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Old 17th August 2006, 07:49 AM   #11
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Checked the tranny today. secondaries are 30Vac. But after hook up to bridge rectifier I get 29.2Vdc. Should have around 42Vdc
if I'm right.

If this ranny is stuffed I'll use my others from my Lm4780 project. They are 300VA 25-0-25, so they'll do just fine.

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Old 17th August 2006, 01:18 PM   #12
macboy is offline macboy  Canada
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Checked the tranny today. secondaries are 30Vac. But after hook up to bridge rectifier I get 29.2Vdc. Should have around 42Vdc
if I'm right.
Yeah, you're right. But you forget to connect some filter caps to the outputs of the bridge right? You need those to get the proper reading. Otherwise the meter will see a rectified sine wave which peaks at around 42 V but with an RMS voltage around 30 V minus the diode drop, which is exactly what you measured. After adding the filter caps, this changes to be a steady 42 VDC.
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Old 17th August 2006, 03:07 PM   #13
AndrewT is offline AndrewT  Scotland
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the 25-0-25 transformer seems a better match.
The 300VA will easily drive two channels, or even three channels. It might struggle with four channels.

Use a separate rectifier and smoothing bank to each channel.
regards Andrew T.
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Old 17th August 2006, 07:13 PM   #14
cpemma is offline cpemma  United Kingdom
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Originally posted by rabbitz
Throwing a larger heatsink at it will work to a certain limit and you can forget about it for a TF package but the TA package is on the limit with a 6R load.

Reducing the voltage is a better way to go and have a look around for the Overture Design Guide spreadsheet at National Semi Conductor. That will give you a guide at what you can and can't do.
The Overture's TF/TA heat sink figures are based on grease only, no insulating washer (not needed with the TF); an average-quality washer will also put the TA out of bounds, a top-quality one you'd need to do the maths very carefully and/or watch the volume control.

However, with a Class B amp in a normal domestic situation, we may make too big a fuss over heatsinking to cater for the maximum design output. Just because it's there, we don't have to use it. And with most music source, we don't.
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