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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 an email to

Query on speaker protector

I am building the Speaker Protector & Muting circuit featured in the July 2007 issue. I’m interested in it because it has both anti-thump capabilities as well as muting and it operates directly from the amplifier power supply.

Some of your stereo amplifiers use just the one transformer whereas the SC480 uses a separate power transformer for each amplifier. May I point out just one small thing in using your Speaker Protector for use with two SC480s?

It appears that the Speaker Protector will be powered by just one of the amplifiers with its 0V return. Won’t it be necessary for the two amplifiers to be joined together via a common 0V return for the Protector to operate effectively with both amplifiers? (G. K., via email).

  • It is true that the Loudspeaker Protector will be powered from only one power amplifier in a separately powered stereo amplifier pair. However, since all such systems will already have a common earth reference via the stereo program source, that should not present any operational problems. It is also true that a power supply failure in one amplifier would cause both speaker channels to be disabled by the Loudspeaker Protector. Again, that is not really a problem.

    By the way, the power supply suggested for a single SC480, in the February 2003 issue, would be quite adequate for powering a stereo system in most situations. There is no real need for separate power supplies for each channel.

    Calibrating the digital thermometer

    I am building the Digital Thermometer published in the August 2002 issue and I have a question regarding the calibration. I am not sure what the "initial offset voltage of IC1" is (detailed in stage 8 of the set-up). Can you please help? Also, the buzzer is going off as I am setting up the unit. Is this normal? (E. P., via email).

  • The initial offset is the output voltage from IC1 (at TP2) when its input voltage at pin 3 and the op amp’s inverting at TP4 are both at ground (0V). For a perfect op amp (IC1), the output voltage will be the same as the pin 3 voltage or 0V (ground). In practice, the output will be different to this.

    The measured offset voltage is used at step 5 of the calibration.

    Query on Circuit Notebook charger

    With regard to the Lithium-Polymer Peak Charger featured in the Circuit Notebook pages of the January 2005 issue, I built the circuit but it did not work. I was able to adjust VR2 for a reading of 8.4V and adjust VR1 for 0.6V on the base of Q1. No output voltage could be obtained upon pressing the "start" button.

    In other words, neither output voltage nor current was obtained. I have carefully checked my wiring, soldering, etc but I am unable to find any errors. (Y. L., via email).

  • Presumably if the transistor has 0.6V on its base then it should switch on and drive the relay. So press switch S1 and check that the relay switches on, as indicated by an audible click and LED1 lighting.

    If this does not happen, then the relay wiring may be incorrect. It should have the 12V relay coil wired between the collector of Q1 and the +12V supply at the input to REG1. The contacts are wired with the normally-open (NO) contact to the LM317 output and the common to the battery pack.

    Adjust VR1 so the maximum base voltage is available at Q1 when S1 is pressed. Transistor Q1 should hold the relay closed. When the current falls to below the C/10 value for your battery, adjust VR1 so that Q1 switches off.

    Op amp substitution in headphone amplifier

    I have built two of the recent Stereo Headphone Amplifier kits (SILICON CHIP, November 2005) successfully but I have a question about the dual op amp used. Is it permissible to use another dual op amp in place of the specified OPA2134?

    I have used an LM833 on one of my kits and it works fine in place of the OPA2134, however I suspect that the kit is now oscillating at a very high frequency. Is this possible and what are the possible solutions, if any? (F. S., Ingham, Qld).

  • The LM833 is not really suitable for this circuit since it is not intended to drive 600-ohm loads, as is the specified op amp. You may be able to stop the oscillation by increasing the 100W resistor at the output of the op amp but this will lead to lower performance than from the published circuit.

    Heating & cooling with the Coolmaster

    I have a question about the Coolmaster. At what temperature do I change the jumpers from cooling to heating? I want to keep my fridge running at 21°C if possible and was wondering if the fridge motor would do this or do I put a light globe or other means of heating in the fridge and run the circuit on heating and cut out the fridge motor altogether? (J. C., via email).

  • Regrettably, there is no easy answer to your question. The problem is that you want to maintain the internal fridge temperature at a level which will be above the external ambient temperature at some times (ie, at night in winter) but below the external temperature at other times, such as during the day in summer. So you really need a system which involves both heating and cooling, not just one or the other.

