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A blast from the past – authentic spring reverb SPRING MODULE Add this spring reverberation module to your guitar, keyboard or organ amplifier and get that great “concert hall” effect. No longer do you have to practice in your bedroom, attic, basement or backyard shed. By turning up the reverb effect you can be transported to the concert hall of your dreams. By JOHN CLARKE 24  Silicon Chip B ACK IN THE “good old days” before digital effects became the vogue for musical instruments, electric guitars were often used with “spring reverberation” to get the echo effect of a large concert hall. Not only did the reverberation sound great but it could also make an average performer sound a lot better. And judicious use of reverb could make a small venue sound much larger and more impressive. But why bother with old technology when digital effects can be so much more flexible, more compact and not subject to any acoustic feedback? b The answer is to that like trying to explain why Hammond organs are so popular in modern bands when digital key­ boards are in so many ways superior. Spring reverb does have a particular “authentic” sound that isn’t quite duplicated by digital effects boxes. And anyhow, this little spring reverb module is cheaper than a digital effects box. A spring reverb unit consists of a box containing two or three stretched springs which are driven at one end by a voice coil – just like a loudspeaker but without the paper or plastic cone. The audio signal travels down the springs and is reflected back and forth and then is picked up at the other end by another voice coil unit. The echo signal can then be mixed with the original signal to produce a range of reverberation effects. For this project, we have arranged for Jaycar Electronics to import a compact 2-spring module which is much more compact than the spring modules used some 20 or 30 years ago. It measures just 264mm long x 52mm wide x 33mm deep. The spring reverb module has two characteristics which determine its overall reverberation effect. The first of these is the signal delay time and this is determined by the springs themselves at 22ms and 27ms. Then there is the decay time and this is typically around 1.2 to 2 seconds, depending on the circuit settings. We have a designed a PC board which fits over the metal chassis of the spring module and the complete assembly can then be suspended within your musical instrument amplifier, whether it is used for electric guitar, keyboard or any other musical in­strument. The spring reverb unit requires an unusual drive circuit. This is because the driving voice coil is an inductor Main Features •  2-spring reverb unit •  Input level control •  Reverb depth control •  Reverb in/out switching •  Wide frequency response and it has an impedance which is directly proportional to frequency. For example, it has an impedance of 8Ω at 1kHz but at 10kHz it is 80Ω. Down at 100Hz, the impedance is only 0.8Ω. To obtain a reasonably flat frequency response for signals fed through the module, we therefore need to apply ten times the signal level at 10kHz than at 1kHz and so on. And while the actual power levels are quite low, the drive current requirements are relatively large and so we have added a buffer stage which can do the job. Block diagram Fig.1 is the block diagram of the circuit. The input signal is applied to attenuator VR1 and then to driver amplifier IC1a which provides the rising frequency response. IC1b is the buffer stage which provides the drive current to the reverb module. The reverb output is then applied to switch S1 and then to recovery amplifier IC2a which amplifies the resultant signal. From there, the reverb signal goes to the depth control VR2. IC2b is a stage which mixes the reverb signal Fig.1: the block diagram of the spring reverb circuit. There is quite a lot of signal loss in the spring reverb module and this is made up in the recovery amplifier. January 2000  25 Parts List 1 PC board, code 01402000, 251 x 51mm 1 2-spring reverberation unit 2 knobs to suit potentiometers 1 push on/push off switch (S1) 2 RCA plugs (one white, one red) 1 1m length of shielded cable 1 500mm length of 0.25mm enamelled copper wire 1 50mm length of 0.8mm tinned copper wire 6 M3 x 6mm screws, nuts and star washers 8 PC stakes 1 50kΩ log potentiometer (VR1) 1 10kΩ log potentiometer (VR2) 1 1kΩ horizontal trimpot (VR3) Semiconductors 2 LM833 dual op amps (IC1,IC2) 1 BC338 NPN transistor (Q1) 1 BC328 PNP transistor (Q2) 1 7815 15V 3-terminal regulator (REG1) 1 7915 -15V 3-terminal regulator (REG2) 4 1N4004 1A rectifier diodes (D1-D4) Capacitors 2 1000µF 25VW PC electrolytic 6 10µF 35VW PC electrolytic 1 2.2µF bipolar electrolytic 1 0.22µF MKT polyester 2 0.15µF MKT polyester 1 .039µF MKT polyester 1 .033µF MKT polyester 1 .015µF MKT polyester 3 .01µF MKT polyester 1 .0039µF MKT polyester 1 100pF ceramic or MKT polyester 1 33pF ceramic 1 10pF