Listening tests concerning the combining of pickups

[CRU JONES]

Nevermind! Ha, I get it! That is the series equation. Good stuff! Thanks Mike!

[Mike Sulzer]

Here is a brief listening test. It consists of flipping the test switch while holding some chords, kind of what David recommended. It starts with a test to confirm that the test setup is working. First, the bridge pickup is shorted out with a clip lead between the switch contact and ground. The chord is neck pickup only, switch open. When the switch is closed it is shorted out, as it should since the bridge pickup is shorted. The next two chords are the reverse situation. The neck pickup is shorted, and the bridge pickup is heard, and then shorted when the switch is closed.

A slight pop is also heard when the switch is closed. This is because there is something less than a mv dc on the pickup, but the shorted one has none. (The op amps use bipolar transistors.) When neither pickup is shorted to ground, the pop is not heard because the dc on the pickups is almost the same. (Both op amps are on the same chip and well matched.)

Following the introductory test are several chords with neither pickup shorted. The switch is flipped several times as the chord dies out. Circuit analysis predicts no change for identical pickups, very little change for pickups nearly the same as these are.

CombiningPickups1.mp3

[David Schwab]

I got a chance to do a quick test. In this test I was curious to see if the tone of one pickup would change when the other was added on a single buffered setup.

I used my '87 Ibanez SR-885LE with two of my Jazz sized humbuckers. The two pickups are not equally wound, the bridge is hotter. Each pickup is wired to a 500K pot, and a single Fender TBX tone control, with the typical Jazz bass wiring. The signal then goes to a JFET buffer based on the Alembic Stratoblaster. It does not have an impedance setting resistor, or input coupling cap.

I wanted a way to get a signal into one pickup, and then the other, while I listed to the results of turning the other pickup's volume up and down. I grabbed our old baby monitor receiver, and turned it on to get some white noise, and held the speaker directly over one pickup. I tested to make sure the other pickup wasn't sensing the noise.

So I listened to the bridge pickup while I mixed in the neck pickup. The tone of the white noise did not change at all. I repeated the test with the neck pickup and got the same results. I was actually surprised by this!

So I conclude that this particular setup shows that there is no change in the tone of either pickup when two are mixed together followed by a buffer.

When I get a chance I'll try it with a passive setup.

[Joe Gwinn]

How different electrically are neck and bridge pickups? If they are identical and are in parallel, there may be little effect.

[fieldwrangler]

Good test, this...

[David Schwab]

How different electrically are neck and bridge pickups? If they are identical and are in parallel, there may be little effect.

They are both humbuckers. The neck is about 7K and the bridge is about 8K. They are in parallel as you would wire a Fender jazz bass, i.e., each going to its own volume control. The controls are wired so turning one down doesn't cut off the other one, as on a Les Paul.

I was listening for a change in the white noise, but heard nothing when I mixed in the other pickup.

Having said that, I still prefer some kind of buffered setup to totally passive mixing. I added the buffer because it sounded better with it.

[Joe Gwinn]

They are both humbuckers. The neck is about 7K and the bridge is about 8K. They are in parallel as you would wire a Fender jazz bass, i.e., each going to its own volume control. The controls are wired so turning one down doesn't cut off the other one, as on a Les Paul.

This implies that there is enough series resistance between pickups to isolate them well enough.

I was listening for a change in the white noise, but heard nothing when I mixed in the other pickup.

White noise is contributed by the amplifier, and not by the pickups, so it may be no surprise that there was little effect on the white noise. How does it sound when played?


If you want to delve more deeply into these interaction effects, you can build a simple SPICE model and play with it. There are plenty of perfectly good open-source and free versions of SPICE available. However, there is also a learning curve, and books are written on how best to use SPICE. But SPICE is [l]the[/i] standard for simulation of lumped-parameter electric circuits using ordinary differential equations. But like all computer simulations, it's only as good as the model one feeds it.

[David Schwab]

White noise is contributed by the amplifier, and not by the pickups, so it may be no surprise that there was little effect on the white noise. How does it sound when played?

I think you missed what I did for the test. I used a baby monitor receiver, which is like a walkie talkie. Because the transmitter isn't on, the receiver produced white noise from it's speaker. I held the speaker over the pickup.

Then I listened as I blended in the other pickup. So the test was only to see if the tone of one pickup changed as I mixed in the other on a single buffered system.

it sounds fine played, but there isn't a lot of mixable range. The pots almost act like switches. They are audio taper and not much happens until you get to 8 or 9.

