What chipamps to active 2-way

[Blueroo]

Hi everybody,

I'm building 2-way monitors with active crossover.
Speakers are: Seas 27TBCD-DXT and ER18RNX.
Cross freq is on a 1,7kHz so the power ratio is about 4:1.
I thought about 100W for woofer and 25W for tweeter.

What chipamps should I go with?
Is LME49830 with mosfets a good choice for woofer-amp? What about tweeter-amp than?

I do not know much about electronics (I've builded couple of amps, know how to solder, etc. but that's it ) so I need projects that are "ready to go".

[AndrewT]

Hi,
forget about average power ratios. It is completely irrelevant to all except average power dissipation in the amplifier and the driver motor.

Look instead at the sensitivity of the two drivers.

If they are 90dB/2.8V @ 1m and 8ohm then both need the same voltage to sound just as loud.
On peak transient signals the same rule applies.

If one is a 4ohm unit and the other is an 8ohm unit then allow for that difference to give the same SPL for the same input/source signal.

If one is more sensitive than the other. eg. 86dB and 89dB then the lower sensitivity needs twice as much power drive to reach the same SPL.

Aim for the same maximum peak transient SPLs to match. That will use sensitivity and voltage to arrive at matched SPLs.

Once you know the maximum voltages to reach your target maximum SPLs then you can select amplifiers to reach those maximum voltages and their associated maximum currents.

Your average powers are used to select the heatsinks and ventilation of the amplifiers and drivers.

[Blueroo]

Does it mean that I should only care about voltage gain of the amps?

My LSPcad simulation shows that my target SPL is 87dB/2.8V, which is sensitivity of my woofer(8ohm).
Tweeter(6ohm) has sensitivity 94dB/2.8V.
So it's 7dB, 5 times in terms of power or 2,24 of voltage gain.

But about BaffleStep compensation? 6dB of equalisation will increase power consumption, doesn't it?

[tangmonster]

"Does it mean that I should only care about voltage gain of the amps?"

yes , but it's also a waste to have a 100W amplifier if you only need 10W.

suggestion:

bass = lm3886
tweeter = lm1875

Tricky part is that you need two different voltage rails for optimum system SPL.

Compare the cost between regulating the higher voltage rails from the lm3886 for lm1875 vs just using two lm3886's with same voltage rails and difference in gain/ attenuation according to speaker sensitivity.

i am willing to bet you will realize that it is cheaper / simpler to just use two lm3886 even though the one for the tweeter will be "underused"

hope my input helps..

[Blueroo]

I should mention that before -> main goal of my project is to sound good

There are plenty of great amps with suitable gain and power for woofer but it's hard to find good class amp (one will match the woofer amp) for tweeter :/

>tangmonster you've got a point.
Anyway, I'm planning to use separate transformers for both amps so different voltages are not a big problem in this case. This is not the cheapest way but the easier one I think.

[AndrewT]

Originally Posted by Blueroo

My LSPcad simulation shows that my target SPL is 87dB/2.8V, which is sensitivity of my woofer(8ohm).
Tweeter(6ohm) has sensitivity 94dB/2.8V.

what voltage at the speaker terminals will the woofer need to develop 100dB SPL in the midband?

What voltage will the tweeter need to develop 100dB SPL in it's own midband?

That will be the ratio of output voltages of the two amps to drive the speaker drivers to the same SPLs.

4.46*2.83 = 12.6Vac will take the woofer to 100dB @ 1m (not particularly loud for peaks, but very loud for average levels).
1.99 * 2.83 = 5.6Vac will take the 6ohm tweeter to 100dB @ 1m. (note that the sensitivity of the tweeter has been measured for 2.8V or ~1.3W for a 6ohm driver.
There's the ratio of maximum peak output voltages you need to take both drivers to the same maximum SPL.
12.6/5.6 = 2.25
The woofer needs an amplifier of approximately 2.25 times the output voltage. In watts that is about 5times. (2.25^2)

If you have 60W available from your chipamp driving the woofer then the tweeter will need ~12W to reach the same SPL. Maximum peak SPL at 2.5m ~99.8dB for a stereo pair. The predicted maximum average listening level ~ 80dB, adequate if you don't party and don't like your music loud.

This has absolutely nothing to do with average power in various parts of the music spectrum. This has all to do with driver sensitivity to reach the same SPL.

[Blueroo]

>AndrewT thank you very much for explanation.

So we've got that one clear:

Originally Posted by AndrewT

The woofer needs an amplifier of approximately 2.25 times the output voltage. In watts that is about 5times.

Let say I'll build 80 to 100W amp for woofer...
What good class ~20W chipamp can I use for tweeter?

[Sebastiaan]

Originally Posted by Blueroo

>AndrewT thank you very much for explanation.

So we've got that one clear:


Let say I'll build 80 to 100W amp for woofer...
What good class ~20W chipamp can I use for tweeter?

If you don't want to use two power supplies (which would be unavoidable with those different power ratings) I would advice you use a bridged LM4780 for the woofer and a LM3886 for the tweeter. Make the power supply voltage low enough so that the LM4780 doesn't overheat and can drive the woofer impedance. This way you can feed both systems from one power supply. A suggestion is 25VDC. for both amplifiers. Gives you around 25 watt 8 ohm for the tweeter and around 90 watt for the woofer. Means you can use a 2x18VAC transformer.

I did this myself for an active speaker, and it works very well, and give the benefit of one power supply for the whole system.

Some people would advice against bridging a chip amp, but my opinion and experience is that it can work really well and reliable, if the power supply voltage is chosen wisely with eye on the lowest to drive impedance for the bridged amplifier. Also make sure you give it a more then adequate heat-sink. (With other words choose both carefully).

With kind regards,
Bas

[Blueroo]

Sebastiaan I don't know if you've read my other posts...

My tweeter has 6ohm, not 8, and I'm planning to use separate transformers for both amps so different voltages doesn't really matter for me.

[Sebastiaan]

Originally Posted by Blueroo

Sebastiaan I don't know if you've read my other posts...

My tweeter has 6ohm, not 8, and I'm planning to use separate transformers for both amps so different voltages doesn't really matter for me.

Dear Blueroo,

I read them But if you go the way I suggest it is not needed to use separate transformers.

With kind regards,
Bas