Lupine Systems Presents...
Build Your Own Guitar Special Effects
Digital Audio Delay Systems
by Spike Tsasmali, Lupine Systems

Build Your Own Digital Delay

Introduction

In an earlier article I demonstrated how to build a simple Bucket-Brigade analog delay. In this article I will present all you need to know to build a basic Digital Delay system capable of delays up to 2 seconds (expandable to 4 seconds of delay). Digital Delays are much much more complex, especially when viewed from a 1970's standpoint. But Digital Delays offer a wide range of features and functions that put them at a much greater advantage than Bucket-Brigade delays or even tape delays. Digital Delays will let you do all of the basic delay effects -- reverb, echo and chorus, as well as time-frame modulation, sampling, repeat, record/playback, time-base correction (speed up or slow down tempo) and feedback elimination. There are other effects Digital Delays can do that are limited only to the user's imagination! Digital Delays also allow for a much faster sampling rate which improves bandwidth. And since there are no "moving parts", there is no tape to wear out and no spring to pick up outside interference. The Digital Delay is the best choice all around for performance, function and flexability despite the complexity of the electronics necessary and the cost that comes with that complexity.

Engineering the Basic Digital Delay

Building a Digital Delay can be a daunting task. Figuring out exactly what you need to do to complete the task is also somewhat illusive. It seems fairly simple, but in reality it takes a lot of electronics to pull it off. Even a basic model requires almost 30 TTL chips to do it!

Modern-day microprocessors could simplify the design considerably, but remember the goal -- to reproduce the same effects as used in the 1970's, done in the same manner as was done in the 1970's. So that means no processor! It has to be done the long, hard way. And for good cause. The sound. Remember, it's the SOUND that's the important thing!

Building the Digital Delay

Fig. 1 shows a block diagram of the Digital Delay project. In this simplified project, all circuits will fit on a single panel of GD153 Ever-Muse PC Board material (blank pre-sensitized PC board material is available from Circuit Specialists. Read the article, "Making Your Own PC Boards" for more information.). You can make your own PC board or you can purchase a pre-etched board from Lupine Systems. There is more information on purchasing a pre-etched PC board at the end of this article.


Fig. 1 -- Basic Digital Delay Block Diagram
Click Image to Enlarge

A Note on Making Your Own PC Board

Making your own PC board can be fun and can save you a lot of money. But it will take time. This project fits on a panel of PC board material which is 11.81" x 5.91" in size! By experimenter's standards, this is a HUGE board! There are over 500 holes (including via holes), most of them .028" in size (that's twenty-eight THOUSANDTHS), many of which are occupied by part leads or chip pins that must be soldered on both sides (only if you make your own PC board because there is no practical way to do the thru-hole plating in a home-brew environment). Making a board of this size and magnitude does not involve any special skills that smaller PC boards don't require; it is more a matter of due dilligence than anything. Patience, dexterity and skills are easily tested on a large board with 500+ holes that must be drilled manually. So be prepared to spend some time on the project and stay away from anything that will give you the "jitters" (like too much coffee...).

The board layout files are part of the Project Package, which includes all of the PC board layouts in both Gerber file format and in EasyTrax format. I highly recommend that you read about and then install EasyTrax before you begin construction. Having EasyTrax on your shop PC will greatly aid you in locating components, tracking down traces and of course, modifying the layout in the event you wish to make changes to the design.

You will also need access to a large-format printer in order to print out the PC board layouts. Large-format printers are printers capable of printing on paper 11" x 17" or larger (SuperB is a large-format size). Graphics printers such as the HP DesignJet 2500 are excellent printers to do this job, as long as you have the proper transparency media and you print using UV-resistant ink. The printout does not have to be solid "black" if printed using UV inks, but rather a very dark shade of "brown" when viewed on a light table. If you use a regular inkjet or laser printer with standard dye-base inks, the darker the print, the better off you will be. The Ever-Muse PC board material is very forgiving -- so much so you'll forget it is a form of photography! But the better your PC board artwork is to start with the better the outcome of the board will be. You can also print out two copies of each side of the layout and carefully overlap them to darken the artwork. If you do this, be careful to align each layer as exact as possible and add an additional 5 seconds (30 seconds if you use a regular fluorescent lamp) to the exposure time.

