Rickresource Rickenbacker Forum

cjj wrote:Hmm, here again, it all depends on what you want to do and how you do the design. You're talking about another filter here, probably. I'll throw a little bit of theory at you. A "low pass" filter, also called a treble cut filter removes high frequencies and passes low ones. Where it starts cutting the high frequencies is a spot called the "cutoff frequency". This frequency is given by the equation:

Fc = 1/(2*PI*R*C)

Where PI is 3.14159265..., R is resistance in ohms, and C is capacitance in Farads (uF is microfarads, or 1/1,000,000 of a farad, also known as 1e-6 or "one times 10 to the -6th power" farads), Fc is cutoff frequency in Hz (cycles per second).

So, you can see, a larger value of R gives you a lower cutoff frequency, or less treble. A higher value of capacitance does the same thing.

So for a 4003's tone pot, it'd be...

Fc = 1/ (2*π*330,000*[4.7e-11]) = 10.261... kHz

Correct?

And a 4002 tone pot...

Fc = 1/ (2*π*500,000*[1e-10]) = 3.183... kHz

Or is it...

Fc = 1/ (2*π*500,000*[1e-10]*10,000,000*[1e-11]) = 31.831... MHz

I'm inclined to believe the first equation! But it subsequently leaves me a bit baffled on the meaning of the cap & resistor on the way to the pickup selector, so I'll have to look more into the meaning of an RC circuit as John Hall put it.

For the 4002 tone circuit, it is really a dual frequency variable tone filter. One filter element is the 10M resistor paralleled with the .01uF cap and whatever portion of the tone potentiometer resistance is in series with them (depends on where the tone control is turned). So as the pot is turned, it's resistance portion and the .01uf // 10M pass different amounts of high frequencies. The other "half" of the tone circuit is the .1uF cap in series with the rest of the tone pot's resistance which will filter at a lower frequency point due to the more capacitance. Smaller capacitor values in series with the signal pass the higher frequencies and filter more mid and low frequencies, so the .1uF will pass more lower freqs, and since one end goes to ground, will filter out more of the bass response of the pickup's signal.

Josh, the schematic you sent to me using the tone portion of the 4002 for a 4003 would work as drawn. But the tone switch shorting out the .1uF turns the tone control into a combo tone and volume control because as the wiper moves towards ground, you are not only changing the frequency spectrum you are filtering, but also taking more of the pickup signal to ground, lowering the amount of signal for amplification. So you wouldn't want that there.
With the second schematic with the taps and the 5M pot on the output, I do not understand what you want to do with the taps. As for the pot on the output, you'd need a dual pot if you use it where you show it, one for each pickup output. It just becomes a second volume control, which is redundant. And you'd only want to use about a 500k pot. This is approximately what the 4005 and 300 series guitars do, but they are mono'd outputs and the 5th pot is used for overall volume and the pickup volumes are used to set balance between the two pickups.

johnallg wrote:For the 4002 tone circuit, it is really a dual frequency variable tone filter. One filter element is the 10M resistor paralleled with the .01uF cap and whatever portion of the tone potentiometer resistance is in series with them (depends on where the tone control is turned). So as the pot is turned, it's resistance portion and the .01uf // 10M pass different amounts of high frequencies. The other "half" of the tone circuit is the .1uF cap in series with the rest of the tone pot's resistance which will filter at a lower frequency point due to the more capacitance. Smaller capacitor values in series with the signal pass the higher frequencies and filter more mid and low frequencies, so the .1uF will pass more lower freqs, and since one end goes to ground, will filter out more of the bass response of the pickup's signal.

Okay... I'm starting to understand the capacitors with some more nuance, such as what they are filtering, but I think I am still failing to reconcile how to separate a lo-pass filter from a hi-pass filter, since most caps on their own just suck off lows to low-mids.

johnallg wrote:But the tone switch shorting out the .1uF turns the tone control into a combo tone and volume control because as the wiper moves towards ground, you are not only changing the frequency spectrum you are filtering, but also taking more of the pickup signal to ground, lowering the amount of signal for amplification. So you wouldn't want that there.

I see... iiipopes had done something like unto that mod to his Bass Tone pot on his 4002 with very positive results, but he probably did it in some way more nuanced than what I scrawled out. He let me fiddle around with it a couple of months ago and I was impressed, but I didn't notice whether it gained volume function...

johnallg wrote:With the second schematic with the taps and the 5M pot on the output, I do not understand what you want to do with the taps. As for the pot on the output, you'd need a dual pot if you use it where you show it, one for each pickup output. It just becomes a second volume control, which is redundant. And you'd only want to use about a 500k pot. This is approximately what the 4005 and 300 series guitars do, but they are mono'd outputs and the 5th pot is used for overall volume and the pickup volumes are used to set balance between the two pickups.