ceramic Resistors (0.25W, 1%) 1 820kΩ 3 1kΩ 1 470kΩ 3 220Ω 4 220kΩ 2 100Ω 4 100kΩ 2 10Ω 6 10kΩ 1 6.8Ω 1W with the input signal. Switch S1 can be a foot-switch which enables or disables the reverb effect. Circuit details Fig.2 is the complete circuit for 26  Silicon Chip The Spring Reverb Module is based on this compact 2-spring unit from Jaycar Electronics. It is much more compact than the spring modules used 20-30 years ago, measuring just 264mm long x 52mm wide x 33mm deep. The two springs provide signal delay times of 22ms and 27ms. the spring reverb module. The input signal is applied through a 100Ω resistor and .0039µF capacitor which attenuate frequencies above 400kHz. This prevents the possibility of radio frequency breakthrough. From there the signal goes to 50kΩ level pot VR1 and then to pin 5, the non-inverting input of IC1a, via a .01µF capacitor. This capacitor and the 100kΩ resistor provide a low frequency rolloff below 160Hz. IC1a provides a rising frequency response by virtue of the 1kΩ resistor and .01µF capacitor connected to pin 6. These pro­vide a rolloff below 16kHz, while the 100kΩ resistor and 100pF capacitor between pins 6 & 7 roll off signals above 16kHz. The result is a response which peaks at 16kHz with a nominal gain of 40dB (100) and rolling off above and below this at a rate of 6dB per octave. Fig.3 shows the actual response of the driver amplifier. Buffer & output stage Op amp IC1b and transistors Q1 & Q2 make up the buffer and output stage. IC1b drives the complementary transistors and they are included in the feedback network of the overall amplifier. The signal from IC1a’s output is fed to pin 3 of IC1b via a 1kΩ resistor while 100% feedback from the emitters of Q1 & Q2 is fed via a 1kΩ resistor to pin 2, giving an overall gain of 1. Q1 & Q2 are slightly forward biased using the 10kΩ and 220Ω resistors at their bases. Their 10Ω emitter resistors apply local negative feedback to Fig.2: the complete circuit details for the Spring Reverb Module. IC1a, the driver amplifier, has a rising frequency re­sponse to compensate for the inductive reactance of the spring reverb’s voice coil drive. IC1b, together with Q1 & Q2, drive the reverb unit while IC2a makes up for its considerable signal attenuation. January 2000  27 AUDIO PRECISION FREQRESP AMPL(dBV) vs FREQ(Hz) 40.000 30 AUG 99 14:19:48 30.000 20.000 10.000 0.0 -10.00 -20.00 50 100 1k 10k 50k Fig.3: this is the frequency response of driver amplifier IC1a which peaks at 16kHz. The nominal gain at this frequency is 40dB (100), the response rolling off above and below this at a rate of 6dB per octave. stabilise their quiescent cur­rent. The 220Ω resistor between IC1b’s output and the junction of the 10Ω resistors allows the op amp to drive the load directly at very low signal levels and it has the effect of lowering the overall distortion of the buffer amplifier. The buffer stage drives the spring reverb via a filter network consisting of inductor L1, a 6.8Ω resistor and 0.15µF capacitor. This filter and the .01µF capacitor connected across the 1kΩ feedback resistor ensure high frequency stability in the buffer amplifier. DC offset adjustment Trimpot VR3 is included to adjust the offset voltage at the output of the buffer stage. This should be as close to Fig.4: signal delay through the spring reverb unit. The top waveform is a burst input signal while the lower trace is the output which contains the original burst and the delayed signal from the springs. One spring provides a 22ms delay while the second spring gives a 27ms delay. 28  Silicon Chip zero as possible so that no DC voltage is applied to the spring reverb input. Any offset voltage here would cause considerable current to flow in the spring reverb’s driver coil due to its very low DC resistance of 0.81Ω. For example, if the offset voltage at the output of the buffer stage was a mere 100mV, the current through the voice coil would be over 120 milli­amps. With VR3 adjusted for minimum output, it should be possible to keep the output offset to around 1mV or so. The two back-to-back 10µF capacitors connected to the wiper of VR3 are there to prevent latch up in IC1 as power is first switched on. Without the back-to-back capacitors, the effect of one of the supply rails reaching 15V faster than the other would mean that VR3 could possibly apply 100mV or more to pin 6 of IC1a and that would cause the op amp to latch up. If nothing else, the effect would be a very loud thump fed to the external power amplifier and speakers. OK. So the spring reverb is being driven with signals which race up and down the springs and then emerge at the output voice coil. The process involves quite a bit of signal loss and this has to be made up in the aptly named “recovery” amplifier, IC2a. After all, as you can imagine, the output signal is probably feeling a little wobbly after going through those Fig.5: this scope shot shows the signal decay from the circuit with maximum reverb