Pickup that catches all harmonics?

[elros]

I was also going to suggest piezo pickups for a catch-all-harmonics type of idea. Because they do catch the whole lot. And if you're using a magnetic pickup you'll lose more signal the closer you go to the perfect spot where all harmonics are present.
And the piezo does not double the frequency, not at all.

And here are some piezo pickup sound clips: Conklin with RMC, Invalid Link Removed, and Invalid Link Removed. (I know I've posted these many times before.... )

 

[Arx]

It seems to me that under a saddle or under the bridge, it is picking up movement in the direction normal to the surface of the piezo film. To and away from the body of the instrument.

For me, there's no need for a scope trace: if it were picking up a doubled frequency, you'd hear it!

The difference under the bridge is that the string tension levers the pressure onto the piezo. Depending how tall and narrow the bridge is, that could be a significant factor.

This would probably be a secondary effect with most bridges. I wouldn't be surprised if it is visible on a scope though.

Regardless, I expect you're right and it's not an issue.

I am going for something different though. It's an experiment as much as anything else, but it sounds very cool so far.

Hopefully I can find a .022uF cap in my junk bins tonight. and I'll try and record some samples.

-Nick

 

[pilotjones]

While I definitely understand the concept of variations on string tension levering down on the saddles, there are a few assumptions made here (which could be right or wrong).
- that the string tension at the terminal nodes does vary in the first place
- that the string is moving in a single-planar fashion
- - if it moved (at least to some extent) in two planes at 90deg, thus producing a circular (or elliptical, if partial) motion, then the tension, given the first assumption to be taken as true, would be constant.

 

[Arx]

And if you're using a magnetic pickup you'll lose more signal the closer you go to the perfect spot where all harmonics are present.

Thanks for the sound clips.

Yeah, of course you'll lose signal, but there's some things that do work in your favour.

The signal level is lower, but you can also get much closer to the strings with much stronger magnets without influencing the sound. I didn't have any problem with output level. In fact, the signal is strong enough that I figure I can get away with a much smaller coil. The next one I try is going to be done with 1/4" diameter 1/2" tall magnets, instead of the 1" tall ones I used this time.

I still need to move the filter up a couple octaves too, which should give me another 12dB across the board. I'll probably have to reduce the gain a little after that.

 

[Arx]

While I definitely understand the concept of variations on string tension levering down on the saddles, there are a few assumptions made here (which could be right or wrong).
- that the string tension at the terminal nodes does vary in the first place
- that the string is moving in a single-planar fashion
- - if it moved (at least to some extent) in two planes at 90deg, thus producing a circular (or elliptical, if partial) motion, then the tension, given the first assumption to be taken as true, would be constant.

I think that's all one assumption, but I understand your point.

if the string moves in any fashion other than a perfect circle, there will be some variance in string tension.

Of course it's very unlikely that the string will end at exactly 90deg phase in two planes, and any small variation there is will be influenced by neck flex as well. It won't stay in that state for long.

It might be fun to mess with. But as I say, that's a later project. Can't get distracted until I finish this one.

-Nick

 

[Arx]

I didn't have much time to work on it tonight, but a couple quick updates anyways.

I replaced the .1uF C3 with a .022uF which moves the whole curve over, and gives me more gain in a useful range instead of just a huge boost at subsonic frequencies.

Now it's a gain of 37dB at 30Hz still with the same 6dB/octave slope. It doesn't sound much different than it did before, but now that I have the right frequencies I can reduce the gain, to a more reasonable level, which should reduce the noise.

It's way too loud now.

I tried to make a recording, but unfortunately the line level of my sound card's XLR inputs is selected between mic and line with a jumper, and I have it set up to use with mics right now. When I hooked the DI from my head up to it, it clipped like crazy.

I think I just need to get/build a cheap mic preamp so I can adjust the levels on my mic, and so that I can have the inputs at line level for the DI.

Samples coming sometime soon, I promise.

-Nick

 

[pilotjones]

Nick

Just thought of something that relates to this.

