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120W public address amplifier Today's public address amplifiers are a real challenge to the designer. They need to combine very high sensitivity for low impedance microphones, mixing for several inputs, tone controls and high power output. The high power design presented here is our response to that challenge. By LEO SIMPSON & BOB FLYNN We have had quite a number of requests for an amplifier design for public address applications. Our first step in meeting those requests was to design the rugged power amp module presented in the November 1988 issue. In this issue, we present the microphone preamplifier and mixer stages and show how to assemble them onto a printed circuit board. Two low impedance microphone inputs are provided and they can be 26 SILICON CHIP connected as balanced inputs via 3-pin XLR sockets or as unbalanced inputs via 6.5mm jack sockets. In addition, there are two stereo line inputs which means that any program source such as a CD player, cassette deck or FM tuner may be connected. Mixing of these four sources (ie, two microphones and two stereo line sources) is provided via four knobs on the front panel. The other front panel controls are bass and treble controls and the master volume control. With 120 watts RMS power output, this public address amplifier will be suitable for a wide range of installations. It could be permanently set up in buildings such as halls or churches or used in temporary installations where AC mains power is available. Performance Traditionally, public address amplifiers have not had low noise, low distortion and a very flat frequency response. They have tended to be very utilitarian affairs with more of an emphasis on reliability than on good sound quality. We have taken a conservative approach to the design of this amplifier, to ensure stability and to prevent RF breakthrough. Even so, the overall performance is rather better than from run-of-the-mill PA amps. +20 ~ +10 TREBLE BOOST BASS BOOST ~I'-- ... / V ' ..... ,/ .,,v -20 20 .......... /"" "'-.... ~ ,/ / ,..... .... -10 ,- I..--' ......... V / -- / ...... _.,,/ --- Fl ~T -............ ' / r--..... ' ...., ... TREBLE CUT BASS CUT Fig.I: this diagram .shows the frequency response of the amplifier with tone controls flat and the effect of each control when at the maximum or minimum setting. Note that there is a small amount of interaction between the controls at mid frequencies. ~ ./ I 100 I I HERTZ Part of the reason for this is the excellent performance from the two toroidal transformers, one for the power supply and one for the 1OOV line output. Not only do both transformers have low external radiation but the line transformer is exceptionally good for its flat frequency response and clean output waveform. If we'd had to depend on conventional transformers to obtain the same performance, the amplifier would have been a great deal heavier and probably bulkier too. All the performance details are summarised in the accompanying specification panel. Preamp and mixer circuitry Now have a look at the complete circuit of the PA amplifier as shown in Fig.1. For each of the balanced microphone inputs we have used a low noise LM833 dual op amp. This is shown in the top lefthand corner of the circuit. The LM833 dual op amp is depicted as IC1a and IC1b which are connected together as a balanced input stage. 1 % metal film resistors are specified in the balanced input stages, firstly to ensure low noise and secondly to ensure good "common mode" rejection. Common mode signals are those extraneous signals which balanced inputs are intended to reject; eg, hum, mains hash and other unwanted interfering signals. 