Regulated supply for LM3876 amplifier

I am in the process of building the 50W amplifier module based on the LM3876, as published in the March 1994 issue. I intend to use this in my study and have the 15W class-A amplifier in the living room. I have read with interest about the 100W Ultra-LD amplifier in the March & May 2000 issues but I like the simplicity of the chip amplifier. Also, I do not need the power.

My questions relate to the power supply for the 50W amplifi-er. I would like to use a regulated power supply. Do you have a circuit or can I modify the class-A amplifier power supply? I understand that regulated power supplies are better and would like to go down this path. Can you assist? (D. F., Sydney, NSW).

There are unlikely to be any benefits in using a regulated power supply with the LM3876. You will not get any better distor-tion because there is no way of separating the supply lines to the small signal stages from the output stage (ie, there is only one supply connection for each rail – pin 1 & 4).

Second, even if you could separate the supply lines, the latent distortion in the LM3876 is about an order of magnitude worse than the Ultra-LD, at high frequencies and about three to four times worse at mid frequencies. You can compare our pub-lished distortion curves from the LM3886 (virtually the same as the LM3876) in the February 1995 issue with the curves for the Ultra-LD amplifier in the March and May 2000 issues.

The point about using regulated supplies with a class AB amplifier is that it only gives quite small improvements. Unless the amplifier is inherently very good, there’s not likely to be a benefit.

Apart from that, a regulated supply is quite a lot less efficient than an unregulated supply and you do not have the benefit of "headroom" whereby the amplifier delivers more power on transients than it does on steady-state signals.

If you’re still keen to improve the power supply for the LM3876, the best way to do it is to double the size of the filter capacitors; eg, increase from 2200μF capacitors to 4700μF.

Finally, if you are determined to persist with a regulated supply, you can upgrade the circuit for the class-A supply.

Ultrasonic pest eliminator for ants

I am currently having a problem with ants around the home, mainly in the ceiling. I have seen a unit on TV that claims to get rid of pests using what appears to be ultrasonics. Has SILICON CHIP ever run any articles or projects on the use of ultrasonics for the removal of pests. If so, could you tell me which volume the articles appeared in. (M. P., via email).

We have published a number of ultrasonic circuits to control the barking of dogs (most recently in July 1999) but as far as we know, ultrasonics have absolutely no effect on insects. In fact, one of the TV current affairs programs did a good job of debunk-ing ultrasonic cockroach repellers several years ago – it showed the cockroaches even crawling all over the piezoelectric tweet-ers.

Replace points for high energy ignition

A few years ago I built the original High Energy Ignition kit as published in the May 1988 issue of SILICON CHIP. Installed into a Mazda 323, the unit has performed flawlessly and I have not replaced the points since.

Recently, I noticed that the "engine speed unit" (tacho?) in the car was not working and found that the dwell extension of the ignition kit was the culprit. Have you seen this before? If so, do you have a modification for the circuit?

I noticed, on your website, that there are a few Notes & Errata on this kit but I don’t have copies of those issues. Can you help me at all? (D. W., via email).

We are bit concerned that you have not replaced the points in all this time. What you will find is that the rubbing block wears down until you have virtually no gap at all. Unless you have already done so, check the points gap and if necessary, replace the points.

Trouble with modified class A amplifier

I have designed my own version of the Class A amplifier with the power transistors and drivers plus power supply filtering on a separate board to the other parts. The boards are joined with pins. However, I am having a problem setting the quiescent cur-rent. Every time power is supplied, the 1.8Ω 5W resistors develop about 10V across them and then cook.

I have been over and over the design and cannot find any faults. I suppose it may be something in the actual layout, though that received much attention during my design. (G. F., via email).

It sounds as though you have a problem with Q10, the quies-cent current setting transistor. If it is open circuit or its base bias resistors are the wrong value etc, it can turn the output stage full on. To check, short the collector and emitter of Q10 with a short clip lead. The current through the 1.8Ω resis-tors should drop to zero. If not, you could have a short some-where in the output stage.

Mitsubishi has no pickup point for tacho

I recently constructed the Digital Tachometer which was published in the August 1991 issue of SILICON CHIP. Unfortunately I could not connect it to my Mitsubishi Lancer because it has an internal ignition coil situated inside the distributor. Could you please give me some advice on this matter?

Also, I have an inductive pickup from my old timing light which I can use to get the input signal from the number one spark plug lead. Is this possible? Or can I modify the circuit to use the inductive pickup?

As my car does not have an RPM meter, I always had problems adjusting the idle speed. Therefore my main purpose is to use this circuit as test equipment rather than as a permanent tachom-eter. (A. G. via email).

The inductive pickup may operate the digital tachometer when connected to the high voltage input. You can only try it.

You also may be able to find the connection to the coil primary by checking the service manual for your car. Many vehi-cles do have a tacho signal output even though the car does not have a tachometer. Information on this can also be obtained from the service manual.