All components used in this project are available from Jameco Electronics or Mouser Electronics. You can also purchase some of the parts at Radio Shack and Circuit Specialists. If purchased new, the total cost of this project will be less than $150 at the time this article is published. You can save some money by using parts purchased from surplus dealers and by salvaging parts from your parts junk box. In any event, the entire project is well affordable for most experimenters and musicians as well.

This project is presented to you by Lupine Systems for educational purposes only and is not intended to be used as a base design for manufacture, and is not intended to implicate a complete and total solution suitable for all needs. All designs remain property of Lupine Systems at all times. Although this is an experimenter's project capable of producing excellent results and the sound quality is fairly high, there remains room for improvements. This project is basic and provides all of the systems necessary to perform the desired task but is left open for you, the experimenter, to modify it to your own personal needs. Feel free to experiment with part values in the amp and conversion circuits or improve quality by working with different types of op-amps. Enjoy the project, and learn how digital delays work in the process.

How It Works

Fig. 2 shows the Schematic Diagram for the Basic Digital Delay Board (click image to enhance/enlarge). Master clock is generated by oscillator module U21 or variable clock generator L1/Q1. Clock source is selected by clock selection switch S2. The variable clock frequency can be tweaked by sliding slide pot R24 or by inserting a ferrous object into the core of coil L1. The 1.024MHz master clock is first divided by 2 to 512KHz and then again by 2 to 256KHz by dual flip-flop U23. The 512KHz clock signal is used by input A/D converters U1 and U2 as the sample clock. Sampling the input controls at twice the system clock rate insures there are no digital artifacts in the playback address generation and stabalizes the delay time setting so that when the playback address is selected the delay time adjustment is accurate. The 256KHz from the clock divider is then sent to the input of counter chip U22 as the main system clock.

Counter chip U22, a 74LS193, and 1-of-8 decoder chip U20 form the system control state machine. This combination of chips further divides the clock by 8, yielding a 32KHz audio sample rate when the fixed clock is selected (this rate will be close to 32KHz using the variable clock generator). The outputs of the 74LS138 decoder chip control the A/D, D/A and RAM read/write sequence in the proper order as to sample, record and then play back the delayed audio.

Although a 1.024MHz oscillator module is specified for the main fixed system oscillator, any oscillator module near or about 1-2MHz can be used. If a higher frequency is selected the audio will be sampled at a slightly higher than 32KHz sample rate. If a lower frequency is selected then the audio will be sampled at a slightly lower than 32KHz sample rate. If you consider 32KHz an ideal sample rate, there is plenty of room for variance so the frequency of this clock is non-critical as long as it falls somewhere between 1MHz and 2MHz. The fastest oscillator tested and shown to work was 12.305MHz. At this frequency, the audio sample rate would be 384.53125KHz! (The formula for calculating the sample rate is Fosc/32). But at this high frequency, the delay time was reduced to only a few milliseconds. If you are looking for high-end performance with wide audio bandwidths AND are willing to sacrifice delay time then the easiest way to obtain this wider bandwidth is to increase the sample rate.

RAM Address generation is handled by chips U9-U12, 74LS193 4-bit binary counters. READ or PLAYBACK address is calculated by subtracting the Delay Time data input from A/D converters U1 and U2 from the raw counter data generated by U9-U12. This subtraction is done by the ALU chain U5-U8, 74LS181 4-Bit Arithmetic Logic Units. Octal latch chips U3 and U4 latch the Delay Time data from the A/D converters. Both the raw counter data and the result of the ALU computation are routed to the inputs of the data selector chain U13-U16. Selected data is then sent to the RAM memory chip U17 as RAM address. Selection between raw counter address and calculated address from the 74LS181 chain is determined by the 3rd bit of the control state machine counter. Therefore, the state machine can implement 4 steps during the record sequence and 4 steps during the playback sequence.

Audio is input to the delay unit via connector jack CN1. Audio is then sent through a resistor divider to the output stage via R7 and the MIX switch. Placing the MIX switch in the ON position will allow the original undelayed signal to be re-mixed in with the delayed/effected signal. When the MIX switch is in the ON position, the delay will automatically "bypass" when powered off.

Input audio is then amplified by 1/2 of op-amp U27, a MC1455 dual op-amp. The amplified signal is then filtered and then presented to the input of A/D converter U25. Conversion is controlled by the system control state machine and reflects a 32KHz sample rate. Digital data from the A/D converter is then placed on the Data Bus and will be written into RAM by the state machine.

