*FAIR USE* Build the EM Distortion Box Build the EM Distortion Box The last project by Charles R. Fischer that you will ever build. By Peter Miller Charles R. Fischer's byline first appeared in EM's pages in February 1987 with his "Build a Slide Pot Ribbon Controller" do-it-yourself project. Between that first contribution and his final offering, "Build the EM Optical Theremin" (May 1995), he produced a steady stream of EM reviews, applications articles, and especially do-it-yourself projects. His "Build the EM Fingerdrum" (March 1992) and "MIDI Mods for Your Minimoog" (September 1989) show Fischer at his DIY best; to appreciate the appeal of his articles is to understand the early roots of Electronic Musician. The Spectra Distortion Generator was Fischer's last invention before he died of a heart attack in July 1996, and he was convinced that its unusual Spectra control would make it his most popular. He had originally intended to market the product to an established manufacturer, but illness prevented his doing so. When Chuck Fischer passed away, he left the prototype Spectra Distortion Generator to his close friend, EM Editor Steve Oppenheimer, who asked me to reverse-engineer it and write this article in Fischer's memory. Steve O. knew that Fischer had wanted you, his fellow electronic musicians, to enjoy his invention, and I was only too happy to comply. Distinctive Design This cleverly designed guitar-distortion box uses the 5534 op amp as its basic building block. (See the sidebar "Parts List" for specs on all components.) One 5534 is used as the input buffer and initial gain stage. A digital inverter is used as the "spectra tone generator," and an adjustable current regulator powers the IC governing the spectra tone. The second 5534 is a combined tone control and output buffer. The Treble control lets you soften the distortion a bit, and the Master control governs the overall output level. First, we need an input buffer to isolate the guitar from the circuit. The input buffer (A1) uses one of the 5534 op amps (see Fig. 1), which is configured to work with a single power-supply voltage by applying a bias to the op amp. The bias is set so the output of A1 is at a DC potential of half the supply voltage. Resistors R10 and R11 divide the power supply voltage, and capacitor C8 filters this bias voltage, which is applied to the noninverting input of A1. Trim pot VR1 is in the feedback loop of the amplifier and therefore sets the gain of the input buffer to match your guitar rig. It has a gain range from unity to 105. Capacitor C9 is used to block the DC component and pass the audio to P1, which is the front-panel Drive control that feeds the right amount of signal into the spectra circuit. Fischer used an MC14049UB digital CMOS inverter IC chip (A3) to amplify the audio. Normally, this IC only knows how to speak digital (i.e., ones and zeros), but Fischer used internal trimmer VR2 to bias the inverter into a somewhat linear mode of operation. The feedback is adjustable, giving you some control over the harmonic content. Capacitor C14 limits the high-frequency response to a reasonable cutoff point. Like C9, C12 blocks the DC potential and passes the audio to the tone-control circuit. In addition, the spectra circuit is juiced up with an adjustable current regulator (IC3) that powers the inverter (see Fig. 2). Don't confuse this with a voltage-regulator chip; IC3 is an LM334Z current regulator. Front-panel pot P2 is the Spectra control, which sets the amount of current fed to A3. This limits the power-supply current, which produces a funky little compression effect. The second 5534 op amp (A2; see Fig. 1) is used as the tone control/ouput buffer. The tone control is a simple circuit that employs a pot (P3) and capacitor (C11) to vary the frequency response of this stage. One end of the pot is connected to the output of the op amp, and the other end is connected to the Spectra stage. The wiper goes through C11 to the inverting input of the op amp. When the pot is set to one extreme, the capacitor bypasses the input resistor at higher frequencies; at the other extreme, the capacitor changes the compensation in the feedback loop of the op amp, thus changing the high-fre quency response of the amplifier. This topology yields a wide range of tone adjustment. Pin 6 of the output buffer is connected through C13 to the Master level pot on the front panel for setting the overall output level. The Spectra Distortion Generator has a true hardwired bypass that uses a small DIP relay (RY1) to switch the distortion out completely. This relay is controlled by means of a Motorola MC1455P timer chip (IC2; see Fig. 2). This is a clone of the 555 timer IC with a few extra bells and whistles. Fischer configured it as a simple relay driver; the precision timing features of this chip are not used here. A front-panel switch (S1) and a 1Ž4-inch phone jack (J4) for a footswitch let you control the relay-driver circuit that switches the effect in and out. A 78L05 +5V low-power voltage regulator (IC1) powers the relay-bypass circuit and the LED power indicator (D3). This project requires a high-quality, regulated power supply for the audio portion; Fischer selected an LM7812T 3-terminal voltage regulator (IC4) for this purpose. It takes the unregulated voltage from a wall-wart power supply (15V to 25V, 200 mA) and maintains a fixed output of +12V, which is required to operate the circuit. When configuring the wall-wart connector (J7), make sure to get an adapter