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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.

Poor damping factor for Studio 350

I have always considered Damping Factor to be of the utmost importance in the design of the highest quality audio amplifiers. I was interested to see the new 350W amplifier module but incredibly disappointed to see a D.F. of only 75. Any reasons?(C. A., Wagga Wagga, NSW).

The reason is due to the InDesign desktop publishing software we use. Unless special precautions are taken when the text is "tagged" for style prior to importing, it can ignore text beginning with the ">" symbol (greater than). The specification should have read ">180 @ 100Hz & 1kHz; >75 at 10kHz, with respect to 8 ohms". So the damping factor is actually pretty good.

Setting a trap for a plant thief

A person who I know has been stealing plants from my backyard and taking them to his house, replanting them and passing them off as his. What I wanted to do is to record him taking them and pass the information on to the police. He has taken over 20 plants and it is very annoying and costly.

I have a mini spy camera and a VCR. The only problem is that I can’t leave the VCR recording 24 hours a day. Do you have some kind of motion detector that would allow my VCR to record only when there was movement? (R. M., via email).

Have a look at the video security project in the September 1997 issue. This used a PIR (passive infrared sensor) to control starting and stopping of the VCR. However, you might want to consider merely using a PIR sensor to control some strategically placed spotlights.

Bridging the SC480 amplifier modules

I’ve built a stereo pair of SC480 amplifiers and a friend wants me to build a couple for microphone use, using the balanced microphone preamp you published in the April 1995 edition.

What I need to know is whether the SC480 amplifier is suitable for bridging. I know nothing about bridging other than the fact that Altronics sell an amplifier bridging adaptor kit – would it suit?

I have on hand a couple of 25W amplifier modules published in the December 1993 edition – are these suitable for bridging? (J. H., Falmouth, Tas).

The SC480s can be bridged using the Altronics bridging kit but you can then only drive an 8-ohm loudspeaker. The 25W amplifier modules are not worth bridging – they don’t have enough power output.

Valve preamp as headphone driver

I was just wondering if I could turn the hifi version of the valve preamp (February 2004) into a headphone amplifier? If so, what is required to be done?(R. D., via email).

Trying to use the valve preamp as a headphone driver is just not practical because the 68kΩ plate load resistor of the second stage severely limits the current that can be delivered.

How to measure damping factor

I have been attempting to work out how one simply measures an amplifier’s damping factor, so that I can optimise the design of loudspeakers. Quoting Vance Dickason, in his book entitled The Loudspeaker Design Cookbook, "Amplifier resistance (Rg) is one of the series resistances taken into account when calculating driver Qt. The easiest method is to use the manufacturer’s advertised damping factor (D), usually measured at 1kHz: Rg = Rd/(D-1) where Rd is the rated driver impedance".

The problem is that the damping factor can be substantially different at different frequencies and at different drive levels. When designing a loudspeaker it is necessary to measure damping at a more relevant frequency and output level, such as 50Hz and high power output, such as is experienced by the woofer for whose Qt I am trying to optimise.

Vance provides a means of measuring an amplifier’s damping factor but I consistently end up obtaining a result with a negative value!

I also noticed that you have measured the damping factor for the new Studio 350 amplifier (January 2004) as 75 at 10kHz with respect to 8Ω. This figure appears completely useless to someone who would wish to drive say a subwoofer with a cut-off frequency of 50Hz.

Can you please provide the damping factor at 50Hz for the Studio 350 and can you please provide a bullet-proof way for me to measure damping factor? (E. W., via email).

See the above answer for the damping factor specification of the Studio 350. Typically, an amplifier’s damping factor will be much the same for frequencies between 20Hz and 1kHz but will taper off above that, partly due to the reducing feedback factor at high frequencies (ie, reduced open loop gain) and the effect of any output coupling filter network.

Damping factor is the ratio of the load impedance to the source (output) impedance of the driving amplifier. You can derive an equation from Ohm’s Law (V = IR; R = V/I) whereby the amplifiers’ output impedance ZO = (VO - VL) x ZL/VL; where VO is the open circuit output voltage, ZL is the load impedance and VL is the load voltage.