LightWave pickups have their optical sensors pretty close to the bridge, and so should get a raw signal with a frequency response something like yours. So, maybe there's something to be learned in however they deal with the tone shaping.

 

[Arx]

Nick

Just thought of something that relates to this.

LightWave pickups have their optical sensors pretty close to the bridge, and so should get a raw signal with a frequency response something like yours. So, maybe there's something to be learned in however they deal with the tone shaping.

Actually, I've already thought about that. they'll actually get a fairly flat signal, since their output is proportional to the position of the string, as opposed to its velocity. This causes a 6dB per octave gain as frequency drops, which conveniently is the inverse slope to what you get when you place the pickup close to the saddle.

I don't know what they've got for eq, but I bet they don't need much. Probably whatever electronics they have is for biasing the led/phototransistors, and perhaps linearizing the output.

-Nick

 

[pilotjones]

Actually, I've already thought about that. they'll actually get a fairly flat signal, since their output is proportional to the position of the string, as opposed to its velocity. This causes a 6dB per octave gain as frequency drops, which conveniently is the inverse slope to what you get when you place the pickup close to the saddle.

I don't know what they've got for eq, but I bet they don't need much. Probably whatever electronics they have is for biasing the led/phototransistors, and perhaps linearizing the output.

-Nick

Hmm, I was thinking that in a sine wave based system, the position and velocity would be basically the same signals, phase shifted by 90 degrees? But that doesn't work out.

 

[Mikey R]

Hmm, I was thinking that in a sine wave based system, the position and velocity would be basically the same signals, phase shifted by 90 degrees? But that doesn't work out.

velocity is the rate of change of position.

d [A sin(B x)] / dt = AB cos(B x)


ha - the old brain still works!

 

[pilotjones]

I forgot about the B term migrating out to the front of the expression on differentiation. Thanks.

 

[Arx]

Hmm, I was thinking that in a sine wave based system, the position and velocity would be basically the same signals, phase shifted by 90 degrees? But that doesn't work out.

Well, if you think about it the right way it makes some sense. You need a certain amount of string movement to give a given output level optically. Say for example, our reference level is 1mm.

If you're instead looking at the speed, the string's going to be moving twice as fast to move that same 1mm at twice the frequency. Alternatively, you could look at it as needing to move twice as far at low frequencies to get the same velocity, which is what really matters to us, since that gives us the basic transfer function between the two pickup methods.

If you were to put an uncorrected optical pickup in a standard pickup location (and assuming it was designed to measure a wide enough range not to clip) you would end up with way too much low frequency.

I'm not sure if it was by intention or luck, but they should have a natural 6dB/octave slope to cancel the 6dB/octave slope which you get below the combed portion of the frequency response (near the bridge)

 

[Arx]

Finally recorded a clip.

I'm holding the pickup by hand, so I can't actually play anything, but here's the sound of the open strings.

I recorded it twice, once with the pickup about half an inch from the saddle, and again with around an inch from the saddle.

There's a couple small pops, and the second B note is clipped badly. I didn't feel like spending a whole lot of time screwing with it right now.

http://arx.ca/projects/headless/BEADG.ogg

Oh yeah, I forgot to mention, that's with a new smaller 350 turn 36Ga coil.
It's nice and small, and I don't hear any hum/noise. I'm almost wondering if I should just skip the split coil and just do a 350 turn single coil.

 

[Arx]

Here's some pictures of the prototype preamp and coil:

 

[Arx]

Here's the current schematic, for anyone who's interested:

Only thing that's wrong is that C3 is actually 22nF, and not 100nF. This puts the corner frequency in the right place, so I'm not wasting 12dB of gain. With the new lower output coil, I think It's at about the right output level.

Here's the frequency analysis of the B string (pickup held above the string about 1/2" away from the saddle):

As you can see, there's lots of fundamental.

