Multi-Delay Project (Part 5)
A six tap, stereo analogue delay constitutes this month's self-construct module for our 19" rack-mounting effects system.
This exciting module for the Effects Rack described in our December 84 issue opens the way to high quality time domain effects including ADT, chorus, echo, vibrato and reverb. Paul Williams reveals the unit's unique combination of features such as a built-in limiter, auto-optimising bandwidth, true spatial stereo outputs and multi-reflection reverberation.
There will still be doubting Thomases among you, so we have pre-empted some of your questions:
Q "All the analogue delays I've heard sound woolly; don't BBDs have a narrow frequency response?"
A Only because the designer is compromising bandwidth to achieve a decent delay time. The Multi-Delay's bandwidth is automatically optimised for any delay setting, giving rise to a bandwidth of up to 12kHz.
Q "But don't BBDs have a very limited dynamic range, and aren't they all too easy to overload?"
A The Multi-Delay has a built-in limiter, making it virtually impossible to overload, whilst giving an equivalent input dynamic range of up to 104dB.
Q "What about all the noise though?"
A The Multi-Delay uses a low noise delay chip, backed up by pre- and de-emphasis to give a typical output noise of -85dBm(A), and no quantisation noise either.
Q "I've got you now, you can't get reverb from a BBD - it just sounds like fast echo - doesn't it?"
A Multi-Delay uses a 6 tap delay line to give 6 different delay times simultaneously, producing a more diffused and natural reverb effect.
Q "I want a true spatial stereo output which is mono compatible - a BBD can't do that - can it?"
A The Multi-Delay can. It produces a sensational stereo field by using three different, unrelated delay times on each channel.
On its own the Multi-Delay can produce artificial double tracking (ADT), reverberation, echo, slap echo, flutter echo, or just straight delay for adding pre-delay to an external reverberation device (or another Multi-Delay!). Using next month's optional Modulation Oscillator however, expands the Multi-Delay's repertoire to encompass true vibrato, a magnificent chorus and some unique key and CV controlled effects. In the reverb mode, three separate delay taps are routed to each output channel. In the echo mode, however, only one tap is used on each channel, but since these two taps have different delay times, even the echo mode produces a true stereo output.
The Multi-Delay will accept inputs from a wide range of sources, from mixer echo sends to DI'd guitars. The built-in limiter means that you don't have to operate close to the clipping level, watching for an LED indicator to tell you that you have just ruined the track! The LED on the Multi-Delay in fact tells you when you are at optimum (limiting) level, giving you the best signal-to-noise ratio. Even after the LED starts to glow, there is still 30dB of headroom left before clipping sets in.
Most effects, with delay times up to 44 milliseconds are achieved with the maximum bandwidth of 12kHz. As the delay time is increased, the bandwidth automatically reduces, giving the maximum possible bandwidth for any delay setting. Figure 1 shows how the bandwidth varies with delay time.
The heart of the Multi-Delay is IC6, the BBD. For those not familiar with these devices, an analogue signal presented to its input is passed sequentially along a chain of thousands of capacitors using FET switches under the control of a clock. The delay time from one end of the chain to the other is thus controlled by, and inversely proportional to, the clock frequency. The Panasonic MN3011 device used here incorporates six taps along the chain of capacitors at non-harmonically related intervals, all of which are available as outputs. In the reverb mode, it is the correct mixing of these outputs which allows a natural sounding reverb with a smooth decay 'tail' to be produced.
Going right back to the audio input, IC1a provides a high input impedance with variable gain. The signal passes from here via the pre-emphasis network to IC1b, whose gain is determined by the negative feedback produced by the Operational Transconductance Amplifier (OTA) IC2a, which becomes active when limiting takes place. To prevent aliases with the clock frequency, the input signal then passes through IC3, a fourth order low pass switched capacitor filter, and thence via IC4b to the BBD chip, IC6. The limiting threshold is determined by the Darlington OTA that buffers IC2c and d which, along with IC4a, perform full-wave rectification via R19 into C10. The current then developed in the emitter of TR1 is injected into the gain controlling OTA, IC2a. Gain reduction is detected at onset by TR2, driving the LED D2.
The cut-off frequency of the anti-aliasing filter and the two reconstituting filters IC9 and 10 is determined by a high frequency clock which is generated from a Schmitt trigger within IC3, using the OTA IC2b to control the charging current into C6, and thus the clock frequency.
The BBD clock is produced by dividing the high frequency clock by eight using IC5. The cut-off frequency of the audio path is thus directly proportional to the clock frequency of the BBD, and hence inversely proportional to delay. The delay time is controlled manually using VR2 to adjust the current control being produced by IC2b. The CV input allows a 0 to +5 volt sweep to cover the full delay range.
