Another active question-

[newey]

We seem to have had a spate of questions lately about buffers, preamps, and various manner of active circuits. This is a change for us, being normally a passive bunch . . .

So I thought I'd chime in with an active preamp question of my own. I have a Guitarfetish active preamp I want to put in my single pup (dual rails HB- 4 wire) Esquire-ish guitar.

I'd like the 3-way lever switch to give: both coils series/ both parallel/ both through the active pre./

Ideally, switching away from the 3rd, active position would also disconnect the battery . . . . No TRS jack needed, just make sure the switch isn't on 3 when the guitar is put away.

Is this possible with a regular Tele switch? Or, would I need one (or more) extra poles?

I've been staring at a schematic for awhile, as I digest turkey and stuffing, to no avail.

[wolf]

I'd say it would not be possible with a conventional tele switch especially if you want a battery disconnect in there.

[sumgai]

newey,

Did I miss something? You want to choose between two coils in series, or two coils in parallel, or two coils going through a pre-amp, is that correct?

Clarification in Aisle 4, please!




sumgai

[JohnH]

newey - I think a standard 2-pole tele switch is only 50% of what would be needed. Series/parallel needs two poles, switching the output active/passive needs one pole and power needs another pole. I don't think theres any lucky clever way to reduce that.

Best I can think of with a Tele switch (and I think this would be very good) would be:

1. Series passive humbucker
2. Passive single coil (needs only one pole not two - personally I like it better than parallel coils, and the tone-cap bypass version that I put on my LP recently is particularly juicy, and also slightly hum canceling.
3. Active with series humbucker

Power would just be engaged at the jack, using a stereo socket in time-honoured fashion, which works fine and it means you can't put the guitar in its case with battery running down.

Power is always engaged when the jack is plugged in, and also the input to the preamp is always connected to the pickups, only the output gets switched. That would all be fine with a JH preamp design, since I make 'em high impedance input (so no pickup loading) and low current draw. Would need to investigate the GF design to see if it was similar.

Other than that, I think you need more poles. If it went to a super switch, you could maybe have series, parallel and single coil, and a couple of versions with active output.

cheers
John

[newey]

Yeah, I figured at least one more pole would be needed. So, a TRS jack and perhaps a superswitch get added to the parts list.

SG- yes, that's what I meant. I was hoping to be able to do this with parts on hand. If I have to buy parts, John's suggestion sounds good.

[newey]

AHA- Found a stereo jack in "Newey's Big Box of Miscellaneous Guitar Crap". There was one that came with this unit, but I used that elsewhere, didn't know I had another one around.

Here's what info GF gives with this unit. I don't know if JohnH can discern anything from this or not.



The little schematic on the right-hand side shows how to wire it as a pedal for true bypass, with in and out jacks, using a DPDT. It probably makes more sense to just put that on a p/p pot and keep the switch for the pup switching duties. This would also then allow the active mode with all three pup selections.

When I first posted this, my goal was simplicity, in particular since I foresee fitment problems. First job is to see if a 9V can be fitted in there. I'm not sure a P/P pot and a battery can both go in there!

[JohnH]

That sounds like a plan newey. I cant tell the input impedance or current draw from that though, but presumably it all will work. Can you read the type of opamp chip that is on it? Wiring it up with a push pull seems like a good idea, then your tele switch can be arranged to give series/single/para.

John

[newey]

Can you read the type of opamp chip that is on it?

I should have clarified, mine is the upper of the 2 pictures, the 25db one. I'm assuming the op-amp is the black chip more or less centrally located, designated "U1". It has no markings on it. The other black square component to the right-hand side says "2.2" and "25V" but I don't think that's the op-amp.

[JohnH]

OK, anyway, there should be quite a nice simple design there with a bit of a kick in it from that preamp.

If I was you, Id be hooking up that preamp outside a guitar with aligator clips to check out what it does. All you need to do is put battery between + and ground, and clip onto a jack cord from the guitar and one to the amp, with all grounds connected.

If you want an estimate of battery life, you could measure the current going in.

John

BTW happy Thanksgiving - it was my favorite holiday when we lived in LA. All the locals stressing out about flying across the cuntry or having to cope with their relatives, with us expats free of our mothers-in-law, four days off work and a free monster turkey from the supermarket!

[newey]

BTW happy Thanksgiving -

Thanks- it was a good one. Today, I've been grazing on leftovers . . .

Good idea to try it outside the guitar, I'll definitely do that this weekend.

