Impedance compensation needed for active speakers?

Another newbie question...

Considering how the non-linear impedance of a driver will result in non-uniform power delivery from amps (specially solid-state ones), is it necessary/preferable to add passive impedance compensation circuits between the power amp and the driver even if I'm going with line-level xo?

I know most full-range lovers can't imagine having anything but straight wire between power amp and driver, but I can't figure out how you're expected to get a uniform power output from the driver without impedance compensation. Maybe it's a simple theoretical confusion I have? I'd be really grateful for any light on the matter. Thanks. :)
 
The sensitivity is, historically, stated as xx dB at 1 W, 1 meter.
However, due to this problem you mention, many specify the driver sensitivity as xx dB at 2.83 VOLT, 1 meter. 2.83V gives 1W at 8 ohm, but the dB/V rating is not sensitive to driver impedance.
This is how you get away with the "simple" solution of source, amp, driver-unit for full range speakers. :)

Jennice
 
phase_accurate said:
The only passive impedance correction of which I expect being useful in active speakers - is a Zobel compensating for the impedance-rise due to the driver's Le.
I'm trying to digest all the responses... I'm not sure I understood the meaning of that earlier post's reference to power factor. I guess changes in phase have something to do with the power output.
And when you said the rising impedance is the only thing that one needs to bother about, what about the resonance peak? Shouldn't that worry the designer, specially for tweeters, which usually do receive some signals at their Fs, even after the xo cuts it down? Won't a resonance peak compensation simply make the tweeter output flatter at the Fs?
 
If you are using an amp that resembles a voltage source (as mentioned by richie00boy) you won't have any response aberrations due to the impedance-rise. Some amps however work better in purely resistive loads, that's where the power factor comes in.
I wouldn't compensate the fs but the rising impedance due to Le (Lvc) only.

If you use a tube-amp(s) things might look differently however.

Regards

Charles
 

Svante

Member
2004-02-01 11:17 pm
Stockholm
If you use a normal solid state amplifier with no crossover between the amp and speaker, impedance compensation is not needed.

Impedance compensation can only be needed when the source (=amp+filter+cables) has an output impedance that is comparable with that of the driver.

Some say that a resistive load is better for the amplifier than a reactive load, but in normal cases, the load rather becomes more difficult for the amp if a compensating network is used even though it becomes resistive.
 

forr

Member
2004-12-01 6:46 pm
Next door
A Zobel network (something around 100 nF + 4.7 Ohm) is usually necessary to ensure the stability of power devices used as followers at the output stage of most solid state amplifiers.

When directly driven from the ouput of such amps, there is no need to compensate for the rise of louspeaker impedance at high frequencies :
- it would be a waste of power
- a solid state amp is less linear when it delivers more current (when the load impedance decreases)

