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Upgrading the SC480 amplifier

With respect to your new SC480 amplifier modules, how can these be connected together or modified to produce 100W. Also, would there be any sound quality advantage in changing the 1μF bipolar input series capacitor to a metallised polypropylene type, as used in crossovers?

(P. R., St Lucia, Qld).

We actually went through the same process way back in February 1988 when we upgraded our original December 1987 design.

The process basically involves increasing the main supply rails to ±50V, changing the output transistors to MJ15003/MJ15004 (TO-3 types), Q1-Q3 to BC556, Q4 & Q5 to BF469 and Q6 to BF470 and increasing the power supply capacitor values and voltage ratings to 63V. Naturally a much bigger transformer is also required.

We think a better and more cost-effective approach would be to build the Plastic Power amplifier from the April 1996 issue. We can supply the issue for $8.80 including postage.

You could change the input capacitor to an MKT type but we very much doubt whether there would be any improvement.

Reproducing old wireless sound

I am a member of an amateur theatre company and we are planning to stage a comedy "Lend me a Tenor", set in Chicago in 1934. There are three or four occasions where the performer is supposed to be listening to the radio, to works by Giuseppe Verdi.

For effect, I'd like to try to approximate the type of reception people may have experienced at that time. I don't want the audience to 'hear' excerpts from Verdi's operas via a CD and a late 1990's full-range, 500W public address system in the theatre - that would be too much of an anachronism.

Clearly the radio systems were AM (ie, limited bandwidth and noisy). In addition to this, receivers were low-powered but with (relatively) efficient loudspeakers. The efficiency came at a cost, though - that of frequency response.

What would the frequency response of the recovered audio have been like? What sort of signal-to-noise ratio might have been expected? I have some ideas about the answers to these questions but am seeking your expert opinion.

I plan to record the relevant tracks, modify the 'system' frequency response to something similar to that you nominate and add appropriate amounts of both white and impulse noise. Hopefully the end product will have a big degree of realism.

(R. W., via email).

Your question is quite complex. In reality, the AM broadcast system was, and still is, capable of very good results. Today, with the very best receivers, you can get a result which is practically indistinguishable (for most listeners) from FM stereo.

In years past, the main limitations were the radio receivers but even the best of those could sound quite good. In reality, the frequency response from a good console radio (wireless!) could be up to 5kHz while the signal-to-noise ratio would be around 40-50dB or thereabouts. A good portion of the background noise would be hum. If you can get to a radio museum, have a listen for yourself.

However, there were other limitations on the sound quality. If it was a live broadcast you need to imitate the effects of the microphones and their placement (they did not have multiple microphones and multi-channel mixers). Some of the old style microphones are still available and sort after by groups who want to produce that "sound".

If the performance was recorded it would generally have been on 16-inch 78RPM transcription discs and these also had a sound all their own because the magnetic pickups of the day had quite a lumpy frequency response and recording equalisations were not standardised. The old turntables also suffered from wow and flutter and there was considerable surface noise, clicks and pops, etc.

To do what you ask and keep things simple, we would be inclined just to limit the frequency response to no more than 5kHz and just see what it sounds like. The illusion would be helped by having the listener actually switch on a radio. You could also take it further and have the sound broadcast via an AM transmitter and then mike the sound from the radio (oops, we mean wireless!)

We described a low power AM transmitter in the January 1993 issue of SILICON CHIP.

Compatibility between DVD players and TVs

We have a Philips 26CE1991 television which when connected directly to the DVD player pulsates its picture light and dark. The DVD player has been checked by two service centres and seems to be OK.

Philips tell me that this TV is too old and we should simply buy a new one. Are there any mods that are known to overcome this problem or are we stuck with the Macrovision protection breaking through?

(E. G., via email).

Our information is that this set was made in 1991 by Sampo for Philips and was designed before Macrovision. It is therefore unable to cope with large chunks of signal being removed.

The only real cure is to build the Doctor Video kit from the April 2001 issue of SILICON CHIP. This copes well with Macrovision 1 & 2 but not Macrovision 3. Otherwise, the only solution is to buy a new set.

Upgrading the 24V/20A speed control

I recently assembled the 10A Motor Speed Control from the June 1997 issue of SILICON CHIP. I did it to teach myself more about DC motor control, as I have started working in the electric forklift servicing field. I increased the controller's capacity to 20A by installing the second Mosfet, as suggested in the kit instructions.

