Ernie Ball Volume Pedal Jr. retrofit

[Neeno]

Hey guys,
some weeks ago I bought my first volume pedal, a EB VP Jr.
I really like the mechanical build quality, but right after plugging it in my rig, I figured out that this design isn't really useable in every spot of my chain.

I soon discovered that the tuner out (connected in parallel to the input) can suck tone depending on the tuner used and the signal chain, in fact using my polytune for silent tuning was pretty much impossible.

After a quick search on the web I found that some companies (ie. JHS and Thru-tone) offer MODS for this kind of volume pedal.

It seems JHS uses their "Buffered Splitter" circuit to buffer the pedal and split the signal for the tuner out, but I don't know anything about the circuit they use.

Thru-tone does pretty much the same thing adding a impedance control to 'manage' the top end (probably their buffer add too much hi-end ?)

I tried to search the web, but I didn't found much information about active designs in volume pedals.
I have some skills with Eagle and I can etch my pcbs to test some prototypes, but I would like to have your input and do something useful for other DIYers like me.

The goals:
- Making a direct replacement PCB with the same dimensions as the original
- Add a buffered splitter at the input to "isolate" the tuner output
- Add a buffer at the output after the volume pot (not sure if it is necessary)
- Make it totally bypassable-back to stock conditions (just in case)

Optionals:
- Leds on the inside (ala JHS)
- Add a input impedance control (not sure if it could be a feature)

Block diagram:

             INPUT -> BUFFERED SPLITTER -> VOLUME POT -> OUTPUT BUFFER -> OUTPUT
                                                       -> TUNER OUT


Now, I would like to ask your help since I don't know what kind of buffer could be the best for this application. I'm experimenting with some of Jack Orman's designs and I've build a couple of his circuits to test them.

I've also tested Mr.Ziltz splitter (which I really like) and seems to be a really transparent option but I would really like to know if someone else is interested in this and I would like to get your input.

Thanks for your help,
and sorry for my english guys...

[midwayfair]

Jacob (JMK) makes a PCB for a buffer-splitter. It's basic but good. And it's tiny.

A buffer/splitter does not "isolate" the tuner out. It just amplifies the split signal sufficiently. An output buffer is unnecessary but not a horrendous idea. The pedal's maximum impedance will be 25K, and that's when it's OFF. Most of the time you're going to be running it between 0 ohms and maybe 10K. That's not enough impedance to worry about. Putting a JFET output buffer would make the impedance ~300 Ohms.

The impedance control can be a DPDT switch to swap the impedance resistors. Jacob notes this in the build doc. Changing them from 10M to 1M would have a big effect on the input impedance. High frequencies are amplified more than low frequencies in pretty much any circuit, so it's not like there was something "wrong" with Thru-tone's buffer. Different guitar pickups may have different impedance requirements.

I'm not sure why you need to make it the "exact" dimensions of the original. There's plenty of room inside a VP.

You could save yourself a lot of trouble, however, my just making yourself a 1590A buffer-splitter box with a tuner out. Put it first in your chain and it'll drive all your pedals, provide a buffer for the VP, keep you from having to drill a quarter inch of aluminum and power your VP, and it can be moved around and used with other boards if you get tired of using the VP. Add a bypass toggle and you are automatically back to your original setup.

[Neeno]

Thank you for your suggestions...

I had a look at Jacob PCB and it seems to be based around which is what I've built using A.Ziltz's micro buffer schematic.

You're right, the tuner out would not be really isolated, in fact I don't want to use transformers, but the buffer will not let the tuner suck the tone or contaminate signal. Right ?

How you determine that the pedal maximum impedance will be 25k ?

Jacob (JMK) makes a PCB for a buffer-splitter. It's basic but good. And it's tiny.

A buffer/splitter does not "isolate" the tuner out. It just amplifies the split signal sufficiently. An output buffer is unnecessary but not a horrendous idea. The pedal's maximum impedance will be 25K, and that's when it's OFF. Most of the time you're going to be running it between 0 ohms and maybe 10K. That's not enough impedance to worry about. Putting a JFET output buffer would make the impedance ~300 Ohms.

The impedance control can be a DPDT switch to swap the impedance resistors. Jacob notes this in the build doc. Changing them from 10M to 1M would have a big effect on the input impedance. High frequencies are amplified more than low frequencies in pretty much any circuit, so it's not like there was something "wrong" with Thru-tone's buffer. Different guitar pickups may have different impedance requirements.

I'm not sure why you need to make it the "exact" dimensions of the original. There's plenty of room inside a VP.

You could save yourself a lot of trouble, however, my just making yourself a 1590A buffer-splitter box with a tuner out. Put it first in your chain and it'll drive all your pedals, provide a buffer for the VP, keep you from having to drill a quarter inch of aluminum and power your VP, and it can be moved around and used with other boards if you get tired of using the VP. Add a bypass toggle and you are automatically back to your original setup.