Mixing two pickups

[Mike Sulzer]

Continuing with the analysis from before the interruption

To get back to analyzing what happens when two pickups are combined passively in parallel, and then buffered, as opposed to individual buffering followed by addition, this is from my earlier post (slightly edited).

So consider a pickup model composed of a voltage source in series with an inductor (which can have a resistor in series if we like) and a capacitor in parallel with the series combination. We put two of these in parallel. For simplicity, detach the capacitors; we will put them back in later after combining the inductors.

Consider one pickup at a time. This means that we short out the voltage source of the inactive one. Now we have a voltage divider consisting of two identical inductors (maybe with resistors in series, but still both are the same). This means that the voltage at the output is half that of the voltage source. This holds true at all frequencies; so if the source varies as a function of frequency, this variation is preserved. If we short out both sources and look back into the network, we have two inductors in parallel which is equivalent to a single inductor of half the value. So the Thevenin equivalent is a voltage source of half the original source and an inductor of half the value of the original. Now we can put the capacitors back in. Note that two capacitors in parallel have half the impedance of a single one, and so the frequency response is preserved. That is, the product of L and C stays the same because the L is halved, and the C is doubled.

The result is that the voltage of each pickup, used alone, is attenuated by a factor of two at the point where they are paralleled, independent of frequency.

Now we want to see what happens when both pickups are on at once. For this we need the superposition theorem: The response to several independent sources is the sum of the responses to each independent source with the remaining independent sources dead. (Close, The Analysis of Linear Circuits) A dead voltage source is one which has been replaced by a short circuit. Note that this theorem is first derived for dc, but then later extended to ac. We apply it for a sine wave of each frequency, making sure to account of the phases of the signals.

The text in the quote above shows that the responses to be added are just that of the pickups themselves divided by two, independent of frequency. So that is what we get when we buffer after putting the pickups in parallel.

If we buffer each pickup first, and then add with a pair of resistors, we get the same thing. So the frequency response is the same whether we buffer before or after combining. In neither case is the frequency response of the pickups altered.

If you combine pickups without buffering and connect to a cable, there is a change in the frequency response as described earlier.

[Mike Sulzer]

Original question

Marko,

If you like the sound of a single pickup alone, you might have to do some work to keep it the same in a buffered system where. If you buffer each pickup individually, then you can put a capacitor across each pickup that has the same value as your cable capacitance. Depending upon the input impedance of your amp and the buffer, you might need a resistor as well.

This might not be as important with some bass pickups as it would be with a guitar.

[Rick Turner]

From Mike several days ago: "If you combine two identical (note identical) pickups in parallel and buffer the result, you have the same frequency response as if you buffer them separately and then combine."

And that is where the argument started.

Now it seems to be a different story, right?

We have to talk real world here...the world in which guitarists and bass players are going to use real cables with real capacitance and have real pots in their instruments and have real shunt loads whether from pickup combining or from the pots or from the load at the amp end of the cable. I could give a lesser s..t about some theoretical frequency response measured (or merely imagined) at the very end of some magnet wire inside a pickup. What I know to be true is that all of us are making pickups and doing wiring for real musicians who hear amazingly subtle...and not so subtle things. When I hear one pickup loading another, I know that frequency response is altered, and no amount of having those pickups be exactly the same can change that. Telling us that such does not exist is disengenuous at best, and at worst it misleads those who want to know more about this stuff.

BTW, I started doing passive pickup wiring in 1966, I started winding pickups in 1968, and I've been buffering pickups and combining the outputs since 1970; that includes separately buffering pickups and then combining as well as combining and then buffering. I know what this stuff sounds like passive or active, and I know enough of the theory behind it to keep moving forward.

Can we agree that indeed, passively combining two pickups results in both loss of volume and an alteration of tone? So call it voltage loss and frequency response change if you will...but what we hear would be described by most musicians in terms of volume and tone...

[Mike Sulzer]

Can we agree that indeed, passively combining two pickups results in both loss of volume and an alteration of tone?

If you do not buffer the result of the passive combination the tone is altered because you are driving the same cable with a different source impedance. I have not changed what I am saying.

You, on the other hand, insisted on a listening test which you thought was important for settling the matter, and you predicted the wrong result. How does that go along with your know-it-all attitude? How does that justify your continual criticism of engineering analysis? Analysis which in no way conflicts with listening as the final judgment, but which helps in finding ways to solve problems.

[David Schwab]

If you do not buffer the result of the passive combination the tone is altered because you are driving the same cable with a different source impedance. I have not changed what I am saying.

What if you are driving the high Z input of a JFET buffer? Now the source impedance is high and you have no cable capacitance, and the tone still changes from the pickups loading each other.

Try it. Wire two pickups to a selector switch, no pots, and into a buffer. Now buffer each pickup separately and then combine them. You can hear the difference easily.

One is not better or worse than the other, but in response to Marko not liking the tone of passive blending, the dual buffer system will get him what he wants.

[Mike Sulzer]

What if you are driving the high Z input of a JFET buffer? Now the source impedance is high and you have no cable capacitance, and the tone still changes from the pickups loading each other.

Try it. Wire two pickups to a selector switch, no pots, and into a buffer. Now buffer each pickup separately and then combine them. You can hear the difference easily.

One is not better or worse than the other, but in response to Marko not liking the tone of passive blending, the dual buffer system will get him what he wants.

