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Ask Silicon Chip

Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097; or send us an email here.

TV signal strength meter wanted

I would like to know where I can buy a kit or a ready-made meter to measure signal strength for TV channels. My neighbour used one from his work to help me locate the best position for my TV antenna. It was quite a whizz-bang gadget, preset for all the TV channels, and he could tell me immediately the dB for all the channels.

Have you people had experience with a simplified version of one of these? Also, where can I get the carrier frequency settings for all the TV channels? (G. B., Melbourne, Vic).

We have not produced a TV signal strength meter. Many installers find that a portable TV is more convenient. Not only does it give an indication of signal strength but it also shows the presence of ghosting and enables the antenna to be oriented for best reception.

TV station frequencies can be found as a PDF file at:

Power reduction to derated transformers

I want to ask if it is possible to use 40-0-40V 300VA transformers for the Mosfet amplifier published in SILICON CHIP, August 2001. This amplifier is essentially a revamp of the old EA Pro One amplifier. In both articles (EA & SILICON CHIP), the design specifies a 45-0-45V 300VA transformer. I have an old EA Pro One and want to restore with it 40-0-40V 300VA transformers.

Can you estimate the output power with these transformers and if the 0.8V quiescent current set-up would be the same? (E. Z., via email).

Power is a square law. Since you are proposing to reduce the supply voltages by a factor of 12% (40/45 = 0.88), the power output will be reduced by 21% (0.79). This will be a barely audible difference of -1.02dB. The biasing will remain the same.

Sniffer probe avoids crystal loading

I have just successfully built the Low-Cost 50MHz Frequency Meter from the October 2003 issue (from a Jaycar kit). May I suggest a follow up article on using this meter, describing probes and servicing techniques for higher frequencies.

In conversation with Bob Young (one of your contributors), I learnt that Bob described the construction and use of a "sniffer" probe for use around radio control transmitters that allows frequency measurements to be made without connection to the circuit under test, avoiding loading circuits that could give an invalid reading.

Another useful probe would be an "active" probe that would increase both the input impedance and sensitivity, especially at the high end of the unit’s range.

I am using a CRO probe with the Frequency Meter but have found that on probe setting ‘x1’ it loads the crystal oscillator circuit in the radio control receiver, causing it to cease operating while on ‘x10’ the reduced sensitivity means the meter reads "Await Signal". (J. K., Broulee, NSW).

We showed how to make a sniffer coil on page 27 of the December 2003 issue. Also you might like to look at the active sniffer probe described in June & September 1988.

When checking crystal oscillator circuits with a CRO probe, try connecting it via a 1kΩ or 2.2kΩ resistor. This can be enough to stop the probe capacitance from loading the crystal, while still letting enough signal through to let you make the measurement. It can be useful when using a scope too.

Phono Preamp For Surround Sound Amplifier

I recently purchased a surround sound receiver amplifier and using it to watch and listen to movies and music DVDs is a real pleasure. My request relates to the missing phono input on the amplifier. I have a reasonable collection of LPs that I would like to occasionally listen to, even if only to amaze my two sons with the big black disks.

Do you have any plans to design a phono preamplifier for a magnetic cartridge which would feed a line input on a hifi amplifier? For some reason, many hifi equipment manufacturers no longer include this in their designs and a project like this would be fun to make and extend the use of most modern hifi equipment. (J. C., via email).

Three designs are relevant: the Universal Preamplifier in April 1994; the RIAA Preamplifier in March 2002 (these are essentially the same circuit but have different PC boards); and the LP Doctor (includes click suppression) in January & February 2001.

Line level switcher wanted

Has your magazine ever described a Line Level Switcher? I have purchased a mini stereo system and it only has one line input and no line output. I have a VCR, DVD, CD recorder, tape deck and a Mini-Disc recorder; as you can see, too many for one input.

I would like these devices to go to the mini stereo system and also be able to record from one device to the other. (R. M., via email).

Have a look at the Video Switcher in the June 1992 issue. It can handle A/V signals from three sources.

LEDs for an LCD video projector

I was intrigued by your article on human-powered LED torches in the February 2004 issue. I have been contemplating building a LED-based home LCD projector. Traditional projectors use very expensive and hot running lamps, necessitating special power supplies and cooling. LEDs seem an ideal solution, if they can be made bright and focused enough.

Using this technology, it should be possible to build a projector for much less than current commercial units. Is this something that SILICON CHIP magazine could investigate? (J. H., via email).

LEDs have a very long way to go before they can replace the halogen lamps used in projectors. We doubt if they will ever be bright enough for that application.

DIY humidity sensor for PICAXE datalogger

I was very interested to read your project on the PICAXE Datalogger in the February 2004 issue. I have made enquiries about various types of sensors from some companies but have found that they are expensive and high end.

Do you know of any publications for the enthusiast that explain how to make your own sensors – eg, relative humidity, leaf wetness, etc? (M. H., via email).

We do not have any information on DIY sensors although some clothes dryers have a humidity sensor based on horse hair. It may be possible to use such a sensor although we are inclined to think that it would be fairly imprecise in its action.

