Amp Impedance vs. Speaker Impedance

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Amp Impedance vs. SpeakerImpedance

Posted: Fri Aug 09, 2013 4:54 pm
Hi, I'm new to SOS, and I need some help understanding a certain concept.

Somebody told me that, when setting up a commercial sound system, the impedance rating of the speakers should be higher than the impedance rating for the amps. For instance, if I have six amps rated for 1 ohm, I should use speakers with an impedance rating that is above 1 ohm.

Is this correct? If so, why is that the case?

Any help is greatly appreciated.

Thanks.

Re: Amp Impedance vs. SpeakerImpedance

Posted: Fri Aug 09, 2013 5:22 pm
Really you would like the output impedance of the amp to be zero. Its not a question of matching to avoid reflections like you get with transmission lines. The actual impedance of speakers varies with frequency. So if your (nominal) speaker resistance is close to the amp resistance you will get a less flat frequency response. In addition, the ability of the amp to damp the speakers will be reduced, which reduces accuracy of reproduction on transients.

(Of course, on a guitar amp, you want both these things to happen since its part of the sound, but not in a PA)

Re: Amp Impedance vs. SpeakerImpedance

Posted: Fri Aug 09, 2013 8:21 pm
Power amps of the sort you use for PA applications are basically current limited voltage sources, and that current limit sets the minimum impedance you can drive.

If for example you have an amp capable of say 80V output and 20A, then the minimum impedance it can drive is 4 ohms (80/20), the amplifier has (within its voltage and current limit) a very low output impedance (A small fraction of an ohm), but it is limited in both its maximum output voltage and the maximum current it can deliver.

Amplifiers normally quote output power into a range of impedances so maybe:
1000W @ 2ohms,
600W @ 4 ohms,
350W @ 8 ohms

or whatever, notice that usually the power increases by a little less then two times each time you halve the load impedance. This is because the amplifier is basically voltage limited, but losses increase at higher currents and the power rail usually sags a bit as well.

Be wary of running at the lowest rated impedance for your amp (Particularly if it is 1 ohm!), cable and contact resistance will have you down there, usually better to series a few things and try to keep the effective load above a few ohms, even if the amp claims to be capable of 1 ohm.

HTH.

Regards, Dan.

Re: Amp Impedance vs. SpeakerImpedance

Posted: Mon Aug 12, 2013 2:12 pm
All very true, but the output resistance is nothing to do with the current the amp can deliver - it is specifically not Vmax/Imax because there is a negative feedback loop that reduces the effective output resistance of the amp. Even if there were not such a feedback loop, Imax can be a limit set by available heat dissipation so that formula won't work.

Re: Amp Impedance vs. SpeakerImpedance

Posted: Mon Aug 12, 2013 7:16 pm
I thought that is what I said in the second paragraph, Vmax / Imax is (subject to SOA limits) the minimum load |Z| the amp can drive, it is not the output impedance of the amp which in anything respectable and solid state is way below one ohm in the audio band (In part thanks to loads of GNFB, country to some more is almost always better until you start to hit loop stability or noise problems).

Regards, Dan.

Re: Amp Impedance vs. SpeakerImpedance

Posted: Tue Oct 08, 2013 4:31 pm
I'm not thrilled with some of the explanation given, perhaps its just my understanding of certain terminology and concepts...?

The link below is a more complete picture, albeit a bit technical. Short answer: Amp output impedance < Spkr Input Impedance (btw Z = R+jX .. and not V/I which is a DC concept)

http://en.wikipedia.org/wiki/Impedance_matching#Loudspeaker_amplifiers

"Impedance matching is not always necessary. For example, if a source with a low impedance is connected to a load with a high impedance the power that can pass through the connection is limited by the higher impedance."

Re: Amp Impedance vs. SpeakerImpedance

Posted: Tue Oct 08, 2013 4:49 pm
C.LYDE wrote:The link below is a more complete picture

I beg to differ, especially since that wiki page specifically concerns impedance matching (in all its variations and contexts) none of which are even slightly relevant to 'normal' solid state audio power amps in typical applications.

Hugh

Re: Amp Impedance vs. SpeakerImpedance

Posted: Tue Oct 08, 2013 7:58 pm
I wonder if it's worth putting up a sticky on this subject. In the time I've been on SOS it's become a 'regular' :roll:

I would just add that these days you would probably have difficulty finding a solid-state amp with an output impedance that was high enough to be realistically measurable!

Re: Amp Impedance vs. SpeakerImpedance

Posted: Tue Oct 08, 2013 9:46 pm
Hugh Robjohns wrote: none of which are even slightly relevant to solid state power amps.
Hugh

My big LDMOS amp for 160-6M would beg to differ, it gets most upset if load Z varies too far from 50+j0 ohms (Which is NOT to say that it has an output impedance of 50 ohms, just that that is the impedance I designed the drain match to drive (Actually I designed for 75+j0 so it can drive a 1.5:1 mismatch to full power).

For audio power amps it is **Mostly** irrelevant, at least for conventional magnetic loudspeakers, I have seen some funny things when driving highly capacitive loads (Piezo ceramic transducers driven off resonance for example), where the phase margin can get very iffy.

Z however does equal E/I, its just that either or both can be (and usually are) complex values.

Regards, Dan.

Re: Amp Impedance vs. SpeakerImpedance

Posted: Wed Oct 09, 2013 9:48 am
dmills wrote:For audio power amps it is **Mostly** irrelevant, at least for conventional magnetic loudspeakers.

Quite so. Clearly there are some very specialised applications where the generality doesn't apply, but for normal audio applications of the type most forum users are likely to come across I'd still go with irrelevant. ;)

Z however does equal E/I, its just that either or both can be (and usually are) complex values.

+1.

H