Digital data stored in the RAM is converted back to analog by D/A converter chip U26. This chip is running in constant voltage mode so no current conversion is required. The low level analog signal is then amplified by the remaining half of op-amp U27, then presented to the input/output mixers R19 and R20. Audio is then mixed in with the original un-delayed signal, retro-inserted back into the delay chain or is output direct from the delay to output connector CN1.


Fig. 2 -- Basic Digital Delay Schematic Diagram
Click Image to Enlarge

Assembly -- Overview and Preliminary Information

Use of a Printed Circuit is HIGHLY recommended. I could not imagine trying to build this kind of project without using a PC board. You can download the entire project package ZIP file (basic.zip 649K) which includes all PC board layout files in industry-standard Gerber file format and in EasyTrax formats, parts placement diagrams, NC Drill files, drill drawings and schematics from the Lupine Systems Download Site. Pre-etched, drilled PC boards with plated-thru holes, solder mask and screened legend layer are available for purchase from Lupine Systems. Contact Us for more information about purchasing these boards.

If you choose not to use sockets for the chips, populate the board with one row of chips at a time, then solder one row of chip legs at a time. This will help cut down on heat damage to the chips, especially when it comes to soldering them on both sides. Most of the bypass capacitors are .300" spacing and can be pre-formed to that size. All other discrete components are .400" spacing. If you are making your own PC board, remember to solder components on BOTH SIDES since you won't have plated-thru holes (you won't have to do this if you use the pre-etched plated-thru PC board supplied by us, more on that later). You will also have to insert a small piece of AWG 24-28 wire through each via hole and solder on both sides. This can be time-consuming but is actually fairly easy to do if you place a piece of anti-static foam underneath the PC board before inserting the via wires. Solder the TOP layer first, then flip the board, pull off the foam, crush the leads flat on the TOP layer then solder the via wires on the BOTTOM layer. Trim both sides smoothly, but don't trim until BOTH SIDES have been soldered. Machine tool sockets are HIGHLY recommended for chips since they solder easily on both sides. After assembly, inspect the board carefully and repeat inspection several times to be absolutely sure all pads on the TOP layer which have traces have been soldered properly.

Assembling the Basic Delay

If you chose to make your own PC board, begin assembly by installing all of the vias first. Be sure to identify ONLY the via holes and not component holes! Insert a short piece of AWG#24 wire through each via hole and solder on the TOP layer first, then flip the board and solder on the BOTTOM layer. Trim only after soldering BOTH SIDES.

If you purchased a pre-etched board from Lupine Systems, you will not have to install wires in the via holes. The pre-etched board has "plated-thru" holes. Each hole is plated through from top layer to bottom layer with a copper-tin plating allowing signals and power to go through from one side to the other. You will also not have to solder components on both sides. But if you made your own PC board, remember when installing components to check to see if there are pads on the TOP layer with traces and if so, these pads must be soldered on BOTH SIDES.


Basic Digital Delay Parts Placement Diagram
Click Image to Enlarge

Install all bypass capacitors, ceramic capacitors and all resistors. Most of the bypass caps are formed on .300" forms; resistors are .400" forms. Trim excess lead wires close to the board. Install a bare wire jumper in place of resistor R23. Leave resistor R22 unpopulated.

Once all discrete components have been installed, begin installing the IC chips. Start off with ALU chips U5-U8 and solder. Next, install the 74LS193 counter chips U9-U12 and decoder chip U20. Solder. Install the row of 74LS157 Data Selectors U13-U16 and state machine counter chip U22 next and solder. Install the far-right column of chips U19, U21, U23 and U24 and solder. Then install the two 74LS374 octal latches at U3 and U4. Solder. Finish off by installing the MC1455 op-amp at U27.

Install a 20-pin low-profile socket in the place of U1, U2, U25 and U26. Install a 32-pin low-profile socket in place of U17.

Install the two 5K trim potentiometers at locations R15 and R16. Then install all upright electrolytic capacitors. Be sure to pay close attention to polarity! Install transistors Q1 and Q2. Observe polarity.

