adding a buffer to a Ernie Ball Volume pedal?

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Beedoola
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Post by Beedoola »

I'm kinda displeased with the tone suck of my EB VP. I was thinking that maybe if I added a buffer it could remove that tone suck? I'm guessing thats what the active volume pedals do? - like the Goodrich, etc. Would adding a buffer eliminate that tone suck? If so, what is a simple, effective buffer to build?

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Post by earthtonesaudio »

It's hard to beat an opamp buffer. Low parts count, easy biasing, and generally better performance than you can get from discrete devices.

"Noiseless biasing" requires 4 parts, but voltage divider (2 parts) biasing is noisy for high impedance.

If you use noiseless biasing, a dual opamp, and tie lug 1 of the volume pot to Vref instead of ground, you get buffered input and output for the same number of components as a single buffer.


I would recommend first starting with the 25k version of the VP, and using a dual opamp for input and output buffers.


There are some other tricks you can do to relieve "tone suck," such as using a treble bleed cap between input and output, but using the 25k plus dual buffers is a "better" fix.
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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Post by Beedoola »

I have the bigger one, the 250k version. That make a difference? What buffer schematic would you recommend?

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Post by earthtonesaudio »

Since you have the 250k version, the tone suck you're hearing is probably worst when you have the volume turned down (and negligible when the volume is up full), and is due to the series resistance of the pot combining with the capacitance to ground of the output cable (which is 10x worse than the 25k version). An output buffer will remove the effect of the cable capacitance, which may take care of the problem.

The other source of tone suck is from the loading that the volume pot places on the (presumably passive) guitar pickups. This problem would be the same at all volume pedal settings. An input buffer isolates the pickups from the loading of the volume pot and takes care of that problem.

I'd guess that you are most likely experiencing the first problem, but it's also possible your guitar's electronics don't like the 250k load of the volume pedal.


I'd recommend an opamp noninverting voltage follower for both input and output buffers, just to cover all the bases. The first one can be biased with a large value resistor, and the second can be biased by the volume pot if you tie lug 1 to the same voltage reference you used for the first opamp. Input and output caps and maybe some pull-down resistors on the outside of those caps to help minimize pops from true bypass pedals that might be in the chain.
As an example, if you use a 1M input biasing resistor, a .01 cap will pass the whole audio range. For the output cap, it should be large enough that the next device's input impedance won't cut bass. 1uF should suffice in most situations, and realistically .1 is probably fine.
Between the buffers, all you have is the volume pot. Lug 3 to buffer 1's output, lug 2 to buffer 2's input, and lug 1 to your 1/2 supply voltage reference.

For that 1/2 voltage reference, a couple 10k resistors between +9V and ground (assuming you're using a 9V supply) and a 100uF cap from their junction to ground is all you need.
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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Post by Beedoola »

ahhh, I'm kind of a noob. Is there a schematic someone for this?

Much Thanks

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Post by earthtonesaudio »

How's this? (pulldowns omitted)
Attachments
volume buffer.png
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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Post by Beedoola »

freakin perfect, much thanks. What Op Amps could I use? Any that are available at Radioshack? - only place open today to get ICs

http://www.radioshack.com/family/index. ... Id=2032279

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Post by earthtonesaudio »

Any "general purpose" opamp will work. I'd go for a dual, low noise, high input resistance type such as the TL082 (which is listed on that radio shack page). Two 741 opamps would also work but will take up more space and be slightly noisier.
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Post by Beedoola »

Hey man, Much thanks on that layout. I'm new to reading schematics and the idea of a "dual" opamp so I bought 2, quickly realizing after seeing the data sheet what your layout meant :slap: :slap: :slap: :slap: I got it working, sounds good. Do the pull-down resistors go in between the in and out and grounds?

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Post by earthtonesaudio »

Beedoola wrote:I got it working, sounds good.
Awesome. When it comes to making music, that's what matters most.

Pull-downs are generally associated with true bypass switching, and their purpose is to drain leakage charge off of the input/output coupling caps while the effect is disengaged. If you have no true bypass effects at the input or output of the volume pedal, you don't need them. If you do have true bypass effects connected to the volume pedal, they might have their own pulldowns and so the volume won't need them. I would advise adding them only if you have noticeable pops when switching effects on/off or if you will be using the volume pedal with other (unknown) pedal systems.
Beedoola wrote:Do the pull-down resistors go in between the in and out and grounds?
Yes. The input pull down resistor attaches to the input side of the input cap and ground, and the output pull down resistor attaches to the output side of the output cap, and to ground.

I'm assuming the volume pedal will not have a bypass switch, but if it does, the pulldowns should remain attached to the effect when it is bypassed (some folks mistakenly put pulldowns on the jacks, which doesn't do much good).
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Post by Beedoola »

There doesn't seem to be too much popping from other pedals, only from my tb looper I made which is right after the Vpedal. Should I add a PDS to the TB looper? If so, where does that go since its just a switch and jacks? - on the switch itself? From the input lug on the switch to ground?

