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A 2-channel gui PART 1: By JOHN CLARKE This high-quality guitar preamplifier is easy to build, with all components and hardware on the PC boards. You can build it with one, two or more channels and each channel has input, bass, mid and treble controls. 30  Silicon Chip As well as the mandatory tone controls, this versatile unit has several other desirable features. These include effects send and return, a line input socket and a headphone socket so that you can practice without disturbing others. There’s also an optional digital reverberation board (we’ll show you how to build that in a future issue). While most people wanting a guitar amplifier would tend to purchase a commercial unit with an inbuilt speaker, they often have mediocre performance, with plenty of hum and buzz, and even pickup of radio stations and mobile radios. They of- itar preamplifier Main Features ten have quite a lot of distortion too, particularly with the unbaffled loud­ speaker and modest power output. Even the headphone outputs are often noisy and distorted. By building this S ILICON C HIP preamplifier and teaming it with, say, our 175W plastic power module (described in April 1996), you can produce a very high quality guitar amplifier. Why would you bother with anything else? This completely new design comprises two PC boards, with the larger one carrying one channel, the mixing for both channels and the regulated power supply. The second, smaller PC board carries two ICs and four pots, for the second channel. Actually, while we are presenting this design as a 2-chan­nel setup, there is no reason why you could not add more chan­nels, just by building the required number of the smaller PC boards. On the main board, the input and controls are arranged in a logical manner with the main input located to the left. Next to it is the level control which adjusts the individual volume from the guitar. Then there are the bass, mid range and treble con­trols. An effects return level pot follows this and then the main mixer • • • • • • • • • Level control. Bass, Mid and Treble controls. Master volume. Effects return control. Balanced & unbalanced line outputs. Headphone output. Line and effects return inputs. Effects send output. Optional Digital Reverber­ ation in effects loop (to be described in a later issue). November 2000  31 Fig.1: block diagram of the Guitar Preamplifier. It has two identical channels (one optional) which are mixed together and then mixed again with an effects return signal (eg, from a reverberation unit). The resulting signal is then amplified and fed to the output sockets. volume control. The headphone socket is located to the right of the volume control. The preamplifier is powered from a 30VAC centre-tapped 5VA transformer. All the rectifier, filter and regulator components are located on the main PC board. Block diagram Fig.1 shows the block diagram for the Guitar Preamplifier. As you can see, it has two identical channels, both with two gain stages with gains of 4.9 and 9.3, respectively. Between the gain stages is a level control (VR1). This allows the signal level to be ad- justed so that following stages are not overloaded. Following the second gain stage are the bass, mid and treble controls. These provide bass boost or cut below 100Hz, mid range boost or cut centred on 1kHz and treble cut or boost above 10kHz. The graph of Fig.2 shows their performance. Both channels are then mixed together in the first mixer (IC3) and this is where the additional channels would be mixed if you wanted them. The output from the first mixer provides an effects send signal suitable for reverberation, fuzz, tremolo or other ef­fects. This mixer output, the effects SPECIFICATIONS Frequency response ��������������������������� -3dB at 20Hz & 30kHz (with tone con­trols at mid-settings) Signal to noise ratio ��������������������������� -86dB unweighted (with respect to 1V output and 50mV input, with input shorted; 20Hz to 20kHz bandwidth); -88dB A-weighted. Total harmonic distortion ������������������� 0.007% at 1kHz and 10kHz Input sensitivity ���������������������������������� guitar input, 10.5mV RMS for 1V output; line and effects return, 1V for 1V out Maximum signal at guitar input before overload ................................... 1.8V RMS Tone controls ....................................... (see graph) Headphone output ............................... 