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Old 14th September 2012, 12:58 AM   #1
pyrohaz is offline pyrohaz  United Kingdom
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Default Philips TDA8920BTH Amplifier

Hey guys, i've got myself a TDA8920BTH amplifier that i've had for a while but i'm only just getting round to using it. It is a pretty nifty piece of kit for how cheap it was! I'm running it off 2x laptop power supplies at 24v (capable of 3a each) and it runs fine and such. Its currently running in BTL mode now on the datasheet, it specifies 174w into 6Ohm at 0.5% THD and +/-27v power supply. After checking with mine, I can get 100w into 4Ohm just before clipping.

Now on the data sheet, it doesn't say whether you can run a 4Ohm speaker in BTL mode but after testing, the module only gets noticeably warm, not extremely hot and after viewing the power supply AC coupled, it doesn't show any major power spikes where the current limiting may be happening.

Why am I only getting 100w out? My power supplies don't seem to be majorly sagging under load either!

Where am I going wrong here? Could it be the current limit? The datasheet specifies this at 8a which clipping into a 4Ohm speaker would be 256w (8*8*4 = 256)

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Old 14th September 2012, 02:43 AM   #2
DUG is offline DUG  Canada
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"I'm running it off 2x laptop power supplies at 24v (capable of 3a each)"

Point 1.

100W / 4 ohms is 5Arms

I would re-examine the power needed to run this amp under 4 ohm load.

Point 2.

24V * 3A * 2 = 144W

Even with a load it was designed for at 90% efficiency that is still only 129W.

Again, I would re-examine the power needed to run this amp under 4 ohm load.

If the amp is expected to limit at 8A, then the power supplies should be able to
1. supply this peak current. (possibly with increased filter supply caps)
2. be able to supply 5.7A continuous (8 * .707)

Can you find two more 24V 3A supplies?

use diodes to combine and you will probably get better results.

Watch the power dissipation.
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Old 14th September 2012, 09:32 AM   #3
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you cannot drive it at 4ohms btl....its what the problem is....
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Old 14th September 2012, 10:38 AM   #4
pyrohaz is offline pyrohaz  United Kingdom
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I do apologise, I was meant to say 24v at 4A instead of 3A (As rated on the base of the power supplies). But I thought that 100w at 4ohms is 5A Peak and 3.5A RMS? (SQRT(100/4)) = peak A
SQRT(100/4)/1.414 = RMS
As far as I know, am I wrong?

Thanks for the help though, its much appreciated
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Old 14th September 2012, 11:24 AM   #5
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Quote:
Originally Posted by pyrohaz View Post
I do apologise, I was meant to say 24v at 4A instead of 3A (As rated on the base of the power supplies). But I thought that 100w at 4ohms is 5A Peak and 3.5A RMS? (SQRT(100/4)) = peak A
SQRT(100/4)/1.414 = RMS
As far as I know, am I wrong?

Thanks for the help though, its much appreciated
Peak is twice the RMS value in a power calculation because the RMS value is squared.

But why complicate things with going with a watt calculation when you know the supply voltage and impedance?

Pavg = V^2 / 2Z = (24*24) / (2*4) = 72Wrms

Apeak = V/Z = 24 / 4 = 6 Apeak

Last edited by Saturnus; 14th September 2012 at 11:38 AM.
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Old 15th September 2012, 12:01 AM   #6
pyrohaz is offline pyrohaz  United Kingdom
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Brilliant! Its down to the power supplies, thanks for clearing that up for me
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