A professional quality stereo audio control system for group and theatre PA use

PARTS COST GUIDE £90 plus case

Nowadays it is usual for a rock band to perform on stage with a high power sound system termed the 'PA'. This is the second part of a two part article and describes designs for an active crossover and a balanced line driver to be used in conjunction with the limiter and peak program meter detailed in the first part. These designs can, however, be installed in existing equipment allowing a fairly cheap upgrade. For those people wishing to build the complete Signal Processor a suitable power supply and constructional details are also given.

Active Crossover

When the scheme of an active crossover is first compared with a passive system the only gain seems to be cost, since more power amplifiers are required. Figure 8 shows the layout of a passive and an active crossover system. Such an active system is said to be 'tri-amped'.

The passive crossover will be a combination of inductors and capacitors which is very hard to design and not very efficient. The reasons for using an active system are summarised as:

(a) Overload of a sound system occurs initially in the bass region. This overload generates high order harmonics which are audible in a full range speaker, but not in a bass speaker since it cannot reproduce the high frequencies properly. As a result quite large amounts of overload may pass unheard. In any case, any small amounts of harmonics that do escape will be masked by the treble reproduced by a different drive unit.

(b) Any passive crossover and speaker network must be designed to remove impedance dips which could cause the amplifiers' protection circuits to operate. Attempts to do this will result in even lower efficiency. An active crossover avoids this since its input and output are defined and, furthermore, the damping of the speaker at low frequencies is much improved because it is connected directly to the amplifier.

(c) The crossover can be designed with steeper slopes if required. The crossover points may be changed at will, in this case by altering 8 resistors, the values of which are easily calculated. No precision inductors or bulky non polarised capacitors are required either.

(d) A tri-amped system offers some degree of protection in event of failure. Simple signal repatching to feed full range into a bass bin may not sound too good, but it is a lot better than nothing. On this point note that a radial horn must never be fed with frequencies below its specified crossover point.

(e) In general, a tri-amped (or even 4-or 5-amped) system will sound louder than the equivalent power into a passive system. This is mainly because the system can be turned up without overload being audible and to some extent because no energy is wasted in a passive crossover element.

Crossover frequencies and roll off

The prototype was designed to crossover at 230Hz and 2kHz, these being suitable bands for most bass, mid and horn treble drivers. The design originally specified 250Hz for the bass crossover, but this was altered to suit component values with no obvious effect. It is possible to use more than 3 bands, although three is most common. Placing a crossover point in the middle of the mid-range should be avoided because experiments have shown that problems do occur due to phase anomalies which are particularly noticeable at these frequencies.

A further bone of contention centres around the optimum rolloff of such a crossover. This design employs filters of 24 dB/octave. Some schools of thought state that the sudden phase change of such a filter is undesirable and a shallower rolloff is better. This may well be true in a domestic installation, but in a PA other factors (alignment of speakers, shape of room) will cause more effect and so it seems desirable to keep the crossover within the smallest frequency band. The characteristics of the prototype crossovers are shown in Figure 9.

Filter design

Since a crossover consists of filter modules, it is necessary to examine such a module. It is not the purpose of this text to give a full description of the operation of filters; such work is covered elsewhere. The basic module used is a second order, equal component value (ECV), Sallen & Key filter. Figure 10 shows the circuits of such filters. Note that the high pass is simply a transposition of the low pass, a property of distinct use in crossover design since the mid-range filter can be derived by transposing the bass and treble filters; the values required being the same.

The break frequency of such filters, f_b, is given by 1/(2πfCR), and the Q factor by 1/(3-K), where K is the loop gain determined by (RA+RB)/RA. In all audio filtering a Butterworth filter should be used since this offers a flat top response and a reasonable actual roll-off. A Butterworth filter is characterised by setting the value of Q to 0.7, or slightly less. Using this figure the suitable crossover points may be chosen. In the prototype, frequencies of 230Hz and 2kHz were arrived at after due consideration of resistor availability and tolerance. A more precise crossover can obviously be made if desired.

The second-order section offers a 12 dB/octave roll-off and so two sections were cascaded to give the desired 24 dB/octave. Figure 11 shows the block diagram of the crossover. Note that the mid-range filter is simply a cascaded low pass and high pass with values corresponding to the transposition of the other edge of the crossover.

The complete circuit can be seen in Figure 12. Each filter section is fronted by a level control which will normally be turned fully up in use and has been provided to facilitate setting up of the PA. A series resistor is included with each potentiometer, which is required to counteract the gain of the filter section. Each filter section is based around LF353 operational amplifiers, chosen again for their low noise and high input impedance. The components used are critical if very precise results are to be obtained, but this is generally unnecessary. Normal 5% resistors were used in the prototype, but the specified capacitors should be used. The output of each filter is fed through a 100R resistor and a 22uF tantalum capacitor before being made available as the output. The PA slave output is also included with the crossoverand this employs an identical output section to the filter elements. R131 allows the mute switch to be implemented. If a partial mute only is required a resistor should be placed in series with the mute switch.