Install voltage regulator chip U28. Be sure to use a heatsink! This part will get quite warm during operation (normal) and without a heatsink it will burn up quickly. Use of silicone or Teflon heatsink compound is also recommended. Use a 4-40 x 1/2" screw and nut to attach the chip and heatsink to the PC board. Next, install the other three voltage regulators U29, U30 and U31. These devices mount directly to the PC board using a 4-40 x 1/2" screw and nut without a heatsink.

Install input/output connector block CN1. Solder. Install power connector J1. Solder.

Finish assembly by installing the four control potentiometers. Install a 5K pot for R17 and R18, a 100K pot for R19 and a 10K for R20. Install a SPDT toggle switch at locations S1 and S2. Use short pieces of scrap wire to hook the terminals of the switch with the corresponding pads on the PC board located directly beside the switch. Wire the pad straight to the lug. Next, install slide pot R24 and tuning coil L1 (see text below). Use a dab of hot-glue to help secure coil L1 in place if necessary.

Once the board has been completely assembled, check for soldering errors, shorts, opens and for missing parts. Then install A/D converter chips U1, U2 and U25, D/A converter chip U26 and RAM chip U17 in their respective sockets.

Table 1
Parts List
Basic Digital Delay Board

DesignatorQuantityDescriptionVendorPart Number
C21147pF Ceramic Disc CapacitorMouser539-GP447
C17,C18,C35,C38,C395220pF Ceramic Disc CapacitorMouser539-GP322
C19,C22,C233.001uF Axial Conformal CapacitorMouser80-C410C102K5R
C401.01uF Axial Conformal CapacitorMouser80-C410C103K5R
C3-C15,C20,C24,C26,C27,C30,C3419.1uF (104) Axial Conformal CapacitorMouser80-C410C104M5U
C411.22uF Axial Conformal CapacitorMouser80-C410C224M5U
C1,C2,C2931uF/50v Radial Electrolytic CapacitorMouser647-UVR1H010MDD
C3312.2uF/50v Radial Electrolytic CapacitorMouser647-UVR1H2R2MDD
C1614.7uF/50v Radial Electrolytic CapacitorMouser647-UVR1H4R7MDD
C25,C31210uF/50v Radial Electrolytic CapacitorMouser647-UVR1H100MDD
C28,C32,C423100uF/50v Radial Electrolytic CapacitorMouser647-UVR1H101MPD
C36,C372470uF/50v Radial Electrolytic CapacitorMouser647-UVR1H471MHD
R141150 Ohm 1/8 Watt Metal Film ResistorMouser299-150-RC
R26,R2921K Ohms 1/8 Watt Metal Film ResistorMouser299-1K-RC
R2512.2K Ohms 1/8 Watt Metal Film ResistorMouser299-2.2K-RC
R2715.1K Ohms 1/8 Watt Metal Film ResistorMouser299-5.1K-RC
R13,R28210K Ohms 1/8 Watt Metal Film ResistorMouser299-10K-RC
R2,R3,R6,R21422K Ohms 1/8 Watt Metal Film ResistorMouser299-22K-RC
R8127K Ohms 1/8 Watt Metal Film ResistorMouser299-27K-RC
R7133K Ohms 1/8 Watt Metal Film ResistorMouser299-33K-RC
R11139K Ohms 1/8 Watt Metal Film ResistorMouser299-39K-RC
R1,R30247K Ohms 1/8 Watt Metal Film ResistorMouser299-47K-RC
R4168K Ohms 1/8 Watt Metal Film ResistorMouser299-68K-RC
R5,R10,R123100K Ohms 1/8 Watt Metal Film ResistorMouser299-100K-RC
R91220K Ohms 1/8 Watt Metal Film ResistorMouser299-220K-RC
R17,R1825K Ohm PotentiometerMouser31VJ305-F
R20110K Ohm PotentiometerMouser31VJ401-F
R191100K Ohm PotentiometerMouser31VJ501-F
R15,R1625K Ohm Trimmer PotentiometerMouser531-PT6KV-5K
R2415K Ohm Slide PotentiometerMouser312-9301-5K
Q1,Q222N3904 NPN TransistorJameco38359CE
U1,U2,U252ADC0820 Analog-to-Digital ConverterJameco10225CK
U3,U4274LS374 Octal D-LatchJameco47634CK
U5-U8474LS181N 4-Bit ALUJameco46973CK
U9-U12,U22574LS193 Binary CounterJameco47036CK
U13-U16474LS157 Quad MultiplexerJameco46771CK
U171628128 128K x 8 Static RAMJameco131810CK
U18,U23274LS107 Dual J-K Flip-FlopJameco46412CK
U19174LS00 Quad NAND GateJameco46252CK
U20174LS138 1-of-8 DecoderJameco46607CK
U24174LS32 Quad OR GateJameco47466CK
U261DAC0830 Digital-to-Analog ConverterJameco14963CK
U271MC1458 Dual Op-AmpJameco23131CK
U28,U3027805T Voltage RegulatorJameco51262CK
U29,U3127812T Voltage RegulatorJameco51334CK
CN11Dual RCA JackMouser161-4218
J112.5mm Power JackMouser806-KLDX-0202-B
L11Tuning Coil--See Text
--420-Pin Low-Profile IC SocketMouser517-ICA-206-S-TG
--132-Pin Low-Profile IC SocketMouser517-ICA-326-S-TG
S1,S22Subminiature Toggle Switch, SPDTMouser108-2MS1T1B1M1QE
--1Heatsink for U28Mouser532-577102B00
--4Screw, #4-40 x 1/2"----
--4Nut, #4-40 x 1/2"----
--4Knob, Black .94" .25" ShaftMouser45KN013
--1Knob, Black for Slide PotentiometerMouser450-3051
--1Power Supply, 12VDCMouser831-PW118RA1203B01
--1Bare PC Board (See Text)Lupine Systems16-334-10A

