An exciting new effects unit of interest to all electro-musicians.
'Panolo' is a term we have invented to describe this easy to build and operate project for the electromusician looking for a new effect. Despite its low cost, the Panolo gives high quality results and can be used in series with any keyboard (organ, synthesiser, etc.) and guitar prior to your amp or mixer.
The Panolo unit is in essence a split phase tremolo unit which also provides additional panning effects as well as conventional tremolo.
The normal tremolo effect simply consists of varying the volume of the processed signal (normally automatically) at a frequency of between about 0.1Hz and 10Hz.
A split phase tremolo is basically just two ordinary tremolo units, but as the gain of one signal is boosted, the gain of the other is reduced. This enables the following interesting effects to be obtained:
1. Conventional tremolo by using only one channel (in and out) of the unit.
2. Conventional split phase tremolo, where the single input signal is applied to both inputs, and is continuously panned from left to right and vice versa at the stereo outputs.
3. Used to process a stereo signal, first one channel and then the other becomes dominant.
4. With separate signals applied to the inputs, but with the outputs mixed together, a mono output is available having each input signal alternating between high and low levels — giving the impression of two sound sources swapping with each other.
This variety of options gives plenty of scope for experiment in the movement of sound sources and will produce exciting effects on stage with a stereo PA feed, exploited on Kraftwerk's 'Computer World' LP and Jean-Michel Jarre's 'Magnetic Fields' (using a special sequencer — see this month's feature).
The block diagram of the Panolo is shown in Figure 1; two voltage controlled amplifiers (VCAs) are at the heart of the unit. By means of a low frequency oscillator the gain of each VCA can be varied at the appropriate rate with the output of the oscillator connected to the control input of each VCA.
The only slight complication is that the two control signals have the same frequency, but must be opposite in phase. In other words, as one signal is positive going the other must be negative going, and vice versa. This is achieved here by obtaining one control signal from the oscillator via an inverting buffer amplifier so that a control signal of opposite phase to the oscillator's output is obtained. The oscillator is also used to operate an LED indicator which flashes at the Panolo 'rate' setting. A switch enables the two inputs to be connected together when a conventional split phase tremolo effect is required.
Figure 2 shows the complete circuit diagram of the Panolo, and the two VCA's (IC3), which are based on an LM13600N dual operational transconductance amplifier (OTA). An OTA is different to an ordinary operational amplifier in a number of respects. The two main differences are that a transconductance amplifier gives an output current that is governed by the differential input voltage, whereas an ordinary operational amplifier has an output voltage that is governed by the differential input voltage. A transconductance amplifier has a third input terminal (the amplifier bias input), and the conductance (gain) of the amplifier is controlled by the bias current fed to this input. In order to use a transconductance amplifier as a voltage controlled amplifier it is necessary to connect a resistor between the output terminal and the 0V rail so that the output current develops a voltage across this load resistor, and the circuit then acts as a voltage amplifier. A resistor is connected in series with the input to the amplifier bias input so that the bias current is proportional to the applied voltage.
In this circuit R11 and R19 are the output resistors, and R8 plus R18 are the series resistors at the amplifier bias inputs. The output impedance of a transconductance amplifier is rather high, and a buffer amplifier is therefore used at the output of each VCA. These amplifiers have internal Darlington Pair emitter followers but require discrete load resistors, viz: R9 and R21.
The circuit is powered by a single 9 volt battery, and the central 0V rail is therefore formed by a potential divider across the supply rails (R14 and R15). R13 and R16 bias the non-inverting inputs of the transconductance amplifiers to the 0V rail, and the inverting inputs are connected directly to this. The input signals are coupled to the non-inverting inputs by coupling capacitors C4 and C6, and series resistors R22, R12, R23 and R17. The resistors boost the input impedance of the circuit to an acceptable level of a little over 16k, and the losses they introduce prevent the VCAs from having excessive voltage gain. It is not possible to reduce the gain of the circuit and produce increased input impedance by using negative feedback over the amplifiers as the feedback would try to maintain the voltage gain of the amplifiers at a certain level, and the desired VCA action would not be obtained.
Although the transconductance amplifiers are used open loop they still give low levels of noise and distortion provided the circuit is not overdriven or used with a very low input signal level. The LM13600N has linearising diodes at the input of each amplifier, and by applying a small bias current to these the overload margin of the amplifiers can be boosted. The bias currents are provided by R10 and R20 in this circuit.
