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2-Channel Guitar Preamp; Pt.2 Digital Reverb This reverberation unit will add “life” to your guitar, making a small room sound much more spacious. It can be used with our 2-Channel Guitar Preamplifier, added to some other piece of equipment or even used as a freestanding unit. By JOHN CLARKE In a live performance, reverberation is naturally caused by the multiple echoes that occur in a concert hall long after the original sound source has died away. These multiple echoes are mainly caused by the sound reflecting off the walls, floor and ceiling of the venue. The sound absorption char­ acteristics of the reflecting surfaces determine the reverberation time; ie, the time it takes for the sound to die away to nothing. Without some reverberation, music can sound dead or flat. Just how much effect it has can be realised when we recall hear­ing the sounds produced by 36  Silicon Chip an organ or choir in a large auditori­ um or church. How lifeless would it be if there were no walls to reflect this sound and add reverberation? However, reverberation is not al­ ways a good thing and too much can affect the intelligibility of speech. Ideally, the amount of reverberation should be made adjust­ able, to suit the particular venue. However, you really don’t have much control over the natural reverberation that exists at a particular venue. What’s more, reverberation can be practically non­ existent in small venues and where there are lots of soft furnishings that absorb sound. As a result, a live performance can seem dull and lifeless but there is a way around this. By feeding the sound through an electronic reverberation unit, you can add just the effect you want to make your performances sound great. In effect, you can be transported to the concert hall of your dreams – figuratively speaking. Digital reverb The SILICON CHIP Digital Reverber­ ation Unit is based on two Mitsubishi M6580P digital delay ICs. These are set up to simulate the different echo effects that naturally occur within concert hall. The overall effect is similar to that produced by a dual-spring reverber­ ation unit such as the one described in our January 2000 issue. However, this new solid-state unit has several advantages over the electromechani­ cal types. First, unlike the spring-based units, it is not microphonic in any way. This can be a problem with spring-based units, since any movement of the unit Fig.1: the block diagram for the Digital Reverberation Unit. It includes two digital delay lines, the outputs of which are mixed with the direct signals in IC3. The delayed signals are also fed back to two input mixers (IC1a & IC1b) and then fed through the delay lines again to provide the decay feature. can cause “spring noise” and lead to unwanted sound. Second, you can alter the delay times to change the effect if desired, something that is impractical on a spring-based unit. Finally, the noise, distortion and frequency response characteristics are much better than the spring reverb units can deliver. As shown in the photos, the unit is built on a single PC board and is easy to assemble. The board measures 173 x 109mm and fits easily into the chassis of the 2-Channel Guitar Preamplifi­er, behind the preamplifier boards. Note that when used with the 2-Channel Guitar Preamplifier, the unit is in the effects loop. This means that the input is driven by the Effects Send output and the output from the Main Features • • • • Dual delay for added effect Direct plus reverb signal mixing Decay and delay time can be altered Reverb and direct signal mixing can be altered December 2000  37 reverb unit is then fed back into the Effects Return socket. The reverbera­tion signal is then mixed in with the main (or direct) signal from the preamplifier stages, as described last month. Alternatively, you can build the unit into a freestanding case on its own. You would have to add a power supply (this could be the same as the one used in the preamp unit) and a couple of RCA sockets