500 1k 10k 5k 20k Specifications Power Output (RMS) 125 watts into 4 ohms; 90 watts into 80; 1 20 watts into 1 OOV AC line Frequency Response 30Hz to 30kHz (-3dB) with line output transformer (all inputs) Input Sensitivity 340mV (Aux 1 , Aux 2) 0 .5mV for balanced microphones 0.5mV for unbalanced microphones Input Impedance 50k0 for line inputs (Aux 1, Aux 2) 6000 for balanced microphones 3000 for unbalanced microphones Harmonic Distortion (0.3% from 20Hz to 20kHz Signal-to-Noise Ratio - 56dB unweighted for microphones - 72dB unweighted for line inputs Stability Unconditional The balanced outputs at pins 1 and 7 of IC1 are fed to IC2, an LF351 single op amp, to provide an unbalanced output at pin 6. This is coupled via a 2.2µ,F bipolar electrolytic capacitor to the Mic 1 gain control potentiometer, VR3. For unbalanced microphones, a 6.5mm socket with integral switch is used. This grounds the pin 3 input of IC1a, effectively converting the circuit to an unbalanced stage with only IC1b functioning. IC4, another LM833 dual op amp, and IC5 provide the balanced/unbalanced input stages for the second microphone input channel. Its output is coupled to the Mic 2 gain control potentiometer, VR5. The two stereo program sources, labelled Aux 1 and Aux 2 on the circuit diagram, are fed into a 4-way RCA phono socket. Both channels of each stereo source are mixed via 1k0 resistors to provide a mono signal which is then fed to the respective volume controls, VR6 and VR7. The signals from the wipers of VR6 and VR7 are then coupled via 0.12µ,F capacitors and 56k0 resistors to the mixer stage formed by IC1a, which is one half of another LM833 dual low noise op DECEMBER 1988 27 :I: ~ ~ n ~ O ~ I i ,.. .0056 - .,. ~ I • -II 220k 5.6pf Lf351 IC5 MIC 2 CIRCUlTRY AS ABOVE -15V I~ 1 .,. l -15V • 2.2 BP BP 2.2 I ':" MIC 2 GAINJ'":.~~ ~ VR5 ~ttf 5k LOG I __Jill __ ">-,.-,,.,,, _ *1% .0015 I •........ ...____________3.9k IC4 LM833 1eon .0015 ~~ ~ AUX 2 ■ .-.-;; I GAiii ,~ ........,,..,.. VR7 ' 50k LOG 1k R ~ ~1k Ry.,. ~1k Ly AUX 1 INPUT I SINGLE ENDED 4 301!"> * Jo';n BA~ANCED MIC 1 INPUT V---2 . 39k 3% ~ E- N- 240VAC CASE -;J,, ! L_: r----: 04 BC556 1.4V 3.9k; ~ I [__: 45.5V 22k: r---. 2.2k ~ 8 ru 120W PA AMPLIFIER I ~~ ~h~ = -r~ ~ ! -.J 3.9ki2.4V :.J seoni-:Lv 07 06 :::r ! -51V +51V r----Wh .,. 0.22I ! 16VW 22 22 16VW J)UT 0.22! SA 0.1 0.1 B 0.22+ 0.1 51V G 0 ECB 0.1 0.1 INmUT GNO ~ 16VW 22 16VW 22 IN 0 -15V GND +15V OUT ffi GND 5 S m/ VIEWED FROM BELOW ELJc 0.22+ 011 1N5404 010 1N5404\· ~ G\,JtJl.. 2s~~34 5A +51V ◄ Fig.2: the circuit features two microphone inputs (either balanced or unbalanced) and two auxiliary inputs. These signals are mixed in IC3 and fed to a power amplifier stage (Ql-Q11) which drives a 100V line transformer. amp, IC6a. The gain of this stage, for each of the two inputs, is set by the ratio of the 56k0 mixing resistors to the 220k0 feedback resistor, from pin 1 to pin 2. This gives a gain of 220/56 or approximately 4 times. Tone controls The output of the line source mixer is then fed via a 2.2k0 resistor to a feedback tone control stage built around IC6b. This is a conventional feedback tone control stage which works as follows. First, assume that bass and treble controls VRB and VR9 are electrically centred. This means that the gain for all signals is exactly unity; ie, flat across the whole audible spectrum. For the treble control, the two .0015µF capacitors in series with each side of VR9 pass only high frequencies so that bass signals are not affected by the setting of the control. Similarly, for the bass control, the .022µF capacitor in parallel with VRB bypasses the high frequencies so that they are not affected by operation of the control. So when the bass control is rotated clockwise, its wiper moves towards the input signal, which means that the feedback resistance path from pin 7 to pin 6 of IC6b is increased. This increases the gain at low frequencies and therefore boosts the bass. The same principle applies with the treble control. Note that