[sumgai]

newey,

You haven't clarified your requirements at all. John gave a list of possiblilities, and they're good, but they still don't address what you said, i.e.:

One combo in series;
One combo in parallel;
and
One combo going through the pre-amp.

What I thought was an obvious question is what combo goes through the preamp?

IOW, it can only be series or parallel, and you haven't picked either one.... yet.

Actually, the answer to that question doesn't matter at all..... The problem is easy to understand, once you realize that you must control (switch) the positive lead of one coil, the negative lead of the other coil, and the output destination of the two combos - either directly out, or to the pre-amp's input. That's three poles right there, so far as I can count.

But my gut reaction is to wonder, and then ask, why not use the pre-amp all the time? What do you gain by having a circuit on-board that is used only occasionally? An option: if you're willing to step up to a superswitch, then we can have SiP, PiP, SooP and PooP, with the last position being total power-down. (Or you can have either individual coil, whatever turns your crank.) That disturbs some of the simplicity you wished for, but only at the construction level - operating the mechanism is still pretty darn simple, IMNSHO.



sumgai


p.s. Be aware that I think strangely. (No! Really!) I can easily use a standard 2P3T switch, and give you a parallel/series/series-with-preamp layout, but the pre-amp circuit is now uncontrolled. It would have to be capable of "self-bypassing", no mean feat in a simple design, but none-the-less, it is possible (and done quite often in industry). The parts count goes way up, the expense goes up even further, and the points of possible malfunction go through the roof. Not for the average bear, I'm sure you'll agree. So it's really a trade-off between this 'Rube Goldberg' engineering feat, or a smarter switch in the first place. Your desired effect may vary.

[newey]

I was leaving open whether it would be series or parallel through the preamp. Ideally, both.

But if I have to buy parts anyway, and if it'll fit, a p/p pot to give true bypass of the preamp seems like the way to go. I don't want to have active on all the time, I like the passive sound of the guitar and don't want to lose it.

If a switch upgrade is in the cards, would one of those Baja 4-way switches work? I wouldn't want the dead position.

[JohnH]

newey - this is just one pick up isn't it? with the Baja 4 way, I think you can get either single or both series or both parallel, obviously intended for switching two SC pups, but equally OK here.

But you might not want two single coils sounds from one humbucker, one is enough. And you can do that with a standard tele switch, with series, single, parallel. Just put a wire link between lugs 1 and 2 of pole 1 and between 2 and 3 of pole 2, then treat it as effectively a dpdt on-on-on, and wire from a standard diagram, usually intended for an on-on-on toggle.

Follow that with tone and volume, then the preamp with true bypass and its a done deal.

If the push/pull doesn't fit, you could add a toggle switch to do the bypass, then if you still want to preserve power when in passive mode, this toggle could become a 3 pole one. Need to be careful when switching power that thumps are not created - and if they are, as resistor in series with the supply and a cap across the preamp power input will fix that.

John

[newey]

And you can do that with a standard tele switch, with series, single, parallel. Just put a wire link between lugs 1 and 2 of pole 1 and between 2 and 3 of pole 2, then treat it as effectively a dpdt on-on-on, and wire from a standard diagram, usually intended for an on-on-on toggle.

That's the way it's wired now. I was willing to sacrifice the SC position if the active could be implemented with the std switch.

We'll see just how much gain this thing produces. If it's too over the top it might be better as a stompbox than onboard.

[sumgai]

newey,

You can always dial back the gain, if it's a bit more than you like.

I've given more thought to this project. Seems to me that I could (as could any engineer) bring in only two transistors to act as switches, that would, in effect, steer the signal from the selector to either the pre-amp, or straight out to the jack. However, that would color the tone somewhat, no matter if they were powered up or not.

A possible alternative would be a mini-relay.... a mechanical unit, not a solid state dealiebob. That would up your current draw, thus lowering your battery life, but at that point, your "simple" system is fully realized.

Minus the relay (I'll draw it all out, if you need me to), here's the schematic for the standard Tele 2P3T selector:



Parallel, series, and series with pre-amp powered up. Is that what you wanted?





sumgai

[newey]

SG- Thanks, that looks like it would do it with a TRS jack for the battery switching.

I could do this today, except the wife has just pulled out 8 boxes of holiday decorations and is casting glances my way . . .

[sumgai]

Mine just points at "The Closet", and imperiously intones "Make yourself one with the ladder", and it's all over but the shouting. Sigh.