for the input and output signals. You also need to add a couple of extra resistors (R3 & R3') to provide the direct signal components (more on this later). Block diagram Take a look now at Fig.1; this shows the block diagram of the circuit. What we’re doing here is first sending the input signal to two mixer stages. These then drive separate delays lines and the outputs from these are then mixed with the direct signals in another mixer stage to produce an output signal. In greater detail, IC1a and IC1b form the input mixers. IC1a then drives IC2 which provides a 32.8ms delay, while IC1b drives IC4 which gives a 20ms delay. The delayed outputs from IC2 and IC4 are then fed to mixer stage IC3. As well as going to IC2 and IC4, the undelayed outputs from mixer stages IC1a and IC1b are also fed directly to IC3. This undelayed signal is important because it provides the audience with the direct signal, before the delayed signals arrive. After all, this is how the sound arrives in a real environment – the direct sound is heard before the reflected signals. However, it’s not enough to simply provide a direct signal and a couple of delayed signals – that won’t provide reverb­era­tion. What we need is a series of delayed signals that gradually decay to nothing. These reverberation (decay) signals are produced by also feeding the outputs from the digital delay chips (IC2 & IC4) back to their respective input mixers. As the signals pass through the delays, they are fed back to the mixer inputs but at a slightly reduced level. As a result, the original signals are repeatedly delayed until they eventually decay to a very low level. The time taken for a signal to decay away (ie, by 60dB) is the reverberation or decay time. The decay rate and the various mixing levels can be easily adjusted by changing resistor values, to produce the required effect. In addition, the delay time for IC2 can be changed in approximate 0.5ms increments from 0.5ms to 32.8ms using 38  Silicon Chip Fig.2: the complete circuit diagram for the Digital Reverberation Unit. Digital delay line IC4 operates with the default 20ms delay, while IC2 operates with a 32ms delay due to the data clocked into its Data input at switch-on. This data is provided by the delay preset circuit (IC5-IC8). December 2000  39 Fig.3: the codes required for IC2. Data (lower trace) from IC8 is transferred to IC2 on each negative edge of the SCK signal (middle trace). During this time, the REQ line (top trace) must be low to enable the follow­ing 12 SCK clock pulses. The positive edge of REQ signals the end of the serial data stream and loads the data in IC2. different linking options for the delay preset control (see Table 3). Circuit details Fig.2 shows the complete circuit details for Digital Rever­beration Unit. It uses eight separate ICs, including the two digital delay chips (IC2 and IC4). Although the circuit sections for IC2 and IC4 may appear to be the same at first glance, there are important dif­ ferences between them. First, unlike IC2, IC4 has its SCK (clock), REQ (re­ quest) and Data inputs (pins 5, 4 & 6) all tied low. As a result, IC4 is reset at power-on to operate with the default delay period which is 20ms. IC2, on the other hand, has its SCK, REQ and Data pins connected to a de­ lay control circuit. This circuit is used to “program” IC2 at power-on so that it provides a 32ms delay. Once this has been done, the delay control circuit goes to “sleep” and takes no further part in the action; it only operates to program IC2 at switch-on. In greater detail, IC1a functions as an inverting amplifi­er. It operates with a gain of -1 for the input signal and has high-frequency rolloff above 19kHz Fig.4: these oscilloscope traces show the two delay times. The upper trace is the input signal while the lower trace is the output after being delayed. The first delay period occurs 20ms after the original while the second delay is some 32ms after the original. 