Crate GX15 OD Channel - In a pedal?

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doompalm
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Post by doompalm »

Hi everyone,
This is my first post. So bear with me. I've built a few pedals - a treble booster, a couple of fuzzes, @dylan159 's Quick Shave and have a reasonable handle on soldering, etc... I'm wondering if anyone can give me any advice on a potential project. I'd like to take the OD channel from an old Crate GX15 amp and put it in a pedal. From what I can see it's a stacked Op Amp type distortion circuit?

Could it work on 9v? And if so what's the best way to make it happen? A bonus would to be to make the distortion a little hairier - sort of like a fuzzstortion. Any info or advice is much appreciated.

GX15ODchannel.jpg
This is a link to the full manual https://www.audioservicemanuals.com/c/c ... -schematic

Cheers....

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bmxguitarsbmx
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Post by bmxguitarsbmx »

Yeah, I have a sweet spot for those old Crate's too. I grabbed this pcb recently and its probably the quickest way grab that tone. Ampeg and Crate were part of SLM and shared a lot of technology, specifically the "flexwave" technology.

https://aionfx.com/project/vh-drive-channel/

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mauman
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Post by mauman »

+1 on looking at the Aion project for ideas including +/- 15V supplies. If you do decide to go with a single +9V supply, you'll need to add a 4.5v reference to substitute for the ground reference on the op amps (pins 5 & 10) as well as the reference for R2 and R4, and optionally C6 and C8. If you keep the input limiter diodes D1/D2 they would connect to +9V and ground respectively instead of +/-15v. If you want a low output impedance you can keep the buffer that currently drives the 3-band EQ and make that your output, after the 250k Level pot. Or if you keep the EQ section as well, I'd add an output buffer after that.

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Post by doompalm »

Thanks! I didn't know about the Aion kit so that's really good to know.

I was thinking of a stripped down version of the circuit, no EQ, just the drive section.

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Post by mauman »

doompalm wrote: 24 Sep 2022, 20:02 Thanks! I didn't know about the Aion kit so that's really good to know. I was thinking of a stripped down version of the circuit, no EQ, just the drive section.
Then I'd go from the input, thru the buffer just before the EQ section (IC pins 12/13/14), add an output cap to block DC and take your output there. Buffer in for high Z, buffer out for low Z, and two drive stages in between, a total of 4 IC sections, so one TL074 would do it.

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doompalm
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Post by doompalm »

Thanks!

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blackboarcult
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Post by blackboarcult »

Just around last week I tinkered with two of those gain stages in series for a project. Did not sound great at all on its own. It seems that pre and post gain stages filtering is essential to get a nice sound out of it. Would love to try those on the topology of the Orange CR120 (instead of abused, overdriven TL072s) but it is far down in my current To-Do list.
Also, I might be dead wrong on it, but I think that if one changes the voltage from +-15V to +9V, Zeners might have to be changed to lower V too in order to achieve the same response...

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Post by doompalm »

By not great do you mean not great as in boring or not great as in broken? I'd want to avoid boring, broken I could work with.

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Post by blackboarcult »

Uninteresting by itself, IMO. I breadboarded it hoping to replace the last gain stage in a Musicman HD150 preamp with something a bit more lively, but it did not give any better results than the original block. Also tried it at the beginning of the circuit, tweaked a bit around, no cigar.
But then again, plain diodes to ground or in the feedback loop are also uninteresting without proper filtering and freq selection :)
I'd say breadboard and experiment! You might end up with something you like...

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