High value control pots

[gs790]

First the question, explanation second.

I'm looking for a 5 Mega Ohm control pot.
Preferably Log taper, and preferably not $30 a pop.
Anyone have a source (I'm seeing a few on ebay but I'm
not seeing a measure on the threaded collar, and it looks a little short)


What's going on is that is that I installed piezo saddles in my guitar and decided (after trying) that I don't want to monkey with an active preamp. It may have been installer error on my part, but what I installed jacked around with the magnetics. I was thinking about cutting my losses and just having massively overpriced saddles, but surely I can make something out of this and the control holes I've already cut A short story made shorter, goodbye preamp and let's hope the 100% satisfaction guarantee gets me my money back (damn it all if they try to claim I fried the board, I had to fight pretty hard on a DOA stick of RAM but I finally got that replaced).

The good news is that the documentation says that it can be used in a passive setup. This makes sense, and I probably should have paid more attention to this in the first place. Yay! They want a 5 mega-ohm control pot. Boo!

They show two passive control schemes.

5 M Ohm pot (presumably log taper) with a 330 pf cap between the middle and 3rd lug (built in tone?)

or

22K fixed resistor into a 250 k volume and a 250 k tone circuit.

I can post the diagrams if necessary. And in writing the post I realized that configuration 2 uses common guitar pot values. Making me wonder, because I checked out during this chapter in physics so I'm just a little bit ignorant of how these things add up, can I use a fixed resistor to make a 500k pot behave like a 5 M ohm pot? Would it just be example 2 sans the tone circuit with a different value resistor? Since I already cut the hole, I also plan to use an ON/ON/ON switch so I can mute either side of the stereo jack or have both on at once. I'm already starting to feel better about a fairly frustrating Saturday evening.

I do suppose I could run this straight to the jack with no controls, but damn it I've got two holes in a solid top guitar (chastise me later) and I need to so something other than have dummy controls.

[JohnH]

I don't know about finding that 5M pot, but theres no way those two control options are equivalent. It would be interesting to see the diagrams, and maybe a link to the manufacturer?

John

[gs790]

I can pull out a screenshot later today, but for the time being

www.graphtech.com/fckeditor/userfiles/File/GHOST%20A-P%20Manual.pdf

pages 9 and 23







I'm also trying to get my head around this impedance thing, how it relates to the signal that is coming out of the pickup, and how it relates to the function of a potentiometer. My reading leads me to understand that impedance and resistance are basically the same construct. Although any time I see a descriptive text use the phrase, "basically the same as," I have to know what they are omitting. So by saying that piezo is a high impedance signal does it mean that it's pushing very little signal?

[ChrisK]

Impedance and resistance are not the same except for purely DC circuits with steady-state signals.

Resistance, along with inductive and capacitive reactance, are the components that make up impedance. Therefore, resistance is only one of the three components of impedance.

If you still have that physics book, check back in (to it).

A piezo pickup is primarily capacitive, with a relatively strong voltage signal output with a significantly weak current output. In essence, they have a very high output impedance.

A passive magnetic pickup in comparison has a relatively low output impedance comprised of the resistance of the coil, the inductive reactance of the coil, and the inter-winding capacitance of the coil. These components are not simply added together, but are combined in a vector sum/phasor relationship. It's called AC circuit theory.


The fact that piezo pickups have a high output impedance requires that they are loaded as little as possible (the multi-meg Ohm pot) and usually require a buffer. Since a piezo is unable to adequately drive the load of a lower value pot, it's also unable to adequately drive a guitar cable where the capacitance will attenuate the high frequencies and the relatively low guitar amp input impedance will significantly attenuate the overall signal.

Also, a guitar amp is unable to adequately process and amplify a piezo signal.


I have several guitars with piezo pickups. In all cases they have an active pre-amp inside the guitar and use acoustic guitar amplifiers, PA systems, or hexaphonic modelers. I have never considered the use of a passive piezo pickup.

Are you sure that you don't want to try to figure out what went wrong with the pre-amp? (We'll help.)