Crossover Wanted For 2-Way Loudspeaker

I have recently purchased a JV-80 loudspeaker kit from Jaycar Electronics and would like to know if it is possible to make a 2-way speaker using one of the tweeters (D26 NC-15-06) and one of the woofers (P22 WP- 01) in a 35-litre enclosure.

If so, can you tell me if it's possible to use the crossover network (CS-2580) supplied with the kit, with suitable modification for a one tweeter and one woofer combination, or do I need to use a completely different circuit?

Also, would one of the ports recommended for the JV-80 kit (ie, 66 x 140mm) be suitable for a 35-litre enclosure? (L. J., via email).

It is certainly possible to build a 35-litre system with one of the woofers and a tweeter. It would then be a 4-ohm system, which may or may not be a problem for your amplifier.

However, you cannot use the existing crossover since it is optimised for an 8-ohm system. As a stopgap, you could try connecting the woofer right across the full signal but still connect the tweeter via the existing crossover components.

Nor can you just use one of the ports as is. The enclosure needs to be designed using a software program such as BassBox to calculate the port size.

Bigger bass for school PA system

I have a question involving a 100V line PA system that is installed in a school sports hall with speakers down one side. A team wants to use them to play up-tempo music for warm-ups and maintaining training intensity. As the speakers are evenly spaced up and down the wall on one side of the courts, evenness of volume is OK but they lack the needed bass punch. So I would like to install a subwoofer that can be taken away when not required.

Is it possible to insert a passive subwoofer crossover directly into the 100V line, feed the high-pass signal to the existing speakers and have the low-pass signal feed a 100V to 8-ohm transformer and onto the sub? For a 100W amplifier, what size transformer would you recommend for the sub tap? Will different taps change the impedance and so the crossover frequency? Or do I need two 100V to 8-ohm transformers; one stepping down from 100V to ‘speaker level’, then through the crossover and, on the high-pass side, a second stepping it back up to 100V?

Or is it best to have an electronic crossover at signal level and bi-amp it – one feeding the existing speakers and the other feeding the sub? Is there any problem with the sub amplifier output being stepped up to 100V and then back again at the subwoofer speaker?

Are there any traps for someone who has enough knowledge to think they know what they are doing but not enough to know they actually don’t? Being a school, cost is going to be a factor so I’d like to go with the least expensive option that works.

By the way, have you considered doing an article on 100V line-distributed PA systems? Ideally the article would at least cover the background of why 100V and why there are 70V and other voltage standards. (T. H., Calwell, ACT).

We would not try any passive subwoofer system. It would be better to take the 100V signal and feed it to a separate powered subwoofer. This would be much easier to set up and disconnect. You might like to have a look at the subwoofer controller project in the December 1995 issue.

Thanks for your suggestion for an article on PA standards, etc. We have already briefly covered the subject in an article entitled Plastic Power PA Amplifier in the March 1997 issue.

Bypass capacitors for Multi-Spark CDI

I have a problem with the Multi-Spark CDI system featured in "Electronic Projects for Cars, Volume 2". Here in Thailand I cannot get the 10μF 63V MKT capacitors for decoupling the DC supply to the transformer (see page 82, Fig.2). Is it OK to change these to 1μF 63V MKT? (P. A., Amper Muang, Thailand).

10μF capacitors must be used rather than 1μF. These are MKT types and are necessary for correct bypassing of the high frequency switching artefacts on the DC supply. Alternatively, low-ESR 10μF 25V electrolytics could be used.

Farnell Electronics sell these MKT capacitors, Cat. 814-155. Refer to:

Dimmer for 900W incandescent lamp load

I have a 900W incandescent lamp load and I wish to place a dimmer on the circuit. The existing switch is on a multi-gang plate so the bulky commercial dimmers are not an option.

My local electronics retailer in Adelaide suggested using a linear pot and a 15A Triac. I purchased these parts along with a heatsink but I am confident that more components are needed. So I need a circuit diagram and parts list to do the job. (S. H., via email).

A 15A Triac is not robust enough for such a load as lamp failure will cause the Triac to blow. Have a look at the High Power Dimmer in the August 1994 issue.

Increased rating for electronic load

In the 50W Electronic Load (SILICON CHIP, September 2002), could the addition of an extra MOSFET in the output stage increase the current capability to 20A? (B. P., Palmerston North, New Zealand).

Increasing the power handling capability of the electronic load (while maintaining reliability) is a little tricky. A second MOSFET in parallel with the existing device will increase power handing. However, it’s not easy to get the two devices to share the load equally.

In fact, we built a 100W prototype using multiple devices but eventually had to scrap the idea, as very large (and expensive) source resistors were required to force equal current sharing.

Having said that, you could try one of these ideas: (1) use a much larger MOSFET (such as one of the "DICE" packaged devices); (2) try MOSFETS specified for audio use. These are designed for use in their linear regions, so are likely to work better in this application; and (3) use matched MOSFETs.


SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws.

Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable.

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