Fullscreen HTML5Med Res (??MB)HTML4

Howl suppression for public address systems Acoustic feedback is a problem with sound reinforcement systems. At a particular sound level, enough of the signal will be coupled back to the microphone so that a positive feedback loop is set up. This experimental circuit reduces this problem by shifting the whole audio spectrum up by 10Hz. By DARREN YATES If you've ever been in a lecture theatre or even at a concert, then you've probably heard the loud squeal that can occur due to acoustic feedback. This effect limits the number of locations where you can place loudspeakers, as well as how loud you can drive them. Acoustic feedback occurs when the resonance of a room at a particular frequency induces a positive feedback between the microphone and loudspeaker. Since theory says we only need a gain of slightly more than unity to start and sustain oscillation, the inevitable effect is that the PA system turns into a high power oscillator at the particular frequency of resonance. 35 0 30 25 a, ~ 2ll --- - - ~ n I n(\ MEANJlESPONg_ How it works This circuit concept was originally a design idea published a few years ago in "Electronics & Wireless World". That circuit used a couple ofMC1495 analog multipliers. We recently decided to take another look at the concept and use some lower cost and more readily available devices such as the LM13600/LM13700. A simplified block diagram of the frequency shifter is shown in Fig.2 while the full circuit is shown in Fig.3. It consists of, firstly, an audio stage using IC1a and IC1b, which incorporates a passive quadrature filt er net- t--- r "' z 0 c.. ~ The common way used by PA operators to overcome this problem is to introduce a dip in gain at the frequency of resonance, using a parametric equaliser or a one-third octave equaliser. This lets the overall gain of the system be raised until acoustic feedback again becomes the limiting factor. In practice , several dips may have to be used to make the system work at a sufficiently high level. This article discusses a different technique. If we take the entire audio frequency spectrum and shift it up by about 10Hz, we change the frequency response relationship of the amplifier and the room so that what used to be a frequency of resonance now becomes a frequency of something like "anti-resonance". To make this a little clearer, the graph in Fig.1 shows a typical room response to a section of the lower audio spectrum. As you can see, some of the p eaks are well above the average room response. It is at these fre quencies that we're likely to get feedback. You'll also notice that every few Hz or so above each frequency of resonance there is one of anti -resonance; ie, a dip. The idea behind this circuit is to shift the frequency spectrum up so that the frequency which is accentuated by the room response is now reproduced as a frequency which corresponds to a dip in the response, or so the theory goes. The end result is that we can increase the gain further before instability occurs. / .-----ix 15 "' 10 \I V OUTPUT 500 AUDIO MIXER AUDIO STAGE 0 520 540 560 580 FREQUENCY= INPUT•+ 1OHz 600 FR EOUENCY (Hz) '-----.-tX Fig.1: this graph shows a typical room response for the lower part of the audio spectrum. Acoustic feedback problems occur at the peaks. XY/101-----' Fig.2: block diagram of the frequency shifter. It shifts the incoming frequency by lOHz. JUN E 1991 5! al Q ,--------------<l>------+12V CJ) 11 r n 0 z n ::r: "O .001 0.1 10k .001 VR3 50k - 12V 0.1 +12V ~ 0.1 202 560k 01 WI,, r270k .,. VR6 100k 270k 270k ~ .I -:120k 270k ... 011'. -1 2V - 12V 01! VR5 50k +12V D1 1N4002 OUT 12VAC PLUG -PACK ,I:- I r +12V 0.1 100 - D2 1N4002 .1 I --- o,Il OUT GNO I I I .... I +V - 12V Fig.3: ICla & IClb generate two quadrature signals which drive the X inputs of 4-quadrant multiplier stages IC2a & IC2b. The Y inputs are driven by a lOHz quadrature sinewave oscillator consisting of IClc & ICld & the outputs then mixed & fed to amplifier stage IC3. - 12V 0.47 OUTPUT V OUTPUT IC2a OUTPUT IC2b Fig.4: trimpots VR3 & VR5 should be adjusted to obtain amplitude modulated waveforms at pins 8 & 9 of IC2, as shown here. work. The two quadrature signals (ie, out of phase by 90 degrees) are then connected to the X inputs of two 4quadrant multipliers, IC2a and IC2b. A quadrature sinewav·e source of about 10Hz is produc ed by IClc and ICld , and is fed into the Y-inputs of the two multipliers (IC2a & IC2b). The outputs are then mixed together and amplified by IC3 , where the final output is then connected directly to the PA system. In more detail, the input signal is amplified by the non-inverting amplifier ICla and an inverted copy is produced by inverting buffer IClb. IC1 is an RC4136, chosen for its low noise and low cost. The passive wideband quadrature network is connected between the two outputs of ICla and IClb. This produces two copies of the input audio signal which are 90 degrees out of phase. IC2 is a dual transconductance amplifier package, the LM13600. Each amplifier is connected as a 4-quadrant analog multiplier. The outputs from the two passive networks are connected via 4. 