5 M Ohm pot (presumably log taper) with a 330 pf cap between the middle and 3rd lug (built in tone?)

No, it's a treble bypass cap to enhance the high frequencies lost when the volume pot is turned down.

....can I use a fixed resistor to make a 500k pot behave like a 5 M ohm pot?

In no useful way.

22K fixed resistor into a 250 k volume and a 250 k tone circuit.

This appears to be a circuit used on the output of the pre-amp to effect volume and tone controls with the same relative impedance of single coil magnetic pickups.


If you combine a 5 Meg piezo pot with a 250K passive pickup pot, the 250k pot will significantly attenuate the piezo signal when both are selected.

If you run the signals separately (tip/ring) as shown in the Graphtech manual, you still have to deal with the cable capacitance and amplifier input impedance.


I do know where to get pots like this (in the U.S.), but would strongly recommend against it.

[gs790]

I suppose there is not harm it trying to trouble shoot as long as I still have the thing.


There is a large body of text below, but I'll put the two possible mistakes that I can identity first, and one global statement.

All black/ground wires faced away from the board, all the colored wires faced inward.

1. While I did solder a jumper between the ground lug and the pot casing, I might have forgotten to solder a jumper between the piezo volume pot and the tone pot. However, since the cavity is fully shielded, I would think the pot would be adequately grounded.

2. From the wiring harness for the jack, there were 2 black wires. I soldered both those wires to the ground lug on the jack. Was one of those wires supposed to go elsewhere? It's not mentioned in the documentation. In fact the documentation omits the part about that harness having 2 black wires.











I'll go through what I know about what I did.

1. I was trying to use a makeshift harness from old motherboard connectors for my toggle. A combination of my trimming the wires too short and the connectors being a little too large, had me scrap the switch at that time. They just kept falling off the board. Without a switch, my understanding is that mag volume does mag and piezo volume does piezo. I never got any piezo coming out. Oh no, was the board trying to output stereo? I was definitely using a mono cable, all signal should have been "intelligently" routed to the tip.

No piezo coming out I guess that something is wrong on that end. Referencing the diagram on page 5, the connector was on the third set of pins labeled P. It's a little hard to see what side is hot and what side is ground. So I tried it both directions. I was unsure if the output wire was coming from the correct spot on the summing board (diagram on page 6 has the output from the end of the block, out of the box it was in slot 6 so I moved that connector to 7th pins).

Let's move to page 15 and address the Mag/Piezo harness. Black (wire adjacent to the green) went to ground on the mag. Green went to center lug of mag (on a side note I noticed a decrease in magnetic volume and fairly significant muddying of the tone [and at that the tone knob turned into a cut when it is turned all the way counter clock-wise, I rarely touch the tone so I cannot remember if this is normal])

Yellow went to the center lug of the piezo volume. Purple went the the third lug and the remaining black wire went to the first lug. Counting 1-3, left to right with the solder lugs pointing north. I did solder a jumper from the ground to the body of the pot. ***I might not have soldered a jumper from the piezo pot to the tone pot*** (This could be a/the problem, but my control cavity is fully shielded, so even if this was a major mistake wouldn't the electrical conductivity of the shielding ground the pot anyway?)

With the black wires facing the edge of the board, the block was connected skipping down 1 pin (I do not have the mid/dark switch)

On to the stereo jack. Diagram on page 16. Blue to tip (I was getting magnetic out) Red to ring. white to SW. Black to ground. The only thing that might have gone wrong here is that the wiring harness had two black wires. I ended up soldering both those wires to the ground lug. Was that other back wire supposed to go somewhere else? Would that explain why I was getting zero piezo signal?



I realize that it is easy to overlook one thing that screws the entire system, but I was as thorough as possible before I took the thing apart and wired it back like it used to be. It might be that the board was DOA, but I'm afraid that I'm going to have a tough time convincing them that I did not fry the board myself (which I have no earthly reason to believe that I did).

[JohnH]

That manual is interesting reading. It seems you have an acousti-phonic intelligent preamp.....

hmmmm.....