Note: "ALS" or "HCT" series chips can be substituted in place of "LS" series chips as long as all of the "LS" chips are replaced with the same logic family. Do not mix "LS", "ALS" and "HCT" chips on the same board.

Winding the L1 Tuning Coil

The L1 tuning coil is part of the variable L/C oscillator circuit. In the original prototype, I used an AM tuning coil, once available at Radio Shack and used in many of their project kits.


Original Radio Shack AM Tuning Coil

But after doing some research I found that Radio Shack no longer sells this coil and nowadays it is very difficult to find. So I figured out the alternative "home brew" version. So if you cannot find one of these nifty tuning coils, you can try alternate coils or make your own. Here's how to make this coil:

You'll need:

  • a Cheap Ball Point Pen, one of those advertising pens will work just fine. As long as it is about 1/4" in diameter. Not one of those "click" types, but the ones with the removeable cap.
  • 30 feet (360 inches) of AWG#24 enameled magnet wire. You can buy it new at Radio Shack or you can salvage this from destroying an old 6-volt wall-wart power transformer (most have this size wire, it's the bigger of the two wires in the transformer)
  • Masking tape, 1" wide
  • 2 (two) #4-40 x 1" screws and 6 (six) #4-40 nuts
  • Drill
  • Hack Saw to cut pen
  • 3/16" drill bit
  • .040" drill bit or Dremel tool with extra fine bit
  • First, dis-assemble the pen. Throw away the ink cartridge, cap and end cap at top of pen tube.

    From the end cap end of the pen tube, measure and cut at 3 inches. Discard the tapered end of the pen tube.

    Draw a straight line down the center of the pen tube from end to end. Use this line as a straight line reference. Starting at the cap end of the tube, measure off 1 3/4" and then 2 3/4". Drill a 3/16" hole completely through the pen tube at these measurements. Next, measure off 1" from the cap end and draw a line around the circumference of the tube. Drill a .040" hole just outside the 1" line (between the 1" and 1 3/4" line up against the 1" line) completely through the tube. Drill another .040" hole 1/16" from the cap end of the tube. Drill this hole only half-way through (only one side of the tube).


    Pictorial of Coil Form and Measurements

    Thread approxamately 2" of wire through the cap end .040" hole starting from the outside (push wire from the outside of the tube through the hole to the inside of the tube, then pull the wire through the hole to the outside). Begin winding the coil around the tube by holding the pen tube in your left hand. Twist the tube to wrap the wire while using your right hand and fingers to guide the wire smoothly around the tube. It does not matter which direction you wind the coil. DO NOT WRAP THE WIRE AROUND THE TUBE -- USE THE TUBE TO "ROLL" THE WIRE INTO A COIL. Continue wrapping wire until you reach the 1" mark. At this mark, hold the wire firmly against the tube and wrap ONE SINGLE LAYER of masking tape around the coil.

