4002 Data

[aceonbass]

Having now finished rewiring Sepp's 4002, and tested it, I've made a couple of conclusions. First of all, Sepp had me wire the neck tone control as a .01uF cap bypass when pulled. This makes quite a bit of difference in the sound by increasing the bottom end frequency response, and is very similar to the current vintage tone circuit in current 4003's, only in reverse since the cap is normally engaged in Sepp's 4002. I'll now probably go back into my 4003 and experiment with bypassing both .01uF caps with jumper wires before removing them all together. I suspect there is really no benefit to having a filter cap of this sort in my 4003, or any 4003 for that matter. I'm also going to experiment with bypassing the 10meg resister too, but does anyone really even know what it's there for? Does it work with the .01 tone cap in some way? I do find I'm getting a clarity in the tone with the pickups wired directly to the tone pots with the 4002 wiring that I wasn't getting when the pickups were run through the pickup selector then to the tone pots, which also had integrated tone caps, but with a .027uF cap for the HB's and a .047uF cap for the RIHS.

[cjj]

I'm also going to experiment with bypassing the 10meg resister too, but does anyone really even know what it's there for? Does it work with the .01 tone cap in some way?

Yes, I know what it's there for and yes, it does work with the 0.01uF tone cap.

The tone cap being in series acts as a high pass filter and cuts low frequencies. The filter changes the behavior of the filter by bypassing some of the signal around the cap. This controls how far down the filter cutoff extends. You can look at the extremes, no resistor (infinite ohms) and a short (zero ohms or no cap at all). In the no resistor case, no DC will ever get through and the high pass filter cuts off completely at DC. In the no cap case, well, there's no filter and everything gets through down to DC. For other values of resistors, you get something in between.

Here's a plot of just the 0.01uF cap, going into a 500k "volume pot" with 2 different resistor values. The green line is a 300k resistor bypassing the cap. The blue line is 50k.

You can see that with 300k, the signal drops pretty fast below 600Hz and gets all the way down to -5.2dB at 20Hz. The 50k (blue) still drops off below 600Hz, but but is pretty much done by 100 Hz and doesn't drop much further, ending at about -1.2dB at 20Hz. Smaller values would flatten the curve more giving more and more bottom end until you hit 0 ohms where it would be flat all the way down...

[cassius987]

First of all, Sepp had me wire the neck tone control as a .01uF cap bypass when pulled. This makes quite a bit of difference in the sound by increasing the bottom end frequency response, and is very similar to the current vintage tone circuit in current 4003's, only in reverse since the cap is normally engaged in Sepp's 4002.

This was done by Scott Pope a few years ago with the same effect. I believe he likes it so much he has the "down" setting as the bypass instead of the stock wiring.

Here's a plot of just the 0.01uF cap, going into a 500k "volume pot" with 2 different resistor values. The green line is a 300k resistor bypassing the cap. The blue line is 50k.

hpf.jpg

You can see that with 300k, the signal drops pretty fast below 600Hz and gets all the way down to -5.2dB at 20Hz. The 50k (blue) still drops off below 600Hz, but but is pretty much done by 100 Hz and doesn't drop much further, ending at about -1.2dB at 20Hz. Smaller values would flatten the curve more giving more and more bottom end until you hit 0 ohms where it would be flat all the way down...

So what's a 10^6 ohm resistor doing? Does it push the hinge down much lower and then it sinks like a rock, or what?

The other thing that bugs me is that the 0.01 µF cap should have an Fc twice as low as a 0.0047 µF cap (like the one used in the 4003 circuit) and given everything else is held constant. So no resistor (0.01 µF cap only) should be less dramatic (again, in the 4003 circuit) than the 0.0047 µF, but I have found that its Fc is something like 150-200 Hz, not up as high as 600. However I don't know the dB decrease exactly. This makes me wonder what else is going on, or if the resistor is actually doing something like raising up the Fc but decreasing the dB loss? Better stop now as I'm well over my head.

[ken_j]

... I do find I'm getting a clarity in the tone with the pickups wired directly to the tone pots with the 4002 wiring that I wasn't getting when the pickups were run through the pickup selector then to the tone pots...