7kQ resistors to the X 0 inputs , which in this case are the inverting pins of IC2a and IC2b . 10Hz oscillator ICl c and ICld form the 10Hz quadrature sinewave oscillator. Th e two 6.8V zener diodes provide amplitude regulation and the amplitude itself is governed by the 100kQ pot, VR4. The two quadrature sinewaves are taken from output pins 10 & 12 and are connected to the Y-inputs of the two multipliers via two 50kQ pots, VR3 and VR5. These are really the inputs to the amplifier bias network, which control the gain of the amplifier and make it act as a multiplier. The output of each amplifier section is then connected external ly to the internal buffers (from pin 5 to pin 7 and from pin 12 to pin 10 of IC2) to provide a low -impedance output. The signals from each of the multipliers are then AC-coup led and mixed together via 100kQ pot VR6. IC3 is a TL071 FET-input op amp connected up as 'a mixing amplifier with a gain of about four. The output signal at pin 6 is then connected straight to the input of the PA system. The power supply is ±12V DC and can be derived from a 12VAC plug pack using a 7812/7912 3-terminal regulator combination. Setting up The setting up procedure is as follows: (1) Adjust VR4 so that the amplitude of the sinewave outputs at pins 10 and 12 of IC1 is about 9 volts p-p (about 3 volts RMS). (2) Connect a lkHz sin ewave source to the input of ICla. Adjust VR1 and VR2 so that the respective outputs at pins 8 and 9 of IC2 are as symmetrical as possible about the centre-line. What you'll find is that these outputs will tend to skew either up or down initially' and as you rotate the pot, they will quickly come into line. (3) Adjust VR3 and VR5 so that you get an amplitude modulated waveform at the respective outputs . The signal envelopes you end up with should be similar to those in Fig.4. As you adjust VR3 and VR5, you should get a normal amplitude modulated waveform and as you turn the pot, the waveform should appear to fold on top of itself, to produce a "bow-tie" effect (as in Fig.4). If you have a CRO handy, connect one probe to the output at pin 8 ofIC2 and the other to pin 9. What you should see are two similar waveforms, except that where one waveform appears to be at a min imum , the other should be at its maximum amplitude as shown in Fig.4. Finally, adjust VR6 so that the output of IC3 has as little or no 10Hz modulation as possible. Because the outputs of the two multipliers have the 10Hz carrier suppressed, mixing the output should give a steady amplitude signal with its frequency equal to the input+ 10Hz; ie, 1010Hz. Note that wh il e this frequency shift concept do es work, in practice we found that it was critical to set up and produced frequency non-l ineari ties of its own. SC PHONE LINE AND MAINS FILTER/SURGE PROTECTOR Protection for •Answering machines •Cordless phones ,,Facsimiles · •Computers •Modems •Telex The Arista model CPEP-1 ensures that lightning, power surges and spikes do not reach your costly communication equipment as it constantly monitors the PHONE line and the MAINS power line. Simply connect to any existing power point, plug in your fax, phone, video, Hi Fi or any other 240 VAC operated equipment and a series of indicators shows the status of the mains power point while several MOV's and gas arrestors prevents access to damaging energy sources. The CPEP-1 is fully Telecom and Dept. of Minerals and Energy tested and approved. Imported and distributed by: .ARIST1\ ELECTRONICS Available through the following retailers: Telegrafax PIL. 305-307 Sailors Bay Road. Northbridge. 2063. NSW. (02) 958 5137 Geoff Wood Elec. 229Burns Bay Road. Lane Cove. 2066. NSW. (02) 427 1676. UV PROCESSING EQUIPMENT e KALEX LIGHT BOXES UV2 Exposure Area 40cm x 27cm $650.00 • Portuvee 4 Exposure Area 25cm x 14cm $225.00 • Portuvee 6 Exposure Area 43cm x 40cm $625.00 3M Scotchal Photosensitive Riston 4600 PCB Materal All prices are plus sales tax if applicable KALEX 1~1.iMc¼r,i ,I,, - , . ., 40 Wallis Ave East Ivanhoe VIC. 3079 (03) 497 3422 (03) 497 3034 Fax (03) 499 2381 ELECTRONIC COMPONENTS & ACCESSORIES e SPECIALIST SCHOOL SUPPLIERS Ju,w: 1991 61