My experience is that that is not what happens. I will try it again when I get the time.

I agree the dual blending is fine for Marko, as I said above, as long as the buffered sound is close enough to single unbuffered pickup that he likes. I have suggested how to make it closer.

[Rick Turner]

Mike, you have bit by bit come around to now saying that pickups loading one another before a buffer does change the frequency response. I've been consistently saying the same thing all along...that one pickup changes the sound of another when combined in parallel no matter whether you buffer the sum or do not. And if you buffer both pickups first, you can then combine the two with no loss of tone...at least no loss attributable to the complex interactions of an LCR network that has multiple inductors, a lot of capacitive interactions, and both parallel and series resistance.

I am not know it all by any means, but I've been at this for a very long time, and I've used my ears very carefully. I've also been lucky to work with some of the best musicians and audio engineers in the biz...folks who live in the real world and listen first, then talk.

Your deaf insistence upon relying so heavily upon textbook...not real world...explanations...which I believe you got wrong anyway...shows us that you are not a hands on, ears open pickup maker/luthier/wiring guru. I listen first, try to explain second. The OP in this thread heard issues in passive parallel pickup combining that he wanted to know more about. I think he's gotten more than he bargained for including a whole lot of misleading information.

[Rick Turner]

And Mike, the result that I predicted is exactly what you reported and everybody has heard.

[Mike Sulzer]

The result you predicted is not what I reported.

And Mike, the result that I predicted is exactly what you reported and everybody has heard.

No, this is what you wrote:

...Just try this...hang a pickup that is not under the strings onto your under strings pickup. Take it away. Now try to tell me the sound didn't change. I do this when lecturing to show exactly this effect of one pickup loading another when two are connected in parallel. You can get pretty close to simulating it exactly by just hanging a resistor with the same resistance as the DCR of the off-board pickup across signal to ground. This shows that most of the negative effect is resistive loading. ...

The sound of loading with the hanging pickup and a resistor equal to the DCR of the hanging pickup are not anywhere close to the same thing. See my post from last night, which states what I heard and also why it is true.

Summary:
1. The two sounds are very different.
2. As explained several times, the change in frequency response when you connect on the hanging pickup results from the halved source impedance interacting with the unchanged cable capacitance.
3. The resistor shorts out most of the output from the pickup because its value is much less than the impedance of the pickup, which is mostly inductive and larger in magnitude than the resistance. The sound also lacks treble.

What you wrote, and what I wrote are not anywhere close to the same thing.

[Rick Turner]

Mike, I'm just through here...

You've gone from claiming that there's no difference in frequency response whether you parallel pickups before buffering or after buffering to now agreeing that the LCR load of one pickup does indeed affect the other. I don't give a rat's ass about WHY at this point, though I know why; I've got my ears well tuned to this stuff. What I object to is your weaseling out of the discussion in such a manner as you have and jumping on the issue of whether a parallel resistor substituted for the pickup yields the same results. Whether the effect is more resistive as I think it may be or whether it's more inductive/capacitive as you seem to indicate is pretty much irrelevant when it comes down to the basic truth, and that is that when you passively combine two pickups...whether they are identical or not...you change the frequency response of BOTH of the pickups and get a band passed sound.

That said, I believe that a majority of the tonal change can be explained by the resistive model, though that is clearly not the only thing going on, nor did I say it was.

Your own ears are now revealing to you exactly the effect of mixing two pickups that I said was happening...and you said wasn't. Give it up, for heaven's sake.

[Mike Sulzer]

Mike, I'm just through here...

Good, I was just thinking the same thing, through with you, that is. I have not changed what I am saying, as you imply I have. Anyone can see that by going back and reading it with the full context. You have not responded to what I wrote, at all, and it seems that you never will.

[Chris Turner]

One is not better or worse than the other, but in response to Marko not liking the tone of passive blending, the dual buffer system will get him what he wants.

On that subject... does anyone have a schematic for this kind of setup, or maybe a link to one? I'd like to try it out.

[Marko Ursin]

I'm making one of these for each pickup.
Discrete FET Guitar Preamp
Hopefully I'll have them on the bass tomorrow.
Marko

[Chris Turner]

I'm making one of these for each pickup.
Discrete FET Guitar Preamp
Hopefully I'll have them on the bass tomorrow.
Marko

I'm new to this kinda stuff, but how do you go about mounting the circuit? I mean, I've worked with breadboards in the past, but it seems like you'd need to make a PCB for this, right?

Sorry for the noob questions.

[RedHouse]

Good, I was just thinking the same thing, through with you, that is. I have not changed what I am saying, as you imply I have. Anyone can see that by going back and reading it with the full context. You have not responded to what I wrote, at all, and it seems that you never will.

It's like nearly every other thread you "participate" in Mike, you'll argue until everybody is sick of it. Oh look the cows are comming home, guess it's time to shutty.

The only explanation I can figure is you must have hearing disability, I mean litterally everyone else can hear these things ...except you ...on your guitar.

You just come on here to argue, it's plain and simple, you agued with Joe Gwinn until everybody was sick of it, argued with me about cable capacitance and then backpedaled after several days of postings (just two prime examples of many) now here you're on here trying to argue with another guy who knows his stuff, dude ...maybe it's time to give it a rest, ya think?
(that wasn't a question)