signals from the two microphones are not passed through the tone control stage. This is normal practice in most public address amplifiers. Fig.2 shows the frequency response of the amplifier and the response of the tone controls. Main mixer Following the tone controls, the signal passes to the main mixer stage, IC3, via a 39k0 resistor. Signals to the mixer from the two microphone stages are fed via 2.2k0 resistors. Signals from the tone controls pass through IC3 with a gain of unity (ie, no gain) whereas those from the microphone stages pass through with a gain of 39kn/2.2k0 or approximately 18 times. Signals from op amp IC3 then pass directly to the master volume control, VR4. This then feeds the power amplifier via a 0.47 µF capacitor. As an optional feature we have included a mute facility on the mixer board. This can be used with microphones which have a press-totalk switch. When the mute switch S1 is closed, it kills the signal from the program source (Aux 1 or 2) and gives the microphone signal a clean background. It works as follows . Normally, Q12 is biased off by the 10k0 resistor connected from its base to the + 15V supply. Because Q12 is not conducting, Q13 has no base current and so its collector emitter impedance is high. When Sl is closed, Q12 conducts and supplies base current to Q13 . Q13 then acts as an audio switch to kill the signal at the input to the tone control stage. Note that because of the 47µF bipolar capacitor in series with the collector, no DC current flows through Q13. This is an unusual use for a transistor but it works quite effectively. In our prototype PA amplifier we - have shown transistors Q12 and Q13 on the board but we have not provided a connection to the microphone inputs. The power amplifier module is identical to that described in last month's issue, so we will not describe the circuit here. Power supply The power supply is based on that featured in last month's issue but includes the additional components needed to provide balanced ± 15V rails for the preamp and mixer stages. These additional components are mounted on·one end of the preamp board which is shown as Fig.3. The AC mains supply is passed via a 2-amp fuse and then double pole switch S2 to the primary of the 300VA toroidal power transformer :;:;:~""' _;~~~ !:.2;!-'""· ~~:-~ r~22M ~ 22µ,F 0 L _ _:~~~=-+--------' Fig.3: here's how to wire up the preamp/mixer board. Use PC stakes at all external wiring points and make sure you don't confuse the two 3-terminal regulators. Note the three wire links. (Altronics Cat. M-3092). Its two secondary windings drive a 35-amp bridge rectifier and two B000µF 63VW electrolytic capacitors to provide balanced ± 51 V supply rails for the power amplifier board. These high voltage rails also feed via 6800 5W wirewound resistors DECEMBER 1988 29 ·~ The close-up view shows the completed preamp/mixer board installed in the chassis. Notice how the two 6800 5W resistors at the bottom lefthand corner of the board are mounted end on. The metal tabs of the two 3-terminal regulators go towards the front panel. to 33V 1W zener diodes. The zeners act as pre-regulators for the 3-terminal regulators. By selecting 33V zeners with a tolerance of ± 5%, the regulators are protected against excessive input voltage; the 7815 positive regulator can withstand a maximum of + 35V while the negative 7915 regulator can withstand - 40V. The ± 15V supply rails from the 3-terminal regulators are further bypassed by a number of 22µF and O.lµF capacitors to lower the output impedance and filter out any hash. The 33V 1W zeners should be 5% tolerance. The two 6800 5W wirewound resistors are stood on end, as shown in the photos. You will need 40 PC pins for all the connections from the mixer board to external components such as the potentiometers. When the mixer board is assembled put it aside and