4002 Data

[cjj]

Who's got either/both 4002 pickup(s) out of a bass??

I've got Sepp's loaded 4002 guard here now John. What readings do you want me to take?

Do you have the pickups? And then, do you happen to have an LCR bridge around to make the measurements? Without one, it's pretty difficult to make 'em cause you have to apply an AC signal and then measure inductance, resistance, & capacitance at that frequency. Then do it again for a series of frequencies that you care about. I used an HP4263A LCR meter and took measurements at 100Hz, 120Hz, 1kHz, and 10kHz of the pickup coils. For the Lo-Z you would need to make the measurements on the Lo-Z coil separately.

As for resistor and cap values, one would really have to do simulations of a lot of different values and decide what does the best job of doing what you want to accomplish. What do we want to accomplish? I'm not sure, really. If it's trying to get the same response as a real 4002, then the only way to do it would be to have an accurate model for the 4002 pickups, which would really mean making measurements of the actual pickups. Then it's a matter of trying different cap/resistor values until you get the response you want, which may never be just like the 4002 because the HB-1 is a different pickup, with different characteristics. But maybe you could get close.

The whole point of the 10M vs. 300k was to try to figure out what the difference is between the schematic, which shows 10M and actual basses, which according to earlier posts in this thread seem to have 300k (one had 560k or something like that, it would be somewhere between 10M & 300k in response). My guess is that they originally used 10M, drew the schematic that way, then found unacceptable low end roll-off and tried smaller values until they found something that worked better. These fancy simulators, while in existence, were much more cumbersome to use (you couldn't just draw a schematic and stick values in), and were VERY expensive, not to mention the hugely expensive computers you needed.

Anyway, no, smaller is not necessarily better, too small and you essentially short out half your tone pot and the series resistor. I'd say that 300k is a good value, better than 10M. Maybe tomorrow I'll try a smaller value to see what it does...

[cjj]

Warning!!! MEGA Techno-Nerd stuff to follow!!!

You've been warned, turn back now!!!!


Edit: Ignore the following, I messed up the circuit!

So Jeff, what are your thoughts on the best way to model a guitar pickup for SPICE simulation?

[aceonbass]

I wasn't really trying to replicate the sound of a 4002 as much as wanting to see if the very different tone control wiring had anything to offer for use with other RIC pickups. At the very least, it seems replacing the cap/resister pack with a piece of wire, and replacing the .1uF caps with pickup appropriate caps (.022uF for HB's and .047uF for single coils) would offer a more straight forward signal flow that might net a better frequency response than stock 4003 wiring. The stock 4003 tone control wiring signal flow is a bit weird, and NOT the way everyone else does it. I'm not saying it sounds bad at all, just that I like trying different things out to see what sounds best. Here's the diagram I drew up for my 4002 harness.

[jps]

So Jeff, what are your thoughts on the best way to model a guitar pickup for SPICE simulation?

I'll get back to you on that!

[cjj]

OK, learning how to use the simulator better, I now know how to do more readable plots for posting.

Anyway, I decided it might be interesting to see how the 4002 wiring with 500k pots and HB-1s compared to 4003 wiring, neck Hi-Gain, 500k pots both with and without the in-line 0.0047uF cap.

First, with the cap:

Here we see the 4002/HB-1 starting at the -4dB point while the 4003 starts at around -11dB. 3 plots for each, with tone down, middle, and up. You can see that on the 4003, the lows are severely cut below 100Hz with the cap.

Here's the comparison of 4002/HB-1 and 4003 without the cap:

Here we can see that the 4003 circuit without the cap has even better bass response than the "magic" 4002 circuit. The 4002 looks like it gives a bit more variability across the range of the tone pot - the 4003 at half and full are pretty close together but the 4002 is further apart. I don't know that this really gives you anything, maybe finer adjustment since the spread is more even across the pot range.

Just for grins, I ran a sim with the 4002 & 4003/no cap circuit using only HB-1 pickup values:

Not a huge difference from the hi-gain. Note that this is not saying that a hi-gain will sound the same as a HB-1. This is a highly simplified view and just shows the frequency response of the circuit, not what harmonics and such are generated by the pickup itself.

[bassduke49]

My head is starting to hurt.

[cjj]

My head is starting to hurt.

Glad I could help!

[cassius987]

First, with the cap:

4002vs4003.jpg

Here we see the 4002/HB-1 starting at the -4dB point while the 4003 starts at around -11dB. 3 plots for each, with tone down, middle, and up. You can see that on the 4003, the lows are severely cut below 100Hz with the cap.

