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Digital Reverb . . . last month’s missing pages An error in the SILICON CHIP editorial office resulted in the wrong pages being printed on pages 42 and 43 of the December 2000 issue, slap bang in the middle of the Digital Reverb article. These are the missing pages. We won’t name the staff member responsible for this appalling error but if you ever visit us, he’s the one with the serious bruises on his neck and the large lumps on his head. As soon as we became aware of the error, we posted the two missing pages in Adobe Acrobat pdf format on our website. In addition, we are publishing the last three pages of the article here, starting from the beginning of the last paragraph of page 41. Our apologies to readers for this unfortunate error. IC1b and IC4 operate in a similar manner to IC1a and IC2 but without the delay control circuit. Instead, IC4 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 requires 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 the regulator, while the +5V output is fil­tered 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 The completed Digital reverb board can be built into the 2-Channel Guitar Preamplifier or built into a separate case and used as a freestanding unit. Take care to ensure that all polarised parts are correctly orientated. 72  Silicon Chip OUT BP 560pF 56k 150pF 560pF .0047F 1k 1k 10F 10k 0.1F 150pF .068F 47F 56k 56k 33 .068F IC4 M65830P 0.1F 0.1F 56k 27k 150pF 33 1 1M X2 56k BP 100F 56k 27k .0047F 560pF 1F 0.1F 27k 150pF X1 OUT 22k 10k 2x 100pF BP 100F 1M IC7 4022B 1 2 x 10F 150 IC3 TL071 R2' 1 10k IC2 M65830P REG1 7805 10F 1F R2 10k 10k 820pF BP 1F SIG GND BP 820pF BP .068F IC5 4060B 6.8k BP .068F 47k 1N 4148 BP 1F 0.1F D1 BP R3' R1' IN .001F 1 1F 10k 1 IC6 4093B 1 1F 1F IC1 TL072 820pF 1 2x 100pF 10F 1F 10F 10k 10k IC8 74HC165 0.1F 1F 10k R1 1 3.3F +15V 15V R3 1F BP 100k _ 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. 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, followed 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. Now for a few preliminary checks. If you have a suitable power supply, con­nect it to the board and check the Table 1: Capacitor Codes          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  No.   2   1   1   8   4   1  13   1   2   1   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 January 2001  73 Fig.7: this is the full-size etching pattern for the PC board. Check your board carefully before installing any of the parts. 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 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 Preamp­ lifier, you can check its operation 74  Silicon Chip simply be wind­ ing 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 headphones or an amplifier. If you wish, you can alter the reverberation characteris­tics by changing 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 reverberation 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 become 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. 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 on page 44 of the December 2000 issue 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 SC supply will be shorted.