Analog audio—in both the mechanical and electronic domains—is slow and easy to understand. The sound of connecting a battery or an amplified kick drum to a loudspeaker emphasizes the keywords impulse, reaction/response time and resonance—all of which can happen at any or all frequencies, from radio and video all the way to light. Impedance is an equal-opportunity vector, equally popular in the data communications realm. Surely, you’ve encountered an SCSI terminator?
Walk into Radio Shack for antenna wire, and a knowledgeable salesperson should ask, “300-ohm or 75-ohm?” In this case, the assumed frequency spectrum includes FM radio and broadcast television (88 MHz and beyond). Digital audio’s S/PDIF interface is equivalent in bandwidth and impedance to line-level analog video (6 MHz and 75 ohms, respectively).
To further study the effects of impedance requires math. Plotting a graph of impedance and phase requires several formulae plus multiple calculations at as many frequencies as possible (enough to represent the audible spectrum, for example). Fortunately, I found the perfect application (“Micro-Cap” from www.spectrum-soft.com), which was available as a demo as a free download. I’d still be ciphering if it weren’t for this handy bit of technology, so I’ll spare you the math entirely this time around.
EXAMPLE 2:
MAGIC CABLE
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Figure 2: Cable capacitance can load down vulnerable output amplifier designs (click for larger view). |
Figure 2 shows the effect of cable capacitance on the frequency response of vulnerable equipment. The “inset,” a schematic of the 38’s output circuit, includes a very guilty 1-kilohm resistor (R117) following the op amp. The purpose of this resistor is to protect the output amplifier from accidental short circuits, as well as to provide a “bias trap,” a filter network designed to stop high-frequency bias leakage that could potentially damage tweeters. (Bias is well beyond hearing range, but a little leakage could potentially become a stealth tweeter eater.)
I didn’t carry a capacitance meter on service calls, but this particular customer chose the cheapest possible cable solution, sending me on a minor detour. Back in the lab, several cable tests yielded a typical range of 50 pico-Farads per foot (pF/ft) to a low of 20 pF/ft, this being for foil-shielded audio cable and wire-shielded computer video cable, respectively. These are acceptable values.
FEED THE KITTY
I fed the Tascam 38 output circuit values into Micro-Cap, the essence of which is a simple RC (resistor-capacitor) circuit consisting of R117, a 1-kil- ohm resistor feeding the interconnecting cable as represented by a capacitor to ground (not shown). The starting value of capacitance was based on 100 pF/ft for 10 feet of cable, incremented in 10-foot steps ending at 100 feet of cable. The resulting capacitance ranged from 1,000 pF to 10,000 pF (or 0.01µF), respectively. (The actual circuit includes L102 and a pair of 470pF caps to “trap” the bias signal.)
A simple RC circuit is a first-order lowpass filter (at audio frequencies) with a slope of 6 dB per octave. (A second-order filter has a slope of 12 dB/octave.) An abnormally high capacitance was chosen to simulate what happens when bad cable alone is interfaced with a vulnerable piece of equipment. Note the “box” indicating 10 kHz being 2.5 dB down, the approximate amount noticed during the house call.
I am not suggesting esoteric audiophile cable, only that the results from the “lab test” should serve as your guide when cable shopping; contact the cable manufacturer for such minutiae as cable capacitance. Also, most modern equipment is not sensitive to cable loading, as was the old Model 38. The solution would have been to add one more op amp per channel to isolate the bias trap from the outside world. Collect schematics for your gear and compare output amplifier circuits with your friends. Who knows, it could be like Pokémon for adult geeks.
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Reprinted with permission from Eddie Ciletti, Tangible Technology, June, 2001
© 2001, All Rights Reserved
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