How important is speaker impedance?

Phoenix · « **on:** 12 Apr 2005, 04:43 pm »

My speakers have an impedance of approximately 2 ohms (+/- 1 ohm from 20 - 20000 Hz - really flat) .
From 20-80 Hz they have 3 Ohm.

How do Bryston amplifiers react to such a low impedance? I never had/heard any problems.
Why does Bryston and in fact almost every manufacturer recommend an impedance above 4 ohms?

Isn't a really flat low impedance-curve equal or better to one above 4 ohms which goes up and down all the time?

THX

Mike Pickett · « **Reply #1 on:** 12 Apr 2005, 08:50 pm »

Hi Phoenix;

A solid state amplifier will attempt to double it's power each time you cut the impedance in half. The limiting factors here are the VA rating of the transformer and the current capacity/number of output transistors. In order for an amp to (for example) deliver 300 Watts at 8 Ohms, 600 Watts at 4 Ohms, and 1200 Watts at 2 Ohms, the transformer would need to be around 10 times larger than if you were only concerned with 8 Ohm performance. This would make the amp in question noisy, heavy, and very expensive. For this reason it makes sense to design an amp that handles a certain range of common impedances easily.

However, the above is assuming continous power being driven into non-inductive fixed loads. Music and real speakers are generally a lot easier on an amp than lab conditions, so most well built amps will probably be quite happy driving a speaker with such a low impedance, as long as you don't actually exceed the amp's current capability.

In response to your last question:

The important factor is the percentage of variation relative to the nominal impedance. A set of 8 Ohm speakers that are +/- 3 Ohms are actually flatter than 2 Ohm speakers that are +/- 1 Ohm, since the variance is less than 40% compared with the 2 Ohm speaker's 50%.
This is really an oversimplification, since the frequency (and direction) of the variation is usually a lot more important than the magnitude.

Mike

thomaspf · « **Reply #2 on:** 13 Apr 2005, 12:32 am »

The one missing factor here is the output impedance of the amplifier.

If you have low impedance speakers with a high output impedance amp (some tube amps come to mind) then you basically have a nice equalizer setup where you attenuate the output at all the freqencies where the speaker has a low impedance dip since you loose most of your voltage over the output impedance at those peaks.

I don't think Bryston lists the output impedance of their amps but I recall reading somewhere on their web site that it is <0.01Ohm. James might have the details for the different models.

The net take away is that other than the high currents mentioned in the previous post there should be no problem driving a 2 Ohm speaker with a Bryston amp if you do not overdo it. I have not experienced this myself but I understand the Bryston amps shut down when they get too hot. The one thing I would check is whether this usage is covered by the warranty.

There is no general rule that a speaker with flat impedance is in any way better than one that fluctuates more. Speakers with a higher impedance are much easier loads since you do not have to carefully select the amplifier but that again is unrelated to sound quality.

Do you mind me asking what type of speaker you are looking at?

Cheers

Thomas

timbley · « **Reply #3 on:** 13 Apr 2005, 02:23 pm »

I read a thread on audioasylum a while ago about autoformers. The guy trying them seemed to think there was a distinct advantage in using one to increase the apparent impedance of the speaker to the amplifier, even if the amp was one that could handle a tough impedance load well. I'd like to try an autoformer myself, but I'd need 6 of them at this point since I'm tri-amping my speakers.

PEB · « **Reply #4 on:** 13 Apr 2005, 03:10 pm »

When the ratio of speaker impedance to amp output impedance decreases, the shape of the impedance curve tends to get "mirrored" into the resulting SPL curve. And this happens for typical dynamic speakers regardless of the driving amp. The higher the speaker impedance, and the lower the amp impedance, then the less this effect shows up.

For example, a 2-way speaker will exhibit an impedance hump ~1kHz due to the crossover, and another one (or two, if vented) at the box resonance frequency in the bass. A tube amp fan might relish the bass response and midrange magic, when in reality the midbass and presence region have both been emphasized 1-2dB by the impedance interaction.

SS amps also exhibit this effect, but generally to a lesser degree, as their output impedances are usually lower.

I offer a "mandatory option" of input impedance compensation for any of my speaker products. This extra circuit is connected in parallel with the speaker cables behind the amplifier. It flattens the crossover hump. I typically do not compensate for the midbass hump, as this complicates the circuit and drives the expense up. I never compensation the gradual rise of impedance in the top octave, as that serves to risk amp instability, and would only roll of the top octave anyway. Note that impedance compensation is specific to the speaker, and not to the amplifier.

To address your issue of low 2ohm impedance, you will find that the amp gain will decrease slightly, causing you to turn the volume up a bit, but you would not realize this until I told you. So it's no biggie. OTOH, the issue of power supply rating and current delivery is real, but still only when you want to really crank up your system. And as was already pointed out, the ratio of max/min impedance (shape) does get mirrored into the SPL response.