40  Silicon Chip due to the 820pF capacitor between pins 1 and 2. The signal from IC1a’s output (pin 1) is fed to a low-pass filter stage con­ sisting of 56kΩ and 27kΩ resistors and 150pF and 560pF capacitors. This filter network in turn forms part of the feedback circuit of an internal op amp at pins 22 & 23 of digital delay chip IC2. In operation, the low-pass filter rolls off high-frequency signals above 15kHz at a rate of 40dB per decade or 12dB per octave. This is done to prevent high-frequency signals from being converted into digital data by IC2, which could cause errors. IC2 samples the filtered analog signal at its input and converts it to digital format using an A/D converter. The inte­ grator components for this A/D converter are at pins 20 & 21. Basically, this RC network provides feedback for another internal op amp. The converted digital data is stored in an internal memory, after which it is clocked out and converted to analog format using another internal op amp stage. The integrating capacitor for this stage is connected between pins 15 & 16 and the output signal appears on pin 15. Another lowpass filter stage on pins 13, 14 & 16 (consisting of 56kΩ resis­ tors and 560pF and 150pF capacitors) removes any digital artifacts. Fig.5: the decay rate is shown in this oscilloscope trace. The top signal is the input while the lower trace comprises the output and the decay of the signal down to zero. The decay is about 0.7 seconds. This was set using a 10kΩ resistor for R1, the decay setting resistor. A 1kΩ resistor and a .0047µF capac­ itor at the output of the filter provide a further rolloff for frequencies above 33kHz. The delayed signal is then fed to pin 2 of mixer op amp IC3 via mixing resistor R2 and a 1µF bipolar capacitor. Similarly, the delayed signal from IC4 is also fed to pin 2 of IC3, this time via R2'. In addition, the delayed signals are mixed back into the inverting inputs of IC1a and IC1b via R1 (R1') and series 1µF capaci­tors. As a result, the signal makes multiple passes through the digital delay chip, to provide the echo effects. The value of R1 sets the decay time; ie, the time it takes for the echoes to fade away. The larger the value, the shorter the decay time. Note that op amps IC1a & IC1b are biased at +2.5V via the 10kΩ resistors connecting to their non-inverting inputs (pins 3 & 5) from pin 19 (REF) of IC2 & IC4 (this is the half-supply voltage for IC2 & IC4). Crystal X1 on pins 2 & 3 of IC2 sets the internal clock frequency and de­ termines the rate at which the digital signal is clocked out of memory for D/A conversion. The associated 100pF capacitors and 1MΩ resistor are there to provide correct loading for the crystal, so that the clock starts reliably. Delay time As mentioned above, IC2’s delay time is set via the REQ, SCK and DATA inputs at pins 4, 5 & 6. To change the delay time, a serial data stream must be applied to the Data input at pin 6 and this is then clocked in at each negative transition of the SCK (serial clock) input. The data stream is then accepted on the rising edge of the REQ (request data) input and includes various mute, sleep and address codes, as well as the delay information. Normally, the SCK, REQ and Data inputs are controlled by a microcon­ troller but we’ve eliminated the need for this by using four low-cost ICs (IC5-IC8). These make up the delay control circuit mentioned above. OK, let’s see how this works. When power is first applied, a 3.3µF capacitor pulls the inputs of Schmitt NAND gate IC6d high and so its pin 3 output is low. When the capacitor subsequently charges via its associ­ ated 100kΩ resistor, the pin 3 output switches high and a short posi­tivegoing reset pulse is applied to pin 15 (Reset) of IC7 via a .001µF capacitor. IC8 is a 74HC165 serial shift regis­ ter with parallel load inputs (D0-D7). The first 8-bits of data are set by the logic levels on the D0-D7 inputs and these are loaded into the register when power is first applied. The loaded data is then clocked out on pin 9 but only when pin 1 (the shift load input) of IC8 is low. The clock signals are derived from