Nickel metal hydride battery charger wanted

I am thinking of modifying the battery charger you published in October 1998 to recharge some metal hydrides. I don’t know a lot about metal hydrides so I was wondering if you have done or are intending to do an article on this topic. (D. C., via email).

We would not recommend the October 1998 design for charging nickel metal hydride batteries. This design was intended for lead acid batteries. Nickel metal hydride (NiMH) batteries are very similar to NiCd types but without the memory effect. They require a charger which will sense the dV/dt change at the end of charge.

We suggest that you have a look at our Multi-Purpose Fast Charger which we published in February & March 1998.

12V neon modulator kit

I want to know a few things about the 12V neon modulator for use with subwoofers (May 1997).

Firstly, I wanted to connect the unit using a guitar amplifier as the signal source. The blurb on the DSE website says that you hook it into the sub-woofer – does this mean that the kit only applies itself to certain signal ranges or it is only triggered by the ranges put out by the subwoofer amplifier? If you connect it straight into a signal source, would it flash for all frequency levels?

Secondly, instead of using one 12V neon tube, would it be possible to connect eight 1.5V LEDs to the unit? Thirdly (and this is just to confirm), any 12V DC power supply should run it, right? (D. C., via email).

You really need to refer to the complete article to under-stand the function of the circuit. We can supply the back issue for $7.70 including postage.

As designed, the circuit will not respond to frequencies above 100Hz but by reducing the 0.1μF capacitor at the collector of Q1 you can improve this. However, the circuit and the neon tube itself inherently do not have a fast signal response - it’s really only intended for bass frequencies.

You could connect a string of LEDs but you need a limiting resistor to set the current. By the way, most LEDs have a voltage drop across them of around 2V so you could only use about five, after allowing for the series limiting resistor. Yes, you can use any 12V supply.

Speed alarm for 1971 VW

I am writing to see if there is an alternative sensor to the one suggested in the Speed Alarm kit. I purchased this item and thought it would solve the speedometer inaccuracy problem in my 1971 VW beetle which has a speedometer in miles/hour.

This kit appeared to have the features I required; ie, a display in km/h. My problem is that it requires a sensor mounted on the drive shaft but as you probably know the VW doesn’t have a normal drive shaft, only rear axles which have a diameter of 25-30 mm. I don’t think these axles rotate at the same speed as a drive shaft and secondly, the diameter of the drive shaft only allows a small number of magnets which I feel may be unsatisfactory.

Can you suggest an alternative sensor? (D. O., via email).

You can use a different speed sensor. The Jaycar photo interrupter (cat ZD-1901) could be used in conjunction with a rotating disk (driven by the speedo cable) which has slots to interrupt the optical beam.

The circuit to connect the internal LED and phototransistor is simple and was used in the rain gauge as published in the June 2000 issue. It comprises a resistor in series with the LED and a resistor in the transistor collector. The collector signal could then be applied to the coil input for the Speed Alarm.

Tacho doesn’t work on Land Rover

I read with interest the article about the new Digital Tachometer in the April 2000 issue. I already have the digital tacho described in the August 1991 issue but I’m attracted by the new design because it is much smaller and easier to fit into a car’s dash.

I originally fitted the older tacho to my Holden-powered Land Rover. The instrument performed faultlessly so long as I had a points-style distributor triggering a SILICON CHIP Transistor Assisted Ignition. I triggered it from the negative terminal of the coil and all was fine.

Then I installed a Bosch reluctor distributor and coil as fitted to Holden motors for a couple of years in the early Com-modores. The tacho thereafter refused to register anything. I checked the tacho in another car; it worked fine.

So far as I know, the Bosch "black box" is merely a potted circuit board containing a big switching transistor and the associated circuitry to drive it from the reluctor pickup. It is a black resin slab with two leads from the reluctor, one to ignition +ve and one to coil -ve. Whether it has the extended dwell feature of the TAI, I don’t know for sure but I think it might.

I gave up as I did not have access to a suitable oscillo-scope to view the size and duration of the voltages around the operating ignition system. A comparison with the readings from a conventional points ignition might have told an interesting story.

Do you have any idea why the tacho refused to work with the reluctor ignition? More to the point, do you think that the new digital tacho will work? Have you actually tested it with an older reluctor type ignition? (P. M., Mackay, Qld).

The older Digital Tachometer should work with reluctor style ignition systems since the pickup is from the coil primary as per the points system. Possibly dwell extension in the reluctor triggering may produce a signal which is too short for the tachometer circuit to operate successfully. You could try in-creasing the value of the .022μF capacitor at the base of Q1 and decreasing the .01μF capacitor value at the junction of the 33kΩ resistor and 10kΩ resistor to ground at the coil input.

The Digital Tachometer described in April 2000 was tested with older style reluctor ignitions.

WARNING! 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.