Yes, I thought I might need two pots rather than one, and that's part of what got me asking more questions here--how to assign functionality. Slowly I am picking up more of the details. Forget about the taps--I threw those on there without thinking much about details. It was a bad idea. I was just thinking, "Well, it'll make it more versatile as a bass to have variable pickup functionality." (Have cake and eat it too?) I like swiss army basses (as stock Ric basses already are). But are you saying that two pots are needed because there are two volume pots or because there are two jacks?

Thanks John, for humoring me and my weird ideas. I appreciate all of the advice I have received without (yet) being expelled for being such a noob!

OK, I did a little looking at the 4002 circuit. One of the things I was doing before was a bit of oversimplification. When looking at these circuits, you can't really take separate parts of the circuit and talk about them completely separately. While it is true that the 500k pot and the 0.1uF cap are a variable cutoff high pass filter, you have to also include what the 0.01uF cap and 10M resistor do as well. Essentially, it all acts as a system, making a more complicated type of filter.

For this one, I cheated, I didn't actually do a complete analysis of the filter by hand. Instead, I drew the schematic in a circuit analysis program I have. I used a frequency sweep from 20Hz to 20kHz and ran the analysis with various values for the 2 sides of the 500k pot (you can't use a pot in the program, you have to use 2 resistors), always keeping the total at 500k.

Anyway, with the 500k pot adjusted so most of the resistance in between the center and the 0.1uF cap (I'll call this the bottom since that's the way the schematic looks), this circuit acts as a high pass filter, cutting off frequencies below 800 Hz, so this would be with the tone know adjusted to the "Treble" end.

With the pot adjusted to the center, the cutoff moves down to 200Hz, adding more bass as you'd expect be turning the tone knob towards the bass end.

When the pot gets turned further down towards the bass end, interesting things start to happen. When adjusted where the we've got 15k on the bottom half (towards the 0.1uF cap) and 485k on the top, we start losing the high frequencies. Essentially, it becomes a band pass filter centered around 200Hz.

As we continue on down towards the bass end, with the bottom part of the pot at 5k, top at 495k, the low frequencies below 200Hz start coming up. The filter is now acting as a low pass filter, cutting off frequencies above 200Hz.

As we get all the way to the bass end, we still have a low pass filter around 200Hz, but the slope of the cutoff has changed such that more high frequencies start coming back in.

Now, I have to say, I was modeling the pickup as an ideal voltage source with a source impedance of 7.5k ohms. The pickup also has inductance, which will affect the circuit as well. I have no idea what the inductance of the pickup is though. I tried a few values and in general, it didn't make a huge difference until the inductance got near 1H (Henry, the unit of inductance). Then, it mostly started losing high frequencies above 4kHz or so depending on how the pot was set.

Anyway, this is an interesting filter. I suspected that the other R & C would make this some sort of band pass or such, but as I said, it's been way to long since I've done a whole lot of filter analysis...

cjj wrote:OK, I did a little looking at the 4002 circuit. One of the things I was doing before was a bit of oversimplification. When looking at these circuits, you can't really take separate parts of the circuit and talk about them completely separately. While it is true that the 500k pot and the 0.1uF cap are a variable cutoff high pass filter, you have to also include what the 0.01uF cap and 10M resistor do as well. Essentially, it all acts as a system, making a more complicated type of filter.

For this one, I cheated, I didn't actually do a complete analysis of the filter by hand. Instead, I drew the schematic in a circuit analysis program I have. I used a frequency sweep from 20Hz to 20kHz and ran the analysis with various values for the 2 sides of the 500k pot (you can't use a pot in the program, you have to use 2 resistors), always keeping the total at 500k.

Anyway, with the 500k pot adjusted so most of the resistance in between the center and the 0.1uF cap (I'll call this the bottom since that's the way the schematic looks), this circuit acts as a high pass filter, cutting off frequencies below 800 Hz, so this would be with the tone know adjusted to the "Treble" end.

With the pot adjusted to the center, the cutoff moves down to 200Hz, adding more bass as you'd expect be turning the tone knob towards the bass end.

When the pot gets turned further down towards the bass end, interesting things start to happen. When adjusted where the we've got 15k on the bottom half (towards the 0.1uF cap) and 485k on the top, we start losing the high frequencies. Essentially, it becomes a band pass filter centered around 200Hz.

As we continue on down towards the bass end, with the bottom part of the pot at 5k, top at 495k, the low frequencies below 200Hz start coming up. The filter is now acting as a low pass filter, cutting off frequencies above 200Hz.

As we get all the way to the bass end, we still have a low pass filter around 200Hz, but the slope of the cutoff has changed such that more high frequencies start coming back in.

Now, I have to say, I was modeling the pickup as an ideal voltage source with a source impedance of 7.5k ohms. The pickup also has inductance, which will affect the circuit as well. I have no idea what the inductance of the pickup is though. I tried a few values and in general, it didn't make a huge difference until the inductance got near 1H (Henry, the unit of inductance). Then, it mostly started losing high frequencies above 4kHz or so depending on how the pot was set.

Anyway, this is an interesting filter. I suspected that the other R & C would make this some sort of band pass or such, but as I said, it's been way to long since I've done a whole lot of filter analysis...