IC8a and b perform the critical mixing of the BBD outputs. When the echo mode is selected, only one output from the BBD is used in each channel mix, as determined by SW1. After passing through the reconstituting filters IC9 and 10, the composite signals are de-emphasised by IC11a and b and then mixed with the 'dry' signals from IC1a using VR5, from which the output signals are taken. When no plug is inserted in the right output JK4, the three right channel delays are mixed via R45 and C16 into the left channel.
The last tap from the BBD is recirculated back to the input via the Regen control VR3 to produce the required sustain or reverb time.
The supply rails are well decoupled by using several capacitors. An analogue reference voltage is produced by R52 and 53 to determine the bias point for the switched capacitor filters and BBD. This is adjusted by means of VR6 to maximise headroom. 'Dirty' noise currents are kept away from the system 0V rail by using a separate digital ground, isolated from 0V by R54.
The double-sided PCB supplied in the kit not only eliminates the need for links, but also provides a ground plane to keep clock noise at bay. As usual in this series, all connectors, switches and pots mount directly on the PCB, so there is no interwiring to do either.
The first step in construction is to insert from the (screen printed) component side, the 49 track pins at the positions ringed on the overlay. This is best done by leaving the 'stick' of pins intact until the leading pin is pushed home, when the stick can be broken away at the second pin in line. These are then soldered on both sides of the board (don't miss any).
Next insert, solder and crop the resistor leads, say ten or so at a time to prevent crowding. Bending the leads outward at 45 degrees prior to soldering will hold the components in place without running the risk of shorting together a pair of pads. Don't be tempted to lay the leads down flat onto the component side of the PCB since this could cause a short to the ground plane.
Taking care with orientation, locate and solder the diodes D1 and 3, and the transistors. The IC sockets come next, making sure that they are pressed down onto the PCB whilst soldering, but leaving the ICs themselves out until later. Now insert and solder the capacitors, taking care with the polarity of the electrolytic types. Before soldering the presets VR4 and 6, lean them back slightly to ease adjustment later. The bus connector and the four jack sockets can then be soldered whilst holding them firmly down onto the PCB. A piece of foam rubber laid on the bench comes in handy for holding connectors and the like in place on upturned PCBs during soldering.
Trim each pot shaft to 8mm from the bush using a hacksaw whilst holding the shaft in a vice. Fit a PC bracket to each pot and locate into the appropriate PCB positions, but don't solder at this point. After determining the correct orientation of the LED, bend its leads down at right angles, 4mm from its body, and locate into the PCB without soldering. Place shakeproof washers on the pots and switches, then offer the front panel up, feeding the pot and switch bushes and LED dome into the appropriate panel apertures. The panel is then fixed in place by means of the pot nuts, which should be fully tightened. Only finger tighten the front switch nuts however, leaving the final securing to the rear nuts, which must be screwed up tight against the rear of the panel. The pots, brackets, switches and LED can now be soldered, after making sure that they are all positioned correctly, and that the panel is at right angles to the PCB.
Spend some time now to check over the assembly very carefully, inspecting all joints using a magnifying glass, or better still, an eyeglass. The assembly is too complex, and the ICs too expensive to skimp on this vital procedure. When you are completely satisfied that all is well, the ICs can be loaded into the sockets, taking care with orientation.
For ICs 3, 5, 6, 7, 9 and 10, additional precautions must be taken to prevent static damage to these MOS devices. There is no need to be frightened of them though; just make sure that they are left in their conductive packing until the last moment. Touch both the conductive packing and the PCB ground plane whilst transferring them and avoid, if possible, touching the IC leads. It is preferable that you are not wearing a nylon sweater, nor is it advisable to go for a walk around on a nylon carpet prior to the transfer!
Finally, fit the knobs with their caps so that the marker line of each covers the scale evenly, then push on the toggle lever covers.
With the Multi-Delay module positioned in the rack, set the presets VR4 and 6 initially to the mid-way point. Set the controls as follows: Delay to 0, Regen to 0, Mix to 10 and toggle switches to 'echo' and 'out'. Switch on the rack power, apply a clean tone of moderately low frequency (from a synth, bass, guitar or signal generator) to the input and make sure that this can be heard by monitoring either of the outputs, at a level determined by the 'input' control.
Switching the unit 'in' should now make no appreciable difference to the sound. Now advance the input control and/or input signal level way past where the LED starts to glow, to the point just before distortion starts to set in. By adjusting VR6, the lower preset, you should find that the extreme settings cause more distortion, but with a distortion-free band near the centre. Set VR6 so that it is in the middle of the distortion-free band.
Now back off the input control so that the LED does not glow, then set Delay to 5 and Regen to 10. Turn VR4, the upper preset, so that the echoes build up, then adjust it slowly anti-clockwise until the echoes start to gradually tail off. Try different inputs and delay times, making sure that the unit is stable in all circumstances.
Feature by Paul Williams
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