Fullscreen HTML5Med Res (??MB)HTML4

Build this stereo microphone preamplifier . . . and get the most out of your tape recordings By DARREN YATES If your dynamic microphones don’t have enough output, try this portable mic preamp. It has low noise & only .008% distortion. What’s more, it runs off two 9V batteries for about 100 hours & is built into a metal diecast case to shield out un­ wanted hum. If you do any outdoor audio recording, then you’ll be familiar with how microphones work and how to get the best sound possible with your gear. The major problem these days is that most microphones are dynamic types. But while these have a superior noise performance compared to other types, they are usually not very sensitive, most being somewhere around -75dBV. By comparison, the older elec­tret and condenser microphones were often as sensitive as -60dBV which is quite a big difference in output. This lower sensitivity for dynamic microphones is usually of no consequence when you are recording sources close-up. Howev­er, if you are trying to record distant sounds, such as bird calls, it can make things quite difficult. Although a recording will still be possible, it will have a great deal more noise due to the necessity to drastically increase the gain of the recorder’s preamplifier stages. Naturally, the results will vary from recorder to recorder but even some well-known professional recorders will introduce noise if the preamplifier gain is advanced too far. The way around this problem is to increase the signal from the mi- crophone before it is fed into the recorder mic input. And that’s the job of the Stereo Microphone Preamplifier described here. It is simply inserted into the audio path between the micro­phone(s) and the tape recorder and provides a fixed amount of gain in each channel. This, in turn, enables much lower set­tings of the gain controls and so drastically reduces recording noise. In the prototype, the gain has been fixed at 10dB but this can be altered for each channel simply by changing a single component. The signal-to-noise ratio is 90dB with respect to 10µV, which is very quiet, while the distortion is just 0.008%. In addition, the unit is built into a diecast aluminium case which not only makes for a rugged design but stops any stray hum from getting into the circuit. The circuit Take a look now at the circuit details – see Fig.1. As can be seen, it is based on a single LM833 dual op amp. Each channel uses one section of the op amp package and this is connected as a non-inverting amplifier. The 2.2kΩ and 1kΩ resistors in the negative feedback network set the gain to 3.2, which is just a tad over 10dB. The .001µF January 1995  65 S1a 0.1 0.47 RIGHT INPUT 1k 22k 8 3 2 IC1a LM833 1 10 16VW 100  RIGHT OUTPUT 22k B1 9V 2.2k .001 1k 0.47 LEFT INPUT 1k 22k 5 7 IC1b 4 100  LEFT OUTPUT 22k 4 B2 9V 2.2k .001 1k S1b 0.1 10 16VW STEREO MICROPHONE PREAMPLIFIER Fig.1: the circuit is based on a single LM833 dual op amp. Note that each section (IC1a & IC1b) wired as a non-inverting amplifier with a gain of 10dB. capacitors ensure that the frequency response rolls off above 72kHz, while the two 1kΩ stopper resistors at the non-inverting inputs reduce the sensitivity to RF noise. To ensure that the preamp can be used with any microphone, the inputs are AC-coupled via 0.47µF capacitors, giving a fre­quency response down to 15Hz. When you consider that most dynamic microphones start at around 60-80Hz, this is more than adequate. The outputs from the op amp stages appear at pins 1 & 7 and are fed to the right and left output sockets respectively. Final­ly, ±9V supply rails are derived from the two 9V batteries via douple-pole on/off switch S2. If alkaline batteries are used, they should provide about 100 hours of continuous operation. The supply rails are decoupled using 10µF electrolytic capacitors. Construction All of the components except for the power switch, the four 6.35mm sockets and the batteries are installed on a small PC board measuring 48 LEFT OUTPUT x 58mm and coded 01111941. Fig.2 shows the wiring details. Before you commence construction, check the board thorough­ly for any shorts or breaks in the PC tracks. If you find any, use a small artwork knife or a dash of solder to fix the problem where appropriate. When you’re sure that the board is OK, begin the assembly by installing