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What causes that Jazz Bass Scoop?

Antisyzygy

Antisyzygy

Dec 8, 2014
3,149
3,133
Washington
#1
I did search for an answer on this quite a few times before, but I've never found a satisfactory one based on circuit design/theory.

Why does that Jazz bass mid-scoop happen when both pickups are set to full volume? What circuit behaviors cause it?

I've heard :

1. One pickup loads the other

2. Comb filtering due to slightly off-phase signals from each pickup

3. Pickups in parallel average their signal together, in series it sums

4. Changing inductance / resistance cause a resonant peak shift (not sure how that ties into a scoop though)

Perhaps all are true at once, but I'd greatly appreciate some sort of more comprehensive engineering or scientific answer to that question. Thanks so much!
 
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#4
lz4005 said:
It isn't a circuit issue. It is an information issue.

You get phase cancellation of mid frequencies when you combine signals from two pickups at different points on the same string.
Click to expand...

Surely that's not all of it though, as for example putting a resistor in between one volume pot and another can "isolate" the two signals? For example 1960s Jazz wiring.

Anyway, thanks and I understand what you mean regarding this being an information issue. That's the comb filtering explanation I've read about before. The problem is I've heard others say different things.

For example I majored in math so I think that way. "Information loss" seems perfectly reasonable as a way to explain it in my mind, but then every once in awhile you see an engineer chime in with something about "loading" pickups, etc. I'm curious if there is a more general explanation tying all that together.
 
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#5
Antisyzygy said:
For example I majored in math so I think that way. "Information loss" seems perfectly reasonable as a way to explain it in my mind, but then every once in awhile you see an engineer chime in with something about "loading" pickups, etc. I'm curious if there is a more general explanation tying all that together.
Click to expand...

Loading happens but it does other things.

From a math perspective it's just plusses and minuses cancelling each other out. One pickup hears a particular harmonic on the upswing of the wave and the other hears it on the downswing. The result is no sound for that harmonic.
 
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#6
It's what they said. Phase cancellation. Plain and simple. Some of the harmonics over one pickup will be 180 degrees out of phase when over the other pickup. Combine them and those frequencies cancel. Gone. Can't be heard.
 
#7
IMO it's a combination of 1 and 4: one pickup is loading the other, and connecting the other pickup's inductance / capacitance / resistance in parallel to the first will shift the frequency response.

Reasons I think this include: turning one of the volumes down slightly introduces some resistance between the pickups (say 20K?) and makes the volume loss / scoop much less noticeable. This favors insertion loss and the frequency shift effect as explanations. (and disfavors the idea that it's purely due to the pickup locations, IMO)

Also, when people connect jazz bass pickups in series, they don't usually find this scoop effect. I have not yet tried this myself, but that also suggests it's got to do with loading when connected in parallel.

You might find this link on pickup response interesting: BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups It's not specifically about two parallel pickups, but it discusses the frequency effect of connecting a resistive and capacitative load across a pickup.
 
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#10
The electrical loading effects are there, but you hear the same scoop with a preamp that actively buffers and mixes the two pickups. So it's there even when the two pickups are isolated from each other in that fashion. So the majority of that tonal magic (for good or ill, depending on the player) is because it's the superposition (comb filtering, what have you) of those two signals together.
 
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#11
lz4005 said:
Loading happens but it does other things.

From a math perspective it's just plusses and minuses cancelling each other out. One pickup hears a particular harmonic on the upswing of the wave and the other hears it on the downswing. The result is no sound for that harmonic.
Click to expand...

tbplayer59 said:
It's what they said. Phase cancellation. Plain and simple. Some of the harmonics over one pickup will be 180 degrees out of phase when over the other pickup. Combine them and those frequencies cancel. Gone. Can't be heard.
Click to expand...

4StringTheorist said:
The electrical loading effects are there, but you hear the same scoop with a preamp that actively buffers and mixes the two pickups. So it's there even when the two pickups are isolated from each other in that fashion. So the majority of that tonal magic (for good or ill, depending on the player) is because it's the superposition (comb filtering, what have you) of those two signals together.
Click to expand...


