4002 Circuitry

[tmossman]

CJ can you post or email your pSpice settings?

[cjj]

So, I'm guessing that most of the 4002s actually used the 100k as this would give a better bass response and better overall output...

This is why Scott Pope did some mods on his 4002. And they really opened the thing up... You could e-mail him a schematic if you like.

Oddly enough, he's the one who said his matched the schematic exactly...

[cassius987]

It may have before, but it certainly doesn't now.

[cjj]

Yes, he did say that he'd added a push/pull switch to be able to bypass the 0.01uF inline cap from the neck pickup, so it's not the same now.

[cjj]

CJ can you post or email your pSpice settings?

Well, I didn't exactly use pSpice. I use LTspice IV, which is free from Linear Technology. It used to be called Switcher CAD as it was mainly intended to help with the design of switching power supplies (which is what I've mainly used it for). Anyway, it's a pretty full fledged spice simulator with a fairly easy to use GUI, so you just make a schematic, add the values you want and then setup and run your sim.

So, here's the initial schematic/setup I used:

To generate the multiple plots, I just replicated this circuit , connected all to the same source, and changed the values of the resistors. There's no "built in" potentiometer, so R1 & R2 are the "pot" I just set them to maintain a total of 500k (well, you can't use zero, so I just use 0.1).

The biggest "guess in the whole thing is the pickup, which is modeled by L1 & R5. Inductors can have series resistance and parallel cap as part of their parameters but I chose to keep the resistance separate for ease of changing it. Anyway, the guess part is that I have no idea what the inductance of a RIC bass pickup is. There's lots of info on the resistance, but this is next to useless without the inductance. I did find a table with inductance values for a number of guitar pickups which included a RIC entry of about 2.5H, but it didn't specify what type it was. I also had a Gibson EB0 bass pickup, the only bass pickup listed, at 65H, so I bumped up the inductance of L1 to 5H figuring a RIC bass pickup might have a bit more inductance.

This shouldn't have a huge effect on the overall characteristics of what the circuit does, it'll just move the freq's around some. It's still reasonable as far as seeing what effect other component values do to the response in a relative way.

I also ran a few sims with the 0.01uF cap shorted. The interesting thing there was that with the pot at mid and min, the cap in or out had very little difference. Only when the tone was full up was there a significant difference (note, I only simulated at full up, mid and min).

[cassius987]

Yes, he did say that he'd added a push/pull switch to be able to bypass the 0.01uF inline cap from the neck pickup, so it's not the same now.

You're right. It was the cap bypass, not a change in resistance (I swear he added or subtracted a resistor or two as well, or at least talked about doing it last time I went to his house). Regardless it really opened the thing up! Maybe something else for you to consider. By the way, I'm 100% positive it was the neck 0.1 uF cap he bypassed, not a 0.01 uF.

[cjj]

No, I'm pretty certain it was the 0.01 uF as he said, "As far as the schematic, my 4002 is right on with the schematic. But I've done one mod to my 4002: added a push-pull so the neck pickup can go straight through without the inline bass cut.". The 0.01 is the inline cap, bypassing the 0.1uF would just turn the tone control into another volume control.

But, realistically, from the simulations I've done, I can't really see a whole lot of value in keeping the 0.01uF cap. It seems that it only has any real effect when the tone control is fairly far up, and then is tends to cut the bass freq's a bit (no big surprise here since it's not a lot different than the 0.047uF in the RIC "vintage" circuit).

Still, I'd like to know a bit more about the inductance of the 4002 pickup as compared with the 4004's HB-1. There is a significant group of people who like the 4001/4003 vintage sound, myself included, though I really like the deeper sound of the modern circuit too. Knowing the inductance values would allow a design of the circuit that more closely matched the 4002 and just might be worth exploring. Looking at traces on a graph and actually hearing the difference are, of course, totally different things.

Also, the pickup placement on the 4002 would be a considerable difference from the 4004. Rickenbrother suggested adding a 3rd pickup in the 4002/4004LK position. I'm seriously considering doing just that. I'm not really sure how to wire it all up, especially if I want to use stereo out as I almost always do. Right now, I'm thinking that triple stacked pots and no switch is the way to go. That way, instead of a switch, you just turn a pickup all the way down to remove it from the mix. Then it's just a matter of which two to pair up on what output. I'm running out of room for switches...

[cassius987]

Well you may be right then CJ... either way I can attest that whatever his push-pull was doing, the difference truly was huge. The fundamental response practically doubled in volume from the neck pickup. It was great for Motown funk. If the 0.01 uF cap doesn't do much I'm not sure why it was such a huge difference.

[cjj]

Well, I'm not conclusively saying that the 0.01 cap doesn't do anything. I guess I stated that a bit wrong. And of course, my simulations are making some big assumptions about the pickups, the real pickups may affect things quite differently.

Anyway, here are the plots for the 4002 tone circuit with the cap bypassed and with it in the circuit:

Vout1, 2, & 3 (green, blue, red) are with the cap bypassed, tone at min, mid, and max, respectively.
Vout4, 5, & 6 (cyan, magenta, and gray) are with the cap in the circuit, tone at min, mid, and max, respectively.

As we can see, there's a huge difference with the tone at full (red vs. gray), especially at the bass end. Somewhere around 650Hz, these lines cross.

For the mid and min positions of the tone control, the traces pretty much run right on top of one another, with a bit of separation at the low end, below 50Hz. Even at the mid setting however, we see a fair amount of difference with the bypassed version having more bass below 120Hz. So yes, there would be a difference in the amount of bass with and without the cap.

Now, one thing to consider here is that the vertical axis is in dB, 10db per grid step. Now, a 3dB change in is doubling the voltage. I'm not really sure how this relates to how we hear things though, but it's probably similar to doubling the volume, I'll have to look into that. In any case, the difference between cap & bypass at mid tone (blue vs. magenta) looks to be a couple of dB so the sound might be quite pronounced, at least below 120Hz. And depending on the real pickup characteristics, this point could well shift quite a bit in frequency (and the difference in level) so an actual 4002 may sound quite different than what these plots seem to imply.

My real aim in doing all this is to find the best way to set up the tone circuitry in my 4004. Having heard about the great sound of 4002s, it seemed a good direction to head. Of course, never having heard a 4002 myself, it's all a bit of a stab in the dark. And then, the 4002 has quite a different placement in its pickups as compared to a 4004Cii, so that will certainly affect what harmonics actually hit the filter circuits. I'm seriously thinking about adding a 3rd HB-1 at the 4002 location just to see how that affects the sound, of course, that's a far bigger mod than just a few caps and resistors...

[cassius987]

While we're talking circuitry, I hope this isn't too OT, but what was the value of the two original pots in your 4004? 500K? The RIC schematic actually lists them as 330K, which really surprised me.

[cjj]

Not too far off topic at all!

The originals were 330k (Volume = 323k, Tone = 328k if you want the total geek answer ). It's a reasonable compromise between 250k & 500k. A while back RIC switched to using 330k all around, most likely because having to stock a single part saves time & money, and doesn't make enough difference in sound to really matter (except perhaps to certain perfectionist techno-geeks )...

[cassius987]

Then it must the HB-1. I've always been amazed at how good the 4004's tone control sounds (I've played two Ciis).