Which Emotiva amp for stereo listening?

[RichB]

You guys still have given the take-home message: a phase angle of 60 degrees (-60 degrees) is very challenging for an amp to drive.

So if the minimum impedance is < 4 ohms from 100Hz - 10kHz and the phase angle is > 60 degrees, don't use an AVR for the amp.

Challenging for Amps: Impedance < 4 ohms, Phase Angle > 60 degrees. That is the take-home message.

From the CPP link to this article:

Understanding Impedance Curves & Phase Angles | Audioholics

From the amplifier’s standpoint there are two things we’re concerned with. The first is heat/power dissipation at the amplifier. In this respect, 45 is a magic number; at 45 degrees of phase shift, an amplifier’s output transistors will have to dissipate double the heat than if the load were purely resistive (i.e. 0 degrees of phase shift). Fortunately, 45 degrees is the worst case scenario for a real world loudspeaker; both above and below this point, the amount of power an amplifier is required to dissipate falls off.

45 degrees is the most challenging.

The article also states:

whether the phase angle is 0 degrees or 60 degrees, the voltage demanded from the amplifier remains the same. As a result you don’t have to worry that an amplifier is going to have to swing loads of extra voltage and current in order to cover the effects of a difficult phase angle.

However, if the twice the heat is generated then power at some point was supplied.

- Rich

 

[AcuDefTechGuy]

45 degrees is the most challenging...whether the phase angle is 0 degrees or 60 degrees, the voltage demanded from the amplifier remains the same. As a result you don’t have to worry that an amplifier is going to have to swing loads of extra voltage and current in order to cover the effects of a difficult phase angle.

So is Stereophile wrong when they say "an 8 ohm load with 60° phase angle is considerably more challenging that a 4 ohm load with 0° phase angle"?

I got this from another source:

Worst case (never achieved with any loudspeaker), is a 90 degree phase shift (the cosine of 90° is zero!). This means that when the voltage across the transistors is at the minimum (turned fully on), the current is also at a minimum. That seems pretty good - zero dissipation can't be all bad. The problem is that the converse also applies, so when the voltage across the transistor is at the maximum, so is the current!

At 45 degrees, the transistor peak dissipation is doubled, compared to the case with a resistive load. This means that for our 100W 8 ohm amp, the transistor dissipation will be 100W instead of 50W - any increase of phase angle over 45 degrees increases the peak dissipated power vs. the power delivered to the load. It is fair to assume that the 'real life' worst case phase angle will be around 60 degrees...

Phase Angle Vs. Transistor Dissipation

 

[AcuDefTechGuy]

Someone recently reported that their Denon 3312 shut off while driving the KEF Q900, which has a minimum impedance of ~ 3 ohms around the 100Hz-1kHz region and the phase angle of ~ -43 degrees around 80Hz.

So I can see why a combination of ~ 3 ohms + phase angle of close to 45 degrees might be difficult for an AVR.

 

[RichB]

So is Stereophile wrong when they say "an 8 ohm load with 60° phase angle is considerably more challenging that a 4 ohm load with 0° phase angle"?

This is not my area of expertise, I do see why this is wrong.

I got this from another source:

Worst case (never achieved with any loudspeaker), is a 90 degree phase shift (the cosine of 90° is zero!). This means that when the voltage across the transistors is at the minimum (turned fully on), the current is also at a minimum. That seems pretty good - zero dissipation can't be all bad. The problem is that the converse also applies, so when the voltage across the transistor is at the maximum, so is the current!

At 45 degrees, the transistor peak dissipation is doubled, compared to the case with a resistive load. This means that for our 100W 8 ohm amp, the transistor dissipation will be 100W instead of 50W - any increase of phase angle over 45 degrees increases the peak dissipated power vs. the power delivered to the load. It is fair to assume that the 'real life' worst case phase angle will be around 60 degrees...

Phase Angle Vs. Transistor Dissipation

Good article. Table 1 shows the power (amp) hits 2 at 45% and then goes down again. It also says this which matches the AH article:

It is not unreasonable to design for a worst case phase angle of 45°, and as shown in the table above, power actually falls slightly above this. At 45° the peak dissipation is as shown in Figure 6 - this is where things can go pear-shaped in a hurry if you underestimate the operating conditions of a real-world amplifier. Transistor dissipation just before clipping is a bit over 120W (with an average of 28W). Any increase of the supply voltage will push the repetitive peak dissipation into the danger zone - especially when the power transistors are at an elevated temperature.

It seems there are worst cases and then real world worse cases

- Rich

 

[Steve81]

It also says this which matches the AH article

Well it should agree reasonably well since Rod Elliot of sound.westhost.com/Elliot Sound Products was acknowledged on the AH article and had considerable input on that section.

With respect to the quote chosen by ADTG:

any increase of phase angle over 45 degrees increases the peak dissipated power vs. the power delivered to the load. It is fair to assume that the 'real life' worst case phase angle will be around 60 degrees...

It's a matter of context. While the chart shows that power dissipation in the amp declines after 45 degrees, the ratio of power delivered to the load versus dissipated in the amp still gets worse. In that context, 60 degrees was estimated to be the worst real world case.

 

[RichB]

If the manufacturer provides it, or the speakers were measured by an independent third party, you would look at the impedance graph. This graph should contain both impedance and phase. This is for speakers.

For amplifiers, a rating of 4 ohms usually means the unit will be stable to 2-3 ohms, but definitely not below. You should also look at the power supply, which is the total capacitance and transformer size. Another factor would be if the capacitance is shared by all channels or if each channel has it's own bank.

I have often wondered how Emotiva amps specifications and numbers achieve similar power ratings into 8/4 ohms with less hardware behind them.

