+-25volts = ??Watts bridged?? - diyAudio
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Old 21st March 2003, 11:18 AM   #1
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Default +-25volts = ??Watts bridged??

I have available +-25volts rails, and want to make an amp of rating 150watts or so per channel, 2 channel.... Is this possible with this low voltage? I was thinking of using the bridge/parallel diagram for the LM3886 ICS.... Not sure how much power this would give... and these parts are EXPENSIVE!!! I would prefer transistor, But don't think I could get this much power from such low rails.... I have no money, and these transformers are cheap... I have no money, but still spend it all..
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Old 21st March 2003, 11:48 AM   #2
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rough 'back-of-the-envelope' calcs would suggest that you would need to be drawing 8.5 amps per rail, per amp !!!

and you would probably need to be driving a 1 or 2 Ohm load to get the power , that would be very hard on the LM3886's even paralleled.
the LM3886 current limiting would restrict what you could drive.

ray
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Old 21st March 2003, 11:52 AM   #3
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But on the LM3886 Data sheet, it says with a +-25volts I can get 55watts into 4ohms.. bridge 2 into 8ohms, and I should get 110watts?? So, why couldn't get 150 using 4 ICs??

BTW, this is the Transforer in Question... I am using 2, one for each rail.. with the secondaries in series..

250VA TOROIDAL TRANSFORMER: 2 x 120V primary, 2 x 9V secondary. No mounting hardware available. The unit weighs 4Kg

Click the image to open in full size.
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Old 21st March 2003, 11:59 AM   #4
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I cant see you getting more than 100W RMS into an 8ohm load even if you bridge / parallel multiple chips. The mathematics is rather simple. Go study the datasheet and you'll see that you will not get ~200W RMS into anything less than a 4 ohm load. To acheive that, you would need to run a few in parallel and provide good heatsinking to keep away from the protection circuit.
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Old 21st March 2003, 12:11 PM   #5
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Well.. umm, umm, umm... I don't really know what I am doing... But why won't bridging 2 into 8ohms give me twice the 4ohm output?
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Old 21st March 2003, 12:15 PM   #6
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Because you run out of voltage to play with.
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Old 21st March 2003, 12:16 PM   #7
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Quote:
Originally posted by AudioFreak
Because you run out of voltage to play with.

Hmm.... I don't get it...

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Old 21st March 2003, 12:21 PM   #8
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You want to use +/-25V and you plan to use an 8ohm load.
Your going to bridge parallel the chips.
IIRC, these chips are capable of swinging to within 5 Volts of their supply voltage so given the configuration we are using, we can swing 40V. 40V swing into 8ohms gives 200W peak or 100W RMS (average).
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Old 21st March 2003, 12:24 PM   #9
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Hmm... but +-25volts gives 55watts into 4ohms... bridge 2, and you have 110watts into 8ohms??

http://david.lewander.com/projects/amp/amp.htm
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Old 21st March 2003, 12:39 PM   #10
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Here is the math approximately:

You have +-25 Volts to play with.

Best case you are going to swing within 3 volts of the rails before the distortion starts to get really bad. That means you are going to swing +-22 Volts, or a maximum excursion in either direction in a bridged configuration of 44 volts

RMS Power = (Voltage/(Sqrt(2))^2
-----------------------------------
Load Resistance

Or in this case (44/1.414)^2/8 = 120 Watts.

If I was being a little more "realistic" with the LM3886, I would say you are not going to swing more than within 5 volts of the rail for good sound which will give you 100 watts.

No matter how many LM3886 you parallel, you are not going to get much closer to the rails. That is a limitation of their internal design and you can't change that.

Your peak current will be somewhere on the order of 50/8 (or higher as your speaker impedance drops lower in all likelyhood). That does not mean your power supply must put out an average of 8 amps, but it does mean sufficient power supply filtering to ensure you can handle the peaks without the power supply drooping.

You are going to use 2-250VA transformers. That does not mean you get 500watts out of them as it is a complex correspondence between VA, how the load behaves, and watts, but I would say they have enough oomph.

Besides, a "good" 100 watts is always better than a crummy 150. A speakers impedance can drop considerably below the nominal, and if your design handles lower impedances well, then you will be surprised how load 100 watts can be.

My primary amplifier puts out only 50 watts into 8 ohms, but it will put 100 into 4 and 200 into 2. My speakers vary in impedance from 3.7 ohms up to about 10 ohms over the audio bandwidth and they have an efficiency of 92db/watt. I love loud music and I never feel like I don't have enough power.

Alvaius
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