Hope this helps.

Phoenix · « **Reply #5 on:** 13 Apr 2005, 04:18 pm »

Hi!

Thanx to all for answering my questions!

Quote

...so most well built amps will probably be quite happy driving a speaker with such a low impedance, as long as you don't actually exceed the amp's current capability.

Highly improbable. With 98dB/W/m you almost never reach the limit of any amp. The 60 watts of the B60 can get neighbour's attention (in the next house) pretty fast.

@thomaspf:

I would really like to tell you which speakers I'm using.
But since this brand has no good reputation here in Germany, I decide not to unveil the brand (silly, isn't it?) - although I think that some (not all - and that's the reason for the bad reputation) of their speakers are exceptional.
In German forums I often experienced that once I told people the brand, they didn't take me seriously anymore. I want to avoid that here.
It's a four way system with conventional drivers except for the tweeter which is a high quality horn.

thomaspf · « **Reply #6 on:** 13 Apr 2005, 06:30 pm »

A circuit that flattens out the impdedance is probably not something you want to apply for any generic combination of speaker and amp.

How does that actually work and how does this change the phase of the signal?

For a random speaker the uneven impedance and phase is optimized for a "flat" frequency response and phase alignment. If you start flattening out those 20 Ohm peak don't you get into heavy equalization again.

Cheers

Thomas

PEB · « **Reply #7 on:** 13 Apr 2005, 07:22 pm »

You are right. As I said, the Zcomp circuit must be developed for the particular speaker, since the hump center frequency, Q, and max Z is specific to the speaker model.

An ideal amp has zero output impedance, and is immune to any non-zero speaker load impedance. So the speaker SPL curve does not become skewed, in spite of a non-flat impedance curve.

Take the non-flat true impedance curve, and a non-ideal amp, and the impedance shape "creeps into" the SPL (amplitude v. frequency) response.

For SS amps, especially those with high current capability, the effect is quite small. So Zcomp is not really necessary. Although, even if it is used, the swinging phase angle will be flattened, just as the impedance amplitude is flattened. In this case, the SPL curve remains unchanged, in spite of the flattened amplitude and phase impedance curve. Even the SS amp appreciates a flatter load.

With high output impedance tube amp, let us say the SPL curve becomes *uncalibrated*, and requires *EQ* to make it as flat as if the speaker were driven by SS power. Rather than take a brute force approach of EQ, flattening the impedance curve via a Zcomp circuit will do the trick, and the tube amp will certainly like a flatter Z load.

It is a simple matter for my measurement program to not only calculate the phase curve, but it can also display the real (resistive) and imaginary (reactive) parts of the impedance. When the real part is kept flat and high in value, the amplifier is *happiest*. Consider that a nominal 8ohm speaker could still have a real part of 1 ohm in a narrow band, where the impedance changes dramatically.

thomaspf · « **Reply #8 on:** 14 Apr 2005, 02:59 am »

Interesting. I guess this has to be done with extreme care in order to avoid disturbing the phase alignment of the typical n-way speaker at the cross over frequencies where you usually find the wildest swings.

Cheers

Thomas

PEB · « **Reply #9 on:** 14 Apr 2005, 02:05 pm »

No, this is not how input impedance compensation works.

I see that you realize that fiddling with impedance by way of changes to the crossover will certainly cause changes to the SPL response. True enough, and the SPL response can be very sensitive to impedance changes with components in particular circuit locations.

For example, the shunt tweeter coil is a pretty sensitive guy. Changing it 10% slides an SPL dip/peak combo up or down, and/or can twist the combo clockwise or ccw. When the coil is just the right value, the dip/peak flattens out and disappears. And of course, the electric and acoustic phase varies during this process.

Think of the crossover as a Frequency Dependent Voltage Divider. This voltage divider is not simply a series/shunt resistor combo, but has all sorts of components. Changing any of the components changes the impedance ahead of the speaker, and therefore changes the voltage delivered to the drivers, so that the SPL response of the speaker accordingly.

But now consider an extra circuit that is placed in parallel with the speaker, but AHEAD of the crossover. Basically it is connected across the +/- terminals of the amp. The amp sees a different impedance load. An IDEAL amp does not care about it, because it will drive the different load with the same gain as before, and independent of frequency. In this ideal case, the SPL response of the speaker also does not change.

But a tube amp with high output impedance has a gain that varies with the load. So the gain is altered by the shape of the impedance curve. If we can flatten the impedance curve, then so will the gain be flattened, and finally the speaker SPL will once again measure as it was originally designed.

IOW, the nonideal amp sees a different load, and yes, the electrical phase will change.

