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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING FET Distortion Effect Devices. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Musician The article discusses amplifying devices that implement the Distortion effect for an electric guitar. This is the most used electro-acoustic effect that guitarists have been using for many years, and its sound is familiar even to a novice player. Currently, there are many types of devices that implement this effect, but they all differ in circuit design and give the sound a different shade. The described devices are similar in sound to the Distortion effect achieved in similar designs on electronic tubes, but they are made on field-effect transistors. Compared to bipolar transistors and vacuum tubes, field-effect transistors (FETs) with a pn junction have a number of positive qualities: high input resistance, low voltage power supply (for ease of use, most of these devices are powered by a compact 9 V battery), low noise, low non-linearity passing characteristics. Perhaps the main drawback of this group of transistors is a significant spread of parameters within even one batch, and this creates certain difficulties when debugging the device. Device nodes The input amplifier pre-amplifies the guitar signal, carrying out at the same time (but not always) frequency signal processing: a decrease in the frequency response at frequencies below 100 ... 700 Hz or a selection of a frequency band in the region of 0,6 ... ). When using PTs, it is more reasonable to use a "tube" approach to the issues of timbre signal processing, i.e., use only simple RC filters to form the timbre.
After the input amplifier, the signal is processed by a limiter. To build a "pseudo-tube" limiter (regardless of the style of the music being played), the cascode amplifier-limiter circuit shown in Fig. 1. Since such a stage is capable of providing high gain, even one of it is enough to get an overdrive with a beautiful and pleasant clipping and high sensitivity. In positions VT2 and VT4 it is desirable to use a FET with a cutoff voltage UOTC = 2...3 V (cut-off should be within 202 ... 2 V). The best gain results in the cascade are obtained when the cutoff voltage of transistors VT5458, VT1 is approximately three times higher than that of VT3, VT201. The diode in the source circuit VT1 is used to increase the maximum value of the input signal to a range of approximately 2 V. The frequency response of the cascade according to the scheme of Fig. 1 at a gain of about 3000 has a roll-off of 6 dB per octave at frequencies above 10 kHz. In the L * 5 position, you should not use a FET with a large input capacitance, for example, 2SK117 and the like, since the cutoff frequency can decrease to 3 kHz. The use of low-noise transistors contributes to the reduction of the level of intrinsic noise. In positions VT1 and VT3, KPZOZA, KPZOZB are most suitable; transistors KPZOZZH with similar current-voltage characteristics in the band 80 ... 5000 Hz, the noise level is approximately 2 ... 3 times higher. Since the voltage gain of the transistor VT1 is small, the correct choice of transistors VT2, VT4, for example, low-noise KPZOZG, is of great importance for minimizing the noise of the entire cascade; its noise EMF does not exceed 0,3 μV (in the band 80 ... 5000 Hz). KPZODZD, KPZOZE, as a rule, have a high interface frequency of excess noise and therefore their use is undesirable (noise EMF up to 1,5 μV). For the same reason, the use of KP302 series PTs is undesirable. Another advantage of domestic PTs of the KPZOZ series is a metal case with a separate terminal, which also helps in the fight against interference. This cascade is insensitive to voltage ripples, but is sensitive to interference from the AC mains, so it must be placed in a metal screen, and the output of the VT1 transistor case is connected to a common wire. The cases of the remaining transistors of the KPZOZ series can also be connected to a common wire.
It is advisable to connect a tone block to the output of the limiter. It can be assembled according to classical schemes. used in devices of well-known companies Marshal, Fender (Fig. 2, a, b shows three-band regulators) or use simpler options that change the spectrum in two frequency bands (diagrams in Fig. 2, c, d).
After tone controls with a large output impedance, it is always useful to install a repeater also on the FET, a variant of which is shown in Fig. 3, as the simplest, with a direct connection to the tone block resistors. On fig. 4 shows a repeater with a current generator and an additional RC frequency response correction circuit. Here, the capacitance of the capacitor is chosen based on the spectrum of the sound of the guitar and the need to limit it from below.
If you intend to use the instrument with a guitar acoustic system (AC), which musicians call a "cabinet", then at the output of this repeater you should set the level control (its options are shown in the following diagrams) and calm down on this. If the speakers are ordinary (broadband), then it is useful to pass the signal from the output of the repeater through a low-pass filter, which attenuates frequencies above 5 kHz. Best results are obtained when using Bessel or Butterworth filters of the third order or more with a roll-off of 18 dB per octave or more. The circuits, as well as the formulas for calculating the cutoff frequencies, of such filters on voltage followers made on the OS are known [1, 2], so they are not given here. Instead of an op-amp, repeaters on the PT can be put in such filters. It should only be borne in mind that the output impedance of the repeaters on the FET is 0,2 ... 1 kOhm, and it is better to choose the values \u47b\u470bof the resistors in the filters in the range of XNUMX ... XNUMX kOhm.