depth. The top trace shows a burst input signal while the lower trace shows the output signal from the reverberation module decaying over a period of 2.5 seconds. Mixer stage The output from IC2a is fed via a 0.22µF capacitor to the 10kΩ depth control pot VR2. This sets the signal level applied to mixer amplifier IC2b via a .033µF capacitor and 220kΩ resis­tor. The input signal to the reverb module is also applied to the mixer amplifier via a .039µF capacitor and another 220kΩ resistor. Since the feedback resistor between pins 1 & 2 is also 220kΩ, the gain of the mixer is set at -1. Frequencies above 22kHz are rolled off by the 33pF capacitor connected across the feedback resistor. The output from IC2b is coupled via a 2.2µF bipolar capaci­ tor and 100Ω resistor. Power supply Power for the reverb circuit is derived from a 30V centre-tapped transformer which is rectified and filtered to provide a ±21V supply. This is regulated to ±15V with REG1 and REG2. The output of the regulators is de­coupled with 10µF capacitors. Also each op amp package has its supply decoupled with 10µF 35VW capacitors. Construction As already noted, we have designed a PC board which fits on top of the spring reverb unit. The PC board measures 251 x 51mm and is coded 01402000. You can start construction by checking the PC board for breaks or shorts between tracks and undrilled holes. Fix any defects you find. The centrally located holes at the far ends Fig.6: the component overlay and wiring connections to the PC board. This mounts on top of the spring reverb unit. Note that the metal cases of the two potentiometers should be connected together and earthed as shown. springs and does need a little time to recuperate. Before the output signal from the spring reverb can get to IC2a, it must first get past S1, the in/out switch. If S1 is switched to the “out” position, the signal from the spring reverb is shunted to the 0V line and that is the end of it. Conversely, if S1 is open, IC2a does its job, amplifying the signal by a factor of 83, as set by the 10kΩ and 820kΩ resistors in the feedback network. To minimise hum pickup from the spring module, the frequency response below 100Hz is rolled off by the 0.15µF capacitor con­necting the 10kΩ feedback resistor to 0V and the .015µF capacitor and 100kΩ resistor at pin 5. January 2000  29 Table 1: Capacitor Codes  Value   IEC EIA  0.22µF 220n 224  0.15µF 150n 154  .039µF   39n 393  .033µF   33n 333  .015µF   15n 153  .01µF   10n 103  .0039   3n9 392  100pF 100p 101  33pF   33p   33  10pF   10p   10 of the PC board need to be drilled out to 13mm so that they clear the neoprene mounting grommets on the spring reverb case. The two holes adjacent to these can be 3mm in diameter. The holes for the PC mounting pots need to be 2mm in diameter and the mounting holes for the regulators should be 3mm in diameter. Start by installing the wire link and all the resistors except for the 6.8Ω 1W resistor. Check the resistor values with a digital multimeter before you install each one or check the colour codes against those shown in Table 2. The two regulators are bolted to the PC board. Bend their leads at right­ angles so that the regulator tabs line up with the mounting holes on the board. Be sure that each regulator is in the correct position before soldering its leads. Next, install the capacitors and take care with the elec­trolytics which must be connected the right way around. Note also that the two electrolytic capacitors adjacent to IC1 and IC2 must have a voltage rating above 30V since they are connected across the 30V supply rail. The MKT types have a value code and these are shown in Table 1. Table 2: Resistor Colour Codes            No. 1 1 3 4 6 3 3 2 2 1 30  Silicon Chip Value 820kΩ 470kΩ 220kΩ 100kΩ 10kΩ 1kΩ 220Ω 100Ω 10Ω 6.8Ω 4-Band Code (1%) grey red yellow brown yellow violet yellow brown red red yellow brown brown black yellow brown brown black orange brown brown black red brown red red brown brown brown black brown brown brown black black brown blue grey gold brown 5-Band Code (1%) grey red black orange brown yellow violet black orange brown red red black orange brown brown black black orange brown brown black black red brown brown black black brown brown red red black black brown brown black black black brown brown black black gold brown blue grey black silver brown This view shows the completed PC board, mounted on top of the spring reverb case. You can either build the completed module into a case of its own and add a power supply or, if there’s room, build it into an existing amplifier. Trimpot VR1, the diodes, PC stakes and transistors can be installed next. Make sure you install the transistors in their correct positions. The two potentiometers are soldered directly into the PC board. However, if you wish to mount them off the PC board this can be done using shielded cable. The shield connec­tion is soldered to the terminal marked GND on the PC board. Cut the pot shafts