IC5, a 4060 binary coun­ter which has a free running oscillator at pins 9, 10 & 11. This produces a clock signal at Q4 (pin 7) which runs at twice the fre­ quency of the signal at the Q5 output (pin 5). Q4’s output is inverted by IC6b which then clocks pin 2 of IC8 and pin 5 (SCK) of IC2. Q5’s output is inverted by Schmitt NAND gate IC6a which then clocks IC7, a 4022 divide-by-8 counter, at pin 14. After two counts, the “1” output at pin 1 of IC7 goes high and is entered into IC8 via the serial input at pin 10 (DS). This high appears at the pin 9 output of IC8 after 10 clock cycles on pin 2. When the “6” output (pin 5) of IC7 subsequently goes high, IC5 is reset and remains that way while ever pow­ er is applied. At the same time, the REQ input of IC2 also goes high, while pin 11 of IC6c goes low to reset IC8. The delay control circuit now remains in this “suspended” state and plays no further role in the circuit operation. The oscilloscope trace of Fig.3 shows the required codes for IC2. The Specifications Delay times ................................................20ms (fixed) and 32.8ms (adjustable) Decay time .................................................0.7 seconds (adjustable) Signal handling ..........................................1V RMS max Signal to noise ratio with respect to 1V ....-83dB unweighted (20Hz to 20kHz filter) Frequency response ...................................-3dB <at> 20Hz & 10kHz Harmonic distortion ...................................typically 0.3% at 1kHz and 1V RMS Data (lower trace) from IC8 is trans­ ferred to IC2 on each negative edge of the SCK signal (middle trace). During this time, the REQ line (top trace) must be low to enable the follow­ing 12 SCK clock pulses (ie, pin 12 of IC5 must be low). The positive edge of REQ signals the end of the serial data stream and loads the data in IC2. IC1b and IC4 operate in a similar manner to IC1a and IC2 but without the delay control circuit. Instead, IC4 Parts List 1 PC board, code 01112001, 173 x 109mm 2 2MHz parallel resonant crystals (X1,X2) 1 500mm length of 0.8mm tinned copper wire 7 PC stakes Semiconductors 2 M65830P or M65830BP (but not M65830AP) Mitsubishi delays (IC2,IC4) 1 TL072, LF353 dual op amp (IC1) 1 TL071, LF351 op amp (IC3) 1 4060 binary counter (IC5) 1 4022 divide-by-8 (IC7) 1 4093 quad Schmitt NAND gate (IC6) 1 74HC165 8-bit serial shift register (IC8) 1 7805 5V regulator (REG1) 1 1N914, 1N4148 switching diode (D1) Capacitors 2 100µF 16VW PC electrolytic 2 47µF 16VW PC electrolytic 5 10µF 35VW PC electrolytic 1 3.3µF 16VW PC electrolytic 10 1µF NP or BP electrolytic 7 0.1µF MKT polyester 4 .068µF MKT polyester 2 .0047µF MKT polyester 1 .001µF MKT polyester 3 820pF ceramic 5 560pF ceramic 4 150pF ceramic 4 100pF ceramic Resistors (0.25W 1%) 2 1MΩ 13 10kΩ 1 100kΩ 1 6.8kΩ 1 47kΩ 2 1kΩ 8 56kΩ 1 220Ω 5W 4 27kΩ 1 150Ω 1 22kΩ December 2000  41 IN OUT OUT 10F 10k 150 1F 56k 33 560pF 150pF 560pF 56k 33 .068F 47F 56k 56k 56k 27k IC4 M65830P 0.1F 0.1F 1 1M X2 56k BP 150pF 27k 100F 0.1F 27k 1k 820pF 10k 1k 0.1F 150pF .068F BP 150pF X1 1 2 x 10F 10k IC3 TL071 R2' R2 10k 820pF BP 1F .0047F 560pF BP 2x 100pF IC2 M65830P REG1 7805 10F 1F 10k 100F 1M IC7 4022B 22k BP 1 .068F IC5 4060B 6.8k BP .068F 47k 1N 4148 BP 1F 0.1F D1 BP R3' R1' .0047F .001F 1 BP 10k 1 IC6 4093B 1 1F 1F IC1 TL072 820pF 10k 10k IC8 74HC165 1 2x 100pF 10F 1F 10F 1 100k 1F 10k BP 0.1F 1F R1 SIG GND R3 1F 3.3F +15V 15V _ 0V IN SIG 220 5W  GND  0.1F 560pF 47F 56k 27k 56k 560pF Fig.6: install the parts on the PC board as shown on this wiring diagram. The ICs all face in the same direction. operates with the default 20ms delay period, as described previously. Mixing IC3 mixes the delayed signals with the direct signals from pin 1 of IC1a & IC1b. The delayed signals come in via R2 & R2', while the direct signals are applied via R3 and R3'. The values of these resistors set the amount of mixing in IC3, while R1 & R1' set the reverberation or decay time. The values chosen will depend on the application of the reverberation unit. When