S2 is the 'mono'/'stereo' switch, and simply connects the two input sockets in parallel when in the 'mono' mode: R22 and R23 prevent possible damage to external equipment should the 'mono' switch position be accidentally selected whilst both inputs are in use.
The low frequency oscillator uses an ICM7555 device, which is the CMOS version of the popular 555 timer IC. The CMOS version is used in this circuit as it gives a lower current drain on the battery, and it does not introduce noise spikes on to the supply lines. The main output of IC1 (pin 3) cannot be used to provide the modulation signal as the waveform here is rectangular, and this would result in the VCAs being switched between two levels of gain, rather than being smoothly varied between the gain limits. However, the output at pin 3 of IC1 is used to operate LED indicator D1 which gives a visual indication of the tremolo rate.
The waveform produced across C1 is roughly triangular, and this is a suitable modulation waveform. IC2b is used as a buffer amplifier between C1 and the control input of IC3b, and the voltage gain of IC2b is controlled by RV2 which is the modulation depth control. Maximum resistance gives maximum modulation depth. S1 can be used to short circuit RV2 and thus switch off the tremolo effect. The tremolo frequency is controlled using RV1, and can be varied from about 7Hz at minimum resistance, to approximately one cycle every seven seconds at maximum resistance. IC2a is used as the basis of the second buffer amplifier, and this is a straightforward unity gain inverting amplifier which is interposed between the output of IC2b and the control input of IC3a, to provide a low driving impedance.
S3 is the on/off switch, and SK5 enables the unit to be used with an external 9 volt power supply (such as the E&MM Synpac, featured in September 1981), whilst at the same time disconnecting the battery.
The recommended case for the unit is a Verobox having approximate outside dimensions of 205 x 140 x 40mm, but any case of around this size should be suitable. The general layout of the unit can be seen from the accompanying photographs, but the layout is not critical and using a different arrangement should present no difficulties.
Figure 3 and 4 shows the component layout and other details of the printed circuit board. Although IC1 is a CMOS device it has built-in protection circuitry that renders special handling precautions unnecessary. IC2 has a PMOS input stage, and does require the normal MOS handling precautions. Use a socket for this device, and do not plug it into circuit until the unit is complete in other respects. Leave IC2 in its protective packaging until it is to be inserted in its socket, and handle it as little as possible. Although IC1 and IC3 are not prone to damage from static charges, they are not the cheapest of devices and it is probably worthwhile fitting these in sockets as well. Note that IC1 has the opposite orientation to the other two integrated circuits.
Use Veropins at points on the board where connections to off-board components will be made. The finished board is mounted on the base panel of the case using 12.7mm 6BA screws, 6.35mm spacers, and fixing nuts. The spacers are needed to prevent excessive stress and possible damage to the board when the fixing nuts are tightened. Figure 3 shows all the point to point wiring of the project, and it is not essential to use screened leads for this (ribbon cable was found suitable when wiring up the prototypes).
A piece of foam material can be glued to the lid of the case so that the PP6 size battery is trapped firmly in place when the lid is fixed in place. The current consumption of the completed unit should be in the region of 12mA. Power is connected to SK5 using a lead terminated in a 3.5mm jack plug, and the tip of the plug carries the positive supply with the barrel carrying the negative supply.
When used to give a split phase tremolo effect, S2 must be set to the 'mono' (closed) position and the input signal can be applied to SK2 or SK3. In this mode the unit can also be used to give a simple tremolo effect with the input applied to SK2 or SK3, and the output taken from SK1 or SK4. S2 is set to the 'stereo' (open) position for other modes of operation. If a wider tremolo frequency range is required C1 can be made larger in value to expand the low frequency range, and R3 can be made lower in value to increase the upper frequency limit. However, this will make it a little more difficult to set the desired tremolo rate, especially at the high frequency end of the range.
If a foot operated bypass switch is required S1 can be replaced with a 3.5mm or ¼" jack socket, and an external foot operated switch can be connected to this. If one channel of the unit has substantially more gain than the other the value of R12 or R17 can be reduced somewhat to increase the gain of the weaker channel to a suitable level.
For stage use, BNC, DIN, XLR or other latching connectors are recommended for reliable connection of the external power; it is important to note that if the unit is powered from an external source via connectors other than the specified jack, S3 must be switched to the off position. Also S1, 2 and 3 will be less prone to damage if rotary or push-button types are adopted.