Not using the entire potentiometer in a buffered volume pedal circuit?

[brand0nized]

While assembling the final parts to my volume pedal project, I realized that I somehow made an error in my calculations somewhere and my volume pedal doesn't rotate the pot to its entire rotation.

I don't want to change the hardware, so what options do I have for the buffer volume circuit? I can have zero resistance at tie down position, or maximum resistance at heel down position.

Any ideas? Also, I'm not sure what value pot to use in the final version.

[midwayfair]

Without knowing what you're using for a buffer (hopefully it's an op amp), if you have buffers on both sides (before and after the pot), the little bit of extra resistance probably won't hurt anything -- you could even take advantage of it and put a cap across lugs 2 and 3 on a small switch for when you need a small treble boost. Similarly, as long as you've buffered both sides of the pot with a good buffer, it shouldn't matter too much what value it is.

I don't know what kind of mechanism you're using to turn the pot, but it might be that not all hope is lost in that regard. Can you show a picture or diagram of it?

[brand0nized]

Without knowing what you're using for a buffer (hopefully it's an op amp), if you have buffers on both sides (before and after the pot), the little bit of extra resistance probably won't hurt anything -- you could even take advantage of it and put a cap across lugs 2 and 3 on a small switch for when you need a small treble boost. Similarly, as long as you've buffered both sides of the pot with a good buffer, it shouldn't matter too much what value it is.

I don't know what kind of mechanism you're using to turn the pot, but it might be that not all hope is lost in that regard. Can you show a picture or diagram of it?

What's the difference between opamp and jfet buffers in context of a volume pedal? I'm guessing I could just build two of the same opamp buffers connected between lug 2 & 3 with 1 going to ground?

This is the mechanism I'm using. It's a string and spring system like Ernie Ball pedals

[midwayfair]

What's the difference between opamp and jfet buffers in context of a volume pedal? I'm guessing I could just build two of the same opamp buffers connected between lug 2 & 3 with 1 going to ground?

For one thing, a JFET buffer will have higher output impedance and it's device sensitive. A BJT will have lower output impedance at the expense of input impedance. Quibbling over a few hundred ohms might seem petty, but an op amp doesn't really take up much room at all in a dual buffer setup and might actually be less than two JFETs (I think you won't even need a cap between stages if you tie lug 1 to Vb instead of ground). Besides, you can also very easily change it to have a slight bump in gain, to correct for the minute loss of volume that comes from buffering, or just as a boost.

You're also still fighting the input voltage issue you'd have in any other JFET pedal (I think the 2n3819 is the highest Vp I've encountered, and it's still well under 4V, so it loses out to an op amp running on 9V). While a JFET will consistently sound better when distorted, your buffer is less likely to be distorted, so it's better to just use the op amp.

Also, at unity gain, my usual complaint about op amps (a little more noise) shouldn't apply.

[brand0nized]

Would this work? I'm doubling the same opamp buffer circuit onto a dual opamp with a 100ka in between. I've never drawn a circuit before so I don't know if this is correct.



The buffer is from here
http://tagboardeffects.blogspot.com/2014/08/buffers.html?m=1

[BaklavaMetal]

why do you need a pot between the buffers?
it would only serve if you want to attenuate the signal
and why use two buffers in series?
if you wanna use two buffers, one for input and one for output, you would connect wah input where the wiper is connected and wah output on the input of second buffer
but, if you want this arrangement, you don't need 1M res to ground before second 100n cap, in fact i would say you don't even need the second 100n cap because 10u cap before pot does the ac coupling

[brand0nized]

why do you need a pot between the buffers?
it would only serve if you want to attenuate the signal
and why use two buffers in series?
if you wanna use two buffers, one for input and one for output, you would connect wah input where the wiper is connected and wah output on the input of second buffer
but, if you want this arrangement, you don't need 1M res to ground before second 100n cap, in fact i would say you don't even need the second 100n cap because 10u cap before pot does the ac coupling

I'm making a volume pedal not a wah, hence the volume attenuation

[brand0nized]

(I think you won't even need a cap between stages if you tie lug 1 to Vb instead of ground). Besides, you can also very easily change it to have a slight bump in gain, to correct for the minute loss of volume that comes from buffering, or just as a boost.

A few questions
1. Where is Vb?
2. Which cap would be omitted?
3. Which resistor would I change to change the output volume?

[midwayfair]

Brandon, Vb would normally be created with a separate voltage divider. Right now, you're creating it with the two 1M resistors after your input cap. Open up, say, the Greenbean schematic to see another way to do it. I think they're 47K in that pedal, but 10K would be better. It creates a more stable bias voltage than the way yours is drawn. It also gives you a "faux ground" -- the op amp's middle point (4.5V) -- which is handy for reducing parts as we'll see in a moment.

