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ppi epx 205, interesting crossover (to me, at least)
ppi epx 205, interesting crossover (to me, at least)
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Old 25th February 2008, 03:11 AM   #1
ppia600 is offline ppia600  United States
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Default ppi epx 205, interesting crossover (to me, at least)

I was able to find the right sized allen wrench today to remove the front and back covers on it today and found something interesting inside. Of course the crossover is designed around the phantom power +/- voltage setup using the din cables directly to the ppi amps, but it also has a tiny power supply section for someone who wouldn't use the crossover on ppi amps.
Now I noticed there is only one transistor in the power supply, it is a to220 style and on the primary side. Then there are of course diodes on the secondary side that I assume feed the audio circuits. What I am wondering is:

1- WHY on earth would they use only one transistor to drive the transformer? That would seem to be a little more difficult to make smooth dc with. (would that be to punish people that aren't using ppi amps with a little extra noise.. to motivate them to buy the ppi's to get better sound??? )

2-If the power supply isn't noisy, WHY would every other power supply I've seen besides this one have two tranistors with opposite polarities driving the primary side? I mean, if it is possible to use only one polarity transistor, why use opposing polarity ones in everything else that requires two different voltage rails?

4-If it would be possible to use only one polarity transistor in a power supply that could produce two opposite polarity voltage rails on the secondary side (in a high powered audio amp for example) , would it be too inneficient at that power level?

Now I'm starting to think I should have kept it and not put it up on ebay, lol. Someone please explain this crossover's power supply and make it seem less special to me so I won't feel so bad when it sells for next to nothing.
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Old 25th February 2008, 05:35 AM   #2
djQUAN is offline djQUAN  Philippines
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1. Audiocontrol does the same thing. they use a regulated supply for low level circuits. assuming the supply has output inductors and the board layout and circuit design is well made, noise is not an issue.

2. it is much easier and cheaper to use push-pull designs.

3. where's three?

4. some pro amps use a flyback switching supply (ask eva) and they work fine. same answer as #2.

there's my 2C.
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Old 25th February 2008, 05:50 AM   #3
Eva is offline Eva  Spain
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Congratulations, you have discovered flyback SMPS. This is the most widely used topology in low power supplies, it uses a single switch and can produce positive and negative voltages. The outputs are always regulated unlike in almost every small push-pull and the ripple from the 12V supply is not fed directly to the outputs unlike in push-pulls. It's just that most car audio designers are stuck to unregulated push-pull even for tiny power supplies because they don't know anything else. Some people just do electronics design as cookbook copy and paste. Designing a flyback requires some more math and magnetics knowledge than a plain push-pull, and you have to design the frequency compensation of the feedback loop too. Of course flyback has drawbacks but they only show up when high currents are involved.
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