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Post by earthtonesaudio »

Pull-downs would work in the TB looper, but on the other hand they'd always be there loading the signal (even when you didn't need them, for example when TB-ing a non-poppy effect). A much better approach is to put the pull-down resistors in the effect pedal itself, if it needs them, so that a) it doesn't cause pops and b) you don't need a special TB looper to keep it from popping.
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Post by Beedoola »

I added some pull-downs to the in and out of the volume pedal - from circuit "in" to circuit "ground" as well as the circuit "out" to "ground". I still get popping from my TB looper - which is right after the volume pedal. Any suggestions?

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Post by earthtonesaudio »

Alright, time for the detective work.

First, though, you should understand and accept the fact that all mechanical switches have some noise, and what we're trying to eliminate here is excessive noise compared to some baseline that you know is normal.


Anyway, the "where's the pop" search starts by listing what pedals you have in the chain, whether they are true bypass, and whether they're on or off. This includes the bypass loop and what's in it. A picture would also help, if possible.

Next comes a little science. Change one variable at a time, noting any changes. Does it pop ONLY when the volume pedal is in the loop, or does it pop if you swap the volume for a Boss pedal, for example?

Once you've isolated the problem pedal, test extensively. Does it pop when turning on, turning off, or both? Is it worse if you hit the switch while playing or with the guitar silent? How about if the pedal's power supply is removed, does it still pop? Does it become less after a few on/off switch cycles? Likewise, is it worse if the pedal has been powered, but disengaged for a long period of time and then switched in?



The pull-down method is not perfect, and in some cases an alternative approach may be better. The reason you get popping is because the input/output caps, being real rather than ideal capacitors, do not perfectly block the DC on the "effect" side of the cap. So a little leakage through the cap charges up to make a DC voltage on the "switch" side of the cap, which makes a pop when you switch it into the signal path. The job of the resistors is to drain away that charge to ground, continuously, and therefore keep the DC voltage on the switch side of the cap at ground potential.
Two factors improve matters: less leaky caps and lower value resistors. The lower value resistor is problematic at the input side, because (especially for a buffer) you want the input impedance to be high. There is some good news. When you use a higher value input bias resistor (for a high input impedance), you can use a smaller cap to get the same frequency response. Smaller caps tend to be less leaky, so you don't need as low a value for an effective pulldown.

The key point is that the DC voltage level on the switch side of the effect's input/output caps should be very close to if not equal to the DC level of the rest of the signal chain. Since there's no standard bias voltage (and many effects use batteries which change their value over time), the only reference that makes sense is ground. When looking for a bypass pop, never assume that the input or output of some other pedal is at the proper (ground) potential. Use your meter and figure out which pedal has some DC leaking through to a point which should be at zero volts.


The "alternative approach" could be a lot of things, but one commonly successful approach is to ground the switch side of the effect's input cap when the effect is disengaged. This is like using a zero-ohm pulldown, but only when you're not using the effect. This allows you to omit the regular input pulldown resistor, and works well as long as the cap is not super leaky. Basically the only chance the cap has to charge up is during the short amount of time while the switch is moving from one set of contacts to the other. I think there are wiring diagrams for this on generalguitargadgets.com, Tonepad.com, Beavisaudio.com, and maybe Muzique.com. Here's the GGG one.
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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Post by Beedoola »

Hi,

Thanks for the lengthy reply. I don't think I explained my setup clearly. The Volume pedal isn't IN the TB loop. The pedal is right BEFORE the loop...

My Chain is: Guitar>Home made dual Fuzz>Red Witch Fuzz God II>Barber LTD>Volume Pedal>TB Looper>etc.

Prior to adding the buffer in the Volume pedal, I got no pop from the TB looper. I tried putting my Boss TR-2 in place of the Volume pedal since it is buffered - All Boss are right? And I got NO pop with the Boss in front of the loop...I added PD resistors - 1M between the Buffer In and Out on the Board and to the circuit ground but no luck.

"Once you've isolated the problem pedal, test extensively. Does it pop when turning on, turning off, or both? Is it worse if you hit the switch while playing or with the guitar silent? How about if the pedal's power supply is removed, does it still pop? Does it become less after a few on/off switch cycles? Likewise, is it worse if the pedal has been powered, but disengaged for a long period of time and then switched in?"

Pops when turning it on and off. Playing while switching may mask the sound.., Its definitely there when its silent. Still pops if the power is removed. No, doesn't become less after switch cycles.

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Post by earthtonesaudio »

If you have a voltmeter, probe both the input and output of the volume pedal. Anything other than zero will make a pop. I'd bet the most likely culprit is the output cap. If you used a 10u electrolytic cap like I drew in the schematic, it's probably leaky, so you can (and should) use a much smaller pulldown resistor (such as 10k) to keep it under control.

That should cut it down considerably. If not, then you might have a seriously leaky cap, so try a smaller output cap (and correspondingly larger output pulldown).
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Post by Beedoola »

Ok thanks, I'll try that. So would probably not need a input pull-down resistor? Just an output?

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Post by Beedoola »

K, so I measured the output and I was getting like 4.5v. I took out the in and out 1m PDs and put in like a 22k PD in the output, seems to be working ok now.

Thanks a lot for the help!!! :applause: :horsey: :thumbsup

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Post by earthtonesaudio »

Awesome! That must be a really leaky cap!
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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