A few months back, I purchased the ultimate unnecessary-but-awesome gadget: the iPad 2.
With the availability of apps like Amplitube, AmpKit, and Garage Band, it’s immediately obvious how this device can be an amazing guitar learning and practicing tool. I’ll talk more about that in another article. But before you can plug in your guitar, you need a special interface…
In part 1 of this 3-part video series, I introduce the project- how to make your own impedance matching, buffered guitar interface for the Apple iPad, iPod touch and iPhone. These iDevices all share a similar headphone/microphone jack specification, so this circuit should work with all of them.
Here’s a great reference image, which clearly shows the differences between the typical A (audio/log), C (reverse audio/log), B (linear), and W (s curve) tapers. Less common are the K, D, and G tapers.
A reader recently asked me a question about the low pass filter in a guitar tone circuit:
Will a 250k tone pot with a .02uF capacitor sound the same as a 500k pot with a .01uF capacitor (all else being equal)?
This is an interesting thought experiment, and the answer is simultaneously obvious and non-intuitive.
At first glance, you might be tempted to look at this standard low-pass filter schematic (borrowed from the LPF wiki), and the associated formula for cutoff frequency as 1/2piRC, and conclude that the two circuits would behave identically (since 250k*.02uF is the same as 500k*.01uF). However, the problem there is that the R in the formula is not the tone pot! That R is really the internal resistance of the guitar, or the resistance of the pickup.
In the tone circuit, the pot actually sits above the capacitor C, but below the branch to the output Vout, as shown at left. So, the formula for the cutoff frequency is more complex. In this analysis by a guitarist/mathematician named Bill, he suggests a formula for the cutoff frequency as follows:
How’s that for insanely non-intuitive?! Bill points out that the lower square root term only works with tone resistances less than about 20k (since otherwise the value would go negative producing imaginary numbers in the square root), thus explaining the often limited useful range of tone pots, and why log taper pots are more useful for tone than linear. Nevertheless, this seems to be an over-idealized formula, since in practice, I do see more variation in the tone pot even at higher resistances. This formula doesn’t seem to capture the full complexity of the reactive network made up of pickup inductor, and overall circuit resistance and capacitance (including cable capacitance).
Ok, so math is clearly the wrong way to think about this!! Too complicated! Back to the original question. Let’s think of it more simply. Imagine you turn both pots down to zero- you’re basically eliminating the variable resistance pot and wiring the cap directly to ground. Of course, the larger capacitance .02uF will sound darker than the .01uF. So they’re obviously not equivalent circuits.
Next up, experiment! Grab a couple pots, caps and some alligator leads and try it out! You’ll find that they do indeed sound quite different. The larger capacitance with the smaller pot resistance sounds darker, no matter how you slice it, when compared to the 500k pot and .01uF cap. Even with both pots up full, the larger capacitance with the smaller pot sounds a bit darker.
This all begs the question, why do guitar manufacturers often pair a 250k tone pot with a .047uF cap, versus the .022uF cap with 500k pots? The former will produce a darker sound both because of the larger capacitance but also because of the increased load on the pickup from the smaller resistance. “Double whammy” as Bill points out at the end of his paper.
Let’s take a look at push/pull potentiometers. Shown here are three specimens- Bourns PDB183-GTR01-504A2, Gibson PPAT520, and AllParts EP 4286-000. These are all 500k audio taper pots with knurled split shafts. The EP4286 has a longer 3/4” shaft, while the others have the standard 3/8” or “short” shaft. This Bourns has the highest torque of the bunch– it’s the hardest to turn, while the Gibson is the easiest, and the AllParts has just a touch of mechanical graininess to the feel.
A push/pull pot is just a potentiometer sitting on top of a DPDT switch. The switch actuator is the shaft of the pot itself, which drives down right into the switch. When you pull the shaft up, you are moving the switch actuator to connect the top half of the switch, and when you push the shaft down, you are moving the switch actuator down to connect the bottom half of the switch. There’s no built-in electrical connection between the pot and the switch – if you want the switch to control the pot’s behavior, you need to connect up some wires (as in the example described at the end of this article).
A while back, I described some problems with trying to use treble bleed on this guitar with three volume pots and a 3-way switch. When turning down one volume pot, that pickup retains brightness, but the other pickups get duller. In this diagram, with middle and neck pickups in a blended switch position, the middle volume is up full and the neck volume is turned down. The middle pickup high frequencies have a path to ground through the neck treble bleed cap. More
Nearly every day, I receive a question like “I have a Custom Slender Flatoblaster with Fleemore Dunkin pickups. What brand and values of pots and capacitors should I buy to get a killer sound for Jazz Country Electro Fusion Metal Ambient Rockabilly?”
I’ve tried to be clear in my posts and videos evaluating guitar electronics, that these things are really subjective and a matter of personal preference. I could tell you that I like CTS brand EP0086 500k audio taper pots for both volume and tone, and Orange Drop 715P .01uF tone capacitors, and these work really well for me. But, you may buy these same components and hate them. More
Today, let’s take a look at some really fancy boutique tone capacitors. You can hear these in the yesterday’s tone cap shootout.
The Luxe Grey Tiger is billed as “a faithful recreation of the famous Cornell-Dubilier Grey Tiger from 1956” and typically sells for about $40.
The Gibson Bumblebee is marketed as being “specially designed to replicate the original parts used by Gibson in the late 1950s”, and typically sells for over $100 for a 2-pack.
Have you ever wondered what special manufacturing and fabrication techniques they use to make these ultra-boutique capacitors?
Well, Steve over at Kernel of Wisdom has taken a knife to the little guys. And what have we here? Inside a Gibson Repro Bumblebee is really a Wesco polypropylene film cap, all wrapped up in black and stripes And inside a Luxe Grey Tiger, we find a General Instruments PIO cap.
Consider that a typical polypropylene film cap sells for maybe fifty cents. Gibson is selling this for about $50, so let’s see— that’s only a about a 10,000% markup 🙂
Here’s an interesting letter from 2004 about the reissue bumblebees from Edwin Wilson, Historic Program Manager at Gibson, as well as another tear-down of the reissue bumblebee.
This is not to say that these caps don’t sound good. However, what is clear to me (as if it wasn’t clear already) is that there is very little reason to spend this kind of money on a capacitor, unless you’ve got money to burn and it gives you warm fuzzies inside 🙂
In the ring this time are a Russian T-1 Teflon, Russian K4Y-9 PIO, Cornell-Dubilier PIO, Goodall PIO, Luxe Repro Grey Tiger, Gibson Repro Bumblebee, Sprague Vitamin Q, Jensen PIO, Sprague Orange Drop as well as a generic brown polyester film and ceramic disc caps.
He also has a blind comparison page (don’t peek at the answer key til after you listen).
In my tone cap material types comparison, there were a couple vintage caps which I couldn’t identify. I referred to them anonymously as the Yellow Cylinder and the Fat Gray Cylinder.
Today, a kind gentleman by the name of Bill Zumwalt was able to give me the clues I needed to track these old caps down and unmask their true identities. The search keywords I needed were CDE and Mustard! More