Parallel/Series Stratocaster: 11 Easy sounds

[Carlos Sanz]

About two years ago, I designed a circuit that could give you all kind of pickups combinations. I explained it here:

guitarnuts2.proboards.com/thread/7923

But it had a problem: a very special selector was needed and you had to open the pots in order to exchange tracks. The conclusion was that a simplification was needed. 

I've been working on it and here it's the work done.

(Update: pickguard was not in the firts post)




These are the combinations:



And this is the schematic:


You can't keep separate Tone Knobs with this mod. A Master Tone is mandatory. In order to use the third knob, I've soldered a PTB circuit (with the adition of a phase switch... I couldn't help it). This is how my pick-guard looks like:



For a little demo, you may watch this video:



And finally, the long document for those really interested:
drive.google.com/file/d/1-zK4y8vfzYUq34w9KG96jcPU2SyklwHC/view?usp=sharing

I really hope that someone gives this a try.

[JohnH]

Hi Carlos, apologies that we left this one uncommented. I think its a case where, with a design such as this which is presented so well and so comprehensively, there's not much to add. And you proved it by building it!

But this does seem like a great design, and you posted everything that can be posted to explain it. The video is a good simple way to see what it does, and also as an intro to series/parallel switching on a Strat.

I like how all the basic Strat sounds are where they should be, and you can ignore the added tones if you wish without complication. The complexity is in the design, so that operation is simple.

You offered a few options for use of the second tone knob. I agree that the PTB bass-cut is probably the best, and it should cut the boominess of some series tones. One other option though, would be to dedicate a tone pot to one pickup, by hard-wiring it directly across the pickup leads. Ideally this would be no-load, and it would travel with that pickup in all combinations. Turning such a control down in a series combo actually brightens the tone by letting treble from the other coil get through more easily.

Also, interested in those 10M resistors. Did you find that they worked to solve a problem that you found in practice?

[Carlos Sanz]

Hey, John! Now I feel a little ashamed. When I told you in another thread that I was expecting your comments here I was somehow kidding… (although I must say that it’s always nice to receive your praises).

One other option though, would be to dedicate a tone pot to one pickup, by hard-wiring it directly across the pickup leads

I had never thought of that! It sounds interesting, indeed.

Also, interested in those 10M resistors. Did you find that they worked to solve a problem that you found in practice?

Yes. As I expected, they minimize the problem. Surely you have notice too that in some parallel/series circuits there is a “pop” sound sometimes when the selector is moved. I cannot give an academic explanation on this. I have not studied this deeply, but I think it happens for two reasons:


- Coils can store energy (as well as capacitors). If the coil is let unconnected when it still has energy, when it is connected again, the stored energy will be given back by the coil

- When any electronic component doesn’t have a voltage reference, it could have an absolute voltage other than zero (ground). When connected, a transient current can flow for an instant creating the “pop” transient sound.

(There is a random component here. You may move the selector many times and only some of them will result in a pop)

The solution, anyhow, is putting a contact of the coil to ground so that:

- There is a path for the stored energy to ground when the pickup is suddenly unconnected from the circuit

- The resistor lets the current flow, but in a limited way.


10 Mega-ohms seemed a good compromise to let the current flow but not influence in the sound. I didn’t make a sophisticated calculation. The varitone circuit has 10M resistors and that’s what I tried in electronic Spice simulation before soldering them. There was some attenuation in the frequency resonant peak, but nothing really serious.


I have one guitar with Kinman noise-cancelling pickups and in previous designs I had detected a small “pop” sound without the resistors. With the resistors it is there only if you look for it. The pop is tiny and I think it’s rather a microphonic effect of the pickguard.


But I’ve mounted this circuit (with resistors) y a friend’s guitar with regular pickups. The guitar is not shielded and he hasn’t notice a pop sound so far. Probably the tiny pops are still there, but they are masked by the background noise.

[JohnH]

Thanks yes I have noticed those occasional clicks, particularly on more complex and series wirings. But its annoyingly slight and intermittent so I've never really concluded exactly what is the cause, whether it needs fixing or how to fix it!

Your schenmatic puts a 10M from coils to ground, which should stop them floating around when disconnected, building up charges that create clicks later. That seems like a credible cause and fix, particularly if it has been found to work.

Or is it more related to all the extra wiring and switch contacts in these more complex circuits?

[Carlos Sanz]

Apr 23, 2019 17:17:41 GMT -5 JohnH said:

[...] its annoyingly slight and intermittent [...]

Yes, it is.

I am a "bedroom headphoned guitar player", so it was absolutely negligible for me. But before presenting the circuit in this forum, I wanted to give a solution so that anyone interested could use the circuit under any circumstances. I didn’t want that anybody could mount this circuit and be disappointed during a critical listening rehearsal with a lot of amplifiers, for example.

Your schenmatic puts a 10M from coils to ground, [...]. That seems like a credible cause and fix, particularly if it has been found to work.

Yes it works, but not definitely. The effect is minimized but not vaporized. Virtually inaudible with a single coil background noise present or with a tube amplifier. But it can be detected in recordings made with hum-cancelling positions and with studio gear if you look for it in the tracks. It’s not always present and when it is, it's so small that you must know where to look.


Probably, it’s not absolutely muted because the parasitic capacitance of the wire turnings of the coil (about 100pF) creates an RC circuit with the 10M-Ohm resistor. The charge/discharge intervals of it are quite short but I think that the stray capacitor may be storing and releasing charge whenever the strings vibrates. Again this is only a hypothesis.

But again, who cares about a little switching sound that can hardly be detected?

Or is it more related to all the extra wiring and switch contacts in these more complex circuits?

Nop. In fact, the resistors are a compromise. If you decide to put them in order to minimize pops (minimized to nothing virtually), then you are loading the pickups with a resistor in parallel. It's the same effect of having a 243K-Ohm potentiometer instead of a 250K-Ohm one. Or a 476K-Ohm pot instead of a 500K-Ohm.

It's almost nothing and many of us have such potentiometers because of the components tolerance, ignoring their actual value.