Amplifier Ignorance

[biggarthomas]

I'm sure that somebody can answer these longstanding questions for me. Some amplifiers are rated at, let's say 200wpc others are rated as 200wpc into 8 ohms and 400wpc into 4 ohms. what differentiates amplifiers that have an 8 ohm/4 ohm designation/capacity? Can I assume that if an 8 ohm operates at 200wpc and a 4 ohm load operates at 400wpc then a 6 ohm load will operate at perhaps 300 wpc?

 

[Bucknekked]

I think that's a pretty good question. I'm pretty sure I know the answers, but there are several members here who know the answers by heart and can express them with vigor. As a rule, I don't tend to believe any generalized power rating : if they just claim 200wpc with no qualifiers, the only takeways from that are they probably aren't going to give you anything close.

I'm going to wait and see what the amp guru's chime in with. That way we both get a great answer

 

[highfigh]

I'm sure that somebody can answer these longstanding questions for me. Some amplifiers are rated at, let's say 200wpc others are rated as 200wpc into 8 ohms and 400wpc into 4 ohms. what differentiates amplifiers that have an 8 ohm/4 ohm designation/capacity? Can I assume that if an 8 ohm operates at 200wpc and a 4 ohm load operates at 400wpc then a 6 ohm load will operate at perhaps 300 wpc?

It's not usually a linear progression from one impedance to another, but if the amplifier rating shows a large increase when changing from a higher impedance to a lower number, it means the power supply and output devices were chosen well. If you see no change, it's either a bad design or it may use an output transformer, like McIntosh's solid state power amps.

The amp's specs shouldn't change much if the impedance changes, but some don't do well with low impedance speakers.

If the speaker load is reduced, the current will increase, so make sure to allow adequate cooling.

 

[PENG]

I'm sure that somebody can answer these longstanding questions for me. Some amplifiers are rated at, let's say 200wpc others are rated as 200wpc into 8 ohms and 400wpc into 4 ohms. what differentiates amplifiers that have an 8 ohm/4 ohm designation/capacity? Can I assume that if an 8 ohm operates at 200wpc and a 4 ohm load operates at 400wpc then a 6 ohm load will operate at perhaps 300 wpc?

If the amp can actually do 200W into 8 ohms and 400W into 4 ohms then you can assume it can do 267.7W into 6 ohms because P=V^2/R, or P=I^2*R. Note that for simplicity, in this formula we assume the load is a resistor when in fact the impedance of a loudspeaker is complex, and frequency dependent.

In practice very few amps will double down because there is always going to be some voltage drop under load. Even with a regulated power supply, you still need the power transistors to carry the higher current when you half the impedance. It is easier to specify in such way that the amp appears to double down by simply understating its output into 8 ohms. I am not saying that a reputable manufacturer will do that, but some might. Example, instead of saying it will do 200W/300W into 8/4 ohms, you can specify it as 150W/300W, of course I am over simplifying things just so you get the idea.

 

[lovinthehd]

The other day I saw an amp rated at X wpc 20-20khz at .0something THD for 8 ohm, then right below it a X wpc rating at 4 ohm but at 1khz at 1% THD....surely a marketing department at work.

 

[Pogre]

If the amp can actually do 200W into 8 ohms and 400W into 4 ohms then you can assume it can do 267.7W into 6 ohms because P=V^2/R, or P=I^2*R. Note that for simplicity, in this formula we assume the load is a resistor when in fact the impedance of a loudspeaker is complex, and frequency dependent.

In practice very few amps will double down because there is always going to be some voltage drop under load. Even with a regulated power supply, you still need the power transistors to carry the higher current when you half the impedance. It is easier to specify in such way that the amp appears to double down by simply understating its output into 8 ohms. I am not saying that a reputable manufacturer will do that, but some might. Example, instead of saying it will do 200W/300W into 8/4 ohms, you can specify it as 150W/300W, of course I am over simplifying things just so you get the idea.

I think I understand what you're saying. They just basically manipulate the specs to make it appear to double down by understating the 8 ohm power output. I get what they're doing, but why? It's still going to be 300w into 4 ohms either way they publish it.