connected to the 2-Channel Guitar Preamplifier, only R1 and R2 are used because the Reverb Unit is in the effects loop. In other applications, however, you may want to include R3 and R3'. In this case, you must use a larger value for R2 so that there will be an audible effect at IC3’s output. Power supply The Digital Reverberation Unit re­ quires regulated supply rails of ±15V and a single supply rail of +5V. The +5V supply for IC2 & IC4-IC8 is derived from 3-terminal regulator REG1. A 220Ω 5W resistor at the input is used to reduce the dissipation in 42  Silicon Chip the regulator, while the +5V output is fil­ tered using several electrolytic capacitors and two 0.1µF ceramic capacitors. The circuit can also be operated from a single +15V supply rail (instead of ±15V rails) if the GND is connected to the -15V rail. In fact, you can use a regulated supply voltage down to 8V, although the 220Ω resistor at the input of REG1 will need to be replaced with a link. Construction The Digital Reverberation Unit is built on a PC board coded 01112001 and measuring 173 x 109mm. Begin the assembly by installing the links and resistors. The resistor colour codes and are shown in Table 2 or you can use a digital multimeter to check each value before soldering it to the board. Note that if you are building the unit to go in the 2-Chan­ nel Guitar Preamplifier, use 10kΩ resistors for R1, R1', R2 & R2' but don’t install R3 or R3'. However, if the board is to be built into other equipment or used as a standalone unit, you must include R3 and R3' (10kΩ) to get a direct signal component. In that case, use 18kΩ resistors for R2 and R2'. The seven PC stakes can now be soldered into place, fol­lowed by the ICs. Take care to ensure that each IC is correctly located and orientated (the ICs all face in the same direction). The convention is that pin 1 is always adjacent a small dot or notch in the plastic body. Diode D1 can be installed next, fol­ lowed by 3-terminal regulator REG1. Again, make sure that these devices go in the right way around. Finally, install the two crystals (X1 & X2) and the capaci­tors. Table 1 shows the codes for ceramic and MKT types. Testing If you have a suitable power supply, con­nect it to the board and check the supply voltages to the ICs. Assuming you are using a regulated ±15V supply, there should be +15V on pin 8 of IC1 and pin 7 of IC3. Also check for -15V on pin 4 of both IC1 & IC3. Pins 1 & 24 of IC2 & IC4 should be at 5V. Alternatively, if you are using a single supply rail (“-” input connected to 0V), there should be +15V on pin 8 of IC1 and pin 7 of IC3. There should also be 0V on pin 4 of IC1 and IC3. In Fig.7: this is the full-size etching pattern for the PC board. Check your board carefully before installing any of the parts. addition, check for +5V on pins 1 & 24 of IC2 and IC4, pin 14 of IC6 and pin 16 of IC5, IC7 & IC8. Note that if you use a supply voltage lower than 15V, the 220Ω 5W resistor will have to be reduced in value or shorted out completely. The input voltage to the regulator needs to be at least 8V. Test & adjustment You can test the reverberation board by connecting a signal to the input (at around 1V RMS) and the output to an amplifier driving headphones or loudspeakers. Check that the sound has the reverberation added and that the signal is undistorted. Alternatively, if the board is built into the 2-Channel Guitar Preamp­lifier, you can check its operation simply be wind­ing up the Effects control. Of course, you will have to feed a suitable signal into the CH1 or CH2 input first and monitor the output using head­ phones or an amplifier. If you wish, you can alter the rever­ beration characteris­tics by changing Table 1: Capacitor Codes o o o o o o o o o Value IEC Code EIA Code 0.1µF   100n   104 .068µF   68n  683 .0047µF   4n7  472 .001µF   1n0  102 820pF   820p   821 560pF   560p   561 150pF   150p   151 100pF   100p   101 Table 2: Resistor Colour Codes o No. o  2 o  1 o  1 o  8 o  4 o  1 o 13 o  1 o  2 o  1 o  1 Value 1MΩ 100kΩ 47kΩ 56kΩ 