    Your best plan might well be to fit a small incandescent lamp or low-power heating element inside the fridge, powered on permanently so it will be trying to raise the temperature above 21°C. Then use the Coolmaster/Tempmaster in normal "cooling" mode, setting it to keep "fighting" the heater and cooling the internal temperature down to 21°C. You may only need a 15-20W lamp as the heating element.

    Fuel Cut Defeater needs MAP sensor

    I have built the Fuel Cut Defeater (SILICON CHIP, February 2007) and would like to install this on a Mazda MX-5 SE. I appreciate that this design is based on another vehicle.

    The instructions indicate that for installation you need to locate the wire from the boost sensor that has around 1.4V at idle. My boost sensor has three wires, two of which have a voltage of around 2.4V and 5V, respectively. Therefore, I am curious if there is some adjustment that I need to make to ensure this device functions correctly. (M. K., via email).

  • The Fuel Cut Defeater is not meant to connect to a boost sensor but to a MAP sensor. The signal from the MAP sensor will vary with engine load and in particular, at turbo boost the voltage will go up to close to 4V. It is this voltage that is intercepted and altered to prevent the ECU seeing the extra boost voltage.

    The MAP sensor has three wires: 5V, 0V and signal. Use the signal wire (the one that varies with engine loading – ie, low voltage at low engine loads and a higher voltage at higher engine loads) for the "MAP in" connection.

    The 3.9V threshold may need to be altered. You can do this by using a 5kW trimpot in place of the 3.9kW resistor at pin 5 of IC1b. Wire one side of the trimpot to the ground track and the wiper to pin 5. Adjust the trimpot to the voltage you wish to clamp at when in boost, to prevent the fuel-cut action.

    Two-way active crossover wanted

    I play in a band and also manage all the PA gear. I have made two 1.5-metre tall linear arrays, that go down to about 100Hz, which I use for vocals and instruments; they perform much better than I expected. The problem is the bass end.

    I have been looking for a 2-way crossover that has a crossover frequency of 100Hz. The obvious choice would be a subwoofer controller but it doesn’t have a high-pass filter at 100Hz to feed the linear array. The next choice would be a 3-way active crossover but that has a midrange and tweeter output and I already have the crossover in the box and I don’t want to buy six amplifiers at $250 each.

    So do you have a simple 2-way active crossover with a 100Hz crossover point? (S. P., Perth, WA).

  • The simplest approach would be to modify our 3-Way Active Crossover (SILICON CHIP, January 2003) to provide 2-way operation. In effect, you would just omit the parts for the central passband section and make sure that the high-pass and low-pass sections had the same corner frequency.

    Using the CD-ROM adaptor in a studio

    I refer to your Playback Adaptor for CD-ROM Drives. I would like to use these in a broadcast studio situation and for that reason require that the software for the adapter do the following:

    (1) Queue to start of selected track so that when play is invoked (START), there is no delay in the drive delivering audio from that track.

    (2) Play the selected track and only that track – ie, the drive will stop at the end of the selected track and will not run into the following track.

    If the experience of the community FM radio station where I work is any indication, these modifications to your existing project (or perhaps a special model) would be very well received. A commercially produced dual CD drive unit at present runs to about $1500 and our station just can’t afford that.

    If you could modify the software to meet these requirements, I’m sure that many other community-based radio stations would build one (or two). The only other proviso would be that the audio quality is up to broadcast standards.

    Your article doesn’t say whether the CD-ROM drives will accept MP3 CDs. Can you please clarify that question? (P. S., via email).

  • To answer your last question first, the CD-ROM Playback Adaptor will not play MP3 CDs (ie, CDs with MP3 files on them). This is because there is no hardware (or software) to decode MP3 files either on the drive or on the board. You will only be able to play audio CDs (native CD audio tracks).

    Second, it is most probable that the ability to queue songs can be incorporated into the source code. You would need to select a track number and then press a "queue" button. This would load the TOC (table of contents) and from that select the starting address of the relevant track. It is then a matter of initiating playback of the track and then "pausing" it as soon as it starts.

