Opinions About Boost Design

[Spacecommandant]

Hi all, I've put together this volume/boost design meant to be used in a volume pedal enclosure.
It sounds good to my ears but I would love to hear any advice about improving the design in any way.
The things I want to keep the same:

The Volume treadle pot is A250k
There is a Max pot that sets the volume ceiling when Volume pot is fully clockwise
There is a decent amount of clean boost available

Thanks

[jimilee]

I like the idea of a boost cap, nice.


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[madbean]

Here's how I would approach it:

- On the power section, move the 47R resistor to the 9v input. This acts as a LP filter on the main power supply so the charge pump works from the lowest noise floor.
- The unused op-amp stage can be a buffer for the bias supply. Not necessary, but it's good practice when practical.

- On the audio side, a slight re-arrangement offers up some possible advantages. Make the Volume pot a voltage divider and the trimpot the max boost control instead of the other way around. This way the volume pot behaves more like the one on a guitar. Add a switch for active or passive input. So, in passive mode it reacts directly with the guitar volume, and in active mode it has an input buffer. Might come in handy with using different style of pickups.

KIM: this is just how I might do it. Your design is just as good so I'm only trying to show an alternate approach to the same idea.

[Spacecommandant]

Here's how I would approach it:

- On the power section, move the 47R resistor to the 9v input. This acts as a LP filter on the main power supply so the charge pump works from the lowest noise floor.
- The unused op-amp stage can be a buffer for the bias supply. Not necessary, but it's good practice when practical.

- On the audio side, a slight re-arrangement offers up some possible advantages. Make the Volume pot a voltage divider and the trimpot the max boost control instead of the other way around. This way the volume pot behaves more like the one on a guitar. Add a switch for active or passive input. So, in passive mode it reacts directly with the guitar volume, and in active mode it has an input buffer. Might come in handy with using different style of pickups.

KIM: this is just how I might do it. Your design is just as good so I'm only trying to show an alternate approach to the same idea.

Brilliant, thank you. If I were to add a jack for a tuner, I've seen with passive volume pedals that the signal at the circuit input is just split, one for audio and one for the tuner. I imagine I could do that with this circuit but is there a better approach in this situation?

[madbean]

For a tuner out, you can modify the design I showed a bit further. Ditch the buffered VB rail and use that op-amp as a separate output. It you tie it the volume pot as shown, you'll still get tuner output even when the volume is all the down. Or, just use the same idea to adapt to your design.

[Spacecommandant]

That's great, I'll try this... thanks again!

[Spacecommandant]

For a tuner out, you can modify the design I showed a bit further. Ditch the buffered VB rail and use that op-amp as a separate output. It you tie it the volume pot as shown, you'll still get tuner output even when the volume is all the down. Or, just use the same idea to adapt to your design.

Trying to reduce white noise-like hiss from the boost section, do you think reducing R3, R4, R5, and the Boost pot is an option?
What is considered to be an acceptable minimum value for these resistors?

[Aleph Null]

For a tuner out, you can modify the design I showed a bit further. Ditch the buffered VB rail and use that op-amp as a separate output. It you tie it the volume pot as shown, you'll still get tuner output even when the volume is all the down. Or, just use the same idea to adapt to your design.

Trying to reduce white noise-like hiss from the boost section, do you think reducing R3, R4, R5, and the Boost pot is an option?
What is considered to be an acceptable minimum value for these resistors?

Reduced resistances will in theory create less thermal noise. Thermal noise probably pales in comparison to what's coming from your pickups. You could try putting a capacitor in parallel to R4 to reduce the highs. Something as large as 100pF, maybe—that would start attenuating around 7.2kHz, which is around where most guitar speakers start to roll off anyway.

[Spacecommandant]

I reworked the schematic but haven't breadboarded it yet... I'd love another set of eyes first.
I decided to change the two inverting stages to non-inverting and use 5532 instead of TL072 which allows for much smaller resistors, and added the 100r resistors at the +input and output pins to help eliminate stray capacitance issues (from what I've read).
I'm unsure what the best value would be for C4.
Are there any ways to improve this?

[jimilee]

I'm curious, why would you run it at 18v if it's just a boost? I mean, I know more headroom. But it seems like unnecessary parts.

Again, I'm just curious, and not being all judgy[emoji1787][emoji1787][emoji1787]


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[Spacecommandant]

I'm curious, why would you run it at 18v if it's just a boost? I mean, I know more headroom. But it seems like unnecessary parts.

Again, I'm just curious, and not being all judgy[emoji1787][emoji1787][emoji1787]


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I guess the idea for this is to have it be a volume pedal most importantly, but with the bonus option of being able to adjust the maximum volume a decent amount above unity gain if need be. Basically, a relatively neutral sounding volume pedal. Maybe the 18V is unnecessary, but I just figured it would eliminate the possibility of nasty clipping with hot pickups.

[jimilee]

I'm curious, why would you run it at 18v if it's just a boost? I mean, I know more headroom. But it seems like unnecessary parts.

Again, I'm just curious, and not being all judgy[emoji1787][emoji1787][emoji1787]


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I guess the idea for this is to have it be a volume pedal most importantly, but with the bonus option of being able to adjust the maximum volume a decent amount above unity gain if need be. Basically, a relatively neutral sounding volume pedal. Maybe the 18V is unnecessary, but I just figured it would eliminate the possibility of nasty clipping with hot pickups.

That makes sense, thank you sir.


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