PA150 (3x LM3886) 2 ohm capability ?

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Hello !

I buy PA150 (3x LM3886) but I don't know if this board is able to load 2 ohm ?

If you know what's parts change to make PA150 2 ohm capability please help me ;) !


In the national.com sheet I don't find any spec on PA150 but the PA100 and the BPA200 have 2 ohm capability version.

thanks in advance for your help ;) !


nicK
 
Depends on the supply voltage. According to AN-1192 PA-100 is 2 Ohm stable with up to ±28 V rails.

Assume ±38 V rails for PA-150. A nominal 2 Ohm speaker can dip down to 1,6 Ohm. 38 V / 1,6 Ohm = 23,75 A. 23,75 A / 3 = 7,92 A, which is just below the 8 A current limit. A bit close for comfort, but PA-150 should therefore be 2 Ohm stable at all supply voltages provided that the heatsink is big enough.

If you assume ±38 V rails, the three ICs have a worst case dissipation of ~150 W, requiring a total thermal resistance below 0,033 K/W. Quite unrealistic, when the IC itself already has 1 K/W (T-type) or 2 K/W (TF-type).

Either keep the rails at or below ±33 V for PA-150 or build PA-250 or PA-300, so that the 150 W are distributed across 5 or 6 ICs.
 
I have a spare PA150 board and 2* 31.5VDC / 3.17A power supplies that I'm currently not using... I also have (2) 4ohm woofers so I'm thinking I can build a nice little 150W sub amp for a 2Ohm load.

Q1) Am I correct in thinking that this would be a good match for 2 Ohms? If I understand the AN-1192 notes correctly, in a PA150 config, each chip should see 6Ohms

Q2) Can I create a dual 31.5v supply by joining the (-) of PS1 to the (+) of PS2 [creating my (0) feed], then using the (+) of supply1 and (-) of supply2 for the PA150's +/- (does this present issues with ground? will I fry the PS's by doing this)?

I also have a 36VCT transformer that I guess could work, but I'm guessing the voltage would be a bit low.

Thanks
 
I also have a 36VCT transformer that I guess could work, but I'm guessing the voltage would be a bit low.

It would be enough for more than 100 W, provided the transformer has enough power. 100 W means less than 2 dB difference compared to 150 W.

I guess I can't edit my post.

Only for a limited amount of time after writing it.

I've discovered that joining the 2 PS's is not as straight forward as I thought and it depends a lot on if, and how they are connected to earth, so I'll leave that alone.

That is right. Usually the negative connection is grounded and you cannot connect two of those SMPS in series without modifying at least one of them.

Anyway 3,17 Apk would not be sufficient for 150 W into 2 Ohm which takes 8,7 Aeff (> 12 Apk) without taking peak current demands into account.
 
It would be enough for more than 100 W, provided the transformer has enough power. 100 W means less than 2 dB difference compared to 150 W.

That's good news. It's a 180VA transformer. So I can still throw this thing together after building a supply.

Anyway 3,17 Apk would not be sufficient for 150 W into 2 Ohm which takes 8,7 Aeff (> 12 Apk) without taking peak current demands into account.

Thanks for your reply.

I guess I'm not understanding the concept correctly... If each chip see's 6Ohms, when paralleled, and I'm using 31.5v rails, wouldn't the current demand be much less? (8A from 31.5V rails would be a 500W supply!)

I thought I was beginning to figure this out :(
 
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Rounding to 30V for sake of ease:

30V / 6R = 5A (each IC)
30V * 5A = 150W (still each IC)
150W * 3 = 450W

It doesn't matter from the math perspective whether it's 3 chips seeing 6R or 1 chip seeing 2R. The important part is you spread out the current draw and the heat dissipation.

You weren't wrong with 500W. But that's peak and you don't really need a power supply that large unless you plan on pushing the amp hard.
 
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Rounding to 30V for sake of ease:

30V / 6R = 5A (each IC)
30V * 5A = 150W (still each IC)
150W * 3 = 450W

It doesn't matter from the math perspective whether it's 3 chips seeing 6R or 1 chip seeing 2R. The important part is you spread out the current draw and the heat dissipation.

You weren't wrong with 500W. But that's peak and you don't really need a power supply that large unless you plan on pushing the amp hard.

Hi, again, Redshift187

I'm certainly not trying to argue or challenge, I'm still confused and trying to understand the logic... The 150w should be spread between the 3 IC's, not each (50W x 3 totaling 150W, not 450W), so why are they drawing 3x the total required current?
 
the 150W per chip is total power 30v * 5A = 150w only if chip is shorted

into a 6 Ohm load 5A the chip dissipates a few watts, the load has the rest.

30v rail 18v output each chip outputs 3A into 6 ohm load
the load sees 18 *3 = 54 watts
The chip dissipates (30-18) * 3A = 36 Watts

therefore the 3 chips should be able to handle a 2 Ohm load.
 
The 3,17 A of your SMPS is the DC current. The effective value of a sine wave is 0,707 times the peak -> 3,17 A(crest) * 0,707 = 2,24 A(eff). That means for a 2 Ohm load, you will reach the current limit at 4,48 V(eff) which leads to ~10 W of output power.

For 150 W into 2 Ohm you need 17,32 V(eff) and 8,66 A(eff) at the amplifier output. That corresponds to 24,4 V(crest) and 12,2 A(crest). You need a few volts more to make up for the losses in the output stage, so a 30 V power supply is okay, but 3,17 A is so far from the required current that you cannot even hope an SMPS may deliver that much more for even short amounts of time.

You are right, each chip will deliver a third of the total, i. e. 50 W into 6 Ohm. Redshift did not account for the effective voltages and currents. His 450 W(crest) turn out to be 225 W(eff) which is what your power supply should be able to supply, if you need 30 V(eff) and 5 A(eff) continuously.

The worst case dissipation is U²/(2*PI²*Rload). For a ±30 V power supply and 2 Ohm that is ~91 W total (plus a little for the chips own consumption) or around 30 W per chip. Real life works in your favour, because you will not always operate exactly at the point of highest dissipation. Real life works against you in that speaker impedances dip below the nominal impedance which means even higher losses.
 
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