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Old 9th June 2011, 06:04 PM   #1
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Default I think my transformer's voltage is too much for LM4780?

Hello,

I bought a 250VA 25VAC+ 25VAC transformer. Parts Express Avel Lindberg transformer. My DC voltage will be around 34-36V after rectification.

I'm just about ready to start mounting everything in my case, but I'm a little bit worried now.

It's a 2.1 Amp, with a 2x LM3886 board driving the two left and right channels, and a LM4780 driving the subwoofer. The speakers on the LM4780 are 1x 8ohm woofer, and 1x 5ohm tweeter.

They have pretty big heatsinks and with that impedance, I do not think they should have an issue handling the +/- 25V. However, my sub is a 4ohm sub and I will be using an LM4780 to drive that.

I've heard that I can reduce the chance of frying my LM4780 by running it in parallel. Is that true? I was originally going to run the LM4780 in parallel because it would give me more output. However, my sub can really only handle 40-60W, not 100W+. So I thought I would just run one channel of the LM4780 and forget about the other one.

However, now I'm hearing that running in mono parallel is better. Is ~35VDC too much for a 4ohm load? What can I do about it?

Thanks.
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Old 9th June 2011, 06:32 PM   #2
tomchr is online now tomchr  United States
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I think you're confusing two separate issues:

1) ABS MAX supply voltage
2) Thermal dissipation

What's the ABS MAX supply voltage for the chips you're using? Is it higher than the max supply voltage you'll have (figure the line voltage is 10 % high for worst case)? If the supply voltage is higher than ABS MAX then you need to reduce it.

Thermal dissipation. Higher output power means higher power dissipated in the chip amp. But this is only an issue if the amp is operated at high output power. The idle power dissipation is fairly low. I suggest running the amp at high output power for an hour while monitoring the temperature of the heat sink. If the heat sink gets uncomfortably hot (60~70 deg C), I'd use a larger heat sink.

~Tom
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Old 9th June 2011, 06:56 PM   #3
preiter is offline preiter  United States
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Quote:
Originally Posted by TheLaw117 View Post
Hello,

I bought a 250VA 25VAC+ 25VAC transformer. Parts Express Avel Lindberg transformer. My DC voltage will be around 34-36V after rectification.

I'm just about ready to start mounting everything in my case, but I'm a little bit worried now.

It's a 2.1 Amp, with a 2x LM3886 board driving the two left and right channels, and a LM4780 driving the subwoofer. The speakers on the LM4780 are 1x 8ohm woofer, and 1x 5ohm tweeter.

They have pretty big heatsinks and with that impedance, I do not think they should have an issue handling the +/- 25V. However, my sub is a 4ohm sub and I will be using an LM4780 to drive that.

I've heard that I can reduce the chance of frying my LM4780 by running it in parallel. Is that true? I was originally going to run the LM4780 in parallel because it would give me more output. However, my sub can really only handle 40-60W, not 100W+. So I thought I would just run one channel of the LM4780 and forget about the other one.

However, now I'm hearing that running in mono parallel is better. Is ~35VDC too much for a 4ohm load? What can I do about it?

Thanks.
Running the two LM4780 channels in parallel will effectively double the impedance seen by each channel. So it would be as if each channel were driving an 8 ohm load. With a 35V rail voltage, that is a good thing.

To drive 4 ohms, you would want a lower rail voltage.

The total power output being too high is not really a concern, just turn it down if the driver starts to make unpleasant noises.
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Old 10th June 2011, 01:07 AM   #4
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Well the LM4780 can handle up to 84V. So that's +/-42V. I would be running them at +/- 35V Same thing goes for the LM3886s.

So I have leeway, but I don't want to kill anything.

My heatsinks are going to be actively cooled, albeit I'll be using 12V fans running on ~8V. So hopefully I'll avoid overheating.

They are computer heatsinks with copper heatpipes.

I also have all of the parts for parallel operation. Some high power resistors etc.

So I guess parallel is the way to go?

Thanks.
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Old 10th June 2011, 07:25 AM   #5
Francec is offline Francec  Australia
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Yep!
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Old 10th June 2011, 01:12 PM   #6
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The datasheet is your friend

http://www.national.com/ds/LM/LM4780.pdf below from page 17. I would highly recommend reading the entire datasheet

Quote:
PARALLEL AMPLIFIER APPLICATION
Parallel configuration is normally used when higher output
current is needed for driving lower impedance loads (i.e. 4Ω
or lower) to obtain higher output power levels. As shown in
Figure 3 , the parallel amplifier configuration consist of designing
the amplifiers in the IC to have identical gain, connecting
the inputs in parallel and then connecting the outputs
in parallel through a small external output resistor. Any number
of amplifiers can be connected in parallel to obtain the
needed output current or to divide the power dissipation
across multiple IC packages. Ideally, each amplifier shares
the output current equally. Due to slight differences in gain the
current sharing will not be equal among all channels. If current
is not shared equally among all channels then the power dissipation
will also not be equal among all channels. It is recommended
that 0.1% tolerance resistors be used to set the
gain (Ri and Rf) for a minimal amount of difference in current
sharing.

When operating two or more amplifiers in parallel mode the
impedance seen by each amplifier is equal to the total load
impedance multiplied by the number of amplifiers driving the
load in parallel as shown by Equation 5 below:
RL(parallel) = RL(total) * Number of amplifiers (5)
Once the impedance seen by each amplifier in the parallel
configuration is known then Equation (2) can be used with this
calculated impedance to find the amount of power dissipation
for each amplifier. Total power dissipation (PDMAX) within an
IC package is found by adding up the power dissipation for
each amplifier in the IC package. Using the calculated
17 National Semiconductor | High-performance Analog
LM4780
PDMAX the correct heat sink size can be determined. Refer to
the section, Determining the Correct Heat Sink, for more
information and detailed discussion of proper heat sinking.
Tony.
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Old 10th June 2011, 01:44 PM   #7
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I'm using 1% metal film...
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Old 10th June 2011, 01:57 PM   #8
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Resistors are cheap. DVMs are cheap. You can match to 0.1% or even far better if you want.

What really matters is junction temperature so be sure your thermal connection to the heat sink is good. Surfaces need to be machined flat. Wavy extruded surfaces are less than optimal. Use good thermal grease, maybe Arctic Silver if you can get it. You can have the greatest heat sink in the world but junction temperature will still be high if there's high thermal resistance between the part and the sink.
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Old 10th June 2011, 02:21 PM   #9
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I'm using Arctic Cooling MX-2, which is better than AS5, so I'm good. I bought some heatpipe computer heatsinks but I'm regretting it now becuse I don't know how to mount them. So I might just go for an extruded aluminum.
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Old 10th June 2011, 03:15 PM   #10
qusp is offline qusp  Australia
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wouldnt tempco be a more important spec in a rather warm power amp? you can match to 0.1% simple, in fact you'll probably find your 1% resistors are closer to 0.1% in reality, but if they drift over temp its not much good is it? plain vanilla metal film dont have the greatest tempco either, although better than some

Last edited by qusp; 10th June 2011 at 03:17 PM.
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