Were you modifing your wiring to run through the selector first? The stock wiring for a 4003 has the tone circuit always wired to the pickup regardless of the selector switch poition. Look Here.

[iiipopes]

This was done by Scott Pope a few years ago with the same effect. I believe he likes it so much he has the "down" setting as the bypass instead of the stock wiring.

Indeed. Just like my guitar mods, the 10meg resistor and the .01 cap are on either side of the DPDT push pull, so when it is down, or push, it is straight bypass from the tone loop to the selector switch, and up, or pull, is stock 4002 wiring, just like the factory diagram. The increased low end to the neck pickup is substantial. I have not performed the mod to the bridge pickup because with less string excursion and less fundamental content anyway, it just isn't worth it.

[teeder]

My head is starting to hurt.

You're supposed to stop reading when the warning signal is lit. That's what I do!

Thanks CJ!

[aceonbass]

Ken....While my tone caps were previously wired up like a Fender, the hot wire from the pickups was wired to the selector switch, just like a standard 4003, with the ground to the back of the pots. The big difference here with a 4002, is that the pickups are wired directly to the tone pots. I believe this, and the straight shot the signal takes through the circuitry, are the reasons for the clarity. CJ....while your very detailed info is somewhat helpful, I'm having difficulty making conclusions from your data. Basically, in a stock 4002 circuit, what value resister is best for bass response, a low value, or a high value, or even a straight piece of wire? Since I'm already liking the basic layout in the 4002 circuit in my 4003, for better frequency response should I just ditch the cap and resister? While I've gotten pretty good at replicating this stuff, and finding out what sounds best to a degree, I don't completely understand it. Especially with the variables in a 4002 circuit that nothing else I've ever seen has. Scott is the one who inspired the push-pull option on Sepp's bass, and has been giving me a lot of good info behind the scenes on this project.

[cjj]

So what's a 10^6 ohm resistor doing? Does it push the hinge down much lower and then it sinks like a rock, or what?

The other thing that bugs me is that the 0.01 µF cap should have an Fc twice as low as a 0.0047 µF cap (like the one used in the 4003 circuit) and given everything else is held constant. So no resistor (0.01 µF cap only) should be less dramatic (again, in the 4003 circuit) than the 0.0047 µF, but I have found that its Fc is something like 150-200 Hz, not up as high as 600. However I don't know the dB decrease exactly. This makes me wonder what else is going on, or if the resistor is actually doing something like raising up the Fc but decreasing the dB loss? Better stop now as I'm well over my head.

The 10^6 resistor does the same thing as the others, but doesn't bypass as much of the signal around the cap. Here's a plot with 50k, 300k, 10M, and no resistor:
The circuits

The response

So, we can see that going from 300k to 10M causes the curve to get very close to the no resistor, cap only response (dk blue & green traces). One thing we see here is that we get a bit more output in the 30hX to 200Hz range, but it's only around 1.2dB, which isn't a lot. At 20Hz though, the 10M resistor is getting near 3 dB below the 300k, which is possibly noticeable, but the 50k is a LOT higher in the 20Hz to 70Hz range.

So, how does this compare to a 0.0047uF cap?

Here lt blue and red are the 0.01uF, dk blue and green are the 0.0047uF, each with 300k & 10M respectively. You can see that they are pretty much the same, just shifted in frequency, with the 0.01uf being lower. The green trace, which is the 10M/0.0047uF cap has Fc (-3dB point) at about 70Hz. The red trace, 10M/0.01uF has Fc at about 35Hz, so yes, the 0.01 is twice as low as the 0.0047uF cap.

The term "hinge frequency," which I think means "cutoff frequency" (it's a new term to me) is fairly misleading in my opinion. It would seem that people think it's where a filter starts to roll off, hence the reference to 600Hz. The cutoff frequency, denoted by Fc is quite well defined as the -3dB point, where the power is down to half it's original value. This often looks like it's pretty far down the curve from where things start to roll off, but that all depends on the range of data in the plot and the scale you are looking at...

[aceonbass]

So....the 10M resister was better then? If this is the case, I'm happy 'cause that's what's in RIC's schematic, as well as Sepp's harness (which is on it's way to him now) and my bass. Since I'm thinking of bypassing the .01 cap permanently in at least my 4003's HB's, what effect will leaving the resister in the circuit have since it's not bypassed? Is it even necessary at all if not using the .01 cap, or is it just redundant, and can just be left where it is since it's already there?