    Basically, the track would be in the play mode but "paused" until another button is pressed to allow the audio to be played from the paused state.

    Regrettably though, we are not in a position to modify the software for this purpose.

    Circuit for measuring injector duty cycle

    I wonder if it would be simple to design a circuit which would give an indication of the duty cycle of fuel injectors? While not giving complete info, it could be of help in refining one’s driving technique as related to fuel economy (A. B., Mackay, Qld).

  • We published a fuel injector monitor in the August 1995 issue. This gave a direct reading of the injector duty cycle.

    Varicap for GPS Frequency Reference

    Can you please advise where I can obtain the BB119 varicap diode or equivalent for the GPS Frequency Reference (SILICON CHIP, March, April & May 2007)? (J. L., via email).

  • It looks as if the BB119 is no longer available, even from surplus suppliers like Oatley Electronics.

    We suggest using a readily available 12V 1W zener diode like the 1N4742 or you can chase up and substitute an SMD varicap diode like the BB202 (Philips, etc) or the ZMV933 (Zetex). The latter devices are very small and will be a bit tricky to use.


    Questions On The 20W Class-A Amplifier

    I have followed with great enthusiasm your series on the 20W Class-A Stereo Amplifier. Congratulations on achieving such a refined design.

    In my audio set-up, the Class-A Stereo Amplifier would be ideal if it could split-off the lower frequencies (<80Hz) for my subwoofer to reproduce and then send the rest via the class-A amplifiers. It seems to me that this would make better use of the 20W to produce sound level, compared to driving woofers as well as the mid & high drivers.

    I note that you featured an Active Crossover design in the January 2003 issue. However, the noise and distortion figures of the crossover would negate the advantages of the Class-A Stereo Amplifier. Do you have any recommendations in this regard?

    Also, I would have thought that headphones would be an ideal way to enjoy the low noise/distortion of the class-A stages. Is this difficult to achieve with the design? And how about an Auto-off feature? This would be nice to have, prompted by your excellent energy feature articles (the 100W light bulb calculations) and your subwoofer controller which
    uses an auto-off function.

    Could the latter be adapted to switch off the Class-A Stereo Amplifier, perhaps? (P. C., via email).

  • There is no easy way of greatly improving the S/N ratios of the Active Crossover, especially since it is based on quad op amps; there is no easy drop-in replacement op amp package which will improve things.

    The best approach is to use a high-efficiency speaker system driven directly from the Class-A Stereo Amplifier. Have a look at the speaker system described in the December 2007 issue.

    A headphone socket can be added by connecting a 330W resistor in series from each speaker output. However you also need to wire the headphone socket so that it switches off the loudspeakers. That might seem simple but the routing of the speaker leads within the chassis is very critical to obtaining the very low hum figures. Auto-off could be done but again, signal routing within the chassis is critical.


    Component Quality In Class-A Stereo Amplifier

    I have purchased the Class-A Stereo Amplifier kit from Altronics. My questions relate to components. I have decided to purchase components to replace those provided with the kit, in particular the capacitors and resistors.

    I have purchased Nichicon KZ and ES parts. These parts are larger and their lead spacing and diameter are different. I could enlarge the PC board holes and bend their leads to make them fit. Please comment on the possible impact on performance from the proposed change (longer signal path, larger components).

    I am researching very low noise resistors. The Vishay Dale RM55 and RM60D (CMF) are potential parts. Again my concern is that they are larger components. Please comment.

    The capacitors supplied have a
    temperature rating of 85°C. The photos of your amplifier show Ruby-con 105°C components. Should I be looking at using these (Rubycon ZA & ZL parts) instead?

    I get the impression that SILICON CHIP does not believe in the "sound of a component" however given comment that appears on the Internet, maybe some components perform better than others? Given the cost and work in assembling the kit, I want the best performance. (P. B., Dee Why, NSW).

  • You are right – we do not believe in component "sounds" unless of course, they are of abysmally poor quality. Virtually all the comment on the internet about component sound quality is made by the ratbag fringe element who have no way of checking the effect on performance of any of their component changes.