45mW into 8Ω 32  Silicon Chip return input and the line input are combined together in the second mixer (IC4a). The effects return level is set by VR6. There is no volume control for the line input since the signal source for this would already have a output level control. The mixer output connects to a balanced output driver com­ prising IC4b and IC4c and this makes the whole system suitable for connection to a multi-channel audio mixer or a remote exter­nal power amplifier. A lot of people won’t need this feature but the extra components and the two op amps in the quad package don’t add much cost. If you want good quality signals over long lines, the balanced outputs are mandatory. The master volume output from the second mixer controls the overall signal applied to the unbalanced line output buffer and the headphone amplifier. This amplifier can drive two sets of stereo headphones. The signal is paralleled in both the left and right channels of the headphones to produce a mono signal. Circuit description Fig.3 shows a 2-channel version of the guitar pream­plifier. It employs nine op amps in five IC packages. All are from the readily available Texas TL07X series, giving low noise and (ie, high impedance). So by providing a low source impedance, we reduce the hum and buzz. Apart from anything else, this makes for a much cleaner sound. The gain of IC1a is set by the 4.7kΩ and 1.2kΩ resistors in the feedback network. This provides a gain of 4.9 (+13.8dB). A 560pF capacitor across the 4.7kΩ feedback resistor rolls off high frequencies above 60kHz. The output signal from pin 7 of IC1a is AC-coupled via a 2.2µF non-polarised electrolytic capacitor to the level pot VR1. This capacitor prevents any DC current flow in the pot which would cause noise every time you adjusted it. Similarly, the 0.22µF capacitor to the pin 3 input of IC1b is there to block DC current. IC1b is set to a gain of 9.33 (+18.4dB) and the 220pF capacitor across the 10kΩ feedback resistor rolls off high fre­quencies above 72kHz. Tone controls low distortion. IC1 is a TL072 dual op amp. The guitar signal is fed to the non-inverting input, pin 5, via a 1kΩ stopper resistor and a 47µF non-polarised capacitor. The 220kΩ resistor sets the input im­pedance so that the guitar pickup provides a good treble response while the 10pF shunt capacitor at pin 5 prevents extraneous radio frequency (RF) pickup. Readers might wonder why we have used such a big input coupling capacitor in view of the fact that the input impedance of the circuit is quite high at 220kΩ. The reason is that the guitar pickup is inductive and therefore its source impedance at low frequencies is quite low. Now we want the minimum noise to be produced by the preamplifier and the way to do that is for it to “see” the lowest possible source impedance. Ergo, we have a large input capacitor. We have taken the same approach in the past with our low-noise phono preamplifier designs. Reduced hum & buzz However, it turns out that the large input capacitor and resultant low source impedance have another bene­ fit – reduced hum and buzz. The reason for this is that most of the hum and buzz on a guitar input is electrostatic The Baxandall (ie, feedback type) tone controls are based on op amp IC2, together with potentiometers VR2, VR3 & VR4. These pots and their associated resistors and capacitors form the feedback between the op amp’s inverting input and its output. Each of the bass, mid and treble networks can be considered separately since they are connected in parallel between the signal input following IC1b and the output of IC2 at pin 6. Furthermore, the wiper of each pot is effectively connected to the inverting input (pin 2) which is a virtual ground. Operation of the bass control is as follows: with VR2 centred, the value of resistance connected between the output from IC1b and pin 2 of IC2 is the same as that between pin 2 and pin 6 and this sets the gain to -1. The .015µF capacitor has no effect since it is equally balanced across the potentiometer. If we move the wiper of VR2 fully clockwise, we get 18kΩ between the input and pin 2 of IC2 and 118kΩ between pin 2 and pin 6. In addition, the .015µF capacitor is across the 100kΩ resistance in the feedback loop. Without the capacitor the gain would be -118kΩ/18kΩ or -6.5 at all frequencies. But with the capacitor, the gain is high only at around 50Hz and as the frequency rises it comes back to 0.1 (ie, overall