[cassius987]

What I didn't realize before is how it sacrifices additional midrange by using the resistor bypass around the cap, especially with the lower values and not so much the 10 Meg. The 10 Meg almost seems a bit pointless side-by-side with cap only. I guess it's all in what you're after but I kind of prefer the cap-only scenario and just picking a big enough value to get the Fc you want (35 Hz may be okay but you could be really safe and use a 0.1 µF cap to lower that down below the audible range).

[cjj]

CJ....while your very detailed info is somewhat helpful, I'm having difficulty making conclusions from your data. Basically, in a stock 4002 circuit, what value resister is best for bass response, a low value, or a high value, or even a straight piece of wire? Since I'm already liking the basic layout in the 4002 circuit in my 4003, for better frequency response should I just ditch the cap and resister? While I've gotten pretty good at replicating this stuff, and finding out what sounds best to a degree, I don't completely understand it. Especially with the variables in a 4002 circuit that nothing else I've ever seen has. Scott is the one who inspired the push-pull option on Sepp's bass, and has been giving me a lot of good info behind the scenes on this project.

OK, bottom line?

The series cap is a "high pass" filter, which means that it passes high frequencies and cuts low frequencies. So, the MOST low frequency option is to bypass the cap and just use a straight wire.

The value of the bypass resistor (10M, 300k, whatever) sets how much low frequency "gets past" the cap. It also changes the shape of the curve, which will change the frequency mix. What does this sound like? There's really no way to know by doing mathematical analysis (which is all these simulations are), especially when you consider the pickup itself. Even knowing all of the parameters about the pickup won't tell you the whole story because pickup placement and the strengths of different harmonics all come into play. Even if you know all of that, there are other factors within the pickup such as eddy currents (electrical and magnetic) that will affect the sound and are next to impossible to model mathematically (realistically, the strength of the Earth's magnetic field where you are will have some affect too).

One question I have is why, exactly, RIC decided they needed to put high pass filters in at all. Maybe they just liked the sound when some of the bass was cut out. Maybe amps of the day didn't handle a lot of bass very well and cutting it helped give a more even sound. Who knows? But I'm guessing it was all done based on what someone thought was the best sound as opposed to any high-tech analysis.

So, what to do? Well, the ONLY way to know how it's gonna sound is to try it out. These simulations and such only give some ideas as to how different circuit values and such will affect things, smaller resistors give more bass, larger less, smaller caps less bass, larger more (for the in-line cap). Adding in the rest of the circuit, including the pickup data still only gives an approximation, but won't really tell you how it sounds.

So, I suppose all of this Techno-Geek stuff is really kind of useless when you still have to experiment and see how it sounds, but at least we know a bit about what different values will do...

[aceonbass]

Then the 300K resister allowed MORE bass around it than the 10M one? See, I thought after reading all that, that it was the other way around. I've always found it difficult to process a lot of abstract info into a usable conclusion. In the end, what I'm after is the widest frequency response with the most "clarity", and to be able to get all of my tone out of my bass. I've been setting the four band EQ on my GK flat for awhile now, using only the volume control, then adjusting the tone settings on my bass. This has been working out pretty well, and even better with the 4002 wiring, with the exception of the slight volume drop when backing off the tone. The volume drop may even be right at this "hinge frequency", because it's right at that point that there's a lot of "edge" to be dialed in or out, and where I find most of the tone settings with either pickup circuit when running both together. I wasn't running both of them together at all before.

[bassduke49]

Definitely hurting.

[cassius987]

I think it's interesting that with the 0.01 µF cap the frequencies lost about the Fc because of the (300k) resistor are probably more detrimental in terms of signal loss than the frequencies lost with the cap alone.

CJ, I know we've already wasted plenty of your time but you can also show us what 1 Meg would look like with those caps? And what a 0.047 µF cap would do? Please???

[jps]

CJ, I know we've already wasted plenty of your time but you can also show us what 1 Meg would look like with those caps? And what a 0.047 µF cap would do? Please???

...and , also what affect all this has using a magnetic horseshoe pickup, too, pretty please!