Efecto de guitarra

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NeoVibe, a Univibe (tm)Workalike
The Univibe is one of the trademark effects used by Jimi Hendrix and Robin Trower, just to name a couple of the more famous users. The Univibe is at its heart, a four stage phase shifter, not much different in concept from the common MXR Phase 90. It is, however, an earlier-technology implementation, built from transistors and not opamps; the “imperfections” ofthe transistor phase shifting stage show through clearly, and produce a unique sound. Note that there are several reissue and clones of the Univibe on the market now. This unit replicates the circuit of the old original as manufactured by Unicord. It is not and should not be confused with any of the reissues or other clones; it is intended only as a DIY project. The term “Univibe” is a trademark ofthe current owner (I think Dunlop), and remains such.

How does it work?
If you can, go to http://www.geofex.com and read “The Technology of the Univibe” for a much more in-depth description of how the original Univibe worked. The Univibe was an interesting way to get unusual sounds out of simple circuitry. The first three transistors (Q1, 2, 3 in the Neovibe schematic) form a feedback preampwith an overall gain of about three, and with complementary outputs at the collector and emitter of Q3. The collector signal goes through capacitor C5 only, and the other through one of the photoresistors. The two are mixed back together at the base of the first darlington-connected signal stage (Q4). The addition of the two would cancel to no signal at all if the signal through the capacitor wasnot phase shifted by the C5. As it is, the two signals add back together to make a single signal whose phase is shifted an amount depending on the value of the photoresistor and the frequency of the signal. The darlington stage (Q4 and Q5) also has complementary outputs, and feeds a similar circuit following it. After a total of four of the variable phase shift stages (one per photocell) thesignal is mixed back to a single signal which is buffered by the last signal-path transistor, Q10, and is available on Q10’s emitter. This phase-shifted signal is then routed to the chorus/vibrato switch where you can select vibrato by NOT mixing any of the unprocessed signal with it (your ear hears the phase shift as a pitch shift) or “chorus” by selecting the signal which mixes unprocessed and phaseshifted signals together. This “chorus” is not the chorus you’d hear from a chorus pedal, but actually more like a phase shifter output. The phase shift networks in the Univibe are staggered in frequency, as each of the phase shift caps are a different value. This is unusual for a phase shifter design and was perhaps done so that there would be a notable amount of phase shift for any audio signalat any point in the LFO sweep. This would be consistent with the “vibrato” setting. The Low Frequency Oscillator (LFO), which sweeps the value of the photocell resistance is also an unusual design. The thing is a phase shift oscillator, implemented oddly. It exhibits a rising output level with rising speed, which seems to compensate for the fall off in response of the incandescent lamp’s responseto rising speed. To get the thing running properly, you’ll need to adjust the idle bias on the lamp. That is the purpose of the PCB mounted trim pot TR1. Some Univibes I’ve seen dispense with the pot altogether, and just use a 150 ohm resistor. Since the light bulb you get may vary a lot, I recommend using the pot and adjusting it for a dim orange glow with the amplitude pot all the way down. I’veincluded typical voltages at various points in the schematic to help with the initial troubleshooting.

Construction Details Unique To The ‘Vibe
For the most part, building the Neovibe board is like other effects boards – image and etch the board, drill the holes and populate it. There are some tips that may help you. For one, all of the resistors and jumpers on the board have lead spacing...
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