    Continue winding the coil by wrapping the second layer on top of the first layer. Repeat the same procedure as outlined above. When you reach the cap end of the tube, put another piece of tape around the coil and begin the 3rd layer. Repeat this process until you have used up all but about 3" of the wire. You should have about 7 layers of coil. If the final layer does not come out even, don't worry, just "stretch" the last layer out so that the windings are even from end to end and go on to the next step. When completed, thread the end of the coil wire through the .040" hole just outside the 1" mark. Once the wire is threaded through the hole, wrap the wire around the pen tube once more to secure the wire in place, then put one more layer of tape around the entire coil. Once the coil is secured, cut the two coil wires to 1" long, then using a piece of fine sandpaper or steel wool, polish off the enamel on the coil wires from 1/8" of the coil to the tips of the wires.

    Tolerance is not critical when winding this coil. If you go over the 1" mark or put a few more turns per layer it is not going to matter. As long as you get it mostly consistent from layer to layer. If the wire breaks while winding, you can splice it as long as you make a clean tack-solder splice (no twisting!). When completed, the coil itself will be about 5/8" round. If the coil is thick at one end or fat in the middle, then rewind the coil. It will not work right if the coil is not evenly wound.

    Finally, with both wires of the coil facing DOWNWARDS, install two #4-40 x 1" screws through the 3/16" holes from the top and secure with a nut on the same side of the tube as the wires. Tighten these nuts firmly but not so tight as to collapse the pen tube. Then thread an extra nut on each screw. Thread this nut down on the screw so that the bottom face of the nut is even with the bottom edge of the coil winding. The extra two nuts are used to mount the coil to the PC board.

    Mount the coil to the PC board. If necessary, adjust the lower two nuts on the mounting screws so that the coil does not touch the PC board. First thread each coil wire through the two holes marked, "L1", then align the two mounting screws with the corresponding holes in the PC board. Fasten coil to the board using the remaining two #4-40 nuts and tighten firmly. Then solder the two coil wires to the pads on the PC board.

    You can make the coil more permanent by coating the coil in epoxy. You can mix up a batch of 5-minute 2-tube epoxy and use a swab or cheap paint brush to quickly spread the epoxy over the entire coil body. You can also mix the epoxy in a "puddle" on a piece of scrap cardboard, then roll the coil in the epoxy. Once coated, allow the epoxy to competely cure before you mount the coil on the PC board.

    Adjustment and Calibration

    Before you can use the Digital Delay, you must calibrate the input buffer amp and set the reference voltage.

    Hook up the delay to your stereo's tape loop. Put on some music and adjust for background listening level. Do NOT engage the TAPE MONITOR at this time! RAP or hip-hop music is good for this test because it has a lot of speech and smooth rythms but any form of music will do.

    Power up the delay. Adjust both pots R15 and R16 to mid-rotation. Using a digital voltmeter, measure the voltage at U25, pin 12 and adjust pot R15 so that the voltage at pin 12 is 2.7V.

    Set both of the DELAY TIME controls and the DECAY control to full counter-clockwise rotation. Adjust the DEPTH control to full clockwise rotation. Set the MIX switch to the OFF position.

    Turn on the TAPE MON on your stereo. You should hear the music play through, but delayed from the original signal. Adjust pot R16 for the clearest, cleanest signal. Then slightly tweak R15 if necessary to clean up the signal more. Adjusting R15 will affect the overall volume of the output as it sets the A/D and D/A reference voltage; be sure not to set this pot too far to the far counter-clockwise rotation (low reference voltage) or the signal will overdrive when louder signal bursts are present. If you set this adjustment too high (high reference voltage) the signal will collapse at low levels and will appear to be "squelched". The near perfect adjustment is centered around 2.7V but may vary slightly due to tolerances and application.

    It is normal for a delay line to restrict some high-end bandwidth. The amount of attenuation is mainly determined by the value of the capacitors used in the output filters. You can modify the output amp section to give more bandwidth by altering the values of the filter capacitors C21, C24 and C34. Modifying these capacitors for more bandwidth may introduce unwanted hiss or white noise so be careful!

    Once the delay has been adjusted, rotate the DECAY pot to mid-rotation and the DEPTH pot to 3/4 rotation. Then slowly turn the DELAY TIME COARSE knob. You should hear the music with reverb, echoes and delay. Play with the DELAY TIME and DECAY pots to get the effects you desire.