unity gain). Thus we have bass boost. Conversely, when VR2 is wound fully anticlockwise, the position is reversed and we get a gain of 18kΩ/118kΩ or -0.15 (-16dB). The capacitor is now on the input side and provides less gain at frequencies below 100Hz but with gain increasing to -1 at AUDIO PRECISION AMP AMPL(dBV) vs FREQ(Hz) 20.000 15.000 13 APR 100 07:25:21 Bass 10.000 Midrange Treble 5.0000 0.0 -5.000 -10.00 -15.00 -20.00 20 100 1k 10k 20k Fig.2: this graph shows the response curves for the bass, midrange and treble controls. The bass boost or cut is mainly below 100Hz, the midrange boost or cut is centred on 1kHz and the treble boost or cut is mainly above 10kHz. November 2000  33 34  Silicon Chip Fig.3: this is the complete circuit diagram, with the optional second channel highlighted on a red background. For each channel, the incoming signal is amplified by IC1a & IC1b and then fed to Baxandall tone control stages based on IC2 and potentiometers VR2, VR3 & VR4. The outputs from the tone control stages are then fed to mixer stages IC3 and IC4a for mixing with the effects return signal. Op amp IC5 and transisistors Q1 & Q2 form the headphone amplifier. November 2000  35 frequencies above 100Hz. Thus we have bass cut. Various settings of VR2 between these two extremes will provide for less boost and cut. The midrange section works in a similar manner except that there is now a .012µF capacitor between VR3’s wiper and pin 2. This, along with the .0027µF capacitor across VR3, gives a band­pass filter, so we either boost or cut the midrange frequencies. The treble control operates with no capacitor across VR4 but with a .0015µF capacitor between the wiper and pin 2 to produce a high frequency boost or cut at 10kHz. The graph of Fig.2 shows the response of the tone controls, with each one individually set to its maximum or minimum settings while the other two are centred. A 39pF capacitor between pins 2 & 6 of IC2 provides a high-frequency rolloff to prevent oscillation which could otherwise occur when the treble control is set for maximum boost. Similar­ly, the 1kΩ resistor in series with pin 2 is there to attenuate RF signals; it stops radio breakthrough. The op amp is also pro­vided with an offset adjustment using VR7 which is Above: these two photos show the fully-assembled PC boards. Note that it is important that the metal contacts on the input jack sockets face in the correct direction, as described in the text. Fig.4: follow this parts layout diagram to build the main preamplifier and mixer board for channel 1. 36  Silicon Chip Fig.5: the parts layout for the optional channel 2 PC board. Note the length of tinned copper wire (shown in green) that’s used to link the pot bodies together. set to mini­mise the DC current flow in bass pot VR2. The outputs from IC2 (in channels 1 and 2) are AC-coupled to the first mixer stage (IC3) via 2.2µF capacitors and 33kΩ resistors. Note that the channel 2 output is also fed to IC3 via a 150Ω resistor which prevents any instability which would otherwise occur with the short length of shielded cable between the two boards. Mixer stages IC3 combines the two channel signals and provides a gain of about -2. Its output is coupled to the “effects send” output via a 47µF capacitor and 150Ω November 2000  37 Fig.6: this is the fullsize etching pattern for the channel 2 PC board. Check all PC boards carefully for etching defects before installing any of the parts. resistor. The capacitor blocks the DC offset at IC2’s output while the 150Ω resistor isolates the output, preventing instability which could occur with shielded (capacitive) leads. The 10kΩ resistor to ground provides a charg­ing path for the 47µF capacitor. IC3’s output is also fed to mixer amplifier IC4a via a 10kΩ resistor. The line input is also applied to this mixer summer via a 2.2µF DC blocking capacitor and 10kΩ resistor. Similarly, the “effects return” signal is coupled to VR6, the effects level pot, via a 2.2µF capacitor and the wiper signal is applied to IC4a via a 10kΩ resistor. The gain of IC4a is -1 for all three inputs. IC4a’s output is AC-coupled to the main volume control VR5 and to the balanced output stage involving IC4b and IC4c. IC4b is a non-inverting buffer which drives pin 2 of the balanced output, while IC4c is an inverting buffer and drives pin 3. The output from the volume control (VR5) is coupled to buffer amplifier IC4d via a 0.22µF capacitor. IC4d provides