FET Preamp - Newbie Learnings

[finastbeans]

Motivation:

Having recently rewired my H-H setup guitar with series/outer coil/parallel options (+ some other goodies), I wanted to find a way to compensate for the volume difference between series and non-series modes.  Although there are some interesting combinations, parallel and single coil options sound slightly woosy by themselves.  Also - I am interested to hear what some of the combos with series mode and non-series mode would sound like with the output of the pickups more closely matched.  I toyed with the idea of putting preamps in the guitar that would engage when necessary - this seemed very complicated.  However, this led to the idea of using one preamp and engaging a series mode volume cut on each pickup.

Here is the original wiring diagram:
http://tinyurl.com/hctw3

Here is the wiring diagram with the series mode volume cut: (still hypothetical)
http://tinyurl.com/kmp8w


Step one: build the preamp and see what the guitar sounds like.  Investigating preamp circuits led me to Don Tillman's Preamp Cable:
http://www.till.com/articles/GuitarPreamp/index.html

Since a phantom power box had to be built, my mind wandered to stompboxes.  As I layed awake at night I thought about all of the times that it would have been nice to mash down that pedal and play a lead out in front of the mix during a performance.  (Instead of my blazing solo tinkling away behind a juiced up rhythm section).  anyway, i reasoned that since i had to prototype the circuit and shand-select some FETs, why not engage the prototype circuit with a footswitch in the very same phantom power box.  and of course i would also need a volume pot on there to dial in just the right amount of "lead boost".

Learning one:
putting a second preamp circuit in series with the first does not necessarily double the amount of gain.  Thankfully I discovered this site (especially the wiki) and learned that i could put a capacitor in parallel with the resistor connecting the FET source to ground.  Since the prototype was made with a trimpot instead of a straight resistor, this was perfect - allowing me to "dial in" the amount of gain i wanted in the second preamp.  and yes, shamefully i have no idea behind the electronic theory that makes this work.

Learning two:
after everything was wired up and working, i brought the thing in the house and hooked it up to my amp.  it really seemed to improve the sound of the pickups as intended (outer coil, parallel options).  unfortunately there was a nasty loud thud every time the footswitch engaged the second preamp.  after hours of wrangling, dispair, more wrangling, some drinking, more dispair, next day...  wrangling, gave up just about to fall asleep - forgot to turn off the soldering iron??? i realized that my clever plan to switch the second battery on with the footswitch (3PDT) was causing the issue.  i just connected the battery directly to the cirtcuit so that it is always on (unless the entire pedal is turned off with the kill switch).  now the switching does not pop or thud. 

Learning three:
the only volume pot i had lying around was a 500K one used in guitars.  i was worried that this would affect tone but it seems to work fine for this application.  i am using it the same way a guitar volume pot would be wired - adjusting the signal between ground and output.  no bypass cap necessary to preserve brightness.  I think if the signal was high impedance that might have been an issue.

I hope this helps others...

TODO:
- i am a little worried about picking up unwanted frequencies - i think shielding the guitar cavity with copper foil should help... anything else i should do inside the stompbox?

- is there a good simple way to build the enclosure such that the 9V batteries can be changed without screwing/unscrewing the box?  I am using a hammond 1590BB clone.

[Gilles C]

About the battery, this is the easiest but not the cheapest way.

http://www.guitarnucleus.com/gnstore/elec/EP0929-023.html

[tcobretti]

I know that this isn't what you had in mind, but I'd try putting a resistor on the series switch to make it as quiet as the parallel mode.  I would guess  that a 2k would be a good place to start, but I am not one of the smart guys around here, so experimentation would be in order.

[Ucho]

I have a very similar "problem": parallel/single-coil pickup combination having less volume than stock HB.

I will build the Fet Preamp Cable by mr Tillman too, and I thought about mounting in the box a 'stompable' volume pot OR a stompable AMZ Mosfet Booster, to have, respectively, a volume decrement (when using HB) OR a volume increment (when using parallel/SC pu), and, in the end, be able to use either pu combination at the same volume, with a little expense of 'foot work'.
In that way I can still have the volume change between pickup combinations if I need (in crunch or light OD, to decrease drive, changing a little the tone, just switching pickup) if i don't push the switch, or a little more boost on HB, or even less volume on SC (I don't think i will need the latter by the way).

Since the Fet Preamp will already give a 3db boost, maybe just a passive volume pot can do the trick... but since I can alway decreas volume via guitar pot, would be better a (AMZ mosfet) boost (I will use another stompbox as 'regular' booster)?
What do you think about it?

[finastbeans]

I know that this isn't what you had in mind, but I'd try putting a resistor on the series switch to make it as quiet as the parallel mode.  I would guess  that a 2k would be a good place to start, but I am not one of the smart guys around here, so experimentation would be in order.

...actually i think that is exactly what i had in mind - check out the second wiring diagram.  i just need to purchase the 4PDT mini-toggles.  that is what i am planning to implement next.

I have a very similar "problem": parallel/single-coil pickup combination having less volume than stock HB.

I will build the Fet Preamp Cable by mr Tillman too, and I thought about mounting in the box a 'stompable' volume pot OR a stompable AMZ Mosfet Booster, to have, respectively, a volume decrement (when using HB) OR a volume increment (when using parallel/SC pu), and, in the end, be able to use either pu combination at the same volume, with a little expense of 'foot work'.
In that way I can still have the volume change between pickup combinations if I need (in crunch or light OD, to decrease drive, changing a little the tone, just switching pickup) if i don't push the switch, or a little more boost on HB, or even less volume on SC (I don't think i will need the latter by the way).

Since the Fet Preamp will already give a 3db boost, maybe just a passive volume pot can do the trick... but since I can alway decreas volume via guitar pot, would be better a (AMZ mosfet) boost (I will use another stompbox as 'regular' booster)?
What do you think about it?

i can do that with mine now (instead of using the stomp switch for a "lead boost") but i wanted to be able to equalize the individual pickups as well.  i'll play around with that and let you know how it goes.

I think that battery thing is pretty expensive - but it looks nice.

[Gilles C]

It's around $10.

Some people use a saw to make a kind of trap in the bottom cover for the battery. But you still need a screw or two to keep the battery cover closed.

Gilles

[tcobretti]

That's what I get for not paying attention.

[bancika]

I like volume drop on paralleled positions. I have wired my custom guitar with two paralleled posistions and they can be used as rhythm/lead switch. But that's just me