Prentiss Effects

This post is an in depth look at the Ernie Ball VP tone suck mod. It works on all passive Ernie Ball volume pedals and can also be applied to other passive volume pedals. I'll go over why people experience "tone suck" (hint: it's because of impedance) and a step by step on how to install a buffer that prevents this.

I've been looking into making a replacement PCB with these mods built in. If this is something interesting to you then please go fill out this form and let me know.

Now, let's get started!

What it does and why it works #NerdTime

In audio level signals, we look at optimizing voltage transfer to maintain our signal. As the ratio drops, our high end begins to roll off as well. The Ernie Ball VP (and JR) is basically the volume circuit from your guitar and an added output jack for the tuner before the volume pot. The problem with this configuration is that the volume pot in the volume pedal puts a load on your signal which combines in parallel with the input impedance of your amp or the next pedal in your chain and lowers your voltage transfer ratio. Let's look at the math of how that all works.

For starters, your guitar has a high output impedance. Sources vary a lot on what that range is, but Jack Orman and Jack Deville seem to agree that it's somewhere between 10k-40k (and they're both really smart so we'll trust them). I'll use 15k for our purposes. Now standard practices tell us that in order to optimize voltage transfer we want the input impedance of the next object in our signal chain to be at least 10x greater than our output impedance. In fact, the higher we can make the input impedance, the better the transfer will be and less high end will be lost. The reason for this is that your guitar's output impedance can be thought of as a series resistance and the following input impedance as a resistance to ground which creates a voltage divider.

So let's say that we normally have a guitar going into a pedal with a 500k input impedance. That gives us a circuit that looks like this:
We can then look at the voltage divider equation to figure out our ratio. Upon inspection, we can state that if our input signal is assumed to be 1V then we really only need to look at the resistance ratio to figure out our voltage transfer ratio. So by plugging in those numbers we get the following:

So we're going to be seeing about 97% of our signal go through. Great! That 3% that we're losing is coming from our high-end because higher frequencies are effected more by impedance. So the frequencies we're losing are ones we can't really hear.

Now if we add the volume pedal into the signal chain we get a different result.

The pot in the volume pedal is essentially a parallel resistance to our pedal's input impedance. This means that our overall resistance value is going to be lowered. If we do our calculation again then we get this result:

Now we're seeing that we're losing about 8% of our signal. At this point we can start to hear some of that "tone-suck" people talk about, but probably not enough for us to freak out about much. But let's add in that tuner.

When we add in the tuner to the circuit we get a new diagram and equation because of all the parallel resistances. The diagram looks like this now:
Let's go through the equation simplifying along the way:

We're now losing 11% of our signal and that high end is really starting to go away.

In order to prevent this high end loss, we add a buffer in between the input and the split. This turns our high impedance guitar signal, 15k, into a low impedance signal, 10-1k (1). If we plug that value into our crazy tuner equation then we the following:

Boom! We have all audible frequencies and then some to annoy the dog with! This is why the buffer is so useful. It lowers your impedance so drastically that it doesn't matter what comes after, you're going to have a great voltage transfer. Tone Suck Gone! Now let's take a look at doing the actual mod.

Step by Step

Now we'll look at the process of modding your volume pedal with a buffer at the input. I put the buffer on a toggle switch when I do it. This lets me turn off the buffer and still use my volume pedal which makes it so I can use that power jack for something else if I need to. Also, some people use their volume pedal with a Y-split TRS cable as an expression pedal. This won't work if there's a buffer in the circuit. So by putting the buffer on a toggle, it still allows you to use it as an expression pedal if you wanted to.

Aside from the standard tools. You will need the following:

• 1x Completed Buffer Circuit (I use a Klon based buffer, but any good buffer will do)
• 1x DPDT Toggle Switch
• 3x LEDs (of your favorite color)
• 1x 330Ω resistor

The resistor and LEDs are not strictly necessary. Once you have the buffer in place, the actual signal benefits of the mod are done. But  let's be honest, you gotta put LEDs in there so people know you have a super modded volume pedal of awesomeness!

Step 1

Turn over your volume pedal and remove the two screws underneath the black faceplate. Then remove the nuts holding the jacks in place so the faceplate can be removed.

Step 2

Take the faceplate and drill a hole between the input and output jacks for the power jack. Then drill a hole directly above for the toggle switch. Be sure to measure the size of each hole first as they will be different. Go ahead and put the toggle switch and power jack in place when you're done, making sure the toggle moves vertically when switched (I'll go over why later).

Step 3

Solder a wire between the top two lugs of the toggle switch. This will allow you to bypass the pedal when the toggle is in the down position. I do it this way so that if you unplugged your power jack to use on a different pedal and your foot slid off the front of your volume pedal, you can't knock the toggle into a position that would kill your signal.

Step 4

Take the circuit board that the jacks are mounted on and desolder the input jack. You may only need to desolder the tip for this to work. Be patient and don't stay on the solder joint for too long or you may burn the PCB trace.

Note: It may be easier to clip off the tip connection from the jack and then desolder the bit still in the pcb by essentially pushing it through with your soldering iron. This does risk burning the PCB but it can be useful.

Step 5

Bend the tip of the input jack so it doesn't make contact with the pcb and solder a wire from the tip to the middle of the right side of the toggle. Then solder a wire from the other side of the middle of the toggle to where the input jack used to be soldered.

Pro Tip: If you don't have a set of soldering "third-hands" then you can clamp the wire down on the jack using some blue tack or a bobby pin.