Luckily I know exactly what that is since I have a diploma in acousti-phonics, bionics and hydroponic intelligence. In fact I have a box of them and can supply them to other scholars like myself, for three easy payments of $9.90 plus shipping (unmounted - framing is extra).

any how

I agree its worth persevering with the active system. I think in some form, it is essential for getting a decent result from a piezo, and particularly if you want to blend in a mag system.

But to investigate, experiment and troubleshoot, how about starting with a direct connection from piezo to jack. ie. like the diagram with the 5M pot, but no pot. Thats the most you can get out of it with no preamp and it will prove whether or not the piezos are working and let you judge the unamplified output and tone for yourself. Try it plugged into various inputs. Id guess that the nearest to sounding decent without using a dedicated preamp might be something like, a short cord, to a buffered pedal (like a tuner or Boss pedal turned off), then into a PA, mixer, stereo or some other full range amplifier.

Once that is proven, you'll know how the levels are and the possible benefits of the preamp. Then get the preamp working outside of the guitar. Once wired up, just touching input contacts should induce a good hum if it is working, then connect in the piezo. The idea is to build up, proving each stage, so you can isolate and fix the problems.

cheers

John

[gs790]

I'll probably be going at it with a multi meter this evening. That will be a lot more simple then trying to jury rig something, especially since there are no bare wires north of the pc board.

I think I just needed to get some space from the project.


Any thoughts on those two ground wires on the jack harness?
Or my no jumper to volume or tone ground blunder?



And John H, were you just kidding or do you really have an acousti-phonic pre-amp for $30? A student discount for a former starving college student that is now a starving member of the entry-level workforce trying to find himself a place to become a starving graduate student/fellow/Phd candidate? The pessimist in me says that if the board is bad, Graphtech won't want to replace it, and that would make me much more likely to finish this thing.

[ChrisK]

Luckily I know exactly what that is since I have a diploma in acousti-phonics, bionics and hydroponic intelligence. In fact I have a box of them and can supply them to other scholars like myself, for three easy payments of $9.90 plus shipping (unmounted - framing is extra).

Uh,

were you just kidding or do you really have an acousti-phonic pre-amp for $30?

No.

He's offering the unwary a chance to buy degrees in "acousti-phonics, bionics and hydroponic intelligence". He can offer these at such an attractive price since he has a "box of them".

Clue #1. Acousti-phonic is not a real word. If it was, it would be redundant.

Clue #2. Bionics - see the 6 Million Dollar Man.

Clue #3. No matter what they tell you, plants just ain't gonna ever be that smart.

[JohnH]

I omitted to mention that I am an Englishman, which would have made my intent much clearer. We work to BS standards.

My underlying point was, while I don't doubt that the GHOST system with its hydroponic preamp is a very fine piece of electronics, the hype that is evident in some parts of the manual leads to a level of doubt about some of the information contained therein.

John

[newey]

We work to BS standards.

British Steel??? ;D

[ChrisK]

At a minimum we will need well-focused pictures of the wiring that you have implemented.

[JohnH]

As Chris says - photos are needed for more help. Its hard enough to diagnose ones own electronics, with full hands-on 3d.

Is it the case that you have not yet gotten a sound from the piezo? As I suggested above, it really would help to get a passive connection to the piezo going first. If the piezo doesn't work on its own, it can't work with the preamp. Break the problem down into smaller parts and solve each one.

John

[gs790]

I got away from this for a few days and damn it all.

100% rookie mistake with a stereo jack.



I've been building computers going on 10 years now. Made 90% of my mistakes in the first 3 boxes. Should have known better, the first one had a few subtle but glaring errors too. My wiring on the jack looked fine. Until I looked closely and followed the rings between the insulation. I mixed up the wires between the tip and switch. Crap.




Though I'm not quite out of the woods yet. Only two of the saddles are making any noise. I'll take a closer look at that tomorrow when I have more time.



-edit-

Okay mystery over. 4 pins on that little summing block are no good. Headache now 98% over.