So you don't get very good blending action. I much prefer the way they blend when each pickup is buffered first.

If you want to delve more deeply into these interaction effects, you can build a simple SPICE model and play with it. There are plenty of perfectly good open-source and free versions of SPICE available. However, there is also a learning curve, and books are written on how best to use SPICE. But SPICE is the standard for simulation of lumped-parameter electric circuits using ordinary differential equations. But like all computer simulations, it's only as good as the model one feeds it.

I have a copy of MacSpice, but I haven't used it yet.

[Joe Gwinn]

I think you missed what I did for the test. I used a baby monitor receiver, which is like a walkie talkie. Because the transmitter isn't on, the receiver produced white noise from it's speaker. I held the speaker over the pickup.

I certainly didn't know this. Where was it posted?

Then I listened as I blended in the other pickup. So the test was only to see if the tone of one pickup changed as I mixed in the other on a single buffered system.

It sounds fine played, but there isn't a lot of mixable range. The pots almost act like switches. They are audio taper and not much happens until you get to 8 or 9.

So you don't get very good blending action. I much prefer the way they blend when each pickup is buffered first.

Don't people use a linear pot here?

I have a copy of MacSpice, but I haven't used it yet.

Get on with it!

[David Schwab]

I certainly didn't know this. Where was it posted?

http://music-electronics-forum.com/t20877/#post174588 #33

I did not record any audio files, it was just a quick listening test. The baby monitor idea popped in my head so I gave it a try.

Don't people use a linear pot here?

I prefer linear, but when I wired this bass up I didn't have any, so I used some audio taper pots I had laying around. I need to replace them soon. Besides not liking the taper, they are dirty. I'm working on a new preamp with dual buffers, active blending and tone controls, so when I'm done, I'll just gut the bass and start over.

Get on with it!

Yeah, guess I need to. I'm trying to find the time to learn that and Eagle.

[Joe Gwinn]

http://music-electronics-forum.com/t20877/#post174588 #33

I did not record any audio files, it was just a quick listening test. The baby monitor idea popped in my head so I gave it a try.

The baby monitor is causing the body to vibrate, so both pickups are driven more or less identically. A small drive coil that can be moved from pickup to pickup may better isolate effects.

[David Schwab]

The baby monitor is causing the body to vibrate, so both pickups are driven more or less identically. A small drive coil that can be moved from pickup to pickup may better isolate effects.

No, I was holding the monitor over the bass with the speaker close to the bridge pickup. It was not touching the body. The bass was laying face up in my lap. I confirmed that the other pickup was not sensing it by turning down the bridge pickup and turning up the neck pickup. I was monitoring with headphones turned up loud.

So in essence I was using the baby monitor speaker as a drive coil.

When I get a chance I'll photograph the setup and make some recordings.

It's like these monitors. The smaller units are the receivers.



What I expected to hear was a change in the sound of the white noise I was inducing into the pickup from the speaker. I figured I'd hear a resonant peak shift.

I was surprised to hear no change.

I'd imagine if the pickups were very different in windings I would have heard a change.

[Joe Gwinn]

No, I was holding the monitor over the bass with the speaker close to the bridge pickup. It was not touching the body. The bass was laying face up in my lap. I confirmed that the other pickup was not sensing it by turning down the bridge pickup and turning up the neck pickup. I was monitoring with headphones turned up loud.

So in essence I was using the baby monitor speaker as a drive coil.

I did understand that the drive was acoustic through the air. I think the entire guitar body vibrates, more or less regardless of where it is being driven. Nor is the "beam" from a baby monitor all that tightly focused.

One test is to replace the baby monitor with a small hammer used to tap the guitar body here and there as you listen. Sort of like tapping plates while making a violin.

[Mike Sulzer]

I did understand that the drive was acoustic through the air. I think the entire guitar body vibrates, more or less regardless of where it is being driven. Nor is the "beam" from a baby monitor all that tightly focused.

One test is to replace the baby monitor with a small hammer used to tap the guitar body here and there as you listen. Sort of like tapping plates while making a violin.

I think the coupling is magnetic.

[David Schwab]

I think the coupling is magnetic.

Yes, from the voice coil in the speaker.

You can hold any speaker up to a pickup and hear what's playing though the speaker, though the pickup. We do it with ear buds connected to iPods at rehearsal sometimes when someone wants everyone else to hear a part and doesn't want to take the time to plug the iPod into the PA system.