start work on the amplifier module. The wiring layout of Mosfet amplifiers is very critical so the printed board is a crucial feature of the design. The printed board measures 163 x 95mm and is coded SC 01111881. As published last month, the board was mounted on a large diecast heatsink with integral rightangle bracket. As used in this PA amplifier it is mounted on one of the side heatsinks with. a heavy gauge aluminium angle bracket. Assembly of · the board is a straightforward matter but it should not be hurried. First, you should closely inspect the board to see if there are any shorted tracks or open circuits in the copper pat- Construction As noted above, we built the new public address amplifier into a rack mounting case. The one used in our prototype amplifier was supplied by Altronics Distributors Pty Ltd who also are the source for the toroidal power transformer and line output transformer. The mixer and preamplifier circuitry is mounted on a printed circuit board measuring 259 x 78mm and coded SCO 1112881. Assembly of the mixer board is quite straightforward (see Fig.3). Take care to ensure that the integrated circuits, electrolytic capacitors and diodes are all installed the right way around. Watch the regulators too and don't swap them around inadvertently, otherwise they'll blow. 30 SILICON CHIP The two input transistors (Ql & Q2) on the power amplifier PCB must be glued together using super-glue. This is done is minimise temperature drift of the DC output voltage. Fig.4: here's how to install the parts on the power amplifier board. Keep the component leads short and make sure that the Mosfet output transistors (Q8-Q11) are electrically isolated from the heatsink. The 2200 resistors shown dotted are mounted on the copper side of the board. -SCREWS Two layers of wire are wound on so that the start is at one side and the finish is at the other side of the bobbin. Bend the start and finish leads at 90° and scrape off the enamel coating before soldering the choke to the board. Heatsink assembly 0 0 PCB I -1 SHAKE-PROOF · - - • - W ASHERS ~• ~-NUTS Fig.5: this diagram shows how the Mosfet output transistors are mounted on the heatsink. Use your multimeter to check for shorts between the case and heatsink after each transistor is mounted. The nuts should be soldered to the PC pattern after assembly to ensure reliable contact. tern. These should be fixed before proceeding further. The PCB component diagram is shown in Fig.4. Fit the small components first, such as the resistors and diodes. Make sure that you don't confuse the small diodes (1N914s) with the 11 V zeners. The fuse clips, trimpots and small transistors can be mounted next. Qt and QZ should be mounted so that their flat faces · are touching. When you have soldered them in place, put a drop of super-glue between them and squeeze them together. The 4.3µH choke at the output of the amplifier is wound with 19.5 turns of 0.8mm enamelled copper wire on an 11mm plastic bobbin. The four Mosfet power transistors are mounted on a 4mm thick extruded aluminium heatsink bracket but with their leads passing through and soldered to the printed board. The assembly is as shown in Fig.5. We used 5mm fibreglass tubing for the insulating bushes. Smear all the mounting surfaces of the Mosfets and the heatsink with heatsink compound before assembly. The transistors are fastened to the heatsink bracket using 12mm 6BA screws and nuts. Solder the nuts to the PCB pattern after assembly to ensure reliable contact. Alternatively, if the nuts are nickel plated or stainless steel, use lockwashers. As each transistor is mounted, use your multimeter (set to a low "ohms" range) to check that its case is insulated from the heatsink. After the nuts have been tightened and soldered, the gate and DECEMBER1988 31 PARTS LIST Power Amplifier Module 1 PCB, code SC01111881, 