PC stakes at the external wiring points (12 in all). This done, install the three wire links, followed by the resistors, capacitors and the IC. We recommend that you check the value of each resistor on your multimeter, as some of the colours can be difficult to decipher. Take care with the orientation of the two 10µF capacitors and the IC. The notched end of the IC is adjacent to pin 1 and must face towards the two 0.1µF MKT capacitors. If performance is critical, you can substitute the OP275GP dual op amp from Analog Devices or the NE5532AN device from Signetics/Philips. These devices are slightly more expensive and not as easy to obtain as the LM833, however. Once the board assembly has been completed, position it inside the case at one end (see photo) and mark out its corner mounting holes. Drill these holes to 3mm, then attach the front panel artwork to the lid of the case and drill the hole for the power switch. The lid should now be fitted to the case and the front panel label used as a guide for marking out the locations of the input and output sockets (two on either side of the case). These holes should be located about 12mm from the top rim of the base. Drill these four holes using a small pilot drill initially, then carefully enlarge them to size using a tapered reamer until LEFT INPUT IC1 LM833 1k 22k 1k 22k 10uF 100  1k -B1 2x0.47 1 2.2k +B2 S1 1k 0.1 .001 0.1 10uF 2.2k 22k 100  22k .001 -B2 RIGHT OUTPUT +B1 66  Silicon Chip RIGHT INPUT Fig.2: install the parts on the PC board & complete the wiring as shown here. Take care with the orientation of IC1 & the two 10µF electrolytic capacitors. PARTS LIST 1 PC board, code 01111941, 58 x 48mm 1 diecast aluminium box, 121 x 61 x 39mm 1 self-adhesive front-panel label 4 6.35mm panel-mount sockets 1 DPDT toggle switch 2 9V battery snap connectors 2 9V alkaline batteries 12 PC stakes 4 15mm x 3mm dia. machine screws 8 3mm nuts 4 self-adhesive rubber feet 1 200mm-length of medium-duty figure-8 cable Semiconductors 1 LM833 dual op amp (IC1) The circuit is housed in a diecast metal case which provides shielding against stray hum fields. Note that shielded cable must be used for the input & output connections to the preamplifier sockets. INPUT OUTPUT L L the sockets are an exact fit. The PC board is secured to the base of the case using ma­chine screws and nuts, with an additional nut under each corner acting as a spacer. Make sure that the board is oriented so that its input terminals are to the right. This done, complete the battery switch wiring, then connect short (40mm) lengths of figure-8 cable to the input and output terminals. The four 6.35mm sockets can now be mounted in position and the wiring to them installed. Note that the centre terminal of each socket must go to the earth pattern on the PC board. On the prototype, the batteries were secured using double-sided adhesive tape. A piece of foam rubber is subse­quently wedged into position when the lid is closed to prevent the batteries from coming adrift. Finally, four rubber feet should be fitted to the bottom of the R R POWER ON PORTABLE MICROPHONE PREAMP Fig.3: here are full size artworks for the front panel & the PC board. Capacitors 2 10µF 25VW electrolytic 2 0.47µF 63VW MKT polyester 2 0.1µF 63VW MKT polyester 2 .001µF MKT polyester Resistors (0.25W, 1%) 4 22kΩ 4 1kΩ 2 2.2kΩ case to prevent scratches to desktops and other surfaces. Testing Before switching on, go over your wiring carefully and check that all is correct. This done, apply power and check the supply rails to the IC. Pin 8 should be at +9V (measured between pin 8 and ground), while pin 4 should be at -9V. The quiescent current should be about 7mA and this can be checked by connecting a multimeter in series with one of the supply rails. If everything checks out OK so far, you can now check that the unit works in practice. To do this, connect the microphones and the tape recorder and check that the gain controls on the recorder can now be set to a much lower level than before for a given sound level. If you need more gain, simply increase the 2.2kΩ feedback resistor in each channel. Finally, if only one channel is to be used, the input of the other channel should have a 1kΩ resistor connected across it. This is best done by soldering a 1kΩ resistor across a spare plug and SC plugging this in as required. January 1995  67