    There is nothing to be gained by substituting capacitors or resistors. There will be no improvement in the virtually unmeasurable distortion or the residual noise

    We would strongly recommend that you build the amplifier with the supplied components in the kit. Do not make any alterations until you have had the amplifier fully operational.

    Then we suggest that you run the amplifier for at least a couple of weeks – the performance is quite superb. Only then, if you really must, make a few component changes to see if there is any audible effect – there won’t be.

    Can CD-ROM Adaptor Control A Hard Disk

    I read the article on the ATAPI drive controller unit (SILICON CHIP, November 2007) but although it talks about controlling a hard drive with a microcontroller, I haven’t as yet worked out whether a hard drive can be used in place of a CD-ROM drive.

    Is it possible to use this controller board to run a hard disk? I want to make a unit that will start playing a sequence of songs indefinitely until the controller receives an input to tell it to stop.

    Secondly, I see it uses a 16x2 LCD display. This is a very common display, yet despite my searching, I cannot find a plastic bezel/window which would enable the mounting of one of these displays into a panel on a cabinet.

    Can you suggest a source? I have tried many companies in the USA, as well as sellers on Ebay and the like. No one seems to make a bezel for this size of display which is really bizarre for such a popular item. (S. W., Auckland, NZ).

  • Since the physical interface that CD-ROM drives use is the same as for ATA hard drives, the same hardware can be used to control a hard drive. The only problem is that the interface will be relatively slow, since it uses only PIO. In fact, the speed of data transfers that can be achieved with this CD-ROM Playback Adaptor (when used with a hard drive) will be of the order of 25kB per second or worse, which is very slow.

    To achieve higher data rates, DMA transfers are used in PCs. This adaptor does not have the relevant hardware needed to implement a DMA mode ATA interface. So, if you wish to use a hard drive with this adaptor, be prepared for low speeds.

    Another problem is that the microcontroller used has very limited RAM. However, if you still wish to use a hard drive with this project it is possible but the firmware will need to be modified. The current firmware is written only for ATAPI devices, not hard drives, although a lot of the low level interface is identical in both cases.

    Note, however, that while this project is useful for experimenting with hard drives, it is not really intended to access hard drives at the speeds necessary for streaming audio.

    We do not know of any bezels for a standard 16x2 LCD module. The only ones we know of are for colour LCD screens. However, almost all LCD modules will have mounting holes for screws which, if you make an accurate enough cut in your case, can be used to mount the LCD so that it is flush with the lid of the case.

    Temperature Sensor May Be Open-Circuit

    I have a question regarding the Fridge/Freezer Temperature Controller in the June 2005 issue. I have built the kit and find that the unit is permanently on. I've double-checked the construction (I bet everyone says that) and can't find anything wrong.

    What I am getting is 8.9V at pin 3 of the comparator chip and if I measure the resistance of the LM335 over a range of 2-20°C it only varies from 35.7kW to around 36.5kW. I'm guessing that this small change isn't enough to give a 2-3V drop across pin 3; in fact, the voltage stays at 8.9V. If I remove the LM335 and replace it with say a 2.2kW resistor, the LED goes out.

    My question is where could my circuit be wrong or is there some instruction missing and should I be adding some extra resistance to the LM335 part of the circuit to help with the voltage drop? (J. P., via email).

  • If you are getting a voltage of 8.9V more or less permanently on pin 3 of IC1, this suggests that the connections to your temperature sensor are either open-circuit or perhaps reversed. Measuring the resistance of the LM335 is not meaningful because it’s very non-linear. In fact it behaves very much like a zener diode, whose reverse breakdown voltage varies in direct proportion to the absolute temperature in Kelvin. And at any particular temperature the dynamic resistance is very low, so that the voltage drop hardly changes over a fairly wide range in current (100mA to say 5mA).

    Notes & Errata

    PIC-Based Water Tank Level Meter, Nov-Dec 2007: we no longer recommend mounting the pressure sensor on the PC board and using the "tube in tank" method for water level sensing. Instead, the sensor should be mounted inside the tank as described on page 86 of the December 2007 issue.

    Alternatively, the sensor can be located in a separate box outside the tank and its input connected directly to the outlet at the base of the tank. The addendum on p89-91 of the January 2008 issue has the details for this method.


    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.

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