Help with gain and EQ interaction

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Eightsevenzero
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Post by Eightsevenzero »

Hey, I'm working on building a D&S II clone and noticed something I'd like to fix. I have the schematic below, I actually own a real one of these and the original does the same thing. When the gain pot is maxed the circuit gets super trebly, but if you back off just a bit, say 5% from max gain the circuit goes back to normal, the sacrifice is you lose some of the distortion of course. I'd love to understand why it does this first, then maybe someone could help me with a possible solution, ideally, I'd like to keep the max level of gain and get the tone to stay the same as it being backed off a bit, but if not, maybe a way to shave off just the furthest extent of gain to make sure the EQ doesn't go all trebly. Thanks!
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mauman
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Post by mauman »

Try adding a cap in parallel to the 100k resistor in the feedback loop of the first half of the op amp. That should roll off some highs as you increase the gain. Something between 22 pF (small reduction in treble) and 470 pF (significant reduction) will probably suit you.
Maxon with added cap
Maxon with added cap
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CheapPedalCollector
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Post by CheapPedalCollector »

Change the 1k5 resistor, increasing it will reduce the treble response too and also affect the clipping. A 10k pot with a 100 ohm resistor in series makes a nice bite control for it. If it lacks output change the 50k pot to 100k.

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george giblet
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Post by george giblet »

The last bit of pot rotation has a lot of gain.

Nothing to do with the mods but that schematic has an error.

The 100k feedback resistor should be 1M0 (on both the original and Reissue)
AFAIK, the original PCB has no 100k's.

I don't have my stash of schematics but I found this for the re-issue.
viewtopic.php?t=24229

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Eightsevenzero
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Post by Eightsevenzero »

Thanks! I caught that when I traced my original.

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george giblet
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Post by george giblet »

Having a closer look at that second schematic, the resistor before clipping diodes is 1k8 not 1k5.
I found a PCB pic of on old D&S II and it definitely has a 1k8 on the board.

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phatt
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Post by phatt »

Something to consider regards that 1k8 Resistor before the diodes is that when they conduct it's shorting the output of the preceding chip.
Some chips can handle very low output loads but a lot do not.
I would try at least 4k7 or even 10k as that will reduce the hi freq crud giving a much smoother sound.
You may even find you don't need the 1nF cap across the diodes. IMO, worth trying :wink:
Just speaking from my experience while BBoard testing many OD circuits.
Phil.

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Eightsevenzero
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Post by Eightsevenzero »

Thanks! I'll give it a shot.

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bmxguitarsbmx
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Post by bmxguitarsbmx »

If you want to decouple the high level of interaction between the amount of top end boost of the first stage and the Gain level, you should move the Gain control. Essentially do what a Tubescreamer does with its gain control. But, since you find the 4k7 + 47nF too bright, modify the 47nF to 56n or something larger.

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george giblet
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Post by george giblet »

Something to consider regards that 1k8 Resistor before the diodes is that when they conduct it's shorting the output of the preceding chip.
Some chips can handle very low output loads but a lot do not.
I would try at least 4k7 or even 10k as that will reduce the hi freq crud giving a much smoother sound.
Increasing the resistor will significantly affect the tone control, making it darker.

Currently tone control 220n + 220R works against a feed resistance of 2.8k (1k8 + 1k0). Upping the resistor will cause more cut and lower the affected frequencies. On a tube screamer the resistor is only 1k (total).

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phatt
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Post by phatt »

george giblet wrote: 11 Jul 2022, 08:33
Something to consider regards that 1k8 Resistor before the diodes is that when they conduct it's shorting the output of the preceding chip.
Some chips can handle very low output loads but a lot do not.
I would try at least 4k7 or even 10k as that will reduce the hi freq crud giving a much smoother sound.
Increasing the resistor will significantly affect the tone control, making it darker.
Yep that is the whole point. [smilie=a_whyme.gif]
george giblet wrote: 11 Jul 2022, 08:33 Currently tone control 220n + 220R works against a feed resistance of 2.8k (1k8 + 1k0). Upping the resistor will cause more cut and lower the affected frequencies. On a tube screamer the resistor is only 1k (total).
Yes you need to tweak it to your liking and Yes you will need to tweak the response in other ways.
just play around with different parts on a Breadboard until you find what works, meantime you learn a lot. :thumbsup

I'm no teck wizz,, but smart enough to simulate while tweaking the circuit. Sims really helped me come to terms with how tone is won and lost.
Phil.

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