Less obvious : with an inductive load at high frequencies, as the current is lagging voltage, the phase margin of a standard Miller compensated amplifier is probably better than with a purely resistive load.

~~~~~~ Forr

§§§
 
re networks

In order to be "unconditionally stable" amplifiers already have an output "Thiele" network, or some varyation of it.
The idea of this is to isolate the amplifiers feedback loop from the reactive speaker load, in the case of no form of passive crossover this is more than adequate.
 
Sometimes it might be beneficial to add a (Zobel-like) RC circuit at the far end of the speaker cable to "terminate" it and alleviate possible RFI interference. (Rising inductance of a woofer or midbass driver could leave the speaker wire unterminated at high frequencies.)
However, I wouldn't think this would be necessary in most situations.

The feedback gain of the amplifier is a variable that might also be a factor in this. (Oops, I see this was just mentioned as I was typing.) :)

Cheers,

Davey.
 
I would ask this question in the solid state forum (assuming you have a SS amp). Any sound quality improvements would be from less distortion from your amp rather than your speakers

I would also phrase it in terms of output stage stability/oscillation.

You are already way ahead of the curve with active XO

To better answer your question, you should state the amp you are using or provide schematics, and list drivers and XO points

A well designed amp should not have output stage instability problems.
 

jomor

Member
2004-02-20 1:36 pm
Athens
Of course most amps act like a voltage source, but its the current that flows in the voice coil which moves the cone. Thus its important NOT to have phase difference between voltage and current at the amplifiers output loop. A inductive load creates phase difference between voltage and current which is not desired. A resistive load helps the amp to control the cone's movement accurately and in time.

Most manufacturers do not compensate impedance (surprise!) for cost reasons.
 

soongsc

Member
2005-03-26 2:31 pm
Taiwan
jomor said:
Of course most amps act like a voltage source, but its the current that flows in the voice coil which moves the cone. Thus its important NOT to have phase difference between voltage and current at the amplifiers output loop. A inductive load creates phase difference between voltage and current which is not desired. A resistive load helps the amp to control the cone's movement accurately and in time.

Most manufacturers do not compensate impedance (surprise!) for cost reasons.

I recently tried a speaker without compensation on different amplifiers. On some, you could hardly see the driver move as I would expect. On some integrated amplifiers, the drivers move like hell, and sometimes you can hear distortion in the music when that happens.
 

soongsc

Member
2005-03-26 2:31 pm
Taiwan
richie00boy said:
Was that with the same source, same source material, same room, same speaker and listening position and same apparant loudness each time you did the test?

Only the speakers were the same. I was just wondering whether it was related with amps or not. My own amps are all FET amps, the other amps at other locations were brands like Yamaha, Pioneer, Maranz. At my location, the drivers rarely move pass xmax even with very low frequency content, whereas at these other locations, I really did not expect that much low frequency content.

The loudness was about the limit of my level of comfort in all cases. All amps had at least 80W capability. Mine had 140W/Ch
 

forr

Member
2004-12-01 6:46 pm
Next door
SOONGSC
"Thus its important NOT to have phase difference between voltage and current at the amplifiers output loop."


As far as amp stability is ok, this does not matter.

Do not forget that at high frequencies, the voice coil inductance decreases and the rise of impedance has an important reistive component.

It is around resonance that there are the greatest phase differences between currents and voltages, and it is very rare that they are compensated at all.

~~~~~~~ Forr

§§§
 
forr said:
SOONGSC
"Thus its important NOT to have phase difference between voltage and current at the amplifiers output loop."


As far as amp stability is ok, this does not matter.

Do not forget that at high frequencies, the voice coil inductance decreases and the rise of impedance has an important reistive component.

It is around resonance that there are the greatest phase differences between currents and voltages, and it is very rare that they are compensated at all.

~~~~~~~ Forr

§§?

Yes, I agree. That is why I wonder why the response of the driver differs so significantly depending on which amp it's connected to.
 

jomor

Member
2004-02-20 1:36 pm
Athens
Sorry, I probably misunderstood the title and got out of topic, I was speaking in general, not just active speakers
The most common speaker topology, the two way, has a large impedance peak at the midrange (slightly below the X frequency). This is usually a high and wide peak (unless the designer overlaps on purpose), which should be compensated. Almost nobody does it, but this doesnt mean that its not important. Phase difference at midrange leads to harsh mids at loud volumes, especially when the amp is weak. I can't see a reason for letting my amp to see a non-resistive 40 Ohm load at 2Khz instead of something close to 8 ohms, unless i find a logical or technical explanation why the phase difference that occurs at the critical midle range is not a problem.

Since the question is limited to active amplification (which means the signal is filtered before the amp), I guess there is no reason to compensate the inductive rise of a woofer, since it will be fed with a signal lets say 3Khz maximum, at which the impedance has risen just by 2 or 3 ohms.

Compensating the woofer's reasonance would also be useful, but it would need very large and expensive components (due to low frequency) so neither manufacturers nor diy-ers do it, as you already mentioned. Apologies for the misunderstanding.
 
re qt

Companies like jbl and peavey both publish in their sound reinforcement manuals that series connection of drivers has the effect of drastically reducing amplifier damping and is not recomended.
The only mechanism that could cause this is the increase in source resistance caused by another resistance in series with the drivers, and the only resistance involved is the other drivers voice coil.
In the case of a sealed box the doubling of Qt causes around 1.5-2db. peaking, hardly noticeable, a reflex box however has 6db. of peaking for a doubling of Qt, and this is audible as well as obvious in measurement.