Issue: 235 Published: 1 April, 2008

Mailbag

Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to submissions to "Ask SILICON CHIP" and "Circuit Notebook".
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Dual mono 20W class-A amplifier wanted

Could you do a follow up article to the Class-A amplifier with details to make it into a dual mono design? I think that would interest a lot of readers, as that is what I intend on doing.

The only problem I have is that the specified transformer is $90. So that makes $180 (one per channel). Could you recommend any other toroidal transformers that will do the job?

Jeff Heath,

Point Cook, Vic.

Comment: the extra power transformer will not be your only added expense. At the very least, you will need two power supplies, two chassis and two loudspeaker protectors. Harbuch Electronics (02 9476 5854) may be able to supply suitable transformers.

Fair go for
tube lovers

As a long-time reader of your magazine from the first issue, I write again for you to reconsider your position on publishing a good tube amplifier design based on the classic Mullard principles.

From "never ever a valve amplifier project stance" at SILICON CHIP we have had everything but, as I recall a project to convert computer power supplies to use in valve projects, a 12AX7 preamplifier, the Mudlark single-ended valve amplifier, a Nixie clock and even a 3-valve radio.

Please go the whole way and give us a valve amplifier project. You may be pleasantly surprised how popular this would be. Not all of us want to build Class-A amplifiers and many of us would like to express our individuality and not purchase mass-produced commercial items.

Don’t forget: nothing too exotic, eg, EL34s, 12AX7, 12AU7, 20-30W per side, negative feedback and on the roll-your-own principle.

Andrew Prest,

Sunshine, Vic.

Project to erase watermark logos

I have been reading the ABC TV forum which has lots of negative views on the use of watermarks in general and the new ABC watermark logos in particular. Australian free-to-air channels are using them now and they are truly annoying to a significant percentage of viewers.

Nobody is listening, it seems and it’s time to act. Watch less TV is one solution. Another is to tape a piece of grey paper to the screen! Yet another is to blot the watermark out electronically.

How about a device that takes in the signal (SCART, CVBS, etc) and inserts a rectangle in the picture, then outputs the modified video signal to the TV? Two front-panel pots could set the position and its hue and luminance could be an average of the picture composition in the area near the watermark.

This could be your most popular project! It would be pretty easy too, with an LM1881 sync separator, some line counters, a monostable or two, a video mixer, etc.

P. K., via email.

Comment: watermark logos are very annoying but there is no easy way to remove them and your suggested cure might be more obvious than the logo. Some stations smear over the logos of overseas networks and then place their own over them (eg, SBS on their news bulletins).

It will also be problematic to do it with component video signals and probably impossible with HDMI signals from HD set-top boxes. We will have a look at the idea though.

We think that your idea of watching less TV may be the best plan and the one that networks would be the most sensitive to.
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