to length suitable for the knobs before installing them. The 6.8Ω 1W resistor has the coil for L1 wound over it; 24 turns of 0.25mm enamelled copper wire. Strip the enamel off one end of the wire and tin it with solder. Wrap this around one of the resistor leads and solder it in place. Then wind on 24 turns along the resistor body. Cut and strip the enamel off the other end of the wire, wrap it around the resistor lead and solder it. Insert the resistor/choke into the PC board and solder it in place. All signal connections to the PC board are made using shielded cable and cables to the reverb unit will need to have RCA plugs fitted to suit the input and output sockets. Bolt the PC board to the spring reverb unit with 4 x M3 screws and nuts but do not connect the RCA terminals to the input and output sockets just yet. Power supply Before you can test the reverb unit you will need to have a suitable power Fig.7: follow this diagram if you are using the 240VAC mains transformer. All exposed mains terminals should be covered in heatshrink tubing and you should use cable ties on the mains wires so that if one becomes detached, it cannot contact other parts of the circuit. January 2000  31 Specifications Frequency response of undelayed signal ........................... -3dB at 22Hz and 19kHz Frequency response of reverb signal ................................. -3dB at 100Hz and 5kHz Delay times .....................................22ms and 27ms (see oscilloscope waveforms) Decay time ............................................. 1.2-2 seconds (depending on signal level) Sensitivity ................................................................................. 34mV RMS at 1kHz Signal-to-noise ratio (reverb off) ........................... -84dB unweighted (20Hz-20kHz) -88dB A-weighted with respect to 1V output Signal-to-noise ratio (maximum reverb) ............... -73dB unweighted (20Hz-20kHz) -76dB A-weighted with respect to 1V output Fig.8: actual size artwork for the PC board. Check your board carefully before installing any of the parts. Frequency response of driver amplifier ...................................................... see Fig.3 32  Silicon Chip supply. If you have one which can deliver ±20V you can use it to power the positive and negative regulators directly. Failing that, you will need to wire up the 2855 trans­former as shown on the circuit. Alternatively, you may be able to pick up the necessary ±15V supply rails from inside your music instrument amplifier or mixer. The extra current drain from each supply rail will be about 50mA. If you can take this approach, you will be able to omit the 15V regulators. On the other hand, if your music instru­ment amplifier has balanced supply rails between, say, ±18V and ±30V, you should leave the regulators in place. If you need to mount the specified 2855 transformer in existing equipment, try to locate it away from sensitive input circuitry. You should be able to pick up the switched mains voltage where it is connected to the existing power transformer input. Using a separate case If you intend to install the reverb unit into its own case, follow the diagram of Fig.7 when running the 240VAC mains wiring. All exposed mains terminals should be covered in heatshrink tubing and you should use cable ties on the mains wires so that if one becomes detached, it cannot contact other parts of the circuit. The metalwork of the case must be earthed. Use a screw, nut and star washer to secure the earth lug to the case. Some metal cases will require the paint to be scraped away from the earth terminal area before a good contact can be made to the case. Testing Now you are ready to test the reverb unit. Apply power and check that the op amps are supplied with 15V. You should obtain a reading on your multimeter of +15V between pin 8 of both IC1 & IC2 and the 0V (ground) line. A reading of -15V should be obtained at pin 4 of IC1 & IC2. Now connect your multimeter set to read DC millivolts across the “to spring reverb input” terminals on the PC board. Adjust VR1 so that the reading is as close to 0mV as possible. You can now connect the RCA plugs to the spring reverb unit and you are ready to test it. You will need a power amplifier and loudspeaker and a suitable music instrument as the driving signal. You could also plug a guitar straight in and avoid the need for a preamplifier. Adjust the depth control fully anticlockwise and adjust the level pot for a suitable volume. Now adjust the depth pot and check that you can hear the reverb effect. You will find that the reverb effect increases as the depth control is adjusted clock­wise. Note that if the reverb effect sounds distorted, you possi­bly have too much signal at the input and this can be adjusted down with the level pot. The transistors driving the spring reverb will also run warm. Too little signal will result in a poor signal-to-noise ratio. The optimum signal level is SC 22mV at the wiper of VR1.