I found this post on AVS:

Outlaw 7500 or Emotiva XPA5

Which compares the following specs:

Power transister output devices per channel:
Outlaw 7500 - 12
Emotiva XPA5 - 6

Total Capacitor Capacity:
Outlaw 7500 - 150,000 mf</STRONG>
Emotiva XPA5 - 60,000 mf

Power Transformer (Power Supply) Capacity:
Outlaw 7500 - 2 X 1.6 kVA</STRONG> (3200 kVA) torroidal with independent windings per channel so acts more like monoblocks
Emotiva XPA5 - 1 X 1.2 kVA

I believe the Outlaw has separate capacitance per channel and I am not sure about the Emotiva XPA-5.

I am not trying to bash the Emotiva is costs half as much.
The Outlaw seems to have twice the hardware so is it safe to assume that the Outlaw may be better at driving loads with high phase angles?

- Rich

 

[AcuDefTechGuy]

While the chart shows that power dissipation in the amp declines after 45 degrees, the ratio of power delivered to the load versus dissipated in the amp still gets worse. In that context, 60 degrees was estimated to be the worst real world case.

So.....................................anything greater than 45 degrees is worst case?

 

[Steve81]

So.....................................anything greater than 45 degrees is worst case?

In that particular respect.

 

[ImcLoud]

What Rich hears: blah blah blah whiskey blah blah blah.


My take: the phase angle of the dangle = the torque of the pork.

- Rich

Rich is that "mmmmmm, pork" or it is "ewwwwww, pork"

 

[AcuDefTechGuy]

In that particular respect.

In which case would a speaker with 4 ohms & phase angel of 45 -60 degrees from 100Hz-1kHz not be considered a difficult load to drive?

 

[PENG]

In which case would a speaker with 4 ohms & phase angel of 45 -60 degrees from 100Hz-1kHz not be considered a difficult load to drive?

In the case when such a speaker has very low power handling capability related to the amp driving it. In that case, it is not difficult for the amp because it would just blow it up and then rest.

 

[PENG]

However, if the twice the heat is generated then power at some point was supplied.

- Rich

Yes but lots of that power would be dissipated in the amp (the power transistors), instead of in the speakers.

 

[Irvrobinson]

Yes but lots of that power would be dissipated in the amp (the power transistors), instead of in the speakers.

Perhaps these questions about phase angle seem to be begging for a discussion of current lagging or leading voltage, and how the resulting power factor affects the power-efficiency of the work being done, which in this case is moving a voice coil. If you non-EE types remember some simple trig and vector arithmetic, try this site: http://www.the-power-factor-site.com/Power-Factor-Triangle.html

 

[PENG]

Perhaps these questions about phase angle seem to be begging for a discussion of current lagging or leading voltage, and how the resulting power factor affects the power-efficiency of the work being done, which in this case is moving a voice coil. If you non-EE types remember some simple trig and vector arithmetic, try this site: http://www.the-power-factor-site.com/Power-Factor-Triangle.html

That's truly a simple way to explain what PF is from the power system stand point. Steve81 did make a good attempt to explain in ways that non-EE type can understand the effect of phase angles between the voltage and current phasors (often referred to as vectors in the non EE world) in speaker loads on amplifiers. I would also recommend people to read the one Steve81 acknowledged in his, it little more technical and is also much more relevant to the topic at hand, and should be understandable by the "EE type".:

Phase Angle Vs. Transistor Dissipation

 

[AcuDefTechGuy]

In the case when such a speaker has very low power handling capability related to the amp driving it. In that case, it is not difficult for the amp because it would just blow it up and then rest.

Wise guy.

 

[AcuDefTechGuy]

Perhaps these questions about phase angle seem to be begging for a discussion of current lagging or leading voltage, and how the resulting power factor affects the power-efficiency of the work being done, which in this case is moving a voice coil. If you non-EE types remember some simple trig and vector arithmetic, try this site: http://www.the-power-factor-site.com/Power-Factor-Triangle.html

Are you kidding?

After 20 years out of college, you think we want to do that for fun? Are you nuts? I can barely use a calculator!

Just give me the short take-home message for dummies.

Phase angle > or = 45 degrees in addition to impedance of < 4 ohms is bad for wimpy amps.

 

[PENG]

Are you kidding?

After 20 years out of college, you think we want to do that for fun? Are you nuts? I can barely use a calculator!

Just give me the short take-home message for dummies.

Phase angle > or = 45 degrees in addition to impedance of < 4 ohms is bad for wimpy amps.

Except that irv may want you to say something about whether that angle is + or - because he seems to believe the claim that capacitive (+) is a bigger deal. IMHO that's not worth complicating your well said 20 words/symbols single sentence that all audioholics, alcoholics or even audioalcoholics can understand.

Note: He may not have a problem since at that kind of angle it isn't going to be good regardless.

 

[Irvrobinson]

Except that irv may want you to say something about whether that angle is + or - because he seems to believe the claim that capacitive (+) is a bigger deal.

It's not so much true with modern amps, but some vintage solid state amps can become unstable with highly capacitive loads. However, I agree, I don't think it's asking too much for people to understand the difference between inductors and capacitors.

 

[mister wiggles]

Hey guys, curious if you have any experience or input regarding Bryston or Aragon amplifiers.

Specifically these models:
Bryston 3B-ST or 4B (gen 2)
Aragon 4004, 8008x3

I had the chance to hook up the speakers to a tube amp this weekend (Melody H300B). The unit is rated at 8.8 watts continuous to each channel, how can one relate this number to other solid state amp specs with ratings of 2-300watts RMS?

 

[cpp]

Capacitance, inductors, phase angle, power factors who cares, all that matters is how a amp performs with a given set of speakers. Listen with your ears not some piece of test equipment. OK back to raking leaves.