 

[PENG]

I think I understand what you're saying. They just basically manipulate the specs to make it appear to double down by understating the 8 ohm power output. I get what they're doing, but why? It's still going to be 300w into 4 ohms either way they publish it.

As I said, I wasn't saying they did, just that they could, so some might. Also as lovinthehd cited, some would use higher THD for their 4 ohms rating so they could boost the number a litte, even Emo has done it, though in a very reasonable manner because 0.2% THD is still relatively low.

https://emotiva.com/products/amplifiers/xpa-gen3

300 watts RMS per channel; 20 Hz - 20 kHz; THD < 0.1%; into 8 Ohms
550 watts RMS per channel; 20 Hz - 20 kHz; THD < 0.2%; into 4 Ohms
800 watts RMS per channel; 20 Hz - 20 kHz; THD < 0.5%; into 2 Ohms

You are right about 300W into 4 ohms is still the same either way but in the odd case, it may be done to appease those who believe "double down" automatically = more stout amp. It is one of those myths that has refused to die.

 

[Irvrobinson]

The other trick amp manufacturers do is to underrate output at 8 ohms so 4 ohm measurements look like a doubling at rated power.

 

[PENG]

The other trick amp manufacturers do is to underrate output at 8 ohms so 4 ohm measurements look like a doubling at rated power.

By the way, remember you thought class A up to 25W for a 300W is high? The Emo 600W monoblock claims class A up to 60W.. There must be some good tricks beside Nelson Pass real class A amps.

 

[Irvrobinson]

By the way, remember you thought class A up to 25W for a 300W is high? The Emo 600W monoblock claims class A up to 60W.. There must be some good tricks beside Nelson Pass real class A amps.

If they have dynamic bias current level I believe it, otherwise 60 watts would have about 200-300 watts of power dissipation at idle... you'd need a fan.

 

[PENG]

If they have dynamic bias current level I believe it, otherwise 60 watts would have about 200-300 watts of power dissipation at idle... you'd need a fan.

It just has a manual selector switch for A and AB. If you leave it in the A position, it will automatically change to AB when it reaches 60W/8 ohms but it is going to run hot for sure.

 

[Irvrobinson]

It just has a manual selector switch for A and AB. If you leave it in the A position, it will automatically change to AB when it reaches 60W/8 ohms but it is going to run hot for sure.

I just read that in their owners manual. I smell fish.

 

[herbu]

I'm sure that somebody can answer these longstanding questions for me.

I can't tell if you're looking for a technical explanation, or just wondering what to look for in selecting an AVR/amp.

Lots of technical discussion already provided.

If you're asking what it really means to you when shopping for an AVR/amp, the answer is more simple. You may have gathered there are a lot of variables and different ways to express power. Unless you are technically savvy, you really can't decipher the truth from their published numbers.

However, you can look for the most common way to report power, and use it to roughly compare products. I think most products will give you an X watts - 20-20k Hz - 2 channels - 8 ohms rating. When comparing product specs, make sure all their qualifiers are the same. If you see different qualifiers, know that maybe a comparison is not valid.

Also realize that power is not the only, and maybe not even the main parameter to consider. Your room size, listening preference, speaker sensitivity, and more are all factors that can make more difference than power.

 

[Swerd]

However, you can look for the most common way to report power, and use it to roughly compare products. I think most products will give you an X watts - 20-20k Hz - 2 channels - 8 ohms rating. When comparing product specs, make sure all their qualifiers are the same. If you see different qualifiers, know that maybe a comparison is not valid.

Adding to what herbu said, in the US, the Federal Trade Commission (FTC) has specific requirements for amplifier power ratings. That's why you see all amplifier power rated as a stated number of watts RMS, per channel while two channels are operated, at 20 Hz - 20 kHz, with THD at a stated % value, while driving an 8 Ohm load.

Any other ratings are up to the manufacturer and may or may not have similar qualifiers. PENG's example (above) of Emotiva amps rated at 8, 4, and 2 ohms shows how one company does this. But others can do what they choose, as long as they meet those FTC requirements.

I guess anyone shopping for an amp or receiver can't know what an amp can do while driving 4 Ohms unless the manufacturer measures & states it. If nothing is said, other than the required FTC ratings, you can assume the manufacturer intends to say nothing else.