To get damping factor, you divide ZL by ZO and this simplifies down to VL/(VO - VL). In practice, you take the difference between the load voltage and the open circuit voltage and divide it into the load voltage (driving an 8-ohm dummy load). Typically, the difference between the load voltage and open circuit voltage will only be a few millivolts so you need a high resolution AC millivoltmeter. A 4.5-digit DMM will do the job.

Depending on the feedback connections in your amplifier, it is possible to have an output impedance which is slightly negative (ie, output voltage increases slightly when the load is connected) but the damping factor is still the ratio of the load impedance to the amplifier’s output impedance.

Note that you can do the test at any power level up to clipping and the result should be the same. Typically, we do the test with a least 10V of output signal, otherwise the small difference in signal levels becomes hard to measure.

By the way, damping factor measurements should be done right at the amplifier’s output terminals, to avoid the effects of connecting cable resistances. Our damping factor measurements for the Studio 350 module were taken right at the speaker terminals on the PC board. Even a short length of connecting wire can affect the result.

Enhancements to wind-up torch

I have just received the February 2004 issue and intend to build the torch described in it. I intend to add an on/off switch and a small solar cell to charge up a large capacitor, as well as having the stepper motor. Do you envisage any problem with doing this?

Are the capacitors out of dead microwave ovens usable? Taking due care of the dangers initially involved of course! (P. R., via email).

There is a problem with charging the battery with a solar panel. This is bound to charge the battery to far more than the forward voltage of the LED and so when you switch the torch on, it will initially give excessive voltage to the LED. It may also exceed the voltage rating of the supercaps if these are used as well.

Capacitors in microwave ovens can generally be re-used but be very careful to make sure that everything is discharged before you attempt any circuit disassembly. These capacitors can retain a lethal charge for months after use, especially if their discharge resistors have gone open circuit.

PIC programmer damage

I recently put together the PIC Programmer from the September 2003 issue. As far as I can see, it works fine but I’ve somehow managed to destroy (I think) a number of PICs (PIC16F628a). I can’t imagine that it is my circuit that is destroying it (it is currently only a couple of LEDs attached to the I/O ports via a 1kΩ resistor on each). All of the tests suggested in the article were passed OK.

Is it possible that inserting the PIC with the programmer board switched on could be causing this problem? This seems to be the case but I can’t verify this as I don’t want to waste any more PICs.

The programmer works as expected for a number of reprogrammings until I get a message "unable to verify pic while programming". The PIC still works but cannot be reprogrammed. Any assistance or ideas would be greatly appreciated. (M. W., via email).

It is possible for PICs to be damaged if they’re plugged into a powered programming socket, despite the on-board current-limiting circuit. However, it seems unlikely that this is the cause in your case, as devices damaged in this way will generally fail to function in-circuit.

The devices may have the ‘LVP’ (Low-Voltage Programming) or ‘CP’ (Code Protect) fuse bits set. Check that both of these fuses are disabled (not ticked) before hitting the program button.

Also, check the resistance between pin 10 of the programming socket and ground (0V). The 4.7kΩ pull-down resistor on this pin ensures that LVP mode is disabled (regardless of the fuse state) when entering programming mode.

Missing capacitor in balanced input

I have an enquiry regarding the Balanced Input/Output Stages for the Studio Series Equaliser, as featured in the December 1989 issue. I noticed that the circuit diagram on page 75 of that issue does not show the 10μF capacitor from pin 7 of the LM833 to the "Output to Equaliser Input" but it is shown on the PC board layout on page 77 of the same issue. It also looks like it is also present in the accompanying photo on page 74.

I have checked through many of my 1990 back issues and have not been able to find Notes & Errata on this aspect. Could you look into this please and let me know? (W. R., Townsville, Qld).

The 10μF capacitor at IC1b’s output should have been shown on the circuit. It is probably best to use a bipolar or non-polarised capacitor instead of the polarised part mentioned on the overlay diagram.

Cybug solar fly does not respond

I’m having trouble with the Cybug Solar Fly, described in the September 2000 issue of SILICON CHIP. Everything except the insulation tape and the heavier tinned copper wire to stabilise it have been soldered onto the PC board but it won’t work. I’ve tried it outside directly under the sun in 30°C weather and it won’t work. I’ve checked that all the polarities are correct three times and have found no error.

The PC board melted slightly while soldering and I was wondering if you could help me find what the error would be? (C. B. via email).

