Ok gurus, answer me this

[daz]

I could easily get to them by lifting the end of the guard on my '72 thinline. Wouldn't have to remove the guard. But do you have a buffer in mind and what voltage are we talking? 3 coins for 9v? Or do you know of a buffer that can run off less? How long will the coins last compared to a 9v?

Getting a really transparent, really small preamp done is not an issue. The entire thing could be smaller than a fingernail. Probably less than a day from concept to ready-to-print PCB layout.

A JFET/bipolar cascade, a FET input opamp, bootstrapped bipolar, lots of choices, and many with low power requirements.

As noted, the ideal place for these is as close to the pickups electrically as your pickup switching arrangement can stand; definitely before controls in the guitar.

Getting the **DC power** to it is the challenge. There's room under the pickguard, even in a strat, for a stack of coin cells, but then you still have to remove the pickguard. That's why I came up with the coin slot battery. If you could find a place to run two small wires from the front (well, OK, or back) side of the guitar, you could disguise a couple of coin cells on the outside.

[R.G.]

If you're interested in trying something out, I'll go mess with it a bit.

Juggling available power and voltage is the biggest issue. I'd pick a low voltage opamp with low distortion and high accuracy, as well as best use of the available power supply. Probably two coin cells for 6V.

I'll go look.

[bob p]

Getting the **DC power** to it is the challenge. There's room under the pickguard, even in a strat, for a stack of coin cells, but then you still have to remove the pickguard. That's why I came up with the coin slot battery. If you could find a place to run two small wires from the front (well, OK, or back) side of the guitar, you could disguise a couple of coin cells on the outside.

I'm in the camp that thinks that putting batteries on-board a guitar is a bad idea. Why not just swap out the Switchcraft jack with something that can provide additional terminals and supply the power remotely?

[R.G.]

It's funny how we are caught by the two faces of the sins of the fathers and the conservatism of the guitar player.

In the first half of the 20th century, there was the phone system. It pioneered all kinds of things, notably for this discussion the 1/4" phone jack. The phone jack/plug system was mandatory for the system of using human (usually female) operators to manually complete all phone calls by plugging and unplugging jacks. The phone jack is GREAT for that, and was nearly ubiquitous.

The magnetic pickup as a magnet wound with lots of wire became the standard for guitars, because it was cheap, low-enough-tech, and amenable to the standards of the audio industry at the time.

The vacuum tube was the only way to amplify, and was expensive, as it was the vanguard of electronics at the time. "Expensive" made the early guitar amps use single ended inputs, not balanced. The inherent high impedance of a vacuum tube input hid the problems with the high inductance of magnetic pickups.

The first working guitar amps became the standard solution, because anything else was more expensive to manufacture. And musicians were broke then, as now. They carried along the 1/4" phone jack the unbalanced cords, and the high input impedance necessities. But guitar players used them.

And became revered. Guitar players now get excited by the words "original" and "vintage", meaning that they can sound just like the heros of the 50s and 60s did, but better. And so we have half a century of refusal to change either the 1/4" phone jack, the single-ended-ness of the cords, and the high impedance of the amplifiers, but now we have the history of a half century of musical equipment that cannot be changed, and a half century of guitarists who feel, with justification, that nothing else can work.

Some of the consequences of this history are that it's really, really, really difficult to do *anything* different, because the need for DC power to do it with out on the guitar, where it's actually most helpful, is headed off by the fact that the history makes it almost impossible to put DC power out there.

The simple technical facts are that getting some electronics out on the guitar offers the possibility to fix some really irritating technical (and sound!) problems in the most-effective, lowest noise, and lowest intrusion way.

Sorry. I feel better now.

Not putting batteries on a guitar is a Good Thing for all kinds of reasons. Putting batteries in a guitar is a good idea because it lets you fix some things. Getting the power there without having to put batteries there is a Good Thing.

Problem is - how can you do that without changing the 1/4" phone jack or mono cord? Hard to get extra wires down a mono cord.

I know!! Phantom power!! Well, that involves a great big inductor inside the guitar. Maybe. Possibly. I'll ponder it a bit. Even phantom power leaves the issues of now trickling DC across four possibly dirty contacts, two each, signal and shield, on each end of a mono cable, and opening up the possibility of clicks and scritches if the plug twists or moves inside the jack.

Hmmmm... a second, parallel power cable that goes into the guitar? Add more contacts and possibly another connector to the guitar? Maybe. A guitarist might be convinced to use cable ties to run another wire in parallel with his audio cable.

