Fullscreen HTML5Med Res (??MB)HTML4

E R U FEAT ECT PROJ The PortaPAL A State-Of-The-Art Portable Public Address Amplifier Features d unit with safe plugpack charger • Portable, 12V battery-powere • High power output eaker with tweeter ut • Inbuilt 200mm (8") loudsp s 6.35mm guitar or aux inp m microphone inputs plu • 2 combined XLR/ 6.35mwit stereo to mono mixer • Stereo RCA line inputs hRCA and 6.35mm jack outlets • 2 line outputs with stereo ut • Level control for each inp • Bass and Treble controlsto extend battery charge with power-down indication • Automatic power-down indication arger on and charging • Ch 14  S ilicon hip re leads ce forCspa rage spa Sto • and corner protectors dle, speaker stand socket han ry • Box includes car siliconchip.com.au Part 1: by JOHN CLARKE & LEO SIMPSON T HIS PORTABLE PA amplifier can be powered from the 240VAC mains or its inbuilt 12V SLA battery. It delivers up to 70 watts and pulls a number of tricks to keep battery current low while still maintaining very high performance. Back in March 2002, we published the “Mighty Midget”, a 70-watt class-H audio amplifier module based on the Philips TDA1562Q power IC. This ground-breaking IC uses special techniques to deliver up to 70 watts from a 12V battery and does away with the need for a DC-DC inverter. At the time, we said the “Mighty Midget” was ideal for use in a portable PA system and now we have followed up with the PortaPAL: a complete system, including mixing for two 600Ω balanced or unbalanced microphones, guitar input and line inputs for a CD player or a cassette deck. This new circuit makes use of the TDA1562Q’s muting feature, to further reduce battery drain and keep hum and noise very low. We’ve also taken advantage of the inbuilt 12V SLA (sealed lead acid) battery to eliminate a heavy transformer and large and expensive electrolytic filter capacitors from the power supply – leading to a considerable cost saving. No DC-DC inverter needed Up until the release of the Philips TDA1562Q IC, if you wanted more than about 16 watts from a 12V-powered PA system, you had to resort to a DC-DC inverter to provide power supply rails of, say, ±40V, to get around 50 watts into an 8-ohm load. Not only are DC-DC inverters relatively complex but even the most efficient designs inevitably lead to a reduction in overall circuit efficiency. With its special class-H operation and bridged amplifier operation, the TDA1562Q not only eliminates the need for an inverter but its class-H operation is considerably more efficient than a normal class-B amplifier which is what would be normally used. So enough of the rave about the TDA1562Q – if you want more information, refer to the March 2002 issue of SILICON CHIP. Now let us have a look at the other features of this great little (actually, not so little) portable PA amplifier. Features Housed in a timber cabinet meas- uring 450 x 280 x 240mm, the PortaPAL amplifier uses a coaxial 200mm speaker which is rated at 50W and has relatively high efficiency of 92dB/ 1 watt <at> 1 metre. It is a 4Ω speaker which incorporates a separate concentrically mounted miniature dome tweeter; that’s where the “coaxial” term comes from. The speaker is specified as a 4Ω model because the amplifier is designed to deliver maximum power into a 4Ω load. All the controls are at the rear of the cabinet. There are two XLR sockets for connection of low impedance (600Ω) balanced microphones. These special XLR sockets also accept standard 6.5mm jack sockets so that unbalanced microphones can be used as well. There is also a pair of RCA phono sockets for connection of a CD player or cassette deck and RCA sockets are also provided for line out signals to a cassette deck, if the proceedings need to be recorded, or to another PA system. This output is also duplicated at a 6.35mm stereo jack socket. There are four mixing knobs for the microphones, guitar and line inputs but there is no master level control, to keep things simple. Bass and treble tone controls are provided and automatic VOX is built in. There are four LEDs on the panel, PortaPAL Specifications 33W RMS into 4Ω (depending on battery voltage) 70W RMS into 4Ω (depending on battery voltage) Line -3dB at 26Hz and 40kHz Guitar and microphone -3dB at 42Hz and 20kHz Tone Controls +13dB and -14dB at 100Hz (see graphs) +11dB and -13dB at 10kHz Input Sensitivity: Line 340mV RMS (for 30W into 4Ω) Guitar 16mV RMS (1.9V RMS overload) Microphone 1.3mV RMS (130mV RMS overload) Signal-to-Noise Ratio: -83dB unweighted (20Hz to 20kHz); input level controls all off (all figures with respect to 33W) -71dB unweighted with microphone level set at maximum sensitivity (-73dB A-weighted) Muting Threshold: <7mW output power Time: 15 seconds (typical) after signal drops below threshold 100ms (typical) unmute when signal applied Battery Consumption Standby (mute) 26mA No signal (unmute) 160mA Battery Charger Charge rate: 1A maximum Charge voltage: 13.8V maximum Dimensions: 500 x 295 x 250mm (including handle and corners/feet) Mass 13kg (including charger plugpack) Output Power Music Power Frequency Response siliconchip.com.au February 2003  15 Fig.1: all of the functional areas of the PortaPAL are shown in this block diagram, with the exception of the mains power supply/SLA battery charger. two at the top righthand corner and two lower down, adjacent to the plugpack charger input socket. At the top, one of the LEDs flashes about twice a second to indicate that the unit is on while the other is the “Fault” indicator. We’ll talk more about this and the charger LEDs later. The only other control is the On/Off switch. Also on the rear panel is the lid for the 12V battery compartment and the battery can be quickly changed over if that is necessary. Power comes from the battery or an external 16VAC 1.5A plugpack. While the plugpack has relatively low power rating (24VA) with respect to the maximum output of the PA amplifier, it is quite adequate to keep the battery fully charged in normal PA operation. In fact, the power supply is really just a battery charger with the battery permanently connected. The inbuilt VOX operates to mute the power amplifier if there is no signal for more than 15 seconds, reducing the standby battery consumption from 16  Silicon Chip 160mA to around 26mA. While it is hard to be precise, we estimate that the inbuilt 12V 7Ah battery should be good for about five to six hours use. In practice, that means you could typically use the PortaPAL all day on battery power. Circuit overview Fig.1 shows the simplified block diagram. Apart from the TDA1562Q power amplifier IC, there are eight low-cost ICs and not a lot else. The microphone signals are amplified in op amps IC1a and IC1b, while the guitar signal is amplified by op amp IC3. The microphone, guitar and line signals levels are set by VR1, VR2, VR3 & VR4 and then mixed and fed to the tone control stages and to the muting circuit involving IC6, D1 & D2. This circuit provides a mute signal to the power amplifier when the audio signal levels are below a certain threshold. Circuit details As already noted, the Portable PA uses two special XLR sockets which also accept stereo or mono 6.5mm jacks. For simplicity, these sockets are shown on the circuit of Fig.2 as separate XLR and jack sockets but remember that they are combined into what look like ordinary 3-pin XLR sockets. In normal operation, using a microphone with an XLR plug, the balanced microphone signals are fed to the inputs of op amps IC1a & IC1b. These provide a gain of 22 from a 600Ω microphone. Both microphone preamps are identical except that the MIC1 preamp has provision for a bias voltage (phantom power) for electret microphones, if required. The use of 1% resistors in the balanced microphone circuits ensures good rejection of common mode signals such as hum and hash. High frequencies above 50kHz are rolled off by the 150pF capacitors across the 22kΩ feedback resistors. The 390pF capacitors shunting the balanced input lines, in conjunction siliconchip.com.au with the microphone impedance, also roll off the high frequencies. Should you insert a 6.5mm stereo jack plug from a balanced mike into the XLR socket it will again be accepted as a balanced signal and converted to a single-ended output. But here is the clever part. We have wired it so that if you insert a mono jack into the socket, the non-inverting input (pin 3 of the XLR) is grounded and IC1a (or IC1b) operates as a normal non-inverting amplifier with a gain of 22. Thus we cater for both balanced and unbalanced low impedance microphones. The unbalanced outputs of op amps IC1a & IC1b are each fed to level potentiometers VR1 & VR2 via a 150Ω resistor and 1µF capacitor. The signals from VR1 and VR2 are then applied to op amps IC2a and IC2b, both of which have a gain of 11. This means that maximum gain for microphone signals is 242. Guitar input The guitar input stage involving the TL071 Fet-input op amp IC3 looks like a straightforward non-inverting amplifier but there are a