Why isn't the neck pickup picking up the slack for the 4003?

[cjj]

First, with the cap:

4002vs4003.jpg

Here we see the 4002/HB-1 starting at the -4dB point while the 4003 starts at around -11dB. 3 plots for each, with tone down, middle, and up. You can see that on the 4003, the lows are severely cut below 100Hz with the cap.

Why isn't the neck pickup picking up the slack for the 4003?

Because I only modeled the bridge pickup.

The neck pickup would be fairly similar to the plots for the hi-gain without the in-line cap...

[aceonbass]

Lots of good info CJ, but I'm having trouble processing it into a useful conclusion. In my 4002 wiring I did use 500K pots for all tone controls and the HB1 volume (these pots actually measured out to 408K), and a 250K pot for the RIHS volume control. Are you saying that the 4003 circuit has better bass response, and that therefore the 4002 circuit has no benefits in anything besides being correct for a 4002? I still wish I could figure out my volume drop

[cjj]

Lots of good info CJ, but I'm having trouble processing it into a useful conclusion. In my 4002 wiring I did use 500K pots for all tone controls and the HB1 volume (these pots actually measured out to 408K), and a 250K pot for the RIHS volume control. Are you saying that the 4003 circuit has better bass response, and that therefore the 4002 circuit has no benefits in anything besides being correct for a 4002? I still wish I could figure out my volume drop

Actually, yes, looking at the data, I would say that the 4003 circuit has better bass response than the 4002 circuit. This is somewhat obvious since it is not trying to send any of the signal through an in-line cap, which will act as a high pass (low cut) filter. The resistor just allows some amount of the signal to bypass the in-line cap, so lower values will allow more to pass and therefore better low freq. response. The 4003, without an in-line cap has no filtering so all low freq's get through.

The tone circuits are really pretty similar. In the 4003, the pot is between the cap and ground, but if it were reversed it would work the same. On the 4002, the pickup is connected to the center of the pot and then the cap to ground. This gives the same arrangement as as reversing the cap/pot positions on the 4003 (a variable resistor in series with a cap to ground) but with the addition of a variable resistor in series (the top part of the pot) between the pickup and the output. The variable resistor/cap is the actual tone control, setting how much of the high freq's get bled off to ground, the same in each case. The the upper part of the pot in the 4002 will interact with the series cap to change the characteristics of the high pass filter, without the cap it's kind of an extra volume control.

Now, how this all sounds, with real pickups, and especially multiple pickups, well, the simulations really can't tell you much. That's something you kind of have to find out by trying it.

As for your volume drop, I did actually simulate having two HB-1s in parallel going to the tone circuit. The response really doesn't look much different than with a single pickup. So, I have no real idea what's going on to cause it...

[johnallg]

My head is starting to hurt.

Don't feel alone, and I have an electronics degree and about 30 years experience.

[cjj]

The more I think about this, the more I think my original model was right. The "voltage source" in the pickup is the coil itself and so, the parallel capacitance of the winding has to also be across the voltage source in the model. Searching the net for info on modelling guitar pickups shows that my first model was essentially correct. The thing that threw me off was the big notch around 6kHz. But is it really there?

Guess it's time to take a real pickup and tone circuit and make some real world measurements...

[Kopfjaeger]

Good, I'm in good company!! I was lost right after Dane mentioned that he had my 4002 pups and electronics. Normally I like to know how things work and I like to think I have a good grasp on the technical aspect of things. Not in this case though. I'm ok with the whole "ignorance is bliss" thing.

Sepp

[cjj]

Wow, lots of good info on pickups, strings and even guitar body resonance here:
http://sem-proceedings.com/25i/sem.org- ... ickups.pdf

And yeah, I was right in the initial pickup model, the cap has to go across the voltage source too. So, now I've got to figure out the best way to measure the actual pickup and circuit to see the real response. Turns out this isn't all that simple. Seems you need to excite the pickup with a signal whose amplitude is inversely proportional to frequency. This is to match the fact that at lower frequencies, the strings will travel further than at high frequencies.

This signal is then used to drive an exciter coil to inject the signal into the pickup the way a string would. The trick here is that the coil must be driven with a constant current source to account for the exciter coil's differing impedance over frequency.

Then you merely sweep the frequency from 20Hz to 20kHz and look at the output with an audio spectrum analyzer.

This is gonna take a day or three...