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Old 30th December 2004, 01:17 PM   #11
Steven is offline Steven  Netherlands
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Quote:
Originally posted by Enzo

Are you running 240VAC into the thing? It looks like it with 120VAC on either side of the bridge with the cap common grounded. If you are running plain old 120VAC to the bridge recto, remember one side of that is already at ground, so that ground in the center of the filters will not cooperate.
Furthermore your earth leakage switch will be triggered and the whole circuit will be disconnected from mains.

Steven
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Old 30th December 2004, 06:03 PM   #12
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Dang. I knew it had to be too good to be true.

Oh well.

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Old 30th December 2004, 10:27 PM   #13
Stocker is offline Stocker  United States
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Quote:
Originally posted by rjon17469

Well, I don't fully agree with that...

The safe operating area of the transistors was 2400 watts (16A @ 150V).

you need to learn how to read the datasheets before you design anything.

at 100microseconds, the SAFE OPERATING AREA is right at 2 amps. 3A 84V is pushing it. Don't be surprised to see things blow up if you specify them that close to their SOA ratings.

In any case, if you try to get more than the specified 50W out of one of these, it will very likely go BANG. If you want to get that kind of power, you need to use LOTS of TO-220 devices... or use TO-247 or TO-3.
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Old 30th December 2004, 10:39 PM   #14
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Quote:
Originally posted by Stocker
you need to learn how to read the datasheets before you design anything.

at 100microseconds, the SAFE OPERATING AREA is right at 2 amps. 3A 84V is pushing it. Don't be surprised to see things blow up if you specify them that close to their SOA ratings.

The SOA datasheet says 4A @ 80V, 3A @ 120V at 100 us.

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Old 30th December 2004, 11:14 PM   #15
Stocker is offline Stocker  United States
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How about derating with temperature? Or do you have a handy pair of blackbody heatsinks for these?

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Old 30th December 2004, 11:22 PM   #16
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I was already planning on doing a case fan, as having amps that pull 15 amps generates some heat by itself.

When I originally posted this, I was looking more for opinions on the general design rather than individual component selection. Granted, the transistors would generate a lot of heat, but with a decent heatsink and a fan, I don't think it'd be too much of a problem.

But I think I'm going the route of a typical unregulated power supply.

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Old 30th December 2004, 11:32 PM   #17
hermanv is offline hermanv  United States
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Clean power helps amplifiers sound better. Your idea is basically a good one but you have significantly oversimplified the design task.

Driving transformers with square waves can be made to work fine. Its done all the time, but if you make the effort why stop at 200 Hz? Typically these things are done at 20KHz to as high as a Megahertz. A significant design and debug task, expect a small pile of burned out stuff before you get it right. Now you need to find a way to keep all those fast edges out of your audio circuits.

Pure sine wave power can also be done but expect to waste AT LEAST 33% of your amplifier rated power as additional waste heat in the sine wave power section. Most of this power is wasted whether or not music is playing!

For less total effort and cost and better results, regulate the DC power fed to your speaker driver output stage. By doing this you waste a little heat but can significantly reduce the total storage capacitor size. You can lower the effective source impedance of your output stage into your speakers, you can lower the noise floor and present your ouput stage with a low source impedance. These are all very good things to do.

Look up a capacitance multiplier, this again converts some power to heat but lowers the effective source impedance a lot and reduces storage capacitor size by a fair amount.

Why do these things work? Here is an imaginary case: Lets say you have decided that your speakers will need 10 Amps and you want a ripple of 0.5V at your power supply. This means (Amps X seconds = Volts X Farads) or 160,000 uF. (60Hz power refreshes the caps every 8.3 mS). With a regulator you can allow 5 V of ripple AHEAD of the the power stage regulator, you can use 16,000 uF and loose 5V in the regulator stage, at 10 Amps this is 50 watts but only while the peak load lasts.

Obviously many trade-offs exsist but regulated power for output stages is often done in expensive power amps because among other advantages listed above it also allows you to lower global feedback since power supply ripple does not need to be cancelled out in the final stage.
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Old 30th December 2004, 11:46 PM   #18
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Two questions though:

1) Wouldn't I need a number of regulators to safely produce a 10A current?

2) Also, what about large transients? Would I then want to put a few caps after the regulators to handle that?

Reece
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Old 31st December 2004, 12:02 AM   #19
Stocker is offline Stocker  United States
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oh boy, high current goodness.

External transistors (pass transistors) can be used to increase output current from a voltage regulator as high as required.

Caps after the regulator would help on transients, yes.

I just noticed that SOA chart's time ratings... the line to look at falls somewhere between DC (0Hz) and 5mS (200Hz) (1/0.005) unless *I* am off my rocker. That makes it 0.4A 100V and 0.6A 80V, with the thing working at 25*C, nevermind the heatsinking... i.e., 50 watts and (much) less.
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Old 31st December 2004, 12:40 AM   #20
hermanv is offline hermanv  United States
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The regulators don't need a lot of gain so one or two transistors will do the job. The transistors will need to be rated to handle peak currents and max voltage so a TO-3 or a couple of TO-220 case devices seem like a decent bet.

There are few transients on the output side of a rectifier the large capacitors mostly take care of them.

And yes there would still be a need for capacitors on the regulator output but their job is different. They don't act as current reservoirs the regulator supplies the big currents between the 60 Hz recharge the smaller caps are needed to keep the regulator honest at say 20KHz.

Also a good capacitor is needed on the regulator reference voltage.

A simple example of a capacitor multiplier, bad because it needs 15 watt resitors, a darlington would help a lot.
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