I would like to know how to modify the kit to handle the much higher currents found in DC electric machines, from a hundred and possibly up to 1000A and also to handle higher voltages such as 24V and 48V. Would it simply be a matter of continuing to add Mosfets in parallel or would the circuit driving the Mosfets need to be upgraded as well.

Also the instructions said the controller could be used to run 12V motors in 24V systems. Could it be used to run 24V components in 48V machines?

(P. R., Jerrambombera, NSW).

While in theory, the specified Mosfets could run with supply voltages up to 48V, it would not be good practice. We would prefer to see much more rugged Mosfets used, with much higher voltage ratings.

Running at much higher currents is also possible in theory but in practice, the very small PC board would mean that the extra Mosfets would have to be off-board and that can lead to problems; you need individual gate drive to each Mosfet and heavy conductors for the commoned Drain and Source connections.

In addition to the above, the 12V regulator feeding the TL494 has an absolute maximum input voltage of 35V, so you would need to add a pre-regulator circuit to ensure that this rating was not exceeded, if you wanted to run the whole circuit at 48V. In summary, the practical limit of the circuit is 24V and not a lot more than 20A.

24V needed for Mustang A/C

I am working on the air-conditioning for an old Ford Mustang and I need a 12V-24V step-up inverter to operate the clutch solenoid. Can you tell me if there is one available or if you have described a suitable circuit?

(K. W., Nevertire, NSW).

The only 12V-24V inverter that we know of is a small one from Jaycar (Cat AA-0264). This is said to double the input voltage (from 6V to 12V or 12V to 24V) and has a maximum output current of 2A; we assume this is at 12V out.

This may not be enough to pull in the solenoid but should be enough to maintain it in the closed position once it has operated. You may need to connect a large electrolytic capacitor across the output to provide the pull-in current. We suggest you try a value of at least 10,000μF at 25V or higher.

Delay for Battery Guardian

I have a quick comment/observation about the Battery Guardian published in the May 2002 issue of SILICON CHIP. I like the solid state and low current draw but I am having problems with it dropping out on start-up, particularly if the load is more than 10A or so.

Would it be possible to delay the voltage sensing for about five seconds so that it does not detect the initial load droop?

(J. H., via email).

The solution is to increase the 10μF capacitor associated with ZD2 to 100μF. This will filter out the voltage droop when a 10A load is applied.

Zero voltage switch for wall oven

Some 15 years ago I redesigned our kitchen wall oven to something akin to 'burst firing' electronic control of the heating elements. I used a combination of an Analog Devices AD595 Thermocouple Amplifier with a ramp-driven 14-pin CA3059 zero voltage switch IC to control a suitably-sized Triac. It has proved to be accurate to ±1° and quite reliable.

Recently though, the CA3059 failed and I have not been able to locate a replacement. Harris and then OnSemi manufactured them but discontinued about a year ago. I was wondering if you could help with the name of a supplier.

(J. N., Tauranga, NZ).

We do not know of a substitute for a CA3059 but if you want a zero voltage switching power control circuit, have a look at the Heat Controller published in the July 1998 issue. This used the readily available MOC3041 zero voltage switching Triac optocoupler.

We can supply the July 1998 issue for $10 including airmail postage.

Audio frequency power meter wanted

I am in need of an audio frequency power meter. I'm thinking of something that reads in dB and mV and takes an electrical input (rather than including a microphone). It needs to be accurate across the audio spectrum.

While the option of a microphone input would be nice, this is a considerably harder proposition, since the characteristics of the microphone would have to be compensated for and accurate calibration would probably require specialist equipment.

(A. M., via email).

We have published two AC millivoltmeters which would suit your application, in August/September 1988 and October/November 1998. We can supply the 1998 back issues for $8.80 each, including postage and the 1988 articles as photostats, again for $8.80 each, including postage.

Braking system for a wheelchair

I am a Year 12 student starting my Major Design Project. My project is a wheeled walker for the disabled.

My current need is for a braking system and an easier way of going about it. I want to create a one-off braking system using electronics and two motors with a simple on/off sequence and a button on each handle to easily apply the brakes.

What do you suggest?

(D. G., via email).

We suggest you build a speed control with inbuilt braking. Have a look at the 50A speed control published in the May 2000 issue. We can supply this issue for $8.80, including postage.

You can buy products mentioned in this article here : AA0264 : 6-14V IN-11-26V OUT VOLTAGE CONVERTER