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How about a “valvesound” amplifier? I would like to suggest a singleended Mosfet transformer coupled amplifier as a project, with a power output of about 25 watts RMS. I’m sure the output transformers could be bifilar wound with a 1:1 ratio. In my opinion, solid state amplifiers that are capacitor coupled are a little clinical. I’m sure the above requested amplifiers will have a smooth warm sound, sound­ ing sweet and open like that of a valve, single-ended amplifier, but not requiring a massive un-affordable power supply as valve amplifiers do. (R. L., Somerville, Vic). • While such a project is feasible, we do not think that it would result in valve-like sound. There are several reasons why valve amplifiers sound the way they do and the first is their moreor-less gradual overload characteristic. The second is that their harmonic distortion is often more or less second harmonic which means that the distortion is musically related to the fundamental. This often leads people to state that such-and-such valve ampli- Big brother is definitely watching you With all the current media interest at present in hidden surveillance cameras, spy cameras, hidden audio devices, thermal cameras, infrared cameras and any other device that can be used for surveillance, I am wondering just what is out there and who are they keeping an eye on. I am hoping you would do a series of articles in your great magazine on what there is at present, what the future holds, how do we recognise it and if possible some projects on their detection. Another possible project I would like to see is a Digital Command Control for model railway locomotives that is very small in size and suitable for “N” scale. (S. F., Strathgordon, Tas). • An article on surveillance cameras would merely illustrate the fact that these devices are now very widespread. In many cases, they are quite 92  Silicon Chip fiers are “more musical”. Indeed, they are more musical but this is not the best high fidelity approach since amplifiers that produce significant harmonic distortion which may be masked also produce inter­ modula­tion which is definitely not pleasant. The third reason why valve amplifiers sound “less clinical” is that they usually have far less negative feedback. The high negative feedback in solid state amplifiers has two results. First, it greatly lowers the inherent distortion of the circuit, leading to a much cleaner (or clinical) sound. Second, it leads to a much lower output impedance and this results in more elec­trical damping on the loudspeaker. This leads to a tighter, less boomy bass. Most solid state amplifiers these days are direct coupled instead of capacitor coupled and this improves the loud­speaker damping factor even further. In effect, while a solid state amplifier can be designed to drive a transformer, it is still likely to have a very clean, high quality sound. The most recent example of this is the 175W transformer coupled design in the March 1997 issue. apparent; you only have to look for them. Most shopping malls have cameras for the main concourses, many large buildings have cameras throughout (including in the eleva­tors), as do factories, petrol service stations, parking sta­tions, toll booths on expressways, shops, banks and clubs. In banks, hotels and clubs the cameras are generally quite obvious while in shops they are usually concealed but there are notices to say that cameras are in use. Believe them. In most cases, the cameras may not be directly monitored but will feed video recorders which run 24 hours a day. In the event of a crime, the tapes are examined by the police and often lead to a conviction. There are also a number of TV programs which run along this theme, with “Real TV” being the main one. In the major cities, video cameras have been used to con­trol traffic flow for more than 20 years. Cameras are also often used in apartment complexes and even homeowners are now using them. We published a doit-yourself article on the subject in the June 1995 issue. The other point to recognise is that all these cameras are watching you, not just somebody else. If you are virtually anywhere in a public place, you could be on camera. On a more cheerful note, one of our contributors is working on a DCC design for model trains. Notes & Errata Audio/RF Signal Tracer, June 1997: users of this project will find that the unit produces a lot of noise and what may sound like “motor-boating” when it is switched to high gain and RF modes and with no connections to the input probe and earth clip. This is normal and is a function of its high gain. As soon as the unit is connected to a circuit the noise drops and the wanted signal will be heard. There is a discrepancy between the circuit on page 40 and the wiring diagram on page 43. The circuit shows the signal from switch S2b coupled to pin 3 of IC2 via a 0.1µF capacitor and pin 2 grounded. The PC board has this reversed, with pin 3 grounded and signal going to pin 2 via the 0.1µF capacitor. The PC board is correct. If the unit is to be used on valve amplifiers, there is the possibility that connecting the unit to a voltage above 100V may blow the LM318 IC’s input protection diodes. To prevent this, we suggest soldering two 1N914 diodes in inverse parallel across the 100kΩ bias resistor to pin 3. These diodes can be installed on the copper side of the PC board. 12V/24V Motor Speed Controller, June 1997: there is a mistake in the text on page 30, third paragraph down. The text states “Make sure that they (the diodes) are connected in the right direction across the motor; ie, anodes to the positive supply line.” The diode(s) should be connected with cathode to the positive supply line, as shown in the circuit and wiring diagrams. Flexible Interface Card for PCs, July 1997: the circuit on page 25 shows 4.7kΩ resistors to the LEDs of the 4N28 optocouplers but 1.5kΩ resistors on the wiring diagram on page 27. Either SC value will work.