Construction

The complete set of components are mounted on a PCB (Figure 13 a & b), including the potentiometers and the mute facility. Note that no earth lead to the power supply is fitted as this connection is made via the screen of the input lead which is connected to the limiter. Assemble the boards (2 for stereo), but do not fit the potentiometers until final case assembly is under way.

Testing

Apply power to the board and connect an input to each filter in turn (since the potentiometers are missing use a croc clip direct to the board). Check that the outputs correspond to the chosen frequency bands - as a guide check: bass - "muffled thumps", mid-range - "transistor radio", and treble - "tinny".

The Power Supply

The power supply is perfectly conventional, generating plus and minus 15 volts.

The Circuit

The mains input is fed, via a 'mains supplied' neon and a 1 amp anti-surge fuse, to the illuminated double pole rocker switch on the front panel. The output from this switch is connected directly to the PCB. A 15-0-15 transformer feeds a small bridge and two large smoothing capacitors (Figure 14). The bulk of the work is performed by 7815 and 7915 regulators, both mounted on 10°C/W heatsinks. Overall decoupling is provided by several 0.1uF capacitors and the 10uF output capacitors. Two LEDs provide a status monitor on the front panel.

The entire 0 volt rail of the unit is decoupled from mains earth by R1. This ensures that earth loops will not be created when an earthed signal source is used, but keeps the 0 volt line at approximately earth potential.

Construction

All the power supply components are mounted on one PCB (Figure 15). Solder in terminal pins at the relevant points and then fit the transformer. Bolt 4½" 4BA pillars to the PCB, having first marked out the correct positions using the transformer as a template. Ensure that a good contact is made with the earthed area. Fit the transformer by bolting it in place, and finally make the solder connections to its terminals. In the prototype, a delta capacitor was fixed to the transformer with a sticky cable tie. The delta was then connected to the spare mains terminals provided, noting that green/yellow is the earth wire. Assemble the rest of the board, not forgetting the wire link. Mica washers and heatsink compound should be used to insulate the regulator tabs from the heatsinks because the heatsinks may actually touch.

The specified mains transformer is a special PCB mounting type. Alternatively it should be possible to fit an ordinary (non-printed circuit mounting) type having similar ratings.

Testing

Cover all the exposed mains voltage terminals with tape to prevent any nasty accidents. Connect a meter between the positive and negative raw supplies (at the top end of the main smoothing capacitors), stand well back and switch on. The meter should read about 45 volts, and if it does not, switch off immediately, and find the fault. Finally check the outputs are plus and minus 15 volts.

The power supply may now be used to check other modules, but to prevent damage it is suggested that a 100R resistor is used in series with the module supply lines until correct functioning is established.

Balanced line output interface

The prototypes have been regularly used with a 25 metre multicore cable feeding 100K impedance amplifiers without any sign of hum or noise pick-up. It may well be desirable to use the processor with balanced line outputs if longer runs are envisaged and hence an optional board was designed for this purpose.

This interface simply inverts each signal, thus the original signal (audio phase) and the new signal (audio non-phase) are available. Stereo jack sockets (or Cannons) should be used for the outputs, wired as follows:

Jack tip - audio phase
Jackring - audio non-phase
Jack body - signal ground

A more detailed explanation of balanced lines appears in the first two issues of E&MM (March and April 1981). Figure 16 shows the circuit diagram for one channel of the balanced line driver. The two channels are incorporated on one PCB.

The circuit board is shown in Figure 17. There are no points to note about assembly, but care must be taken to ensure that ground loops are not created as it is wired up. The board has two earth points for left and right grounds, these should be connected to the PA Slave screens which are also connected on the crossover board. The screens from the three crossovers should not be connected at the interface, dummy circuit pins are provided for these connections. The audio non-phase output screens should be connected to the ground at the interface and also to the body of the jack socket, whereas the audio phase screen is only connected at the jack socket.

Overall Construction

The prototype was housed in a Classic 17 x 9 x 3½" instrument case. The front and rear panels were spray painted black, and finished with white Letraset.

The panel drilling details are shown in Figures 18 (front) and 19 (rear). Care must be taken when drilling the holes for the LED display, since any faults will show clearly. The panel should be carefully marked with a scribe, and each hole centre punched with a very sharp punch. A power drill will make drilling easier and it is worth buying a new ⅛" drill bit just for these holes. After drilling the display holes, put a small countersink on the rear to help guide the LED in. The holes to mount the display should be front countersunk. The rest of the holes are not as critical, although take care when making the mains switch cutout because the clearances are very tight. The back panel offers no problems.

Once the drilling is complete, two ½" pillars should be bolted to the front panel in the display mount holes. The holes should then be filled with plastic padding, allowed to set and smoothed off. Prime the panels with a metal priming spray paint, and then follow this with several coats of matt black spray, remembering to spray several light coats. It is a good idea to let this set hard for at least 24 hours before lettering it according to the legend. Note that the display was not calibrated except for the 0dB light in order to keep the panel simple. A letraset line or full calibration can be added if required, although it has been found that the colour change provides an adequate indication. The panels should then be varnished and left again for 24 hours or more before attempting final construction. Assemble the back panel first. Fit the IEC socket, neon, fuseholder and finally the jack sockets.

