how to drive a 0.4 ohms load?

[dr.power]

Hi guys,

I have desired to design and make a line or planar sound source using 10 or more (if needed) common 4 ohms speaker (I have more than 20 of them).

The main problem which I am dealing with is finding an amplifier circuit which is able to handle those speakers. Saying the speakers are all in parallel then the total impedance of them would be only 0.4 ohms. I don't know how to drive a 0.4 ohms load.
any idea please?
I have more than 12 of LM3886 power amplifiers which are able to deliver almost 68W into a 4 ohms load. But I do not know how to use them for 0.4 ohms load.

Can you guys help me out and let me know what to do?
I know that there are a lot of experts in this forum which are able to assist.

Thanks beforehand.

 

[crutschow]

Use one amp dedicated to each speaker.

 

[audioguru]

PA systems use a transformer at each speaker. The transformer has taps for different amounts of output power. Some audio stores sell transformers designed for homes.

 

[dr.power]

Use one amp dedicated to each speaker.

Thanks crutschow,

Actually I already thought of this, but indeed I concern of the Phase difference between the speakers by using dedicated amplifiers for each. I am not sure but I guess that by doing that, I will end up with an array of speakers which are not in phase.

 

[dr.power]

PA systems use a transformer at each speaker. The transformer has taps for different amounts of output power. Some audio stores sell transformers designed for homes.

Thanks audioguru,

But how to drive the transformer for 10 speakers?
Furthermore I do not know if a transformer is the right solution. I do not know how to say, But for any transformer "Pin" is equal to "Pout", so What do I lose If I use a transformer instead of a dedicated amplifier designed to work by a 0.4 ohms load? Hope it makes sens.

 

[dr.power]

Actually I do not like to lose the Maximum of power for each speaker.
I want each speaker works at its MAX of operating status when needed.

 

[rogs]

You can use the 'max power' of each speaker, if you choose your distribution transformers correctly.

Have a look at this : https://en.wikipedia.org/wiki/Constant_voltage_speaker_system which puts some detail on the concept suggested by Audioguru.

 

[dr.power]

You can use the 'max power' of each speaker, if you choose your distribution transformers correctly.

Have a look at this : https://en.wikipedia.org/wiki/Constant_voltage_speaker_system which puts some detail on the concept suggested by Audioguru.

Thanks but yet it does not make any sense to me.

Suppose We have got a big 50W speaker. Are we able to play it at its Maximum of power by just a 1W amplifier connected to a suited transformer?!

An output transformer is not something more than an impedance matcher. Actually it is not able to generate and creates POWER.Hope my mean makes sense?

 

[crutschow]

Thanks crutschow,

Actually I already thought of this, but indeed I concern of the Phase difference between the speakers by using dedicated amplifiers for each. I am not sure but I guess that by doing that, I will end up with an array of speakers which are not in phase.

Don't understand why you think they would not be in phase. Each identical amp fed by the same input would have negligible output phase shift difference between them.

 

[carbonzit]

If you want multiple speakers, you can wire them in series-parallel. They don't all have to be in parallel, you know. Even with an odd number of speakers (like 10), you can come close to matching the amplifier's output impedance.

 

[Nigel Goodwin]

If you want multiple speakers, you can wire them in series-parallel. They don't all have to be in parallel, you know. Even with an odd number of speakers (like 10), you can come close to matching the amplifier's output impedance.

Exactly - simply wire the speakers in series/parallel - it's standard practice (fears of damping factor problems are just that, totally ungrounded fears).

As for worrying about phase problems with different amplifiers, that's nonsense as well - PA systems commonly use dozens of different amplifiers with no such problems.

 

[audioguru]

If you want multiple speakers, you can wire them in series-parallel. They don't all have to be in parallel, you know. Even with an odd number of speakers (like 10), you can come close to matching the amplifier's output impedance.

Speakers in series lose the excellent damping of resonances provided by the extremely low output impedance of a solid-state amplifier. When vacuum tube amplifiers were used many years ago the old speakers had built-in damping because the output impedance of an old vacuum tube amplifier was set by its output transformer to be the same impedance as its load (4 ohms, 8 ohms and 16 ohms). The output impedance of a modern amplifier is 0.04 ohms or less.