number of interesting wrinkles. First, the guitar signal is coupled in via a relatively large value of capacitor, 47µF, especially when the input load resistor is also high at 470kΩ. This is because are aiming for two separate outcomes. We have specified the high load resistance of 470kΩ to ensure optimum high frequency response with the relatively high inductance of typical guitar pickups. With such a high load resistance, you might wonder why we have used such a large input coupling capacitor. After all, to maintain a flat response to below 20Hz, all you need is a 15nF (0.15µF) input capacitor. Why use 47µF, 300 times bigger? The answer is that the inductive guitar pickup represents a low source resistance at low frequencies. In order to minimise noise, op amp IC3 needs to see as low a source resistance as possible. Ergo, we use a big capacitor. IC3 is set for a gain of just two. This is adequate for any guitar (when the following gain is accounted for) but it also means that this input can handle line input signals of up to 1.9V before overload occurs. Following potentiometer VR3, the siliconchip.com.au Construction will be fully described next month but to whet your appetites, at top we show the completed amplifier assembly ready for mounting in the box, while immediately above is the separate SLA charger board. By the way, this could be built independently as a high performance SLA battery charger. guitar signal is fed to op amp IC4a, which has identical gain to IC2a & IC2b. Stereo line inputs Stereo line inputs (eg, from a CD player) are mixed to a mono signal with 2.2kΩ resistors and fed to potentiometer VR4. All of the signals from the four potentiometers are then mixed in IC5a which has gain of unity. Note that the input resistor from VR4 is 10kΩ rather than 15kΩ to make up for a slight gain loss in the resistive mixing of the stereo line inputs. IC5a drives the tone control stage involving IC5b and this has its output signal fed to three places: the line outFebruary 2003  17 18  Silicon Chip siliconchip.com.au siliconchip.com.au February 2003  19 Fig.3: total harmonic distortion versus frequency at 12W using the microphone input. put to RCA and 6.35mm jack sockets, the muting stages involving IC6 and the power amplifier involving IC8 and IC9. All of the op amps in the circuit, with the exception of IC4b and IC6, have their non-inverting (+) inputs biased from the Vref line which is at +6V. This is derived from the +12V line by a voltage divider consisting of two 10kΩ resistors with the centre point bypassed by a 100µF capacitor. The bypassed supply is then buffered by op amp IC4a to provide the Vref line. This means that all op amps will have symmetrical clipping at overload, to maximise the output signal. All op amp outputs, with the exception of IC6b, will sit at +6V (or half the battery voltage). Muting stages As noted above, we have incorpo- rated VOX into the circuit to mute the amplifier and cut current consumption when no signal is present. This muting function is provided by dual op amp IC6. Op amp IC6a is a non-inverting stage with a gain of 471 by virtue of the 470kΩ and 1kΩ feedback resistors. The 22pF capacitor rolls the gain off above 15kHz, while the 10µF capacitor in series with the 1kΩ resistor rolls off signals below 16Hz. The amplified signal from IC6a is then fed to a diode pump circuit consisting of diodes D1 & D2 and 1µF & 10µF capacitors. Hence, the peak level of the signal from IC6a will be stored in the 10µF capacitor which is continuously being discharged via the 1MΩ resistor across it. The 10µF capacitor is monitored by IC6b which is connected as a Schmitt trigger. A 10MΩ resistor between pin 5 and pin 7 applies a degree of positive Fig.5: distortion versus frequency at 30W but using the line input. 20  Silicon Chip Fig.4: total harmonic distortion versus frequency at 30W using the microphone input. feedback to give hysteresis. This makes the comparator output switch cleanly between high and low, to prevent the possibility of parasitic oscillation at the switching points. The inverting input of IC6b is set at +3.4V using the 100kΩ and 39kΩ resistors across the 12V supply. When power is first applied to the circuit, the 10µF capacitor between the 12V supply and the inverting input to IC6b is initially discharged and therefore pulls pin 6 high, causing pin 7 to be low. Pin 7 of IC6b is connected to the mute (mode) input, pin 4, of the power amplifier, IC9. So at power-up, the amplifier is muted. Once the audio