Dan Kropp's Sonic Nuance Tuner + DI (TDI) Mk2 Pedal The Sonic Nuance TDI is a combination isolation transformer and tuner. This pedal will electrically isolate your instrument from the rest of your gear. With the mute engaged, it also acts as a tuner. It also has a very high quality transformer-generated microphone level output. A run through the test bench will give some hard facts as to how well this pedal stands up to expectations. Internal It only took a few seconds to get into the inside of the case, which is cast aluminum, approximately .08” thick. The top and sides are one piece, with a removable bottom that is machined for a snug fit, and even has cast reinforcements. Four Philips-head screws hold the bottom panel in place. Inside the case, as shown in Fig. 1, there are two circuit boards. Two nicely trimmed ribbon cables connect everything together. The boards are laid out nicely to avoid extra wires and keep ribbon cables as short as possible. Everything in the case is board-mounted, except the large foot switch, which is securely mounted to the case. The XLR output and both ¼” jacks are attached to the case, as well as to the board. The attention to detail is very good. There is even a nyloc nut holding the circuit board place. Operation The large foot switch on the top toggles the operation of this box from pass-through mode to mute/tuner mode. In pass-through mode, the output ¼” jack is resistively tied to the input, and the XLR output is a microphone-level duplicate of the input. In mute/tuner mode, the ¼” jack and XLR output are muted (and physically disconnected from the input). If there is power to the box (either via phantom power from the XLR connector, or from the DC adapter input) and the power button is on, then the tuner circuit will be active, as well. The isolation transformer and mute mode will work without power supplied; the tuner will not. Also, without power, there is no feedback on what setting the footswitch is on. Trial and error is the only way to determine if it is in pass-through or mute mode. If there is power applied and the power switch is on, then the LED display will be active. In pass-through mode, there will be a “P” displayed. In mute/tuner mode, the note that is detected on the input will be displayed and the series of red and green LEDs under the display will indicate in tune, or the amount that the note is sharp or flat. It is worth noting that in mute/tuner mode, the display will continue to show the last note it could tune after the input to the box goes away. For this pedal, the operating range for the tuner is 20.1Hz to 660Hz, so the impedance test was changed to use a 660Hz sine wave at 100 mV as the input. The measured input impedance is 67.1 kOhms. As an added check, the impedance was also checked and 20Hz and 300Hz, which resulted in less than a 2% variance across frequencies. Fig. 2 shows a guitar sample wave fed into the pedal. The input and output are indeed almost identical, and the smaller amplitude wave is the XLR output. A close examination of the wave showed a less than 1 mV negative shift on the output with identical amplitude. Fig. 3 and Fig. 4 show a breakdown of the component parts of a sine wave fed into the pedal and an equivalent breakdown of the outputs. Both graphs show the output and input levels for the target frequencies to be identical. There are very minor differences in some of the component levels of the input and output, but keep in mind that these are logarithmic scales, so the differences that are shown are in the micro- and nano-volt range. Fig. 5 is the THD for frequencies from 20Hz to 1kHz. There is slightly higher THD+N across all frequencies on both the ¼” output and XLR. Once again, keep in mind the scales, here. The XLR output hovers around 0.01%, while the output and input hover below 0.001%. These are all very low numbers and unlikely to be noticeable. Impedance In pass-through mode, this pedal will influence the input impedance seen by the instrument. Even though the signal passes straight through the pedal, there is an element of impedance put into the circuit by the transformer that creates the signal for the XLR connector. Because the transformer is connected in parallel across the signal wires, it will always lower the overall input impedance seen by the instrument. The amount of parallel input impedance added to the circuit is directly dependent on the input impedance of the load on the other end of the XLR cable. In summary, as far as impedance goes, this pedal effectively puts the XLR connector in parallel with the ¼” output jack, creating two parallel loads for the instrument. For testing, a 660Hz sine wave at 100 mVrms was used as stimulus. The ¼” output jack was connected to a 98.7 kOhm load, and the XLR was connected to a 612 Ohm load. This combination created an input impedance of 42.1 kOhm, as seen by an instrument. If the XLR connector load was disconnected, leaving an open circuit, then the input impedance seen by the instrument was 71.1 kOhms. Through some math, the impedance of the pedal in parallel with the load on the ¼” jack output is 73.1 kOhms with a 612 Ohm XLR load attached, and 262.1 kOhms with and open XLR circuit. Tuner The range of the tuner is advertised to be from E0 to E5, which is 20.602Hz to 659.255Hz. The accuracy is listed as +/- 2 cents initially, and then +/- 1 cent, as the amplitude of the note decays over time. All note frequencies are based off A4 at 440Hz. For this test, a constant sine wave was used as stimulus. Because the sine wave amplitude did not decay, the +/- 1 cent accuracy was never initiated. All results were in the +/- 2 cent range. During regular use, it may be useful to have the accuracy get more exact with a degrading input amplitude, but to keep consistent test results, the test stimulus must be kept at a reproducible standard level. Through testing, the lower limit that is readable by the tuning circuit is 20.1Hz. Anything lower, and the tuner does not register a note. That should be plenty of room to tune E0, since 5 cents flat on E0 is still 20.542Hz, which is well within the tuning range of the circuit. There is more headroom on the other end of the scale. At A5, which is 880Hz, the tuner still provided accurate results. Anything higher than A5, and the tuner will still read that there is a note, but the accuracy drops off to the point of not being usable. Random notes were selected to check the accuracy of the tuner. The following is a sample of the results from checking notes. E0, C2 and A4 show in tune from -1 to +1 cents. D#3 shows in tune -1 to 0 cents. A5 shows in tune -2 to +1 cents. All notes tested fell well within the advertised +/- 2 cents accuracy. On all tested notes, once the note was showing out of tune, one red light would illuminate dimly, at first. Then, as the note went more out of tune, the red light would come to full brightness and then another light would illuminate. This worked the same on the sharp and flat side. Conclusion This is one seriously sturdy and well-built pedal. Inside and out, this pedal is made to hold up and perform for a long time. The pass-through transformer provides very clean electrical isolation and the tuner accuracy is right where it should be.
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