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Old 15th December 2004, 04:26 AM   #1
Stocker is offline Stocker  United States
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Talking High power regulated, quasi-comp.

I just scored an extremely well regulated 250W power supply that will yield 28.43VDC unloaded and is rated up to 10 Amps (with forced cooling). I also have loads of NPN power transistors. I am thinking a marriage of the two is a great idea. I like the idea of quasi-comp. output stages, and classA is not my favorite.

So: I am hoping to build a quasi-complementary output, 2 or 2.1 channels class B or AB amplifier. With that sort of current available, what sort of power should I shoot for, per channel?

But more importantly, what would YOU do?
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Old 15th December 2004, 12:55 PM   #2
paulb is offline paulb  Canada
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28VDC single-ended won't get you much power unless you're driving very low impedance speakers. Best of luck.
Can you get another power supply? Then you could build a nice Class A.
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Old 15th December 2004, 01:35 PM   #3
blmn is offline blmn  Brazil
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I sugest a bridge power amp driving 4 ohms (or less) loads.

Since you have enough current available, you can achieve a lot of power with this topology.

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Old 15th December 2004, 01:46 PM   #4
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linear or switcher ?

if it's a linear supply and "extremely well regulated" you may want to get rid of the regulation -- there is a lot of overhead in a commercially designed supply which can be dispensed with -- as an example,the HP 6129C power supply, designed to produce 50V @5amps has secondary rails of +/- 75V. It really is an amplifier in disguise.

disconnect the rectifiers and examine the transformer secondary voltages -- then report back.

switchers are different -- but the transformer can often times be unsoldered and rewound for the secondary voltage you need -- of course you will have to modify the control loop, etc.
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Old 15th December 2004, 08:52 PM   #5
Stocker is offline Stocker  United States
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Jackinnj:
Linear regulation, I think. I haven't reverse engineered it yet and the model number escapes me at the moment. At a glance, it's an opamp driving a TIP31 to a VRef, driving a 2n3055, driving six! 2n3773s. TO-3 Rectifiers apparently. The main filter caps (3x13000uF ! ) are rated for 50V, I haven't measured the secondaries directly yet. It's rated to be 60% efficient... and 0.03% regulation. The noise unloaded was 0.04mVAC. That level of regulation really makes me happy... it's even got a smashingly good spec. with a sudden 50% load increase. But the efficiency... oh boy. I'll have to think about that. The transformer is rated for about 600+VA input, so maybe 500VA out wouldn't be out of order... well now there's another bug in my ear...
If I took out the regulation, there's a half-dozen 150W output devices right there. Oh boy, more NPNs

Blmn:
I was thinking of "or less" myself! ... So let's get the calculator... 28V, rails, bridged into 2 ohms... would be...
Then a couple of low-power channels for stereo.

PaulB:
Chances of getting another of these for free range from slim to none. It's a $180 supply new AFAIK, and this one was apparently dropped and retired due to a dented chassis. ClassA is all good except for the Texas summers. I don't need a 500W heater on my bench when it's 110*F outside!
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Old 16th December 2004, 03:01 AM   #6
Stocker is offline Stocker  United States
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The regulation is so good because it's based on an LM723CN regulator. That regulator is useable for positive or negative voltages and inspires me to look for a higher-voltage type of similarly-great regulator for the *big* transformer I have for a dual supply for The Lab. Anyway, the LM723 has a max input voltage of 40VDC so there is potentially up to that much but likely less than 35-38V.
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Old 16th December 2004, 07:00 PM   #7
blmn is offline blmn  Brazil
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I think, if you put 50vpp over the load, near 160w continuous, drawning near 10 amps from the power supply.

If you use 4 ohms loads you can build a 2x80w amp.

Regards
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Old 16th December 2004, 11:28 PM   #8
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I know at this stage of design it's all numbers grabbed from the air... but where did you come up with 50Vpp? I think you are maybe thinking of bridged "normal" amps with 28V rails but please confirm this for me.

And, how do 50V and 10A combine to make 160W? (really, no kidding what do you mean by this? )
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Old 17th December 2004, 10:36 AM   #9
blmn is offline blmn  Brazil
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Yeap, Im thinking about bridging the amp, so if you consider Vcc equals 28.5 you can consider (2 x 28.5v - some losses) over the load (peak to peak). So, just do the math and you will find near 17.7 v RMS over the load.

If you consider near 55% the efficiency of the amp, over 28.5vcc you will find near 10A of current.

regards,
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Old 17th December 2004, 09:56 PM   #10
Stocker is offline Stocker  United States
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OK, this is good stuff...

50Vpp =approx. 35VRMS... a 4 ohm load is 2 to each side...

But then I am nearly out of current! an ampere or two only left! (VERY round numbers here! )
I guess that's still, say, uh, about 10-25W left over for the other two channels, each and a little headroom.

25x2 + 160x1... not shabby. I will revive this thread next week after I measure the transformer's secondaries...the power supply is in my garage and the "safety" setup for testing is here at work. wasting that 40% of the transformer's output is starting to bother me more and more, especially putting some possible ballpark numbers on power out of the amplifier

Blmn, thanks alot for your input. I would appreciate it if you will stick this out with me.
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