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Supersonic Sounds plc Internal Correspondence Not to be quoted in published work May 2000 Supersonic Sounds plc Design Department Technical Report Design ideas for the “Anti-Heckler” microphone pre-amplifier Michael Prior-Jones Summary This report details work done to date on the “Anti-Heckler”- a microphone pre-amplifier intended to be used by people speaking in public. It features a sine wave test tone generator and peak-detecting output level meter. The circuitry has been designed to minimise the use of operational amplifiers through the use of bipolar transistor techniques. This has been done to avoid a supply difficulty with the TL071 opamp, but if this passes, I would recommend the replacement of the transistor circuitry (particularly the differential input stage) with the higher-quality opamp alternative. Much work has still to be done to test the system and check that the individual subsystems do not interfere with each other. Page 1 of 12 Design Ideas for the “Anti-Heckler” Microphone preamplifier. Supersonic Sounds plc Internal Correspondence Not to be quoted in published work May 2000 Supersonic Sounds plc Design Department Technical Report Design ideas for the “Anti-Heckler” microphone pre-amplifier Michael Prior-Jones Contents 1 Introduction ...........................................................................................................................3 1.1 Background ...................................................................................................................3 1.2 Requirements ................................................................................................................3 1.3 Product Specification ..................................................................................................4 1.4 Development Plan .......................................................................................................4 1.5 Report Structure ...........................................................................................................4 2 Design considerations ..........................................................................................................5 2.1 Microphone input stage ..............................................................................................5 2.2 Line output stage..........................................................................................................7 2.3 Sinewave oscillator .......................................................................................................8 2.4 Output level meter .......................................................................................................9 3 Conclusion .......................................................................................................................... 10 3.1 Recommendations .................................................................................................... 10 3.2 Final Circuit Design .................................................................................................. 11 4 References ........................................................................................................................... 12 Page 2 of 12 Supersonic Sounds plc Internal Correspondence Not to be quoted in published work May 2000 Supersonic Sounds plc Design Department Technical Report Design ideas for the “Anti-Heckler” microphone pre-amplifier Michael Prior-Jones 1 Introduction The “Anti-Heckler” is a new proposal for a microphone pre-amplifier, giving a line level output intended for use with power amplifiers like the “Discotomax”. It also incorporates a number of other features designed to assist the user in setting up a public address system. 1.1 Background This product grew out of a proposal from the Sales Department to make a microphone pre-amplifier intended for use for public speaking and lecturing. This would be partnered with an appropriate power amplifier and loudspeakers to make a compact public address system. A number of requirements were identified by Sales through consultation with our customers, and these are detailed below. 1.2 Requirements The requirements identified were: amplification of a conventional 600 balanced-line microphone to line level. provision of a sine wave test tone to allow the system be set up single-handedly. provision of an output level meter to give a visual indication of sound level. Page 3 of 12 Design Ideas for the “Anti-Heckler” Microphone preamplifier. 