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Amplifier stability with very low impedance, high efficiency mid/tweeter section???

I've been looking for this information everywhere but can't seem to find a clear cut answer.  I understand that a very low minimum impedance like 2Ohms is a very difficult task for most amplifiers to drive and may even damage it.  But it's also my understanding that this is a current, not a voltage problem.  In other words, say we had an MTM speaker that was used ONLY as a midrange from 200hz up, so it wouldn't have to play bass where most current is required.  And say it also has a very high efficiency of 97db but also a very low impedance of 2 ohms.  Would this be a problem for most amps?  Could it damage the speakers? I'm thinking no since the amplifier wouldn't have to put out much voltage or current to output sufficient SPL.  But what's the actual answer????

poseidon1500
atmasphere's avatar
atmasphere
11,615 posts
 

 This is simply for a speaker I'm thinking of making myself, not for an existing product.  Once I model it I'll have a better idea of the impedance curve. 

@poseidon1500 To prevent the array from being low impedance you can put the midrange drivers in series rather than parallel. The efficiency does not change although the sensitivity does. A lot depends on how you design the crossover and the selection of the midrange drivers but that becomes a simple engineering issue. It would allow the speaker to sound smoother (all other things somehow being equal...) regardless of the amp used.

poseidon1500 OP
15 posts
 

Audioman58, 

I've never listened to a Coda amp but would love to!

Ralph, 

I should say the main reason I want to use two midrange drivers is to match their output level to the tweeter for the same given power input.  But I'm going by the notion that two drivers in parallel will result in a 6db boost in output.  That is 3db from the doubling of drivers and 3db from the doubling of power.  From my understanding two drivers in series will result in the same output as one because voltage for each will be cut in half (-3db) and the doubling of drivers will bring it back up again by 3db.  I also don't know the exact explanation between efficiency and sensitivity other than one is expressed in percentage and the other in SPL/power/distance. 

Either way, last night I modeled the drivers in a typical parallel crossover and it turns out the lowest they dip is 3 ohms.  Not as bad as I thought.  

waldenpond
17 posts
 

Ralph (atmasphere)- Thank you for all you clear and concise info. 

Question: What about the potential use of autoformers to improve impedance matching? ,,. e.g., Paul Speltz' autoformers:, quoted here (not a recommendation, I I have no connection or financial interest, just an example):: 

It is well understood the importance of getting a well matched amplifier for your speakers, or vice versa; getting the proper speakers for your amplifier. Now you can make virtually any amp/speaker combination work well together with the ZEROs.

The ZEROs are the simplest type of transformer called an auto transformer or autoformer. They are used to multiply the impedance of any speaker so that it "feels" like the optimum load for the amplifier being used. They are very helpful in matching speakers to amplifiers. By adjusting the speaker's impedance, both the damping factor and the maximum power transfer of the amp/speaker can be tailored for best sound. Almost all amplifiers sound better when driving a higher impedance speaker, simply because amplifiers produce less distortion when driving less current." 

 

Of course, more wires & connectors, etc.

Thoughts? 

Thanks, 

Ken

 

 

 

rolanda's avatar
rolanda
10 posts
 

There is another, very serious issue that no-one has mentioned.  An amplifier will often - usually - run into stability issues when it is loaded by a reactive load.  An ESL looks like a capacitor to an amplifier, and that means it’s stability margin is affected.  This can cause oscillation at worst (frying things), or at the least sound degradation.  At low frequencies, speaker drive units are non-linear, which means that the impedance they present varies according to what they happen to be doing at the time.  The effect is that they can act as a large inductor, which again can drive an amp into instability.  When an amp does go unstable, it’s output will become momentarily independent of the input, and it can take a little while for it to get itself together again afterwards.

Generally it is better to run an amp into an ESL with some series resistance or inductance such as 6 feet of zip cord to avoid these issues.  These issues may also be the cause of speaker cables sounding differently - the reactive aspects of the wiring are not normally considered.

Back in the 70’s it was these issues addressed by Julian Vereker  that made the Naim amplifiers so exceptional.  A 60W amplifier capable of providing 8A was unheard of (and is still very unusual).  Nowadays amplifiers such as the Sanders Sound Magtech are designed to avoid reactive load problems as of course they have to work with his ESL’s.

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