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Old 18th March 2004, 05:40 PM   #1
sobazz is offline sobazz  Denmark
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Default OPA549 supply questions

I'm in the process of drawing pcb layout for a couple of two channel OPA549 based amps. They will be put inside some small speakers along with active crossovers and bsd correction. The circuits will be powered from 18-0-18 120VA transformers. How much output power can I expect (2 OPA549 per toroid)?

However, I have several questions:

- The OPA549 has poor PSSR. How should I deal with this? There's no possible way to include active regulation of the supply due to limited space. The OPA549 is chosen cause I have some spare ones laying around.

- The active crossover will be powered from the same supply as the OPA549s. Three OPA134s will be used - or maybe I'll make a 24db/okt with one OPA134 and two 2134. I don't know yet. These opamps have fairly good PSSR, but the rail ripple will be rather significant because of the OPA549s, I suppose. If I'm wrong, please tell me. And the rail voltage is to high for OPA134. Should I actively regulate the supply down to about +-15v (would require heatsinking of the regulators, which the limeted space doesn't allow)? Or could I just use some active rail splitters to split the positive or the negative supply with ground as 0 to +-12v?

I hope you understand what I mean... Maybe I should draw a schematic...

EDIT: forgot to mention.. each OPA549 will have a seperate pair of 4700uF electrolytics.
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Old 18th March 2004, 05:48 PM   #2
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OPA549 2* at 120VA 2*15V transformer (my computer speaker amp)
measures with load resistors on each channel:


1kHz

10 Ohm
0,02%THD ~ 10W
0,33%THD ~ 13W

5 Ohm
0,05% THD ~ 22W

3,33Ohm
0,2% THD ~ 31,5W

1,8 Ohm
0,22% THD ~ 43,5W

caps are for each rail (+ and -)
2700uF
2*1000uF
3,3uf Film
0,1uF Film
in parallel.
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Old 18th March 2004, 05:54 PM   #3
sobazz is offline sobazz  Denmark
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I suppose I can expect around 30/8ohm with 18v transformer (= 25v rail). What is your gain?
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Old 18th March 2004, 05:59 PM   #4
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gain is 10.
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Old 18th March 2004, 07:37 PM   #5
sobazz is offline sobazz  Denmark
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Thank you for the numbers, Till.

While I'm at it, I may just as well add another question to the list. As noted above, each OPA549 will have a seperate pair of electrolytics and bypass capacitors, but will share rectifier bridge. What would I benefit from using seperate rectifiers apart from lowering the stress on the rectifier if only one is used, considered the application (bi-amping of a two way)?
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Old 18th March 2004, 08:02 PM   #6
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this is a philosophical question, better diskuss with those people who spend their time listening to differences of different brands of resistors. I use on transformer and one bride rectifier for the + rail and one for the - rail made from some discrete diodes for both channels. I would build depending on the parts you allready have on hand.
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Old 18th March 2004, 08:11 PM   #7
sobazz is offline sobazz  Denmark
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Hmm, thanks. I only have to 25A rectifiers at the moment, and 2 35A. The latter are overkill and they are physically too large.

What is the benefit from using a rectifier for each rail? I've seen people claim, that seperate rectifiers for each amp will provide some of the benefits from using completely seperated supplies (including seperate transformer). I suppose this is voodoo, or?

EDIT: In my Harman/Kardon HK680 seperate bridges for the two channel are used along with seperate electrolytics. But not two bridges for each supply.
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Old 18th March 2004, 08:36 PM   #8
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You will find enough diskussion and good reasons in this board by using the search function. I will not give comments whats sounding better etc. One bridge for the positive and one for the negative side is needed to have some real ground inbetween (0V). In case you use only one bridge for both sides and make a virtual ground by two resistors and some caps, this ground will go up and down with against the load, not really stable ground. So better use one bridge for + and one for - . Separate bridges for each channel are not needed. I use 30A diodes bwt.
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Old 18th March 2004, 08:46 PM   #9
sobazz is offline sobazz  Denmark
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You don't need a resistor based voltage divider if the transformer has two secondaries or a center tap...

I appreciate your advice.

But I'd stick with 25A. Above 25A they are only available in 30mm square packages... space issue
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Old 18th March 2004, 08:48 PM   #10
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yes. go for it, and think about doing it the other way in case you are not confident with the sound.
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