The front panel is next. First clean out the display LED holes and the PSU LED holes with the ⅛" drill, turned by hand from the front. This will remove the surprisingly thick accumulation of paint in them. Fit the mains switch and then the auxiliary function switches. Fit the power supply LEDs and glue them lightly in place with super glue, which incidentally is ideal for this purpose. Fit the display as described in the display section, not forgetting to test the bottom board before assembly is completed. Finally, fit the crossover boards by fitting the potentiometers with nuts partially tightened to the front panel using two ⅜" washers as spacers and then offering up the crossover board and soldering the potentiometer pins into the board. The potentiometer nuts can then be fully tightened. (Note: If long shaft pots are used - don't forget to trim them before assembly). After both boards are fitted, join them together with two 6BA pillars by means of the spare mounting hole, a 1" 6BA bolt and a ¼" 6BA bolt.

Drill the relevant mounting holes in the bottom of the case, the layout of which is shown in Figure 20, which also details the point-to-point wiring. Drill four holes for rubber feet and an extra hole to allow the case to be earthed. Fit ½" pillars into all the holes, and then bolt the boards in place.

Start wiring up the unit with the mains connections. First, the earth from the IEC socket is connected to the chassis by means of a solder tag and is then connected to the power supply board and the front panel. The live and neutral are connected to the mains switch, using proper blade connectors if required. Remember to connect the live via the fuse. Finally, the output of the mains switch is wired direct to the PCB. It is a good idea to test that all is well at this point.

Next, the power supply should be connected to all the modules. The ±15 volt rails go to each board (limiter, display, 2 x crossover and line output boards), but earth connections are only made to the limiter and display drivers. Also wire the power supply LEDs, taking care with the polarity.

The auxiliary switches should be wired next. Since the wires are very short, unscreened cable may be used, but single core should be avoided at all costs for reasons of reliability. The two unscreened wires from the limiter board to the relevant display drivers should also be added.

Finally, the signal wiring. Do not forget to mount C100 and C200 on the input socket in series with the input. Connect the screen at both ends to the socket earth and the low end of the input potentiometer. The wiper of the potentiometer is connected to the limiter board, the screen also being connected to the low end of the potentiometer, thus forming an earth connection from the limiter PCB to the input socket. The outputs of the limiter are then wired to the relevant crossover boards and again the screens are connected at both ends to earth the crossover board. The outputs from the crossover board are then wired to the balanced line driver or direct to the output sockets.

Remove all loose bits of wire and check all the connections, particularly checking the earth for continuity. The unit is now ready to be tested.

Final testing and setting up

As the individual boards should have been tested before final assembly, no real problems ought to be encountered. Apply an input signal and check that all the outputs are as expected and that no stereo reversal has occurred during the final wiring.

All that remains to be done is to calibrate the PPM and set up the limiter. If a scope is available, set the presets on the PPM boards so that the orange LEDs light with 2.17 volts peak to peak output from the limiter. If a scope is not available an analogue multimeter can be used instead, by measuring the voltage on the output of the precision rectifier (not the PPM drive) while playing the Blondie track 'Hanging on the Telephone' - the meter should be peaking at 0.95 volts when the LEDs peak on the orange.

The limiter is set up by playing a music track and adjusting the presets on the limiter so that no peaks exceed the orange LED. The Elton John track 'Funeral for a Friend' makes an excellent test for limiters. On listening to the output the action of the limiter should be clearly audible, particularly on the sudden snare drum about half way through this track.

The processor is now complete. Seal the presets and assemble the case. The unit should be left on for 48 hours at least, preferably kicking it at regular intervals to soak test it. Don't rush it into service though, a few hours testing it properly could well save considerable embarrassment in the middle of a gig since failures are most likely to occur within the first few hours.

The PA System Controller in Use

Obviously, every user of a system such as this will derive their own test sequence after a PA system has been rigged.

A test tape which is carefully recorded so that the peaks are exactly at 0dB makes a convenient sound source. The first job is to set the input level to the controller with the mute control in action. Once the levels are set, the output levels are checked to be zero and the mute switched off. The various parts of the PA are individually checked, starting with the TOP of each side. This is because a reversed connection may switch bass and treble; the result of feeding a treble horn with a 100 watts or so of bass is probably the most efficient way of blowing such a horn. Starting at the top end will avoid this, since any reversals will become quickly apparent. Once the correct operation is established all the levels are turned up and fine adjustments made with a graphic equaliser.

Once the setting up is complete the limiter is usually switched in, particularly if the group are unknown. Care must be taken, since most people tend to mix up, rather than down, and it is not unusual to obtain a 10dB increase over an evening.

An additional point often overlooked (even by professional groups) is to keep the backline levels down. For a group not used to playing with a PA it is natural to turn everything up to 10 (with the exception of the quality control which always seems to be set at zero), but this will not allow a clear sound to be obtained. The only way to convince a group of this is to demonstrate it and show them that the PA should take the full force of the sound rather than the backline.