95 x 163mm 1 aluminium angle bracket, 4mm thick, 32mm wide, 170mm long, drilled for four T0-3 power transistors and to match the PC board 4 3AG fuseclips 2 5A 3AG fuses 6 PC pins 1 plastic coil bobbin, 12mm diameter x 11 mm long; Siemens B65672-B-T1 or equivalent (or 4.3µH aircored choke; see text) 4 T0-3 transistor mounting kits Semiconductors 2 2SK 1 34 Mosfet transistors 2 2SJ49 Mosfet transistors 4 BC556 PNP silicon transistors 1 BC548 NPN silicon transistor 1 BF4 70 PNP silicon transistor 1 BF469 NPN silicon transistor 9 1 N4148, 1 N914 small signal diodes 2 11V 400mW or 1W zener diodes 2 1 N5404 3A silicon diodes Capacitors 1 22µF 16VW PC electroyltic 1 0.47µF 16VW PC electrolytic 1 0.27µF metallised polyester (greencap) 4 0.22µ,F metallised polyester (greencap) 1 .001 µF metallised polyester 1 39pF ceramic Resistors (0.25W, 5%) 1 x 27k0, 3 X 22k0, 2 x 18k0 0.5W, 2 X 3.9k0, 2 X 2.2k0, 1 x 1kQ, 1 X 6800, 4 X 2200, 2 X 680, 3 x 120 1 W, 1 x 5000 trimAOt (Bourns Cermet horizontal source leads of the Mosfets can be soldered to the PCB pattern. The four gate resistors are then soldered in place, on the copper pattern side of the PC board. Now closely inspect all your work for correct assembly and soldering. Make sure there are no blobs of solder bridging out tracks. As a 32 SILICON CHIP mount, 0.2 x 0.4-inch), 1 x 2000 trimpot (Bourns Cermet horizontal mount) 1 4 7 µ,F bipolar electrolytic capacitor 4 1 on 0.25W resistors Mixer Board Hardware & Power Supply 1 PCB, code SC01112881, 259 x 78mm 40 PC pins Semiconductors 3 LM833 dual low noise op amps 3 LF351, TL071 Fet-input op amps 2 33V 5% 1 W zener diodes 1 7815 15V positive regulator 1 7915 15V negative regulator Capacitors 2 220µF 35VW PC electrolytics 4 22µF 16VW PC electrolytics 2 2 .2µF bipolar PC electrolytics 2 0 .12µF metallised polyester (greencap) 6 0. 1µF ceramics or green caps 1 .022µF greencap 4 .0056µF greencap 6 .0015µF greencap 1 33pF ceramic 1 5.6pF ceramic Resistors (0.25W, 5%) 1 x 220k0, 2 x 56k0, 2 X 39k0, 2 X 27k0, 1 x 18k0, 2 X 12k0, 4 X 6.8k0 1 %, 4 X 3.9k0 1 %, 4 X 2.2k0 1 %, 3 x 2.2k0, 4 x 1 kO, 2 x 6800 5W wirewound, 4 x 3010 1 % , 2 X 1 800 1 % Potentiometers 2 1OOkO linear potentiometers 2 50k0 log potentiometers 1 1 OkO log potentiometer 2 5k0 log potentiometers Optional mute facility 1 BC558 PNP transistor 1 BC548 NPN transistor final check on your work, connect your multimeter (set to a low "ohms" range) and test for shorts between the supply rails and the OV rail. There is a trap here - flyback diodes DlO and Dl 1 will show a low resistance for one connection of the multimeter and a high resistance for the reverse connection. 1 3-unit rack mounting case with extruded aluminium side panels, Altronics Cat. H-0418 1 300VA power transformer, 70V centre-tapped, Altronics Cat. M-3092 1 160VA 1 OOV line transformer, Altronics Cat. M-1124 2 3-pin XLR chassis mounting sockets 2 6.5mm mono jack sockets, chassis insulated, with changeover switch 1 4-way RCA phone socket panel 1 red binding post terminal 1 black binding post terminal 1 3-core mains power flex with moulded 3-pin plug 1 cordgrip grommet to suit power flex 1 4-way insulated barrier terminal block 2 solder lugs 1 35-amp bridge rectifier, Altronics Cat. FB-3504 2 8000µ,F 63VW electrolytic capacitors 1 3AG 2A fuse and chassis mounting fuseholder 1 neon illuminated DPDT 240VAC-rated rocker switch 6 20mm black anodised aluminium knobs 1 40mm black anodised aluminium knob 4 plastic PCB supports Miscellaneous Twin-shielded cable, figure-8 shielded audio cable, hookup wire, solder, heatsink compound, screws, nuts, washers. With the power amplifier module complete, carefully check your work for misplaced parts and faulty soldering. Do not apply power at this stage. Next month, we will show you how to assemble the two modules and the power supply in a rackmounting case. ~