georgehifi,
Why did you create a post, delete it, then create a new post making it look like you agree with the post you deleted?
Why did you create a post, delete it, then create a new post making it look like you agree with the post you deleted?
Showing 11 responses by atdavid
When some of the "impedance" number were being thrown around in this thread (i.e. < 1 ohm), I was not paying enough attention to realize that figure was for EPDR, equivalent peak dissipation resistance. This is a figure that calculates the peak power loading on the output devices in a linear amplifier when current/voltage are not in phase, it is Not the impedance or a measure of peak current. EPDR does not apply to a Class-D amplifier. It effectively has no merit in the discussion of a Class-D amplifier. For a Class-D amplifier, the impedance curve is sufficient. Even from a power dissipation standpoint, in a Class-D amplifier, the FETs are not operated in the linear region, so you don’t have the same device loading. |
This is a class-D thread. No one is pushing anything. I am stating, quite clearly, that your "push" of EPDR as a relevant value is meaningless w.r.t. Class-D amplifiers. Given that EPDR is meaningless, we don't need to consider 0.9 ohm loading as you have pushed for the Alexia as it has not meaning within a Class-D discussion. The standard impedance which does not dip below about 2.5-2.6 ohms, is the relevant value for discussion. BJT has nothing to do with a Class-D discussion, but since you bring up EPDR, EPDR is most damaging to a BJT amplifier in real world conditions because of secondary failure that MOSFETS, and tubes do not experience. |
Just to restate, georgehifi has been pushing the EPDR "number" for certain speakers as a reason why Class-D will not work unless it can drive <2 ohms, as they have an EPDR (Equivalent Peak Dissipation Resistance) < 1 ohm. EPDR is a term that was developed to represent the peak power dissipation in output devices of linear amplifiers with reactive loads. It does not represent higher peak current or lower impedance, it represents higher power dissipation on the output devices that are operating in the linear region. As Class-D devices do not operate in the linear region, EPDR is not a useful criteria for them. |
Now if only there were Class-D amplifiers that drove into 2 ohms without any trouble like the op asked ... oh, there are. Cherry Class-D do. Mytek will as well. There is nothing "sexy" about them, they just do the job they are designed to do. Obviously you have heard them with difficult to drive speakers? |
The whole basis of your reasoning is that if the power does not double from 8 ohms to 4 ohms, and then again from 4 to 2, that it couldn't possible work right. There is no way you can make that determination without knowing exactly why a specific power limitation may exist at 2 ohms, and you definitely cannot use whatever logic you may think applies to a linear amplifier to a Class-D. |
jetter, I have no skin in the game for Class-D, AB, or A. I am just the guy that actually understands what EPDR is, and why it does not apply to Class-D amps, and why it is particularly harsh to BJT amps, the type that a certain person pushes as the holy grail for low impedance, citing EPDR. I and Noble100 have given examples of amplifiers proven to work, and work well at 2 ohms. Here is an independent post from DIYAudio. That "max-current-limit", something a Class-D amplifier can do on a cycle by cycle basis, explains why you can design a Class-D amp with a derated 2 ohm output power without impacting sound quality. The statement about SOA (safe operating area) goes back to the EPDR, or why it is not applicable to Class-D. You will find similar posts on the Internet from people with a good understanding of the technology. There is another thing that comes into play: https://www.diyaudio.com/forums/class-d/43426-class-output-impedance.html |
You are applying criteria that could be used as a measure of a Linear amplifier to a Class-D amplifier which can current limit cycle by cycle, hence the current limit and power limit of a Class-D amplifier can be decoupled rendering your statement irrelevant depending on the Class-D implementation.
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