signal monitored by IC6a is of sufficient level, IC6b’s output will go high and the power amplifier will be unmuted. Muting indication is provided by Fig.6: power versus distortion when driven by the mic input. Maximum power here is 42W at 10% distortion. siliconchip.com.au Fig.7: power versus distortion when driven by the line input. The distortion is lower than Fig.6 because of the lower gain from the line input. IC7, a CMOS 7555 timer, which drives LED1, the power/ standby indicator. Initially when power is switched on, transistor Q1 is off and so pin 4 of IC7 is pulled high via the 10kΩ resistor connecting to the 12V supply. This allows the 555 timer to run and it flashes LED1 on and off. The rate of flashing is set by the 10µF capacitor connected to pins 2 & 6 and the associated 100kΩ and 10kΩ resistors. Note that the 10µF capacitor is tied to the +12V supply rather than 0V, as in a normal 7555 timer setup. The reason for doing this is so that pin 3 of the 7555 will be low when power is first applied and the LED will light immediately and then flash. If the capacitor was connected to 0V (as in a conventional 7555 circuit), the LED would be off for almost one second before flashing on. In effect, the 10µF capacitor is charged via the 10kΩ and pin 7 and then discharged to +12V via the 10kΩ and 100kΩ resistors. Since the ratio of the charge/discharge resistances is 1:11, the LED flashes with about the same duty cycle (on for 70ms, off for 740ms) and thereby keeps current drain to a minimum when the amplifier is muted; LED1 draws about 400mA. Fig.8: the tone control action in the “flat”, “max boost” and “max cut” settings. siliconchip.com.au February 2003  21 The main PC board includes most of the electronics: the power amplifier (the large IC attached to the heatsink), the mixer, tone controls and so on. Input “daughter boards” attach to this main board. The power supply is also on a separate board. When pin 7 of IC6b’s output goes high to unmute the power amplifier, transistor Q1 is switched on and it pulls pin 4 of IC7 low. This forces the pin 3 output low and LED1 is lit continuously. So LED1 is on continuously in normal operation and it flashes when the amplifier is muted. Power amplifier IC9 is the TDA1562Q power amplifi- The specified 200mm 4Ω woofer from Altronics has a separate concentrically mounted plastic dome tweeter and has quite a wide overall frequency response. 22  Silicon Chip er which can deliver up to 70W under music power conditions, depending also on the state of battery charge. The circuit presented here is very similar to that first presented in the March 2002 issue of SILICON CHIP. The main difference is that here we are using the mute (pin 4, Mode input) facility, as described above and the diagnostic output at pin 8. This is used to drive LED2. It will show when the amplifier is clipping, if there is a short at the output, if there is an open circuit load and if the amplifier has gone into thermal shutdown. If you want a full description of the TDA1562Q, you will need to refer back to the March 2002 issue. For those readers who have not seen that issue, we will briefly the describe the circuit operation. The TDA1562Q actually contains two power amplifiers which drive the 4Ω speaker in bridge configuration and its inputs are balanced. So we drive these balanced inputs (pins 1 & 2) with signals that are 180° out of phase. Hence, pin 1 of IC9 is driven directly from the output of IC5b (albeit via two capacitors) while pin 2 is driven from the output of IC8, a TL071 op amp connected as a unity gain inverter. The two 4700µF capacitors at pins 3 & 5 and pins 13 & 15 of IC9 are “lift supply” reserves for when momentary high power levels are required. Both amplifier output terminals pass through Zobel networks, each comprising a 2.2Ω resistor and parallel inductor shunted via a 220nF capacitor. The components are included to guarantee stability (ie, stop any tendency to supersonic oscillation) when driving reactive loads. Power for the circuit comes from a 12V 7 amp-hour battery which is fed via switch S1 and a 7.5A fuse. Diode D3 is included should the battery be connected the wrong way around. If that happens, the diode will conduct and blow the fuse. Next month In March, we will present the charger circuit for the PortaPAL. This can be built as a general-purpose charger, as well as the power supply for this amplifier. We will also present the full construction details of the PortaPAL PA SC amplifier. siliconchip.com.au