 

[mvw356]

afaik harmonics tend to appear at certain spots of the strings, so therefore it deosn't matter what pickup you have. what matters is how much area the pickup covers under the strings. derek smalls lakeland will probably excel at picking up harmonics, that is if the the magnetic field doesn't kill them in the first place.

 

[Arx]

afaik harmonics tend to appear at certain spots of the strings, so therefore it deosn't matter what pickup you have. what matters is how much area the pickup covers under the strings. derek smalls lakeland will probably excel at picking up harmonics, that is if the the magnetic field doesn't kill them in the first place.

Yes, but all the harmonics are present at the end of the strings, hence the idea for my project.

Yes, the size of the pickup does have an effect, though it's somewhat opposite to what you would expect, in that higher order harmonics (which admittedly aren't too important) won't get picked up by a large pickup.

A very large pile of pickups like that will have a really weird response.

If you look at the response I measured from my single small coil, you can clearly pick out all the harmonics in a fairly smooth consistent pattern. I don't think you'll get this by any other setup of magnetic pickups, however outlandish (though I'd love to be proved wrong. )

Something is eating the 5th, 10th, and 15th harmonic. Maybe the neck has a resonance there.. I'll have to look at the response of the other strings in a more controlled measurement to find out.

-Nick

 

[pilotjones]

Something is eating the 5th, 10th, and 15th harmonic. Maybe the neck has a resonance there.. I'll have to look at the response of the other strings in a more controlled measurement to find out.

-Nick

And the 20th is missing, too. Any chance this is an artifact of the choice of bin size/sample frequency choice for the FFT rather than the signal itself?

 

[Arx]

And the 20th is missing, too. Any chance this is an artifact of the choice of bin size/sample frequency choice for the FFT rather than the signal itself?

That's a likely possibilty. It's not really of any consequence though. I'm sure it's not my preamp, so it's either a non-existent artifact or simply a characteristic of the bass I used to test it.

 

[Arx]

I finally got back to working on this some more.


I built a closer-to-final working prototype of both the preamp and coil, and mounted them on a cheap bass I picked up as a guinea pig to hack on.

The preamp circuit is the same as in the earlier schematics, with the exception that the coupling caps are .68uF instead of 1uF. It shouldn't make any audible difference, but I wanted to stick with film caps, and the price difference was pretty huge between .68 and 1.0.

Also, I added that 2nd chip, which is a rail splitter. It allows running off of a single supply voltage. The primary advantage is that I can switch the power on using the jack, like most active basses, instead of requiring a dpdt power switch. Another upside is that it might allow it to operate with reduced headroom off a single 9v battery, though I plan to run at 18v

I've switched to using surface mount components too, which helps keep things nice and tiny.

The new pickup is single-coil, about 225 turns of 32Ga magnet wire on a plastic bobbin I machined. The magnets are 3/8" x 3/8" NdFeB rare earth magnets. Coil resistance is about 22 Ohms.
I sized it to require no routing. It's just screwed to the top of the bass.

My initial tests are going well.. It seems the E string has a little more output than the others, probably due mainly to the higher mass. I'll probably make an adjustable version in the future. String height makes much more difference with this configuration, because I'm running the magnets really close to the strings, as compared to other pickups.

I didn't have any 9v batteries handy, so I just used a 14v wall wart from my junk box. (about 20v output, since the load is so tiny)
I haven't bothered to mount the preamp board under the pickguard or anything either, so during the recordings, nothing is shielded. a pair of 36 Ga wires come out of the pickup, and run down to the floor, where the preamp board was sitting. Ground was not hooked to bridge, strings, etc.

The noise immunity appears extremely good, as expected. I'll still test it under some tougher circumstances, but it looks like there's no need to add a second coil

Excuse my clumsy playing...

http://arx.ca/projects/Pickup/mypickup_EADG.mp3
http://arx.ca/projects/Pickup/mypickup_test.mp3

Let me know what you think.

If I were to make something like this commercially (with many refinements, of course), would anyone be interested? I'm not taking orders or offering anything right now, just curious.

-Nick