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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 silchip@siliconchip.com.au

Class-A power supply & 80V capacitors

I am excited about the SILICON CHIP 20W Stereo Class-A Amplifier but I have a quick question. I have two beautiful and new 33,000mF 80V electrolytic capacitors. Can I use them in the 20W Stereo Class-A Amplifier power supply or is the rating 80V rating too high for the capacitors to be effective? The capacitors have an ESR (equivalent series resistance) of 5 milliohms. (L. L., via email).

  • You can certainly use your 80V capacitors. In general, having a higher than specified voltage rating is never a problem unless there is a limitation on physical size.

    Getting The SC480 To Drive 2-Ohm Loads

    Is it possible to add high-current output devices to make the SC480 amplifier (SILICON CHIP, January & February 2003) stable into 2-ohm loads? The power supply is no issue as I am using a 50kHz switcher for a car amplifier.

    I realise I would need to redo the PC board and make the high current tracks a bit thicker. Could it work with ±40V rails and with three pairs of MJL21193/4 (two less than the Studio 350) and something with more collector current than the MJE340/350s to drive them? I’d really like to see an article on how you draw the reactive load lines up when you design amplifiers.

    I am currently using two of the April 1996 modules at home; they’re great. (S. P., Carrum Downs, Vic).

  • In fact, just changing the output pairs on the SC480 to MJL21193/4 should make it OK to drive 2-ohm loads. No other changes should be necessary although we would prefer to see heavier copper tracks for the main current paths.

    You will also need to leave out the Polyswitch output protection devices. Instead, we would recommend the Loudspeaker Protector from the July 2007 issue.

    For interest, you can plot the 2-ohm load line on the load-line graph for the April 1996 modules and then take an approximation of the equivalent 1.4 + j1.4-ohm reactive load line on the same graph to demonstrate that two pairs of MJL21193/94 transistors should comfortably handle a 2-ohm load.

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