I think it best goes directly into a mixer or headphone amp.

I also tried an NJM4556 and could not tell a difference from the TL072, but R3 should be adjusted to 470k when using a 4556 or 4558. I tried various clipping diode arrangements and must say I am rather taken with the very simple red LEDs. Nice and crunchy and the right amount of headroom for this circuit. Q1 can be any medium to high gain NPN.

I originally planned to release this together with the PCB layout but it takes me forever to do those and Mark (https://www.diystompboxes.com/smfforum/index.php?topic=126135.0) forced my hand ;) I promise, I'll do a layout and sound clips someday but it may take a while. Busy times...

Cheers,
Andy
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

iainpunk

Re: Angry Grouse: An amp-ish overdrive for guitar and bass
« Reply #1 on: January 04, 2021, 06:34:45 PM »
that clipping in tone control idea is genius, i wish i came up with that myself!
do i understand correctly that if you want to put this through an amp, you can leave out the tone shaping after the last opamp?

cheers, Iain
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
『snailpilled』

Mark Hammer

Re: Angry Grouse: An amp-ish overdrive for guitar and bass
« Reply #2 on: January 04, 2021, 06:37:29 PM »
Intriguing.  So does stage 2 result in more clipping as the tone controls are set for more boost, and less when set for cut?

And just to shine a light on my naivete, is "n33" equal to 330pf?

Fancy Lime

Re: Angry Grouse: An amp-ish overdrive for guitar and bass
« Reply #3 on: January 04, 2021, 07:16:07 PM »
that clipping in tone control idea is genius, i wish i came up with that myself!
do i understand correctly that if you want to put this through an amp, you can leave out the tone shaping after the last opamp?

cheers, Iain
Yes, the tone shaping is probably redundant in that case.


Intriguing.  So does stage 2 result in more clipping as the tone controls are set for more boost, and less when set for cut?

And just to shine a light on my naivete, is "n33" equal to 330pf?
Yes, boosting bass or treble distorts them more, cutting cleans the respective frequencies up. So you can tighten up the bass or make it fat and fuzzy. Obviously, this makes the tone controls highly interactive with the gain control. I was a bit worried that the effectiveness of the tone controls would suffer to much in such a design but it turned out that this only becomes an issue at high gain. At medium gain, bass or treble cuts quickly drop the respective band below the clipping threshold with red LEDs, this getting back their effectiveness. Boosting shapes the tone only at lowish gain settings but shapes the character of the clipping at medium gain. The controls are thus a bit unusual but I found them rather more intuitive than I has expected.

And yes, n33=330p, M10=100k. I like the fact that you can express any standard R or C value with just two digits and one letter. It's the sort of neatness I enjoy. Don't tell my wife, she already thinks I'm crazy.

Andy
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

bluebunny

Re: Angry Grouse: An amp-ish overdrive for guitar and bass
« Reply #4 on: January 05, 2021, 04:19:31 AM »
So you sit through ten months of a pandemic and then two great overdrives come along at once!  Thanks Andy.  8)

BTW, expect Rob along at any minute.  The word "Grouse" will be like a magnet...    ;)
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Ohm's Law - much like Coles Law, but with less cabbage...

Fancy Lime

Re: Angry Grouse: An amp-ish overdrive for guitar and bass
« Reply #5 on: January 07, 2021, 05:18:04 PM »
Rob's been demagnetized, it seems... BTW, I forgot to mention what D1 and D2 are for. Since this thing is designed to be used like a dirty single channel amp, I wanted it to play nice with boosters up front. Kick on the treble booster for a "lead channel", sort of thing. The diodes at the input protect the TL072 opamp from straying out of it's common mode input range, which sounds, sham, "interesting" but not exactly great in all circumstances. If you use an opamp that does not suffer from phase reversal, D1 and D2 are not necessary.

Cheers,
Andy
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.