Hmmm.... trickle charged rechargeable batteries inside the guitar? Supercaps that are not really batteries?

Hah!! We can beam in microwave power, using hidden antennas to pick it up~!!

Help me here. I'm sure there are things I haven't thought of.

[R.G.]

... But do you have a buffer in mind

I do now. Looks like there are some new opamps that will work OK at low(ish) noise and only 6V. I think that's the place to start. Target circuit is about a fingernail. We used to state silicon chip size in nano-acres, but that's probably not a good unit for this.

and what voltage are we talking? 3 coins for 9v? Or do you know of a buffer that can run off less?

I think two 3V cells will do.

How long will the coins last compared to a 9v?

Good question. A 9V battery is about 450ma-hours. A 2032 lithium coin cell is about 200ma-hours. The buffer I have in mind uses about 1ma, so the coin cells will go 150-200 hours of active-on use. A 9V would go more than twice as long, but then it's big.

[rjb]

Ideally the buffer amp would be installed between the pickup and the volume control. That way any inductive weirdness from the pickup/load relationship become moot and there is a lower impedance to the actual resistive divider and capacitive cable shield.

...But can anyone point me to a known good transparant buffer? (small simple circuit so it can be made tiny enough to fit easily)

I've, ahem, never tried this myself... but if you don't want to go with an op-amp, I'd suggest Googling (self-biased) common-drain JFET amplifier (AKA source-follower)- a simple unity-gain circuit using one JFET.

The input impedance is essentially the value of your gate resistor- usually shown as some high value, like 10Mohm.
I think that for your application, you want to emulate what the pickup would "see" with the volume at 10 and the guitar plugged into an amp through a longish cable. So, I would try 200Kohm (250K || 1M) or 330K (500K || 1M), parallel with maybe 1000pF (20 ft old-school coax @ 50pF/ft).

If you decide to permanently install the buffer, you might want to replace the volume pot with a lower value. I guess.

[rjb]

A 9V would go more than twice as long, but then it's big.

Would a 9V battery fit through the F-hole?

[loudthud]

Not Rocket Science

Here's a couple of quick schematics. One shows phantom power to the guitar, the other is my preamp from the Reagan years.

OK, now I'm getting pissed off. I tried to attach files, it looks like they uploaded but they arent there. I tried again but nothing. Then I get an error message to drag files to the post area. ARG!

[bob p]

Hmmmm... a second, parallel power cable that goes into the guitar? Add more contacts and possibly another connector to the guitar? Maybe. A guitarist might be convinced to use cable ties to run another wire in parallel with his audio cable.

Dual cables aren't anything new.

Wolf 2.0 used dual cables for send/return in 1977. This picture was taken in 1978.

[bob p]

I'm not uploading. I'm stealing the other guy's bandwidth.

[bob p]

originally installed in the 1970s.

[R.G.]

Yep, all it takes is being willing to do it.

I'm good with that.

That opamp circuit is a good place to start. I'd do some different biasing for lower noise. It's probably about 10ma of current drain. That doesn't matter if you're using a cable, but it will eat batteries pretty quickly.

I set myself the design goal of getting the amplifier under 1ma for longer battery life. I think a single opamp circuit can get to about 475uA with some of the TI LinCMOS units and still be acceptably quiet.

I've also messed some with a discrete buffer. Latest sim run shows -6db points at 12Hz and 400MHz (!), and over 1MHz into a typical cable capacitance guess, using about 100uA from a battery. That gets a 150ma-Hr battery to 1500 hours of on-time, about 62 days if you never switch it off.

It looks pretty promising.

But hey, if a guitarist is willing to run a second cable, great. Makes the design much, much easier to not do the power and noise budgets.

[bob p]

that buffer design is what, 35 years old? yes, the modern low voltage stuff would be a much better option.

the Garcia setup used an on-board 9V battery, which some tech probably changed pretty often. the send/return lines used a pair of TRS jacks, and the system was designed to drive a dry signal to the effects rack at full volume, bring the wet signal back, and run the wet signal back out through the guitar's volume control. AFAIK they never bothered with remote power.

I'm wondering why they just didn't use a locking DIN connector and a single multi-conductor cable.

[R.G.]

That would have been a good idea. I suspect that DIN wasn't all that well known then, or that Jerry's tech didn't know much about it. Or that any of a number of happenstances did or didn't happen.

I also suspect it would be hard to get most guitarists to change/add to a DIN on their instruments.

Like I said, the problem is not the technology. It's what the people will accept or believe.

[kleuck]

Old Musicman preamps built around LM4250 draw nothing, battery can last several years.