 

[audioguru]

Thanks but yet it does not make any sense to me.

Suppose We have got a big 50W speaker. Are we able to play it at its Maximum of power by just a 1W amplifier connected to a suited transformer?!

An output transformer is not something more than an impedance matcher. Actually it is not able to generate and creates POWER.Hope my mean makes sense?

If you have 10 speakers and play 50W from each one then obviously the amplifiers must provide a total of 500W.

An amplifier rated for an 8 ohm load might blow up if the load is 4 ohms or less because it is almost a dead short and the current will be much too high. You could use 5 amplifiers spec'd for 100W into 4 ohms and connect two 8 ohm speakers in parallel to each amplifier.

 

[Nigel Goodwin]

Speakers in series lose the excellent damping of resonances provided by the extremely low output impedance of a solid-state amplifier.

In theory yes, but in practice it's not a problem - with probably all big PA for bands done in that way, where top quality is VERY critical.

 

[audioguru]

In theory yes, but in practice it's not a problem - with probably all big PA for bands done in that way, where top quality is VERY critical.

Here is the frequency response of a typical 12" guitar speaker, the Celestion Blue. It is no where near hi-fi since it sounds like an old AM radio with no bass and no highs. It has a screeching peak from 1.8kHz to 5kHz.

If two are connected in series like is normally done then the damping is reduced so the output at the resonance of about 75Hz is increased.
The rear of the enclosure is completely open unlike a properly designed hi-fi speaker.

 

[Nigel Goodwin]

Here is the frequency response of a typical 12" guitar speaker, the Celestion Blue. It is no where near hi-fi since it sounds like an old AM radio with no bass and no highs. It has a screeching peak from 1.8kHz to 5kHz.

If two are connected in series like is normally done then the damping is reduced so the output at the resonance of about 75Hz is increased.
The rear of the enclosure is completely open unlike a properly designed hi-fi speaker.

What has that got to do with professional PA systems?, which are essentially multi-kilowatt HiFi systems.

 

[carbonzit]

What has that got to do with professional PA systems?, which are essentially multi-kilowatt HiFi systems.

Just another one of Audio McDuck's non sequiturs. Ignore it.

 

[audioguru]

I worked for many years for a sound systems company and with many concert and stadium sound systems. I also worked for a PRO-Sound manufacturer. None of the speakers were connected in series. Each speaker had its own high power amplifier.
The cheap PA and music systems for large stores used a cheap sound system with a transformer at each speaker.

Maybe old speakers designed for vacuum tube amplifiers do not sound different when connected in series because vacuum tube amplifiers do not have the extremely low output impedance of a solid state amplifier so the old speaker provides its own damping of resonances.

 

[carbonzit]

OK, McDuck: I challenge you to tell us how the scheme pictured below could possibly have a problem due to having two speakers in series (this assumes the amplifier is happy with an 8-ohm load):

**broken link removed**

(This is actually currently relevant to me, as I'm advising a friend who's installing 8 speakers in the ceiling of a hall. I gave him a wiring diagram to show how to wire the speakers in series/parallel to get a 4-ohm total impedance out of the 8-ohm speakers.)

Now, granted, it all depends on factors such as whether the speakers are in the same enclosure or not, what kind of acoustic coupling exists between them, etc. Such things may make all the difference in the world to audiophools (meaning the types who actually buy gold-plated connectors and such). But to the rest of us, where these speakers can all be considered to be independent drivers, it matters not.

I look forward to your explanation.

 

[audioguru]

A modern amplifier has an output impedance of only 0.04 ohms or less. So it damps the boomy resonance and other resonances of a speaker. You don't want a speaker to sound boomy. You want the speaker to stop moving when the amplifier output stops.

With an identical speaker in series then the damping impedance is maybe 40 ohms or more at resonance so the speakers get almost no damping.

Maybe cheap little speakers in the ceiling of a hall don't matter about damping. Or maybe they sound better with poor damping.