But for an ideal amp, the gain remains unaltered, and this phase change is not the same as the phase characteristics within the crossover circuit itself.

Clear or mud?

thomaspf · « **Reply #10 on:** 14 Apr 2005, 08:37 pm »

I stay a little sceptical.

If your device in whatever way it is connected does anything to the phase of the signal that the speaker sees in a frequency dependent way then this will impact careful tuning of the crossover not just from an impedance standpoint but more so from the phase alignment of the drivers. I am not quite clear yet of how this device impacts the phase.

Cheers

Thomas

PEB · « **Reply #11 on:** 14 Apr 2005, 09:38 pm »

As you are entitled to be.

Perhaps it is too difficult for me to explain adequately.

The thing is, the extra circuit is in parallel with the speaker, but BEFORE the speaker crossover, i.e., closest to the amp. Ideally, there is no voltage divider effect for an impedance string (Zcomp) placed in parallel with another load (speaker w/ crossover).

It is when the output impedance of the amp itself is nonzero, then there will be a greater or lesser degree of voltage divider action. By flattening the overall impedance curve, the amp voltage does not vary. It is still *lowered* by V-divider effects, but at least it would not vary with frequency.

Or, look at it this way. Take a speaker with flat SPL, but non flat impedance. Connect it to an amp with high output impedance. The shape of the impedance curve causes a varying voltage divider effect so as to make the speaker slightly less flat, and by the shape of the impedance curve. If we can make the load to the amp flatter, then the speaker will once again measure flat.

Yogus · « **Reply #12 on:** 15 Apr 2005, 08:59 am »

I think the "plus" models of the DB1, FB1, TB2 uses some form of impedance compensation.

thomaspf · « **Reply #13 on:** 15 Apr 2005, 03:41 pm »

Oh, are you basically saying this is basically just a passive real resistor with no inductivity or capacity?

That can't be because putting that in parallel would be lowering the resistance no matter what. So this must be something else.

How does is affect the phase of the signal being passed to the speaker terminals? Does it not impact the phase at all?

Cheers

Thomas

PEB · « **Reply #14 on:** 15 Apr 2005, 07:06 pm »

No, the circuit is simply an R-L-C series string that is placed in parallel with the speaker.

The 3 values are adjusted until the center frequency, minimum Z, and bandwidth match that of the crossover impedance hump. When combined in parallel with the speaker's impedance, the net result is flattening of the total impedance seen by the amp.

The amp sees a more resistive load, and a near-zero phase angle over the band that is flat Z.

The phase relationships within the speaker crossover network remain unchanged. This is because the Zcomp circuit is *ahead* of the speaker network.

If necessary, I can post some graphical data.

thomaspf · « **Reply #15 on:** 15 Apr 2005, 07:50 pm »

I have no idea what ahead means in this context.

At the output terminal of the amplifier you are loading a complex circuitry. When you add any R-L-C elements to that circuitry you end up changing the signal that the speaker sees at its inputs. The amplifer will hopefully output a flat phase and the crossover designer assumes this. If the input phase is changed in any way the result will be wrong.

How does your R-L-C string not change the phase that the speaker sees?

I am not really an analog designer and probably just don't get how this works.

Cheers

Thomas

PEB · « **Reply #16 on:** 15 Apr 2005, 08:21 pm »

*Ahead* means between the amp and the speaker. So it is (= means speaker cable)
Amp = Zcomp = Speaker

Normally, a voltage divider puts the series component ahead of the parallel component. If Zcomp were placed across the driver terminals (for example an RLC string across the tweeter terminals in order to damp the Fs resonance), then the drive voltage delivered to the driver would vary with frequency differently than without it.

But now put a circuit in parallel ahead (before, i.e., between amp and speaker). If the amp has zero output impedance, then there is NO voltage divider affect, and so the speaker's SPL curve remains unchanged.

If the amp's output impedance is NON-ZERO (tube amp), then there is now a slight voltage divider effect. Consider the amp's output impedance to be a small series resistor *before* the speaker. The speaker's shape of the impedance curve gets superimposed onto the SPL curve, although not in a direct one-for-one ratio. So the SPL curve becomes *decalibrated*, or slightly less flat.

The best way to overcome this is with the Zcomp circuit. Yes, the amp's internal electical phase will change slightly, as will the SPL curve become recalibrated.

But for a zero (or near zero) output impedance (SS amp), there is no change, even with the presence of Zcomp.

thomaspf · « **Reply #17 on:** 15 Apr 2005, 11:55 pm »

I see we are not communicating.

Besides from working as a voltage divider your circuit also changes the phase?

Doesn't any frequency dependent phase change even at equal voltage impact the SPL response of the speaker at the cross over region as well due to the interaction of the multiple drivers that the crossover is trying to integrate.

Cheers

Thomas