Note: If you had to cut the connection to the jack then you will still need to bend it a bit. When you're soldering the wire to the tip, scratch the metal a bit with a knife or some sandpaper to help the solder flow better. You may also want to do the bobby pin trick to keep the wire from sliding around while you're soldering.

Step 6

Find where you want to place your buffer circuit inside the pedal and tape it in place using double sided tape. I place mine above the input jack on the faceplate. Then solder the input and output wires from the buffer to the remaining two lugs on the same sides as each input/output jack.

Note: At this point if you don't want to add the LEDs, you can solder your power and ground buffer connections to the power jack (make sure to connect your power jack's ground to the chassis ground or you'll get hum) and you're done.

Step 7 (Adding LEDs)

Pick three LEDs (same color is usually best) and a 330Ω resistor. I personally love green and blue LEDs for this, but I've seen some awesome purple ones too. You will be putting one in each bottom corner and one in the top center (angled slightly upwards). We're going to take advantage of the ground connections from the jacks in the bottom two corners and solder the bottom LEDs to them in the next step.

Step 8

In the bottom left corner, take the negative end of your LED and push it through the ground connection of the jack while heating that joint with your soldering iron to make the solder flow. Get the LED right up next to the solder joint and make sure that it's a firm connection. Repeat this process in the bottom right corner. 

Step 9

Take the negative lead for the LED in the top center and do the same process you did for the previous LEDs, but push just enough of the negative lead through in order to center it. 

Step 10

Solder one side of your resistor to the positive ends of your LEDs. I find it helps to angle the LED leads towards the center of the pedal and then bend the resistor lead to wherever it needs to go. Also, another time where blue tack or the bobby pin trick can be helpful. 

Step 11

Connect a wire to the other side of your resistor and run it to the power jack. Solder in the power connection from the buffer and the resistor at the same time to make it easier. 

Step 12

Connect your power jack's ground connection to the buffer ground and the chassis ground (you can use one of the input jack's like you did with the LEDs) and you're all done! 

There you go! You now have a tone suck free volume pedal with some awesome LEDs to prove it.


References

1. Jack Orman - http://www.muzique.com/lab/pickups.htm
2. Jack Deville - Mr. Black Pedals Blog - Buffers, Impedance, and Other Internet Lore
3. Endolith - How important is impedance matching in audio applications
4. Impedance Bridging -  https://en.wikipedia.org/wiki/Impedance_bridging
5. Voltage Divider Tutorial - https://learn.sparkfun.com/tutorials/voltage-dividers
6. Parallel Resistor Calculator - http://www.sengpielaudio.com/calculator-paralresist.htm
7. Voltage Divider Calculator - http://www.raltron.com/cust/tools/voltage_divider.asp

The Mod

Parts:

• Joyo Ultimate Overdrive
• 500K Log Potentiometer (link has a drop in replacement for the board mounted one)
• 1K5 Resistor
• 470K Resistor
• 100nF Capacitor

Schematic

Here is a schematic of the changes we'll be making. We'll then go step by step through how to perform the mod.

Steps 

When desoldering, it will probably be easiest to do it from the underside of the board. I recommend marking which components to remove on the underside. Then you can put the board back into the enclosure and use the enclosure to stabilize the board while you desolder.

1.  Desolder the drive pot and replace it with the 500K Log pot. This will give you a smoother  transition through the gain range of the pedal. Alternatively, you can solder a 330K resistor across the outside lugs of the gain pot per the original mod.
2. Solder the 1K5 resistor to the outside lugs of the Tone pot. (You may find it easier to do this on the under side of the board)
3. Solder the 470K resistor to the outside lugs of the Volume pot. (See note in step 2)
4. Remove C9, a 1uF capacitor, and replace it with the 100nF capacitor.
   Note: If you have a board version prior to "2010.11.24" then you'll need to replace C7, which is a 10uF, instead. 
5. If you're board has a version date of "2010.11.24" (which all the recent ones do) then you'll need to remove the germanium diode and turn it around. The silkscreening is wrong and means that only one half of your signal will be clipped if you don't change this. I put sockets on the underside so I can easily swap out different diodes to taste. 
6. Optional: The wires from the footswitch to the main board are cheap and prone to breaking. I recommend replacing all of them with some good wire so it doesn't die on you when you're playing live. 

What it does

The mod is fairly simple as far as theory goes. The original mod by Freekish Blues uses 3 resistors in parallel with the pots to lower their values and thus limit their range. By using a 330K resistor, the gain pot goes from 1M down to 250K. I prefer to just swap it for a 500K Log pot since this is the preferred value on the OCD. Also, the log taper makes it a smoother transition across the gain range so it doesn't need to be limited to just 250K. The volume pot gets reduced in the same way from 500K to 250K.

The tone pot is a bit more interesting. It forms a simple low pass filter with the 100nF capacitor. With the pot turned all the way up the cutoff frequency is originally about 16k. By reducing the value of the pot with the parallel resistor, we get a different cutoff frequency. The 1K5 resistor takes our tone control down to a value of 1.3K which is pretty cool since we can't just go buy a potentiometer of this value. By doing this the cutoff frequency becomes roughly 122K. Now, to be fair, you can't hear about 80% of those frequencies...but it is brighter. Also, since the tone pot is a log taper, you're going to have a really wide range of usable settings across the whole knob.

Lastly, the capacitor change just limits how much bass is allowed to pass through the circuit before hitting the tone control.

I hope you found this a helpful bit of info. If you did then leave a comment telling me what you found most useful so I can focus on those things in later posts. Also feel free to leave a link to any pics of your mods.