1.3 Product Specification During discussion with the Sales Director, more detailed specifications for the design were agreed upon and these are detailed below: Microphone input at 1mV peak-to-peak with 600 input impedance. Differential amplifier to handle balanced-line microphones and reduce noise when working with long microphone cables. Internal sine wave oscillator at 400Hz ± 7.5% Provision to switch between the microphone and the sine wave oscillator. Low impedance output to power amplifier at 775mV r.m.s. Output signal level meter. Due to supply problems with the TL071 operational amplifier chip, the decision was taken to minimise the use of op-amps in the design, and use discrete transistor circuitry instead. 1.4 Development Plan This report details the initial considerations and circuit designs, backed up with laboratory experiments. Further work has to be carried out in prototyping and modifications may be necessary to the circuit designs as they appear here. 1.5 Report Structure Section 2 describes the design process for each of the circuits within the overall system. Section 3 gives the full circuit diagrams and raises issues that need to be further investigated before the product can go into production. Page 4 of 12 Design Ideas for the “Anti-Heckler” Microphone preamplifier. 2 Design considerations The design process is made considerably more complicated by the requirement to use bipolar transistors rather than op-amps wherever possible. The bipolar transistor is considerably less ideal than the op-amp, and consequently considerably more mathematics has to be used in the circuit designs. The circuit is divided into a number of subsystems, which are shown in the block diagram of figure 1. Microphone input stage Line output stage Switch Sinewave oscillator Level Meter Figure 1- Outline block diagram of system 2.1 Microphone input stage The microphone input stage is a differential amplifier, to reduce common-mode noise introduced by the long microphone cable. A long-tailed pair is used to implement this stage using a matched pair of bipolar transistors. These may be discrete components, matched by hand (a laborious process) or an IC. The circuit has a gain (determined experimentally) of approximately 43dB (130 times) over a bandwidth of 100Hz-10kHz, which is acceptable for speech. Our 1mV pk-pk signal is thus amplified to 130mV pk-pk, which is 90mV r.m.s. The output level is 775mV r.m.s., and thus we need a further gain stage of 8.6 (19dB) to reach the correct level. The circuit is adapted from Tew, Howard & Garner1 (page 51) and has a common-mode rejection ratio (CMRR) of 49dB, which is likely to be adequate for this application. Large values of CMRR are desirable to remove noise picked up on the microphone leads. The CMRR may be increased by substituting a current source for RT, but my opinion is that this is unlikely to be necessary. The signal from the microphone will arrive on screened twisted-pair cable, terminated in an XLR connector. Pin 1 of the XLR should be connected to amplifier ground, and pins 2 and 3 connected to the differential inputs. The 620 resistor provides a close impedance match with the cable and microphone (which are 600), to maximise sound quality. The DC blocking capacitors C1 and C2 are effectively in series, and form a high pass filter with RIN. The values given give a –3dB point at 50Hz, giving us plenty of bandwidth. The output impedance of the differential amplifier is equal to RC i.e. 6k8, and thus a buffer stage should be introduced to avoid loading problems. This is implemented using the emitter follower circuit shown in figure 3. Page 5 of 12 Design Ideas for the “Anti-Heckler” Microphone preamplifier. Vcc (15v) RC 6k8 C1 10F Vout (output) TR1, TR2 BC182 Noninverting input RB1 10k RB2 10k RIN 620 0v Inverting input C2 10F RT 6k8 Vee (-15v) Figure 2- The differential input stage Vcc (15v) RB1 100k Vin (input) Cin 1F RB2 100k TR3, BC182 Vout (output) RE 6k8 0v Figure 3- emitter follower buffer for differential amplifier The emitter follower has approximately unity gain, but a very high input impedance and a very low output impedance. It will adequately buffer the output of the differential amplifier. The biasing has been arranged to be at the same level as the differential amplifier, so no dc blocking capacitor is required between them. A further gain stage is now required to bring the microphone signal up to line level. This will be the common emitter amplifier of figure 4. It has an approximate gain of 9.1 times, which will take the signal up to line level. Page 6 of 12 Design Ideas for the “Anti-Heckler” Microphone preamplifier. Vcc (15v) RB1 91k Vin (input) RC 7k5 Cout 10F TR4, BC182 Vout (output) Cin 1F RB2 10k RE 820 0v Figure 4- the common emitter stage The output resistance of this circuit is equal to RC i.e. 7k5. An additional stage of buffering will be added to lower the output resistance before sending the signal out to the power amplifier. The blocking capacitor on the output stops the master volume fader from affecting the biasing. 