You need to do some basic circuit checks. For example, try covering D1 or D2. Does that cause the associated comparator input to go low? If so, what happens at the comparator outputs? Can you turn on the motors by shorting out the respective Darlington transistors?

By this process, you should be able to find what’s wrong. Our tip is a missed solder joint or a component soldered in the wrong way.

4-station telephone intercom

I need to build an intercom using normal Telstra type phones with up to four stations. Have you ever published a project that could do this? (J. A., via email).

We published the 10-station Interphone in August, September and November 1992. You can obtain the PC boards from RCS Radio (Phone 02 9738 0330). Or there is a much simpler PICAXE phone intercom published in June 1992 (no PC board). We can supply these issues for $8.80 each, including postage.

Remote volume control works in one direction

I have just completed assembly of the PIC-controlled remote volume control described in the June 2002 issue of SILICON CHIP. I am using it with the Jaycar AR1073 remote.

The unit does not work at all in the ‘clockwise’ (increase volume) direction. That is, it will not ‘step’ up with the channel button, nor will it rotate with the volume button or return to a preset position with the ‘unmute’ function. The ‘mute’ and ‘acknowledge’ LEDs say that signals from the remote are being received OK.

In the reverse, the unit works quite OK in all modes. I have done all the usual checking of component values, placement, etc and the voltage measurements are OK.

I must be doing something wrong, but for the life of me I can’t figure out what. Can you guys help?(B. B., via email).

Check the placement of transistors Q1, Q2, Q3 and Q4. We suspect that two are incorrectly placed.

Induction loop receiver for headphones

Would you please publish a circuit for a pocket or purse-size hearing assist device, with headphone outlet, to pick up voice signals radiated from deaf-aid loops installed in theatres, airports, etc.

A tone control would benefit people whose hearing impairment is in a defined frequency range. As well, small, modern headphones could be appealing to users.(J. A., Magnetic Island, Qld).

"Electronics Australia" described just such a project in the October 1995 issue. We can supply a photostat copy for $8.80 including postage.

HT supply for valve receiver

I read the article on the 12AX7 valve preamplifier in the November 2003 issue with interest, as many years ago I had built a number of similar circuits for guitar preamplification and general audio applications. The part of the article which really appealed to me though was the method of obtaining the HT via a DC-DC converter.

For some years, I have had a project on the back burner to recondition an old military aircraft receiver of the type commonly available years ago. And as transformers capable of supplying suitable HT voltages at a current of 115-120mA seem to be relics of the past, I have puzzled over ways to provide a solution at an affordable cost.

The article infers that the DC-DC converter in its present form should be capable of supplying about 40mA, dependent on the plugpack. I am wondering what modifications to the published circuit would be necessary to provide an HT of 250-260V at a current of 115mA. I have a number of 12V 40W and 60W transformers which, with the provision of rectifier and filtering components, should be suitable for the 12V DC required.(R. K., via email).

It turns out that we have been very conservative in the design of the converter and it should provide much more than 40mA. In fact, with the right driver stage, the transformer core is capable to delivering about 100 watts. To get 115mA at 260V, you would need to bypass REG1 and fit Q3 with a much better heatsink.

Smaller transformer for Studio 350

I have a question regarding the Studio 350 amplifier that was described in the recent issues (Jan-Feb, 2004) . Can a 50-0-50V 300VA toroidal transformer be used? I don’t mind the reduced power output. Could you tell me what it would be on a 300VA transformer instead of the 500VA as stated in the article. (E. Z., via email).

You can certainly use a 300VA transformer and it will probably be quite adequate when playing normal program material. Naturally, the 4-ohm continuous power output will be reduced though.

Power supply for Jacob’s Ladder

I recently built a Jacob’s Ladder kit (SILICON CHIP, September 1995) and I was wondering what kind of 12V power supply was required and the current output necessary? (D. S., via email).

A standard DC power supply will probably not be able to drive the Jacobs Ladder successfully due to the high peak currents required. The supply would need to deliver at least 5A at 12V. This is why we recommend using a 12V battery.

You could use a 12V battery charger (with, say, up to 5A charge) in con-junction with a battery if you did not want the battery to go flat over time. Alternatively, you could use the 12V output of a discarded PC power supply.

Zap Protection For Jump Starting

I have been told that if I am jump-starting a car with a flat battery, then I must use jumper leads with "anti-zap" protection to connect to the vehicle with a charged battery.