    Using the Digital Delay

    In this section I will assume that you already know what a Digital Delay does, what it is used for and pretty much what you can do with one once you get one. So all you need is a quick review of the controls and their function.

    But before I explain all of the controls, I must mention the following:

    This unit is designed to work with LINE LEVEL signals. It is not intended to be used as a pedal capable of receiving microphone or pickup level (high impedance) signals. If you wish to use this device directly with a microphone, guitar or other "pickup"-type musical instrument, you will first need to send your instrument's signal through a pre-amplifier.

    This unit will work directly with mixer boards, keyboards, powered microphones, audio systems such as CD-players, computer sound generators, synthesizers and any other instrument or music source that produces line-level signals (low impedance).

    It is OK to "play" with the settings on the Digital Delay and figure out for yourself all of the effects you can do! There are many creative things that digital delays can offer. Grab the controls and twist away! It's all in your imagination!

    Controls and Their Functions

  • MIX Switch. This switch allows the original un-delayed signal to be mixed in with the delayed signal at the end mixer and controlled by DEPTH control pot.
  • DEPTH Control. This control sets the amount of delayed signal re-mixed with the original signal. It is used when doing echo, reverb and chorus.
  • DECAY Control. This control sets the volume of the remixed delayed signal with the original un-delayed signal.
  • DELAY TIME -- COARSE AND FINE. These two controls set the delay time. Use the COARSE adjustment to set the delay time first then fine tune in on the exact amount of delay you want using the FINE control.
  • CLOCK SELECT. This switch selects between the fixed clock generator and the variable clock generator. Use the fixed clock for most effects. Use the variable clock when you wish to do flanging effects. You will also find that the L/C oscillator will yield longer, deeper echoes and lengthier deep repeats. This is because of the slightly slower clock rate and the pulse width symmetry difference the variable clock has that the fixed crystal-controlled symmetrical clock does not have. The overall amount of delay time obtainable is a function of the clock rate and the amount of memory -- the slower the clock and the more memory the more delay. But a slower clock means a slower audio sampling rate. The variable clock will not sample the audio slower than about 28KHz. Slower rates can be obtained by varying the value of the L1 coil, but sampling slower than 28KHz may introduce "aliases" and noise in the sample.
  • FLANGE. This slide control slightly varies the variable sample clock rate. Sliding this control back and forth will emulate the "flanging" effect (finger against reel-to-reel tape reel "flange"). You can also vary the variable clock by sticking a steel or iron object into the tuning coil (like a small screw, screwdriver tip or cut piece of metal coathanger).
  • Additional Information

    You can double the amount of memory the delay has by simply removing the jumper at R23 and installing a jumper at R22. Doubling the memory will yield double the delay time.

    If you choose to make this modification be aware that the Delay Time controls will only let you adjust to half the total amount of generated delay. This is because the Delay Time potentiometers and their associated A/D converters only produce 16-bits of data. Installing the R22 jumper adds a 17th bit to the address. The effect is longer echoes and much lengthier repeats despite the fact that you can only adjust within the first 2 seconds of a total 4 seconds of delay. Try this modification; you won't even notice the lack of adjustment.

    Parts and Ordering Information

    Here are links and contact information for the recommended parts suppliers. You can purchase all of the parts used in this project from the suppliers listed below (not all from one, see the Parts List for supplier information), or you can shop around on the Internet for better prices and sources. Surplus outlets are especially useful when purchasing TTL logic chips, sockets, resistors and capacitors.

  • Circuit Specialists
    • Phone: 800-528-1417

  • Digi-Key
    • Phone: 800-DIGI-KEY

  • Jameco Electronics
    • Phone: 800-346-6873

  • Mouser Electronics
    • Phone: 800-831-4242

  • Radio Shack
  • Professional factory produced, pre-etched, punched, plated-thru and screened bare, unpopulated PC board with green solder mask is available from Lupine Systems.

  • Basic Digital Delay PC Board
    • Part Number: 16-334-10A
      Price: $42.95 USD plus shipping

  • Shipping Charges
    • One, Two or Three Boards: $8.00 USD Domestic Shipping Only
      Add an Additional $1.00 USD for each Extra Board. Domestic Priority Mail Shipping Only

  • We're Sorry, but this project is not being offered as an assembled ready to use product at this time. Please do not inquire into having us build one for you!
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