27kΩ 22kΩ 10kΩ 6.8kΩ 1kΩ 220Ω 150Ω 4-Band Code (1%) brown black green brown brown black yellow brown yellow violet orange brown green blue orange brown red violet orange brown red red orange brown brown black orange brown blue grey red brown brown black red brown red red brown brown brown green brown brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown yellow violet black red brown green blue black red brown red violet black red brown red red black red brown brown black black red brown blue grey black brown brown brown black black brown brown red red black black brown brown green black black brown December 2000  43 Table 3: How To Set Different Delays For IC2 Using Linking On IC8 Delay 0.5ms 1.0ms 1.5ms 2.0ms 2.6ms 3.1ms 3.6ms 4.1ms 4.6ms 5.1ms 5.6ms 6.1ms 6.7ms 7.2ms 7.7ms 8.2ms 8.7ms 9.2ms 9.7ms 10.2ms 10.8ms 11.3ms 11.8ms 12.3ms 12.8ms 13.3ms 13.8ms 14.3ms 14.8ms 15.4ms 15.9ms 16.4ms 16.9ms 17.4ms 17.9ms 18.4ms 18.9ms 19.5ms 20.0ms 20.5ms 21.0ms 21.5ms 22.0ms 22.5ms 23.0ms 23.6ms 24.1ms 24.6ms 25.1ms 25.6ms 26.1ms 26.6ms 27.1ms 27.6ms 28.2ms 28.7ms 29.2ms 29.7ms 30.2ms 30.7ms 31.2ms 31.7ms 32.3ms 32.8ms Pin 12 GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 44  Silicon Chip Pin 13 GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND + + + + + + + + + + + + + + + + GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND GND + + + + + + + + + + + + + + + + Pin 14 GND GND GND GND GND GND GND GND + + + + + + + + GND GND GND GND GND GND GND GND + + + + + + + + GND GND GND GND GND GND GND GND + + + + + + + + GND GND GND GND GND GND GND GND + + + + + + + + Pin 3 GND GND GND GND + + + + GND GND GND GND + + + + GND GND GND GND + + + + GND GND GND GND + + + + GND GND GND GND + + + + GND GND GND GND + + + + GND GND GND GND + + + + GND GND GND GND + + + + Pin 4 GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + GND GND + + Pin 5 GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + GND + the delay of IC2 and the values of resistors R1, R1', R2, R2' and R3 & R3'. The table shown on the main cir­cuit (Fig.2) indicates the ranges that can be used for the resis­tors. As mentioned in the text, the re­ verberation decay times can be made longer by decreasing the values for R1 and R1'. However, these resistor values cannot be made too small, otherwise the feedback signal will exceed the input signal and the circuit will be­ come unstable. The R2 & R2' mixing resistors determine the reverberation signal levels applied to the final mixer (IC3). Similarly, R3 & R3' set the undelayed (direct) signal levels. Note that when used with the 2-Channel Guitar Preamplifier, the reverberation unit is in an effects loop, whereby the signal is mixed in with the main or direct signal. This means that R3 & R3' are not required in this situation. However, if the reverb unit is connected as an in-line effects unit, resistors R3 & R3' must be included to provide the direct signal. A value of 10kΩ works well with 18kΩ values for R2 & R2'. If you’re prepared to experiment, you can substitute trim­pots for these resistors so that you can adjust the reverberation unit to your liking. This done, the trimpots can be measured using a multimeter and replaced with fixed value resistors. Changing the delay Finally, the delay time for IC2 can be changed by alter­ing the connections to pins 3, 4, 5, 12, 13 & 14 on IC8. Table 3 shows the connections required for each possible delay time. Note that the initial setting has all these pins connected to +5V. To make changes here, you have to cut the thinned track sections connecting these pins to the +5V track (ie, the track connecting to pin 16 of IC8). You then have to apply a solder bridge to connect the disconnected pins to the GND rail (on either side of IC8) instead. Make sure that none of the pins connects to both +5V and GND or the supply will be shorted. That completes the PC board assem­ bly. In Pt.3 next month, we will show you how to install it in the 2-Channel Guitar Preamplifier case, along with the two preamp boards and the power SC supply.