Wow... just wow. Thank you for this analysis, I hope you continue to add to it if you have time. What a tribute to the skill involved in the design of that circuit!

CJ, nice using the circuit analysis - I don't have that capability. I broke it into two tone circuits to try and let Josh see how the parts work. I saw the hi/lo filtering, but wasn't sure how the 10M resistor would interact and suspected bandpass but wasn't sure at all. Good analysis.

Josh, IF you were to put 500k pots on the output like in the second drawing, you'd need two because the pickups are treated seperately by the circuits until a mono cable is plugged to the mono jack, which ties them together into a mono source. But the two pots would just be volume setters and take out half the volume, so what's the point, you already have volume controls there. See?

johnallg wrote:Josh, IF you were to put 500k pots on the output like in the second drawing, you'd need two because the pickups are treated seperately by the circuits until a mono cable is plugged to the mono jack, which ties them together into a mono source. But the two pots would just be volume setters and take out half the volume, so what's the point, you already have volume controls there. See?

Right, I'm already on the same page with you. Thanks!

I have an AmeriKit motion detector that involves a lot of soldering onto a PC board... it comes with four essays about electronic theory, one that deals with filtering wavelengths. So hopefully I'll walk away from that knowing more. The kit is about as old as I am! Hopefully everything will still be workable.

If you want a decent book on electronics, The Art of Electronics By Horowitz & Hill is a really good one. It starts off at a level suitable for those with no prior exposure to electronics and takes you through just about everything you'd want to know. It's really a good book!

As for the circuit simulation program, here's where you can get one for free. It's really pretty good, I used to use it a lot back when I was designing switching power supplies, but it's good for lots of other circuit analysis as well:

http://www.linear.com/designtools/software/#Spice

Just click the "Download LTspiceIV" link. It will ask you to register for an account, but also gives you a "No thanks, just download" option. There's also links on that page to the user's guide and demo circuits.

Just to make sure I understand this on a practical level as well, I illustrated what I think the wiring would look like. (4002 players, feel free to upload actual pictures.)

First, a highlight of what I'll illustrate, based on the schematic:
Second, my illustration:
This illustration seems to fit with my understanding of how RC filters are setup. It designates the RC filter as a Hi-Pass, since the cap is in the way and the resistor is grounded to the pot. I am still uncertain about a couple of things: C2 is not grounded and I don't know if the leads need to be soldered into the same lugs or not (if such a thing is actually industry practice, I wouldn't know). I am still trying to put the pieces together about how the movement of the wiper changes the tone from Hi-Pass to Band-Pass to Lo-Pass but I am getting there. It's behavior according to CJ's program just a lot more nuance than I would expect.

Thanks again for everyone's contributions to this thread.

Josh, the 10M resistor does not go to the pot body (ground) - it goes to the center tab (wiper) of the pot along with the red wire from the pup you show.

johnallg wrote:Josh, the 10M resistor does not go to the pot body (ground) - it goes to the center tab (wiper) of the pot along with the red wire from the pup you show.

Ohhh... that makes more sense in terms of explaining the functionality. Thank you John.

Correction:


...I was scoping things out over here: http://www.rickresource.com/phpBB3/view ... es#p353651
and it looks as though the capacitors in the wiring are green metal film... at least the 0.01 uf. The 0.1 uf are unfamiliar to me by their looks and I can't even see the resistors. Can anyone comment on the materials?

cassius987 wrote:...I was scoping things out over here: http://www.rickresource.com/phpBB3/view ... es#p353651
and it looks as though the capacitors in the wiring are green metal film... at least the 0.01 uf. The 0.1 uf are unfamiliar to me by their looks and I can't even see the resistors. Can anyone comment on the materials?

OK, in this picture from that thread: the resistors are the whitish things with stripes on top of the pot just beneath the pickup on the right. The other caps are the yellow square things near that pot and the right most pot. I'm not sure what types of caps they are, probably polyester or polystyrene, but it shouldn't make much difference what type you use. Ceramic types are a bit less linear in their capacitance over frequency, but you probably wouldn't notice it.

I use Sprague Orange Drop caps - from guitar parts sites there are limited values, but the .01 and .1 are common values.

http://www.guitarpartsresource.com/elec ... orange.htm for the caps
http://www.guitarpartsresource.com/elec ... tspots.htm if you need pots
Locating a 10M resistor will be a little tricky unless you want to buy a large quantity.

johnallg wrote:I use Sprague Orange Drop caps - from guitar parts sites there are limited values, but the .01 and .1 are common values.

http://www.guitarpartsresource.com/elec ... orange.htm for the caps
http://www.guitarpartsresource.com/elec ... tspots.htm if you need pots
Locating a 10M resistor will be a little tricky unless you want to buy a large quantity.

I got 10M resistors for 99 cents from RadioShack. Now that I look at the picture they look like the same type!

Do I have the wiring diagram correct? Looking at that photo it looks like the capacitor (0.01) and the resistor are wiring into the same wire, not two separate wires like I drew out.