the unbalanced output which is suitable for driving an amplifier. IC4d also drives the headphone amplifier which com­prises IC5 and transistors Q1 & Q2. output so that two sets of headphones can be driven simultaneously. Note that only one socket is provided on the PC board. Power The op amps for the guitar preamplifier require a ±15V supply and this is provided using two 3-terminal regulators. REG1 produces a +15V regulated supply while REG2 provides the -15V rail. Headphone amplifier Op amp IC5 is combined with a complementary transistor output stage to drive the headphones. The transistors are within the feedback network of the op amp and so the overall distortion of the stage is low. The complementary transistors are operated in class AB and are biased on via diodes D1 and D2 to reduce crossover distor­tion. The overall gain of the headphone amplifier is set to 3.2 by the 2.2kΩ feedback resistor between the amplifier output and pin 2 of IC5 and the 1kΩ resistor to ground. Two 68Ω resistors connect to the Construction There may appear to be a lot of circuitry in the Guitar Preamplifier but it is easy to build, with all of the parts on two PC boards. The main PC board is coded 01111001 and measures 234 x 76mm. It carries all the parts necessary for a single channel preamplifier, including the mixer, output stages and the headphone amplifier. The second PC board is coded Table 1: Resistor Colour Codes o No. o  1 o  3 o  1 o  2 o  1 o  2 o  2 o 14 o  3 o  1 o  2 o  4 o  3 o  2 o  2 38  Silicon Chip Value 220kΩ 100kΩ 68kΩ 33kΩ 27kΩ 18kΩ 12kΩ 10kΩ 4.7kΩ 2.2kΩ 1.2kΩ 1kΩ 150Ω 68Ω 33Ω 4-Band Code (1%) red red yellow brown brown black yellow brown blue grey orange brown orange orange orange brown red violet orange brown brown grey orange brown brown red orange brown brown brown orange brown yellow violet red brown red red red brown brown red red brown brown black red brown brown green brown brown blue grey black brown orange orange black brown 5-Band Code (1%) red red black orange brown brown black black orange brown blue grey black red brown orange orange black red brown red violet black red brown brown grey black red brown brown red black red brown brown brown black red brown yellow violet black brown brown red red black brown brown brown red black brown brown brown black black brown brown brown green black black brown blue grey black gold brown orange orange black gold brown Table 2: Capacitor Codes o o o o o o o o o o o Value IEC Code EIA Code 0.22µF   220n   224 .015µF   15n  153 .012µF   12n  123 .0027µF   2n7  272 .0015µF   1n5  152 560pF   560p   561 220pF   220p   221 150pF   150p   151 39pF   39p   39 10pF   10p   10 01111002 and 142 x 58mm. This board accommodates only the input preamp­lifier and tone control stages for the second channel. If you require additional channels, then it’s just a matter of adding the extra boards. Before installing any of the parts, check the PC boards for shorts or breaks between tracks. You should also check the holes sizes for the pots and 6.35mm jack sockets, to make sure these parts fit correctly – they require 2mm holes. Figs.4 & 5 shows the assembly details for the two PC boards. Begin by installing 15 PC stakes at the external wiring positions on the main PC board, then install the resistors and wire links. Table 1 shows the resistor colour codes but it’s also a good idea to check their values using a digital multimeter. The five ICs can go in next, taking care to ensure that they are all correctly orientated (ie, with their notch­ed ends towards the pots). Also, make sure that IC1 is a TL072 and that IC2, IC3 & IC5 are all TL071s. Now for the capacitors. As always, make sure that the electrolytic types in the power supply (1000µF and 10µF) are installed with the correct polarity. The BP or NP (bipolar or non-polarised) values can be installed either way around. Table 2 shows the IEC and EIA marking codes for the smaller capacitors. Transistors Q1 & Q2 and diodes D1D6 can be installed now. Don’t get the transistors mixed up – Q1 is a BC337 (NPN), while Q2 is a BC327 (PNP). Similarly, take care with regulators REG1 (7815) and REG2 (7915). Each must be installed in its cor­rect location, with its metal tab facing towards its adjacent 1000µF filter capacitor. Next, install trimpot VR7 on the Fig.7: full-size etching pattern for the main PC board. November 2000  39 Parts List Main PC board 1 PC board, code 01111001, 234 x 76mm 1 6.35mm PC-mount