Nuts and Volts - December 2008

why I am having blown high and low output transistors. I plan to put MUR160 fast diodes across the FET; my question is: Will one amp be sufficient when I am chopping at 20 kHz? The MUR160 diodes are rated 35 amps for 8. 3 ms.

— Anonymous

AWith a forward current of (maximum) 3. 5 amps, the one amp diodes should work okay. But, if you had a 1 μS delay between turning off one half bridge and turning on the other, the fast diodes would not be needed. Even with the fast diodes, there should be a 50 ns delay.

STEREO AMP

QI recently upgraded my old stereo to a five channel home entertainment system. Although the amp is rated at 100 watts per channel, it puts out about half that. I’d like to build a stereo amplifier that would take the output from the front two speakers of my five channel system and drive the 200 watt front speakers. Preferably one that could be switched in and out and one in a kit. Thanks.

— Ed Bixby

MAILBAG

Dear Russell,

Re: Vibrator Replacement, September 2008, page 28.

Although it's possible to replace the 6X4 rectifier with a solid-state replacement, I would suggest that the B+ voltage reading be taken before and after the modification. Double-check that you're not exceeding the voltage rating of the filter capacitor.

Early electronic design had

AQKits part number K4005B ( http://store.qKits.com) looks like what you want. It is rated 2X100W into four ohms, but it can be bridged to provide 200 watts mono into eight ohms. You will need to buy two to have 2X200W. The price is $159.95 which seems quite reasonable.

From the size (350 x 62 x 85 mm), I am sure it is PWM, not linear. A power supply kit is also available.

Figure 9 shows how the two inputs of the right and left channels are connected for bridging and how

I would switch

generous (by today's practice) headroom in component selection values, but the leap from a 0.7 volt forward voltage drop device from a 20 volt forward drop per section device may push the original filter design pretty close to the maximum rating of the capacitor. Keep up the good work!

— John Mattesini

Response: Thanks for the feedback, John. I had not thought of that problem.

the output to take the added amp out of the circuit.

The driving amp will be essentially no load when the 200 watt amp is in, and the 200 watt amp will be no load when it is not connected to the load. I don’t think this will damage the amps, but you should check with the manufacturer to be sure. NV

■ FIGURE 9

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