LM4780 : good choice for 3 way active speaker ? - diyAudio
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Old 5th April 2013, 06:52 AM   #1
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Default LM4780 : good choice for 3 way active speaker ?

Hi,
I am planning to make a 3 way active speaker (inspired by the PMC IB1S).
I have been looking around for cheap chip amps that would suit such a speaker and i am currently considering the following (in each speaker) :
- 1x stereo LM4780 (2x60 W RMS) : tweeter + mid
- 1x bridged mono LM4780 (120 W RMS) : bass

Questions :
- Is this a good choice?
- Would it be better to use a 3883 on the tweeter & mid ?
- I've read that the bridged LM4780 gets real hot & needs a massive radiator + a fan. Any way to avoid the fan ?
- Can i use one single power supply for both the bass + tweeter & mid ? (1 PSU per speaker)

Thanks a lot.
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Old 5th April 2013, 10:33 AM   #2
AndrewT is offline AndrewT  Scotland
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I would use 3 off 3886 all running off the same PSU for a 3way.

Select a bass speaker with the same sensitivity and impedance as the other drivers and you don't need extra power and don't need different PSU voltages.
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Old 5th April 2013, 07:48 PM   #3
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What we need to know here is what the impedance of your drivers is. And ideally their nominal efficiencies.

I am initially inclined towards Andrew's suggestion, but you are missing the opportunity of having vastly higher peak output. You can likely do without the fan but let's hear the answers first.
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Old 7th April 2013, 12:25 AM   #4
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Thanks for your replies, much appreciated.

Tweeter: 6 ohms, 92dB
Mid: 8 ohms, 88.5dB
Bass: 40 ohms, 90dB

Thank you!
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Old 7th April 2013, 04:53 PM   #5
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why would same impedance and efficiency mather at all?
one of the best features of an active 3 way x-over is the easy handling of sutch.
just before every poweramp a buffer is never a bad idea, and supposedly this buffer stage may not be a real buffer (not unity gain) to help overcome the difference between drivers.
Go for stereo only setup, that is good down to 4 ohm rated drivers.
Most probably it will supply more power than Your drivers can handle.
And match the sound levels just before the poweramps.
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Old 7th April 2013, 05:09 PM   #6
frank1 is offline frank1  United Kingdom
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Quote:
Originally Posted by RickDangerous View Post
Thanks for your replies, much appreciated.

Tweeter: 6 ohms, 92dB
Mid: 8 ohms, 88.5dB
Bass: 40 ohms, 90dB

Thank you!
You wouldn't normally use bridged chip-amps to drive a 4 Ohm load as each amp will "see" an impedance of 2 Ohms.
Is it too late to get 8 Ohms bass drivers?
Frank
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Old 7th April 2013, 06:47 PM   #7
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Quote:
Originally Posted by frank1 View Post
You wouldn't normally use bridged chip-amps to drive a 4 Ohm load as each amp will "see" an impedance of 2 Ohms.
Is it too late to get 8 Ohms bass drivers?
Frank
Nothing is too late, i havent bought anything yet, this is still in the thinking stage.
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Old 7th April 2013, 06:58 PM   #8
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Quote:
Originally Posted by Arty View Post
why would same impedance and efficiency mather at all?
one of the best features of an active 3 way x-over is the easy handling of sutch.
just before every poweramp a buffer is never a bad idea, and supposedly this buffer stage may not be a real buffer (not unity gain) to help overcome the difference between drivers.
Go for stereo only setup, that is good down to 4 ohm rated drivers.
Most probably it will supply more power than Your drivers can handle.
And match the sound levels just before the poweramps.
It is indeed my impression that going active helps to not suffer from impedance differences but i could be wrong.
But how do you use one stereo amp with two 3 way active crossovers? I imagine it then has to be an analog active crossover ?
I was thinking of using a WAF-Audio Najda digital active crossover (at least for R&D, and possibly in the final product, not sure yet), which has 2 inputs and many outputs.
So for R&D i need one mono amp per channel (6 therefore).
I have no objection to using one mono amp per speaker in the final product (or 1 stereo amp for both) with an analog active crossover, but is that possible? Does such a crossover have just one input and one output (both of which must be at line level because they must be before the amp) ?

Thanks

PS Any good books to recommend?
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Old 8th April 2013, 10:12 AM   #9
AndrewT is offline AndrewT  Scotland
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If you are proposing a 3way active then you need 3 amplifiers.

If the sensitivities are 92, 88.5 & 90 as has been shown, then I would use the same amplifier for bass and treble and either a half power amp for the treble requiring a pair of lower voltage secondaries, or just use the same power on the same secondaries.

To me it seems that a single transformer with a single dual polarity PSU is the simplest way to achieve a 3way active solution.
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Old 8th April 2013, 08:27 PM   #10
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That 90dB seems a touch high, even for a 4 ohm unit. How far down does it go? Do you have the Thiel Small figures.

But if you are going to use it anyway - and yes the impedances shouldn't much matter - I would suggest you use the second 4780n in parallel mode. It won't give you the peak headroom that you would get with a bridged arrangement but it will have the advantage of each section being driven by effectively the same amplifier. You may also be able to hoik up the voltage on this one since it won't be working that hard (now seeing 8 ohms effectively, and on a pretty narrow bandwidth). In fact, while you can be conservative on the others, 30 to 35V, you get a good deal more latitude with the two in parallel. And no, you will not need a fan.

So, those are the three amplifiers. Effectively identical, but one able to deliver twice the current.


Edit. IIRC you actually do have the possibility of using two different supplies on the one chip (which if I'm right is where I would go) but you can use just one to begin with. I would definitely use a separate supply for the bass section. None of these have to be huge and the choice is mostly about what you think your system deserves. 160VA or 220VA would be around my starting point. But i would use the same total reservoir capacitance on each.

Last edited by ChristianThomas; 8th April 2013 at 08:32 PM. Reason: additional PS
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