stereo jack socket 1 6.35mm PC-mount switched mono jack socket 3 grey knobs (bass, mid & treble) 2 yellow knobs (level & volume) 1 blue knob (effects) 1 400mm length of 0.8mm tinned copper wire 18 PC stakes Semiconductors 1 TL072, LF353 dual op amp (IC1) 3 TL071, LF351 op amps (IC2,IC3,IC5) 1 TL074. LF347 quad op amp (IC4) 1 7815T 15V regulator (REG1) 1 7915T -15V regulator (REG2) 1 BC337 NPN transistor (Q1) 1 BC327 PNP transistor (Q2) 4 1N4004 1A diodes (D1-D4) 2 1N914, 1N4148 switching diodes (D5,D6) Capacitors 2 1000µF 25VW PC electrolytic 5 47µF NP PC electrolytic 7 10µF 63VW PC electrolytic 6 2.2µF NP PC electrolytic 1 1µF NP PC electrolytic 3 0.22µF MKT polyester 1 .015µF MKT polyester 1 .012µF MKT polyester 1 .0027µF MKT polyester 1 .0015µF MKT polyester 1 560pF ceramic 3 220pF ceramic 2 150pF ceramic 2 39pF ceramic 1 10pF ceramic Potentiometers 3 10kΩ 16mm log pots (VR1,VR5,VR6) 3 100kΩ 16mm linear pots (VR2,VR3,VR4) 1 10kΩ horizontal trimpot (VR7) main board, followed by the 6.35mm jack sockets (two on the main board, one on the chan­nel 2 board). Note that there are two types of 6.35mm jack sock­ets – mono and stereo. The mono 40  Silicon Chip Resistors (0.25W, 1%) 1 220kΩ 3 4.7kΩ 3 100kΩ 1 2.2kΩ 1 68kΩ 2 1.2kΩ 2 33kΩ 4 1kΩ 1 27kΩ 4 150Ω 2 18kΩ 2 68Ω 2 12kΩ 2 33Ω 15 10kΩ Parts For Second Channel 1 PC board, code 01111002, 142 x 58mm (116 holes) 1 6.35mm PC-mount switched mono jack socket 3 grey knobs (bass, mid & treble) 1 yellow knob (level) 1 250mm length of 0.8mm tinned copper wire 6 PC stakes Semiconductors 1 TL072, LF353 dual op amp (IC1) 1 TL071, LF351 op amp (IC2) Capacitors 1 47µF bipolar PC electrolytic 2 10µF 63VW PC electrolytic 3 2.2µF NP PC electrolytic 1 .22µF MKT polyester 1 .015µF MKT polyester 1 .012µF MKT polyester 1 .0027µF MKT polyester 1 .0015µF MKT polyester 1 560pF ceramic 1 220pF ceramic 1 39pF ceramic 1 10pF ceramic Potentiometers 1 10kΩ 16mm log pot (VR1) 3 100kΩ 16mm linear pots (VR2,VR3,VR4) 1 10kΩ horizontal trimpot (VR7) Resistors (0.25W, 1%) 1 220kΩ 3 10kΩ 1 100kΩ 1 4.7kΩ 2 33kΩ 2 1.2kΩ 2 18kΩ 2 1kΩ 2 12kΩ 1 150Ω version is used for the input socket on each board and can have either two or three sets of switched contacts. Note that the jack plug contacts should be on the righthand side and LE the switch contacts on the left, as viewed from the front of the board. If not, they will have to be repositioned by gently prising the contacts out of the plastic body and reinserting them the correct way around. The PC board photos clearly indicate the orientation of these contacts. The stereo socket is used for the headphone output and has three sets of switched contacts. Its terminals can be on either side of the socket body. It should be installed as shown on Fig.4. The pots can all be installed now. Place the 100kΩ linear types (B100k) in the bass, treble and midrange tone control positions (VR2, VR3 & VR4) and install the 10kΩ log (A10kΩ) pots in the remaining positions. This done, connect the pot bodies together by soldering each one to a length of tinned copper wire. You will need to scrape away some of the passivation coating on each pot body before soldering the wire in position, otherwise the solder won’t “take” to the metal. The idea here is to prevent hum pickup by ensuring that the pot bodies are connected to the chassis earth when the PC boards are installed in a metal case. This, of course, assumes that at least one pot makes good contact with the case (it may be neces­ sary to scrape away some of the paint around the holes to ensure this). Preliminary checks If you have a power supply with regulated ±15V rails, you can carry out a few preliminary checks on the completed PC boards as described below. If not, you can leave this step until after the unit has been fully assembled into the case with its power supply. First, apply power and check the power supply rails on both PC boards. There should be +15V on pin 8 of IC1, pin 7 of IC2, IC3 & IC5, and on pin 4 of IC4. Similarly, you should be able to measure -15V on pin 4 of IC1, IC2, IC3 & IC5 and on pin 11 of IC4. If everything checks out, switch off and connect your multimeter between TP1 and the 0V supply pin on the main PC board. This done, set the meter to the mV range, apply power and adjust VR7 for a reading of 0V (or as close to this as possible). Now do the same for the smaller PC board. That’s all for this month. Next month, we’ll describe the digital reSC verberation board.