Planet Z

I was reading the news on Zite this morning, and serendip’d upon this article in The Strad which describes a double-blind test with violinists, comparing several modern violins to those made by Stradivari and a del Gesu.  The scandalously shocking surprise was that many accomplished players unknowingly preferred the modern instruments over the high-value vintage rarities.

Now, I’m not a violin player, but I can appreciate a story about challenging the preconceptions of value in hyper-expensive vintage items over well-crafted lower-priced contemporaries.  It’s not so much that they were pitting the modern luthiers against the celebrated ancients;  these were doubtless all well crafted instruments.  The experiment was really challenging the human perception and preconceptions of the study participants.

It’s an interesting philosophical/psychological question whether it’s even possible for the human brain to be objective about anything.  There is a disconnect between perception and reality—our brain is an imperfect interface to the world around us, doing its best to interpret the signals it receives, and occasionally totally falling down on the job.

I’m sure you’ve seen these great mind-bending optical illusions, like the spinning dancer above (which way is she really spinning?  I see her spinning counter-clockwise, but my wife and kids see her spinning clockwise), or the Ebbinghaus Illusion at left (yes, the orange balls are actually the same size).

But have you heard any good auditory illusions lately?  Try these:

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The new Mobile In iOS guitar interface and Mobile POD app from Line 6 claims to put the POD sound library in your iPhone or iPad.

This interface connects to the 30-pin dock, which as I described in my DIY iOS interface project, is the higher-fidelity lower-noise way to get audio into your iPhone/iPad.  This is because there is a high-quality 24-bit 48kHz D/A in the interface itself, and the signal is transferred digitally into the iDevice.

Again, my main concern with this type of device is that it hangs off the bottom of your iPhone/iPad on that delicate dock connector, with the guitar cable plugged into that.   It makes me nervous that the inevitable kick-the-cord accident will brick your $600 iPad.  Also, the iPhone/iPad can’t charge while this is plugged in, so you have to make sure you’re fully charged up.

At $79.99, this is cheaper than the other digital guitar interfaces (Apogee Jam, Sonoma GuitarJack, etc).   The iOS app is free, but is tied to the hardware.  You currently can’t use the app with another interface.

Intriguing, but I’m not rushing out immediately to get one.  Are you?

I wrote about my love of looping back in 2010, when the new JamMan Solo and JamMan Stereo were announced. 

Now, I’ve finally replaced my old Oberheim Echoplex Digital Pro with a shiny new JamMan Delay.

This unique pedal combines a looper with a nice delay modeler.   It doesn’t do everything my old Echoplex did, but it’s a nice leap forward in technology- with stereo recording, tons of memory (and SD card expansion), stereo recording, USB connectivity and software librarian, not to mention some really nice delays.  I’ll talk about the features I’m missing in another post.

But before we get to happiness, let’s first talk about a major manufacturing issue, nearly a total deal-breaker.   If I hadn’t figured out a solution, I would have had to return the JamMan Delay and seek looper nirvana elsewhere.

When I first powered up the JamMan Delay and plugged it in to my Vox AC15, I was greeted with a faint but persistent “beep beep beep” sound, in time with the flashing tempo LED.  

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It’s an interesting tale of intrigue, the discovery that this old dusty Vox UL730 belonged to George Harrison.  It was used during the recording of Revolver and Sgt. Peppers.

You want that sound?  Buy this amp at auction!  It’ll only cost you about $100,000.

Update 1/4/2012: I went back to see how much the amp sold for, and it turns out the auction was withdrawn- the mystery owner decided to keep it.

Read more about this amp at the vox showroom.

… November 29. Rest in peace, George, 10 years now gone …

Update: 10/19/2011, added links to part #s at radioshack, mouser and mammoth.

Here’s the exciting conclusion to the series, following up the intro in Part 1, and the electronics course in Part 2.  

The main goal here is to cram all the parts into the narrow confines of the jack, so we don’t need to use an external box or enclosure. 

In addition to the cable and jack parts listed in Part 1, here are some of the things you’ll need:

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Following up on Part 1, it’s time now to get into the heart of the project:

In this video, I talk about how JFET’s work, then work up a circuit diagram (as shown at right). 

Then, I prototype the circuit on the breadboard of my Radioshack Electronics Learning Lab, and finally play through the circuit to show how it sounds.

In Part 3, I’ll demonstrate how to assemble the circuit so that it fits entirely inside the jack.

Here are some useful resources and background reading:

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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.

I’ll show you why a simple unbuffered cable interconnect will sound terrible- because of the significant impedance mismatch between a passive guitar circuit and the iDevice mic jack.  Also, the iDevice provides a 2.8V DC on the mic input to drive a microphone preamp, and as you’ll hear, this voltage totally screws up your guitar circuit. 

This isn’t intended to be an ultra-high-fidelity interface. But trust me, it sounds good and costs very little. And the principals learned in this simple electronics project are the same as those required to make a guitar boost pedal like the Fulltone Fatboost.

In parts 2 and 3, I’ll explain all the electronics and show you how to assemble the interface, but for now, let’s get started:

And here’s some of the things you’ll need:

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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.

Source: Keiko corporation website

Here’s another excellent article on guitar pickups, courtesy of Pete Biltoft at Vintage Vibe Guitars.   Thanks Pete for the permission to post this here!

August 2011

DC resistance measurements are widely used as a gauge of the “output” of passive magnetic pickups. This use of DC resistance is both technically incorrect and often misleading; to find out why, read on…

In its most basic form a single coil magnetic pickup consists of a permanent magnet or set of magnets typically housed in an electrically insulating coil form (commonly referred to as a bobbin) and a coil of electrically insulated wire wound onto the bobbin (see Figure 1).

Figure 1. Simplified drawing of an unwound and a wound single coil pickup bobbin.

In operation, the vibrating strings of the instrument (which contain iron and nickel, both magnetic materials) cause the magnetic field of the pickup magnet(s) to fluctuate.

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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.