If your volume pedal doesn't have a switch, you don't need the 1M before your input cap. That's a pulldown to drain off any voltage that leaks out through the cap.

After you create the Vb point, you can omit the 10uF and 100nF caps, as well as all the 1M resistors, between pins 1 and 5, and tie the pot's pin 1 to Vb instead of ground. It will still be a normal volume control that way, but since you aren't connecting it to ground, you don't need to decouple the cap to keep the op amp working properly.

in fact i would say you don't even need the second 100n cap because 10u cap before pot does the ac coupling

If the pot is going to ground, he would need to isolate pin 5 from the ground connection, so the other cap would still be necessary as drawn.

why use two buffers in series?

So that the volume pot doesn't load the guitar or the following circuit. EDIT: even if he were building a wah pedal (not sure how you got that impression), it would still be a good idea to buffer the volume pot from the output of the wah to avoid affecting the cutoff frequencies. And once you're using an op amp, you might as well make use of the other half of the op amp and buffer the output.

[BaklavaMetal]

2. My mistake, i thought ypu were building a wah, prolly had few too many beers
Vb is between 1M resistor which run from Vcc to +input of opamp then another 1Mres from +input to ground

I believe you could ommit the 100nf cap before 2nd opamp input, but i may be wrong.
3. If it's a vol pedal, then the pot is used for volume change, no res change necessary

[brand0nized]

Brandon, Vb would normally be created with a separate voltage divider. Right now, you're creating it with the two 1M resistors after your input cap. Open up, say, the Greenbean schematic to see another way to do it. I think they're 47K in that pedal, but 10K would be better. It creates a more stable bias voltage than the way yours is drawn. It also gives you a "faux ground" -- the op amp's middle point (4.5V) -- which is handy for reducing parts as we'll see in a moment.

I'm looking at the Greenbean schematic, and I'm not sure what is going on to create Vb.

After you create the Vb point, you can omit the 10uF and 100nF caps, as well as all the 1M resistors, between pins 1 and 5, and tie the pot's pin 1 to Vb instead of ground. It will still be a normal volume control that way, but since you aren't connecting it to ground, you don't need to decouple the cap to keep the op amp working properly.

On my original schematic, I would be omitting everything between pin 1 and 5 but the potentiometer (the circled parts)?

[Guitarmageddon]

Vb is created with a voltage divider at the junction of R18&19 (top, near #4)
I'm no EE, but isn't it a bit of overkill to buffer the output of a volume pedal? Why not just use a single opamp and minimum of parts?
As for the lack of full rotation, that's not a bad thing, saves busting the pot. Maybe try using a larger value pot with a series resistor to take up the difference.
One other thought, Proel volume pedals have a pot on the side that sets the heel down level, so you can treddle btween full volume and a pre-set minimum (fully off or just a lower volume)- quite a handy feature.

[brand0nized]

Vb is created with a voltage divider at the junction of R18&19 (top, near #4)
I'm no EE, but isn't it a bit of overkill to buffer the output of a volume pedal? Why not just use a single opamp and minimum of parts?
As for the lack of full rotation, that's not a bad thing, saves busting the pot. Maybe try using a larger value pot with a series resistor to take up the difference.
One other thought, Proel volume pedals have a pot on the side that sets the heel down level, so you can treddle btween full volume and a pre-set minimum (fully off or just a lower volume)- quite a handy feature.

I have no idea why I would use two, actually, but it seems like it won't hurt to have it.

I plan to use this between drives and delays so I can do swells, so I need heel down position to be zero volume.

Good point about messing up the pot, didn't think about that. But in my design, I have stoppers to stop the foot rocker right where the pot reaches maximum rotation.

[midwayfair]

I'm no EE, but isn't it a bit of overkill to buffer the output of a volume pedal? Why not just use a single opamp and minimum of parts?

I'm no EE either, but turning down the volume pedal slightly does add series resistance, which could load down a following circuit. A single op amp is just as big as a dual op amp, and all you're doing it connecting two more pins and using Vb for the volume control's ground. The parts count is identical.

Brandon, you're correct on the circled parts. Needless to say you should breadboard it first to make sure you're happy with it.

[brand0nized]

Brandon, you're correct on the circled parts. Needless to say you should breadboard it first to make sure you're happy with it.

Yup, plan to breadboard, but I need to order parts because I'm not deep enough into guitar electronics to have opamps laying around.

Is Vb created with just the two 10k resistors or with D5 and R17 also?