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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 silicon@siliconchip.com.au

Flea-power Ultra-LD module

Kits for the Ultra-LD Mk.3 module represent great value; better value than the SC480 (SILICON CHIP, January & February 2003) with upgraded output transistors. Which prompts the following question: how low a supply voltage will the Ultra-LD Mk.3 work on (with reduced output power, of course) – ±20V?

Do any other changes need to be made to allow it to work on reduced supply voltages? (I. F., via email).

• The Ultra-LD amplifier should work OK with supply rails down to ±20V but you would be severely limiting the power output. Having said that, you could also dispense with one pair of output transistors and the associated 0.1Ω resistors. The fuse ratings should be reduced as well, down to 1A or 2A.

Drill speed control for home-brew grain mill

I want to use an electric drill to drive a small roller type grain mill. I am a home-brewer. The design speed is ideally 200 RPM but up to 300 RPM is OK. My existing older model Bosch drill has a 1000 RPM low-speed range but does not have any variable speed control. A decent 2-speed drill will set me back $200 or more.

I suspect that I might be better off making a 10A Universal Motor Speed Controller, Mk. 2 for $100 from Jaycar (as described in your February 2009 issue) and use it with my existing drill. I also presume that your controller would be an improvement on the variable speed control supplied with most of the drills sold over the counter. Would you please comment on my suggestion before I make a commitment. (P. B., via email).

• 200 RPM is really too low to be practical with a drill that is intended for use at 1000 RPM. A better approach would probably be to adapt a battery drill/screwdriver which can be used at low speeds and deliver high torque. These are quite cheap to buy; much cheaper in fact than the kit for the 10A speed control at $100.

100V line adaptor needs more output

Some time ago, I set up 100V line speaker cabling throughout our church buildings to enable audio from the sanctuary or other locations to be heard elsewhere if it was needed. The system works well with a 100V line output amplifier driving up to six 30W Redback column speakers with more than adequate volume. I have also set up VGA over Cat.5 video to the same areas.

I built the 100V-to-Line Level Adaptor described by Ross Tester in the May 2006 issue of SILICON CHIP, to put the 100V signal into another amplifier where there was already a PA mixer/amplifier system set up. The problem I encountered is that despite having very high volume levels on the 100V line system, the line level output from the adaptor requires near maximum amplification to be sufficiently loud.

Recently, I connected the adaptor to the audio line input of a large-screen monitor to trial using only a monitor with its own audio rather than monitors with audio from separate speakers. I cannot get enough volume to hear the audio adequately. Audio from other sources is fine, with more than adequate volume.

Is there any way I can modify the SILICON CHIP 100V to line level unit to get more volume or will I need to provide some additional amplification of the signal before connection to the monitor?

• Just reduce the 100kΩ resistor. Try a value of 33kΩ, for example.

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