If you need to play "in line" in any block of the Distortion effect, it is useful to add a "cabinet emulator" that forms the frequency response of a certain type. It can be assembled entirely on field-effect transistors, for example, according to the circuit in Fig. 5. At the sources of the FET, the voltage should be +4,5 V. To reduce the sensitivity to pickups, the transistor case should also be connected here to a common wire. The advantage of using a FET instead of an op amp here is the softness of the overload limit, which is symmetrical when using complementary FETs. To overload the device, the input voltage must be at least 4...5 Vp-p. On transistors VT1, VT2, a band-pass filter is assembled with some semblance of "resonance" in the 100 Hz region. You can shift the center frequency of this filter, for example, down, by proportionally increasing the capacitance of capacitors C1, C2. The height of the low-frequency "resonance" can be reduced by increasing the resistance of the resistor R3, and the cutoff frequency of the filter will also decrease. On transistors VT3-VT6, a fourth-order low-pass filter is assembled with a rise in the region of 3 ... 4 kHz and a slope of 24 dB per octave at frequencies above 5 kHz. The frequency response of this node is similar to the characteristics of emulators in the well-known Sunsamp GT2 and Marshall Speaker-simulator devices. In the "upper middle" region, the timbre can be softened by reducing the capacitance of capacitors C3 and C5 by 1 times and increasing the capacitance of C5 and C4 by the same amount. Practical device options The circuits below are various combinations of the previously presented cascades used in conventional processing units.
Let's start with a relatively simple device circuit (Fig. 6), which generates an output signal similar to the output signal of a two-stage tube limiter, in which the first tube amplifies the signal and the second one limits. The functions of the input amplifier and limiter are performed by one stage with transistors VT1, VT2 and VT3, VT4 in a cascode switching circuit (as in Fig. 1). The capacitance of the capacitor C2 is selected "to taste" for a specific instrument. The degree of overload is controlled using a variable resistor R2 - from virtually undistorted sound to a good, juicy overdrive. A simple repeater is added at the output of the cascade, and then the signal timbre is regulated by the usual "Marshall" timbre block. The R12R13 divider reduces the output signal level by an order of magnitude and at the same time reduces the influence of the input resistance of the subsequent device on the operation of the tone block. Such a device will be very useful for fans of non-heavy styles using the "correct" guitar speakers. It reproduces the "Hard" sound of the seventies well.
The next version of the device, the diagram of which is shown in Fig. 7 creates a much "heavier" sound without losing its musicality. To get a denser sound at the input, a preamplifier was added to VT1, VT2. The maximum input signal of the stage is up to 1 Vp-p. As VT1,5, it is desirable to choose a FET with a cutoff voltage of 2 ... 4 V. A simple buffer stage for two FETs is added at the output. The initial drain current of VT3 must be less than that of VT0,5. When using the types of FET indicated in the diagram, this requirement is almost always satisfied (for KPZOZZH, usually the initial drain current is 0,8 ... 0,8 mA, and for KPZOZA - 2 ,XNUMX...XNUMX mA). Diodes VD3, VD4 limit the signal at the input of the second amplifier to 1 V peak-to-peak. Eliminating these diodes results in an overload on the input of the second amplifier and a much less musical sound at the output. In addition, the C4R6VD3VD4 circuit forms an attack of the sound, since for small signals the cutoff frequency of the C4R3R6 high-pass filter is close to 70 Hz, and for large signals, the diodes VD3, VD4 shunt the filter resistors and increase the cutoff frequency, thus forming a clear attack. Do not be alarmed by the presence of back-to-back diodes in the circuit; so often they are included in tube preamps and eminent manufacturers: take, for example, the tube Marshall 900 preamp (only there diodes are included in a group of five, but they limit the signal in the same way). Further, the signal from the distortion control engine is fed to a cascode amplifier with a dynamic load, assembled on VT5-VT10 transistors. The maximum gain of the cascade is reduced to 700 by installing a lower resistance resistor R7 (in comparison with the resistor R3 in Fig. 6), but the main quality of the cascade - smooth limitation - is preserved. The cascade gain is adjusted using a variable resistor R8 in the range of 20...700. The sound from both devices can be significantly improved if an emulator is added to their output, assembled according to the scheme of Fig. 5. Literature 1. Johnson D., Moore J., Moore G. Handbook of Active Filters. - M Energoatomizdat, 1983.
Author: D. Pustovoi, Moscow; Publication: radioradar.net
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Comments on the article: a guest Has anyone done these distortions on the PT and how are they in business? a guest I put together the circuit shown in Figure 7. All fire!
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