with the correct polarity (tip = negative). Diode D1 protects the circuits from reverse polarity, and C1 and C2 smooth the incoming power. Straightforward Construction Fischer used a RadioShack 276-188 prototyping board with 44 edgeconnector fingers, 22 on each side of the board. These connections are labeled with numbers on one side and letters on the other. In the accompanying schematic diagrams, the rectangular boxes call out the appropriate edge-connector pins. The card also has foil traces on one side, which are used to supply DC power and ground paths. The board unplugs from the chassis (J3) for repairs or modifications. A connector for the front-panel pots was made from a header-plug ribbon wire and a 16-pin IC socket (J1). In the accompanying schematic diagrams, the square boxes indicate the pin numbers of this connector. The components of the circuit are mounted by inserting the leads through the board and soldering them to the foil traces and pads (see Fig. 3). Jumpers are used on the component side to make connections between traces in order to form the whole circuit. Note that RadioShack has discontinued the 276-188 board and offers no replacement. However, you can substitute the Vector 3677-2 prototyping board from Allied Electronics (tel. 800/ 433-5700; Web www.allied.avnet.com). The enclosure is a plastic prototyping case. I found some suitable cases in the Jameco catalog. The controls and jacks are positioned as shown (see Figs. 4 and 5). Metal Shredding Like any other guitar-effects box, the Spectra Distortion Generator can be placed in the signal chain at the beginning (i.e., between the guitar and the input of the amplifier) or in an effects loop. However, line-level loops will overdrive this unit, and it won't sound right, so use it only with guitar-level signals. I invited some guitar players to evaluate the sonic qualities of the Spectra Distortion Box. A typical comment was "It has a fairly wide range of solid-state-type distortion textures, very reminiscent of 1970s styles. Depending on the settings, you can get a sound similar to the MXR Distortion+. Other settings make it sound like the old Sunn Beta amplifier." This makes a lot of sense, because Sunn also used a digital IC biased for linear operation in its overdrive circuits. The really cool departure from these traditional distortion sounds is the Spectra control. By setting just the right amount of Drive and Spectra distortion, you get that MXR Distortion+ sound with a funky compressor built in. It reminds me of the Neil Young tune "Hey Hey, My My," but Fischer's Spectra Distortion Generator has qualities not available in any of my stomp boxes. What's going on to produce this effect? When the digital chip is driven to draw enough current from the power supply, it causes the regulator chip to start limiting the current. In addition to changing the waveform in some unique ways, it also limits the amplitude, thus compressing the signal at the same time. Of course, as is usually the case, you can have too much of a good thing. We found some settings that are not very usable in our application, but the Fischer Spectra Distortion Generator is definitely worth experimenting with. Don't Hold Back If you know nothing else about Chuck Fischer, just keep in mind that he was a stubbornly independent man who refused to follow the crowd. He even insisted on having his middle initial in his byline&emdash;Charles R. Fischer&emdash;just because he didn't want to be confused with another electronics writer named Charles Fischer, whose work he disliked. This determination to follow his own muse, come what may, was at the root of Fischer's creative talent, as well as his wisecracking, irreverent personality. (His San Francisco Bay Area "country" band was once booked into a quiet town festival, and once onstage, unveiled itself as the loud, protogrunge Jim Jones and the People's Temple, throwing packs of Kool-Aid to a stunned audience!) It should come as no surprise, then, that he preferred to invent gadgets with unusual twists, like the Spectra Distortion Generator. So enjoy building this legacy from one of EM's classic authors&emdash;then crank it up and let 'er rip! If you don't send the dog and cat running for cover and freak out your roommates and neighbors, you haven't gone far enough. Peter Miller has specialized in the repair of electronic musical instruments for more than 30 years. He has owned CAE Sound since 1980 and has designed custom audio electronics for groups such as Tuck and Patti, Counting Crows, and the Grateful Dead. parts list Integrated Circuits A1, A2 NE5534 A3 MC14049UB IC1 78L05 (package code TO 92) IC2 MC1455P (Motorola) IC3 LM334Z IC4 LM7812T RY1 LR4512 relay Resistors (1Ž4W, 5%) R1 270‡ R2, R3, R10, R11 10 k‡ R4 1 M‡ R5 1 k‡ R6, R7 20 k‡ R8, R9 2.2 k‡ R12 47‡ R13 100‡ Capacitors C1 1000 µF, 25V C2 22 µF, 25V C3 50 µF, 25V C4, C5, C12 1 µF, 50V C6, C7 0.1 µF, 100V C8 10 µF, 16V C9 47 µF, 16V C10, C15 1 µF, 25V tantalum C11 0.0056 µF, 100V C13 10 µF, 20V tantalum C14 680 pF, 100V Potentiometers P1 100 k‡ audio P2 5 k‡ linear P3 100 k‡ linear P4 10 k‡ audio VR1 100 k‡ trim pot VR2 500 k‡ trim pot Diodes D1 1N4001 D2 1N4148 D3 Red LED (package code T1 1Ž3) Connectors J1 16-pin IC socket J2 2-pin (AMP) J3 44 ckt PC card edge connector J4 Switchcraft #11 (Remote jack) J5 Switchcraft #12 (In jack) J6 Switchcraft #11 (Out jack) J7 2.5 mm (Power jack) Miscellaneous S1 SPST push on/push off PWB1 PC board (Allied Vector 3677-2) Case Jameco or equivalent