| amo |
Hi-
Just got my sample of the lm3875 chips, and the sample of MUR860 is on the way :) For this project, I decided to use 1 transformer to power both channels, but since it will be a Plitron, it will have 2 seconderies. I need to get 30 watts per channel, at 6 ohms from this amp. Based on the data sheet for the chip, I will need about 30 volts at 3.15 amps, per chip..... So, which transformer do I need? Will the 400 VA, with 30+30 volt secondaries, at 6.45 Amps, work for this? If so, what is the best way to split up the seconderies between the bridges and the channels? Thanks. |
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| elizard |
30v secondaries will give you over +/- 40VDC which is too much for this application
something along the lines of 22-25V on the secondaries will be better suited |
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| GregGC |
| quote: | Originally posted by amo
Hi-
Just got my sample of the lm3875 chips, and the sample of MUR860 is on the way :) For this project, I decided to use 1 transformer to power both channels, but since it will be a Plitron, it will have 2 seconderies. I need to get 30 watts per channel, at 6 ohms from this amp. Based on the data sheet for the chip, I will need about 30 volts at 3.15 amps, per chip..... So, which transformer do I need? Will the 400 VA, with 30+30 volt secondaries, at 6.45 Amps, work for this? If so, what is the best way to split up the seconderies between the bridges and the channels? Thanks. |
400 VA is fine but 30-0-30 VAC is too high. It'll give you 42VDC wich is to close to the max. alowable. I use 22-0-22. which gives me 32VDC per rail. |
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| amo |
| Not sure how you got that, but you are the experts.... So I need right around 22-0-22, and at how many amps? |
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| officeboy |
| You get the voltage by basically multiplying the Transformer voltage by 1.414. That gets you the DC voltage after rectifying. So 22VAc*1.414=31.1V DC |
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| GregGC |
| quote: | Originally posted by amo
Not sure how you got that, but you are the experts.... So I need right around 22-0-22, and at how many amps? |
Anything between 20-25VAC/rail and 200-400VA (having in mind the min 30W/chanel). |
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| officeboy |
BTW, 31V DC (22VAC) with a LM3875 into a 6ohm load will use about 4.3A per channel, and output 55W per channel.
You could still hit your target of 30W into 6ohm if you went with an 18-0-18 transformer. (25VDC = 35.5W and 3.5 amps) |
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| arold19 |
| Why 40 v is not Ok ??? On the data sheet, it's written 20 to 84 V supply voltage !!!! |
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| GregGC |
| quote: | Originally posted by arold19
Why 40 v is not Ok ??? On the data sheet, it's written 20 to 84 V supply voltage !!!! |
..... which means 10 to 42VDC per rail. If you have 10% mains variation the amp wont like it and nether will you. |
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| arold19 |
| what do you mean by 10 % ???? |
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| SkinnyBoy |
| quote: | Originally posted by arold19
what do you mean by 10 % ???? |
fluctuation in the mains voltage.... so the voltage may rise 10% which means the secondary voltage would rise by 10% too.. if you're using 40 volt rails, and that mains increases by 10% you will now have 44 volt rails.... which are too high....
or maybe you were refering to 10% variation.. as in +-5% (cos 10% is a lot :p even still... 5% gives you 42 volt rails..... |
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| arold19 |
What do you think it will happen if somebody use a 42 V rail ????
Me, I have the choose, I can use 13 v or 26 ....
(13 * 1.4) - 0.7 = 17.5 V
(26 * 1.4) - 0.7 = 35.7 V
But now, I think I will have more that that because the transfo are made with more voltage at the output that it is written, maybe it will trop at my number when it will be on load.... |
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| elizard |
| quote: | Originally posted by arold19
What do you think it will happen if somebody use a 42 V rail ????
Me, I have the choose, I can use 13 v or 26 ....
(13 * 1.4) - 0.7 = 17.5 V
(26 * 1.4) - 0.7 = 35.7 V
But now, I think I will have more that that because the transfo are made with more voltage at the output that it is written, maybe it will trop at my number when it will be on load.... |
84V is the maximum supply voltage (+/- 42V). You'll be running the chip at its limits, so it might die, or it might not last as long as you expect it to. |
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| arold19 |
If I have 44 volt at the output of my power supply, do you think that it will drop below 40 v ????
If no, I will burn my two LM3875 !!!! |
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| Nuuk |
| quote: | | Not sure how you got that, but you are the experts.... |
You may benefit from a read of THIS. |
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| amo |
| quote: | Originally posted by Nuuk
You may benefit from a read of THIS. |
Yeap.. Should have read it. :smash: |
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