Anyone know how to put a gain control on a DOD FX868?

Frequent asked about building blocks: gain stages, buffers, clipping configurations, ...
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Musician78
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Post by Musician78 »

I did search.

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JustinFun
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Post by JustinFun »

Do you mean an FX86?

Schematic is here:
http://www.freeinfosociety.com/electron ... php?id=346

Are you modding, or building? The latter would be easier I expect.

I'm not great on the tech side, but if it was me I'd start on the breadboard and play with pots replacing the 220k and 100k resistors from the inverting input to output on the first two opamp stages.

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Post by mictester »

Musician78 wrote:I did search.
Remove the 120k that goes from the output back to the inverting input of the second op-amp. Put a 220k pot - wired as a variable resistor - in series with a 10k resistor, and connect across the 120k position. The resistance will then be variable from 10k to to 230k. The other gain-setting resistor in that stage is 470Ω, so you'd be able to vary the gain from 21X up to 489X. That range might be too big, so you could pad down the 220k pot with (say) 330k, and perhaps increase the 10k up to 22k or even 33k to increase the minimum gain. The trick is to experiment!
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Post by Musician78 »

Sweet, thanks!!! How do you get the damn thing out? I've remover the bottom, and the bolts and washers for the switches, and the one screw holding the board in place and it won't budge.

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Post by Musician78 »

Wow. How can you look at the schematic and see exactly where I need to put it? Amazing! I only had a 100k pot that would fit, so the range is between 7 and 9, but I can go from blues to full blast in thAt range. Much more manageable now. Thank you!!!! :)

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Post by Musician78 »

Now, if I add another resistor is series, will I get that same range from say, 7 - 12 instead of 7 to 9? Would the resistor go to the wiper or one of the other lugs?


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Post by JustinFun »

Musician78 wrote:Wow. How can you look at the schematic and see exactly where I need to put it? Amazing! I only had a 100k pot that would fit, so the range is between 7 and 9, but I can go from blues to full blast in thAt range. Much more manageable now. Thank you!!!! :)
In Mictester's case, education and lots of experience.

However, it's not that hard to get on the first rung. Get to know classic 'building block' schematics; Tubescreamer, Distortion plus, RAT, Fuzz Face, Electra Distortion. There are articles on the net to help if you google. After that you should be able to recognise that the heart of this schematic is based around an op-amp with diode clippers to ground. In fact it looks a lot like a RAT.

In fact, if you're interested, have a look now. Find the RAT schematic. Put is side by side with the DOD schematic. Can you see the clipping stage in each schematic? Now can you see how the gain control in the RAT is implemented? And how you might duplicate that in the DOD?

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Post by mictester »

If I was adding that control to the pedal, I'd use (say) a 100k pot and put (perhaps) 56k or 68K in series with the pot. The gain of that stage would then be stock with the pot halfway, and you'd be able to vary it downwards and upwards either side of the stock value. Here's how it works:
Gain Mod.png
Gain Mod.png (1.95 KiB) Viewed 182 times
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Post by JustinFun »

And if you want to know the why, rather than the how, you could do worse than read the 4 articles here:

http://gaussmarkov.net/wordpress/catego ... s/op-amps/

You'll find the answer in article 4 'divided negative feedback', but you'll need to read the other 3 for it to make sense.

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Post by Musician78 »

mictester wrote:If I was adding that control to the pedal, I'd use (say) a 100k pot and put (perhaps) 56k or 68K in series with the pot. The gain of that stage would then be stock with the pot halfway, and you'd be able to vary it downwards and upwards either side of the stock value. Here's how it works:
Gain Mod.png
I tried putting a 20 k resistor in series with the pot and all it did was increase the starting gain. The gain knob worked same as before, 7 - 9 with 9 being stock and nothing added from 9 - 5. Just the stock distortion.

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Post by mictester »

Is your pot 100k? If so, try a 68k resistor in series. If it's a linear pot, halfway up will be just about the stock gain for the stage. You should then be able to turn the gain up to more than "11" in one direction and down to almost clean in the other.
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Post by Musician78 »

I don't have a 68k but I do have 3 20ks I can run in series and be 8k short.

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Post by mictester »

Musician78 wrote:I don't have a 68k but I do have 3 20ks I can run in series and be 8k short.
That'll work.
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Post by Musician78 »

It didn't work. It made very little difference between 7and5 and that was max distortion. I'm thinking it isn't supposed to be fucked with.

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