I appreciate that the vehicle’s electronics must be protected from voltage surges but I cannot understand how a 12V battery can give out any more than a nominal 12V. Could you please explain (a) where these high voltages come from; and (b) whether I can retrofit my existing jumper leads with whatever it takes to neutralise these nasty voltages.(M. H., via email).

Jumper leads with anti-zap pro-tection don’t always work. You would be advised to closely follow the instructions in your owner’s manual for jump starting. These should minimise any risk to your car’s electronics.

Jump starting can certainly cause spikes to be generated. The starter motor itself is a large inductive load and its solenoid and commutator can generate considerable hash and spikes. The problem is made worse by the car’s flat battery – because it is flat, it has a higher internal impedance than normal and so it is less able to suppress spikes across the supply.

Playmaster 30+30 amplifier overheating

I built the Playmaster 30+30 amplifier as described in EA magazine in April 1992. It uses the Philips TDA1514A power amplifier ICs.

There are a couple of problems I’m having with this project. There seems to be very little low end (bass) from this amplifier. You need to have the bass control on maximum for there to be any kind of appreciable amount of bass - and it’s still not quite enough.

But the biggest problem is that one of the power supply filter capacitors keeps failing. The 5600μF on the positive rail starts to bulge on the top until it eventually splits and leaks gunk all over the PC board. One tell-tale sign is that one of the power amplifier ICs runs considerably warmer than the other. It seems to be pulling more current than it should be.

I swapped the ICs over, thinking there was an internal fault with the one that was warming up but the fault did not move with the IC. All voltages around both ICs are OK. I’m at my wit’s end as to what the problem could be. Some suggestions would be greatly appreciated. (M. O., via email).

From the symptoms, it seems likely that one of the TDA1514s (the hot one) is oscillating supersonically. This is causing high ripple current on the positive supply which is overloading the positive rail 5600μF capacitor and causing it to fail.

Check that the Zobel network is OK (R30 & C24) and that all capacitors are correctly soldered into circuit. In particular, check C22 (3.3nF) and C20.

PIC programmer problems

I’m having a problem with the PIC Programmer described in September 2003. I built the circuit on my own PC board as I used a simpler (standard 7805-based) power supply to avoid sourcing the LP2951. The programming voltage is fine, until the PIC is inserted, when the pin 4 voltage drops down. Any ideas? (L. W., via email).

We assume that you haven’t modified the Vpp generation circuitry and that you’re programming an "F" (flash memory) series micro. As mentioned in the article (under the "Vpp Check" heading), you can use the "Enable MCLR" box in the "Hardware Check" dialog to switch Q7 on and off and examine the operation of this particular part of the circuit.

There should be about +13.6V on the cathode of ZD1 in either condition. If not, check the voltage on the cathode of D4. It should be about +17.8V. The MAX232, D3 & D4, as well as the associated 1μF capacitors, generate the required high voltage. Anything more than a few mA load on this supply will overload it, causing the voltage to drop down.

If all is working properly, only about 0.5mA will be drawn from the high voltage supply, determined by the 1.2kΩ resistor between the base and emitter of Q3 (Q3 & Q4 act as a constant current source for ZD1 & D5). In operation, the voltage drop across this resistor should measure about 0.6V.

The problem is most likely around Q3 to Q7, ZD1 & D5. Check that you haven’t accidentally exchanged the PN100/PN200s, etc.

Power-Up doesn't work with TV

I built your Power-Up module from the July 2003 issue and am pleased with its performance on my computer. It is fabulous for powering all those plugpack peripherals such as modem, scanner, printer and speakers.

Now I want to build another unit to power the audio equipment when I turn on my TV. The problem is that the PowerUp is not sensitive enough to reliably detect the difference in current drain between standby and normal operation of the TV, even with careful adjustment of the sensitivity control.

Is it possible to make the unit more sensitive to smaller current changes? (P. T., via email).

We suspect that your TV set draws considerable current when in standby and so the current detection circuit is being overloaded.

To fix this, try reducing the value of the 470kΩ resistor at pin 2 of IC1a to say, 100kΩ. At the same time, you will need to increase the value of the 2.2nF capacitor to 10nF, to maintain the same roll-off frequency.

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.

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