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Near field studio monitors Could you please explain to me what is meant by the term “near field studio monitors” when applied to speakers suitable for use in a recording studio. How do these speakers differ from good quality hifi speakers? Has SILICON CHIP ever described a kit for near field studio monitors or a speaker kit that would be suitable for this application and if not, could this be done? From what I have read, these monitors may be suitable for use with a PC fitted with a sound card and amplifier, where high-quality, high-level sound is required. As these sorts of systems are becoming popular, it may be a good opportunity to produce or revisit a kit after which the transistors again died, taking the fuses with them. With a circuit diagram in front of me and limited access to an oscilloscope, I’m stuck – not willing to sacrifice another two transistors for three minutes of music. I have included a copy of the circuit diagram and indicated the offending components. If you have any suggestions, I’m listening. (J. D., Blackburn South, Vic). • There are a number of possible reasons as to why your ampli­fier’s output stage is not working as it should. The first ap­proach should be to measure the voltages marked on the circuit with no load attached. It is quite possible that the degree of heating in the 5W (marked 2W on the circuit) resistors is normal. If the associated volt­ages (+15.4V and -15.7V) are correct, then it should be OK. However, the most important thing to do when you have re­placed the output transistors in a power amplifier is to set the quiescent current using, in this case, trimpot R130. The relevant voltages for this are shown at the emitters of Q109 and Q110 (ie, +0.618V and -0.589V). Therefore, you should connect a digital multimeter across the 220Ω resistor between the emitters of Q109 and Q110 and adjust trimpot R130 until the voltage is 1.207V. Leave the amplifier operating for at least half an hour after doing this 94  Silicon Chip approach to this type of speaker system. I appreci­ ate your comments and look forward to hearing from you. (R. C., Stockport, SA). • In audio parlance, “near field” refers to the response of transducers at very close proximity, in the pressure region. Hence “near field” microphones are used by vocalists (where they almost swallow the microphone). We would assume that “near field studio monitors” are intended for use in the confined spaces of recording studios, at listening distances of less than one metre. If this is so, such speakers are unlikely to be suitable for use in domestic living rooms. We have contacted a number of audio equipment distributors on this question but none of them have been able to give any information. adjustment and redo it if the reading has increased. The equivalent quiescent current through the output tran­ sistors can be calculated by dividing the total voltage between the output transistors (+0.012V, -0.007V) by the total resistance (0.66 ohms). This gives a quiescent current setting of 28 mil­liamps. Wireless microphones need muting Over the years, the magazines have designed bucket loads of FM wireless microphones but they are not much good for singers because they usually don’t have enough dynamic range. They aren’t very good for announcements on PA systems either be­cause they must be on all the time, even when you aren’t speak­ing. If you turn them off, you get random radio noise coming out the speakers (unless you are using an expensive tuner). So why have we all built one of them? Probably because they are really “neat” gadgets. How many of us are still using them? Not very many, I suspect. I’ve heard that FM stereo is transmitted with a signal frequency to signify that it is stereo (around 16kHz). My sugges­tion is to add this frequency to one of your FM microphone cir­cuits and then get a tuner with a stereo LED and use the LED output to switch a relay which disconnects the speakers when the mic is off. This stops noise from coming out the speaker when the mic is off or there is no audio. The relay could also be used to switch on music when the mic is off. Can you make a project out of it? It just seems like a logical improvement for the old FM mic! (C. P., Coromandel Valley, SA). • It is true that quite a few such circuits have been pub­lished over the years and that some are much better than others. The design featured in the October issue of SILICON CHIP is quite good although it does not incorporate your idea of muting. FM stereo signals are transmitted with a pilot signal of 19kHz at ±7.5kHz deviation of the FM carrier (ie, 10% modula­tion). We would be reluctant to use this pilot signal as a mute control as you suggest because it would automatically switch the tuner into stereo whenever it was present. This is not a good idea for wireless microphone applications since tuners always have an inferior performance in stereo mode compared to mono mode. What you really need is a VOX circuit to switch off the microphone when it isn’t being used but which leaves the trans­mitter section operating to keep the tuner “quieted”. We shall see what we can do. Note & errata Solar-Powered Electric Fence, April 1993: C4 should be increased from 10µF to 470µF to improve the supply decoupling and prevent erratic operation of the inverter circuitry. UHF Remote Switch, December 1989 and August 1990: in some cases, the MC145028 decoder (IC2) may not operate correctly since the specified oscillator components cause it to operate at 770Hz which is outside its recommended frequency range of 1kHz to 400kHz. The solution is to change the timing components so that the oscillators operate at about 2kHz. For IC1 in the transmitter, replace the resistors at pins 11 & 13 with 220kΩ and 100kΩ resistors respectively and change the .01µF ceramic capacitor at pin 12 to a .0022µF polyester type. For IC2 in the receiver, change the resistors at pins 7 & 10 to 39kΩ and 180kΩ respectively. The capacitors at pins 7 & 10 are unchanged.