Cool, that makes perfect sense. I've just read other explanations that seemed to hint there was something else to it as well. Thanks guys!

Here's a link that helped me understand what's going on with pickups and harmonics : Response Effects of Guitar Pickup Position and Width

I looked that up one time when I noticed a 51' P bass couldn't pick up certain harmonics on the open string, whereas a modern split-pickup-style P-bass does seem to get them. I believe it was the 4th harmonic.

So then it seems like with series wiring there doesn't seem to be the same effect (perhaps it's just not as noticeable). Is that because in parallel the signals from each pickup are averaged, and in series the signals are more-or-less summed?

I suppose the resonant peak shifts back in the series situation as well.
 
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#13
Funkinthetrunk said:
I dunno, but I like it.
Click to expand...

Me too. I switched to P-basses for various reasons but I miss my Jazz. I do have a SB-2 that gets a mid-scoop when both pickups are on full, but it doesn't have quite the same growl as my Jazz did. It's more piano-like, and a bit glassier than a Jazz bass sounds like.

I ended up trading my MIA Jazz I had for 13-14 years for a SB-1. No regrets in terms of the bass I got, it's now my main player, but I do miss that Jazz bass sometimes.
 
#15
It's really quite simple.

FIrst, measure the ohms emanating from pickup A, and subtract the ohms being rebroadcast from pickup B along the connecting wire. Then, divide the difference by the square root of the impedence of the capacitor. This measurement of harmonic resonance is then multiplied by the coefficient of the specific molecular weight the grounding material. You then have the mathmatical model and explantion for what we called "scooped tone".

It's as easy as Pi. Really.
 
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#16
Phase cancellation as has been said. I THINK also that the distance between the coils determines (at least in part) the frequency of the scoop.

In a humbucker when the coils are side by side you lose a bit of the highs. On a typical jazz bass you lose some mid-mids (maybe 500-700hz I'm guessing). On a bass where one pickup is really close to the neck and one is really close to the bridge I believe that what you get is a lower mids scoop (say 300-500 hz).

The scoop generally sounds pleasing on bass but you have to be careful not to scoop too much in a live setting. Using both pickups and then also scooping with the amp can cause the bass tone to lose definition. I usually solo one pickup when playing live.

Would love to see the scoop frequency proved. I have tried to use the various freeware out there but can't figure it out. Just going by ear with my triple pickup basses.

Brunette.jpg


JPJ and JJJ.jpg
 
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#17
MattZilla said:
quick, somebody dismount their bridge pup, resolder it, jam with both vols maxed and the bridge pup dangling in the breeze and come back with a tone report!
Click to expand...

You're joking a bit, but what happens is a nasal ALL mid honky thin nasty kind of a tone because your pickups are out of phase. I believe strats have this configuration available via a switch (maybe it's just some of them?) but I am not a guitarist to speak with authority on the matter.

I know, I'm great at parties. :(
 
#18
InhumanResource said:
You're joking a bit, but what happens is a nasal ALL mid honky thin nasty kind of a tone because your pickups are out of phase. I believe strats have this configuration available via a switch (maybe it's just some of them?) but I am not a guitarist to speak with authority on the matter.

I know, I'm great at parties. :(
Click to expand...

Some Alembics use a phantom coil in the middle (basically a pickup with no magnets) to cancel hum without losing the single coil tone. The Audere NC system does this using mini coils in the control cavity. Brilliant concept but has not really caught on for some reason.
 
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#19
Doner Designs said:
Some Alembics use a phantom coil in the middle (basically a pickup with no magnets) to cancel hum without losing the single coil tone. The Audere NC system does this using mini coils in the control cavity. Brilliant concept but has not really caught on for some reason.
Click to expand...

That's pretty slick. Maybe it is expensive to manufacture or requires changes to other parts of the circuit?
 

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