2.2 Line output stage Vcc (15v) Vin (input) RB1 100k CIN 1F TR3, BC182 RFader 10k log. RB2 100k COUT 1F Vout (output) RE 6k8 0v Figure 5 - the emitter follower line output stage This circuit (figure 5) forms a master volume control as well as reducing the output impedance of the system to a low level (approx. 50) to drive the power amplifiers. The blocking capacitors keeps the DC bias current from interfering with other parts of the circuit. Page 7 of 12 Design Ideas for the “Anti-Heckler” Microphone preamplifier. 2.3 Sine wave oscillator The 400Hz test tone is to be generated by a Wein bridge oscillator (from Angus, Tew & Howard2, page 86) built using an opamp. The circuit shown in figure 6 oscillates at a frequency of 408 Hz, which is well within our 7% limit (372-428 Hz), even allowing for variations in resistors and capacitors. This circuit has not been exhaustively tested, and some adjustments may be necessary to the values of R1, R2 and R3 to make the circuit oscillate reliably. The output from this oscillator is a 10v peak sine wave, which must be attenuated down to 775mV r.m.s. (a factor of 9, or –19dB gain). This is done by the potential divider of R4 and R5. R6 390k C1 1nF R7 390k TL071 ±15v supply + - C2 1nF R1 3k3 R3 10k Figure 6- The sine wave oscillator Page 8 of 12 R2 1k R4 82k R5 10k Vout 0v Design Ideas for the “Anti-Heckler” Microphone preamplifier. 2.4 Output level meter An output level meter gives a visual indication of sound level, and can provide an early warning of amplifier distortion. The classic “vU meter” is simply a calibrated voltmeter across the output of the amplifier, and is difficult to use because the meter needle moves very rapidly. This can make it very difficult to see the level in fast-paced music, or spot short transient peaks. Peak detector meters (sometimes called Peak Programme Meters or PPMs) were invented by the BBC to avoid this problem. Early models were electromechanical in nature, but improvements in technology have made it possible to build similar systems electronically. Figure 7 shows a peak detector circuit taken from Angus, Tew & Howard2 (p. 52). The circuit forms a half-wave rectifier, which charges the capacitor. During negative half-cycles, the diode does not conduct, so the capacitor’s only discharge path is through the 1M resistor. Consequently, the capacitor charges rapidly during peaks and discharges slowly in between, making the display on the meter much easier to read. The opamp is arranged to buffer the signal, and compensate for the 0.7v drop across the diode. TL071 ±15v supply Vin + R1 10k C1 10nF 0v Figure 7- the peak level meter Page 9 of 12 R1 1M V Design Ideas for the “Anti-Heckler” Microphone preamplifier. 3 Conclusion The circuit designs presented here still need further work- some have been tested in the laboratory, and others have not. The system will need testing as a whole to ensure that all of the subsystems work together correctly and meet the specifications. The system also needs a power supply (±15 volts DC) and this will need to be tested with the circuits described above to ensure that mains hum and other undesirable interference does not enter the sensitive audio circuitry. 3.1 Recommendations The circuit should be packaged in a small desktop case, preferably with a sloping top, incorporating the power supply and all connectors, and with the volume fader and test tone switch mounted on top. All connectors and controls should be clearly labelled. If the supply situation on operational amplifiers improves, significant benefits would be gained by replacing the transistor circuitry with the higher-quality opamp alternatives. The differential amplifier in particular would benefit from the use of an opamp to give higher common-mode rejection ratio and lower noise. I would suggest the use of the LM833 or OP-37 low noise opamps. Page 10 of 12 Design Ideas for the “Anti-Heckler” Microphone preamplifier. 3.2 Final Circuit Design Vcc (15v) R5 6k8 R9 91k C1 10F TR1, TR2 BC182 Noninverting input R1 620 R12 7k5 R6 100k R2 10k TR3, BC182 C3 1F R3 10k C510F TR4, BC182 To switch, below C4 1F R1010k R7 100k R8 6k8 R11 820 0v Inverting input C2 10F R4 6k8 Vee (-15v) R22 390k C7 1nF R23 390k C8 1nF R17 3k3 From C5 above TL071 ±15v supply Vcc(15v) Switch R13 10k log. + - R19 10k R14 100k C6 1F R18 1k R20 82k R21 10k TL071 ±15v supply R15 100k TR5, BC182 Vin + C9 1F R16 6k8 Line out 0v Page 11 of 12 R24 10k C10 10nF R25 1M V Design Ideas for the “Anti-Heckler” Microphone preamplifier. 4 References 1. Tew, Howard & Garner (1994) “First Year Laboratory Scripts, Spring Term 2000”, page 51 2. Angus, Tew & Howard (1992) “First Year Laboratory Scripts, Autumn Term 1999”, pages 86 and 52 Page 12 of 12