Amplifier Power Question

[bang4bucker]

the thing is even between 2 amps rated @100 watts may not have the same driving power against difficult load.

P (watt) = Voltage x I (current)

the amp who has more I (current) will be better at driving difficult load like speakers with low impedance at low frequencies and big negative phase angle (capacitive load). the bass is one who suck power the most.

say you have avr with 100 watts @ 6 ohms and a power amp 100 watts @ 4 ohms. your power amp will have more driving power against power hungry speakers. but like i mention before, it's the bass that need more current so if you put a high pass filter in your speakers and subwoofer is handling the bass, you may skip the power amp and leave it all to avr.

 

[killdozzer]

say you have avr with 100 watts @ 6 ohms and a power amp 100 watts @ 4 ohms.

Aaah, but this is again two different amps. The first one might demand 140 Watts for a 4 Ohm load. What if it was two 100Wpch into 8Ohms with satisfactory THD.

@slipperybidness hit the nail on the head but didn't elaborate:

You need HEADROOOM--you need the power on reserve to hit ~20dB+ peaks on the dynamic range!

Yes, this 'power on reserve' you speak of, I know you even build some amps, how is this power on reserve achieved. Other than just having a stronger amp, can one amp have more power on reserve than another with same basic specs?

 

[PENG]

the thing is even between 2 amps rated @100 watts may not have the same driving power against difficult load.

P (watt) = Voltage x I (current)

the amp who has more I (current) will be better at driving difficult load like speakers with low impedance at low frequencies and big negative phase angle (capacitive load). the bass is one who suck power the most.

say you have avr with 100 watts @ 6 ohms and a power amp 100 watts @ 4 ohms. your power amp will have more driving power against power hungry speakers. but like i mention before, it's the bass that need more current so if you put a high pass filter in your speakers and subwoofer is handling the bass, you may skip the power amp and leave it all to avr.

Sorry, don't mean to nitpick, but respectfully, I would like to mention the stated power formula is not correct for a reactive load such as typical loudspeakers that often have resistance and inductive and/or capacitive reactance. You obvious understand the principles involved, just that the way the formula stated would otherwise contradict your narrative.

 

[bang4bucker]

Aaah, but this is again two different amps. The first one might demand 140 Watts for a 4 Ohm load. What if it was two 100Wpch into 8Ohms with satisfactory THD.

might be. but less likely. there are amps which have lower power rating at lower impedance because less robust power supply like typical entry level avr's. and most entry level avr's are rated @ 6 ohms which i don't think is suited for challenging load.

as for 2 amps rated 100 watts @ 8 ohms : the only way to choose between them is to try them with specific speakers at specific volume setting. if they're both fine that means the speakers aren't demanding and/or both amps are robust enough for that kind of load. also, the THD measurements at different load can be very different compared to @ 8 ohms because speakers are not passive resistor used to measure amps.

@ PENG : that formula is the one that used as amps measurement because they use passive resistor. that's where they rate their amps. but we all know speakers are not passive resistors.

 

[killdozzer]

might be. but less likely. there are amps which have lower power rating at lower impedance because less robust power supply like typical entry level avr's. and most entry level avr's are rated @ 6 ohms which i don't think is suited for challenging load.

as for 2 amps rated 100 watts @ 8 ohms : the only way to choose between them is to try them with specific speakers at specific volume setting. if they're both fine that means the speakers aren't demanding and/or both amps are robust enough for that kind of load. also, the THD measurements at different load can be very different compared to @ 8 ohms because speakers are not passive resistor used to measure amps.

Not the answer to my question. I'm not trying to listen to two amps and decide which one to buy. I want to know what would make one amp 'cope better' with the speaker's load. Same speakers, two different amps of lower power - let's say 2x25Wpch into an 8Ohm load of a 90dB sensitivity speaker. Is it possible for one of them not to 'run out of power' at difficult transients and power demanding parts of a song? If it is possible, what would make it possible? What part of an amplifier can have this 'power on reserve' as @slipperybidness described it?

 

[ski2xblack]

Is it possible for one of them not to 'run out of power' at difficult transients and power demanding parts of a song? If it is possible, what would make it possible? What part of an amplifier can have this 'power on reserve' as @slipperybidness described it?

Possible? Yes. How? More capacitance in the power supply. A dual rail or adaptable supply rails (class G/H) is another approach.

Same speakers, two different amps of lower power - let's say 2x25Wpch into an 8Ohm load of a 90dB sensitivity speaker.

When good, clean power is cheap, why limit yourself to amps of only 25 watts? With power, too much is just enough.

 

[Irvrobinson]

Yes, this 'power on reserve' you speak of, I know you even build some amps, how is this power on reserve achieved. Other than just having a stronger amp, can one amp have more power on reserve than another with same basic specs?

Some amps have more conservative specifications than others. Some amp designs also have less tightly-regulated power supplies, which allows them to demonstrate significantly higher short-duration output levels (tens to hundreds of milliseconds) than they do in FTC testing. Also, some amps, like older NADs, are specifically designed to have higher short-duration "peak" output ratings.

 

[ski2xblack]

Those old NADs are an example of amps that use class G supplies (dual rail) and very conservative ratings.

 

[slipperybidness]

Sorry, don't mean to nitpick, but respectively, I would like to mention the stated power formula is not correct for a reactive load such as typical loudspeakers that often have resistance and inductive and/or capacitive reactance. You obvious understand the principles involved, just that the way the formula stated would otherwise contradict your narrative.

That formula is indeed correct, but it is only showing the real (i.e. resistive) part of the load. What you are hinting at but didn't quite state: The amp will also see the reactive load, thus we need to be looking at APPARENT POWER, which is the sqaure root of the squared sums of the real power and the reactive power.

But, that really complicates this discussion more than is necessary right now.

 

[slipperybidness]

Aaah, but this is again two different amps. The first one might demand 140 Watts for a 4 Ohm load. What if it was two 100Wpch into 8Ohms with satisfactory THD.

@slipperybidness hit the nail on the head but didn't elaborate:



Yes, this 'power on reserve' you speak of, I know you even build some amps, how is this power on reserve achieved. Other than just having a stronger amp, can one amp have more power on reserve than another with same basic specs?

The quick and easy answer is that you run the power rails at a higher voltage to get more power.

When you do that, you will need higher voltage caps, and bigger heatsinks, etc.

You need a stiffer power supply. Typically that means more filter capacitance, bigger transformers to avoid hysterisis and saturation, bigger heat sinks.

The power supply is one of the most critical parts of a good amp design.

 

[Dave Blount]

The quick and easy answer is that you run the power rails at a higher voltage to get more power.

When you do that, you will need higher voltage caps, and bigger heatsinks, etc.

You need a stiffer power supply. Typically that means more filter capacitance, bigger transformers to avoid hysterisis and saturation, bigger heat sinks.

The power supply is one of the most critical parts of a good amp design.

........ and those are the kind of amps that very often are the ones to exhibit the ability to 'dbl down', a quick way to usually tell if the given amp has 'balls' !

 

[ski2xblack]

Yes and no. Keep in mind what Irv said regarding regulated supplies vs. less well regulated supplies. The regulated supplies allow you to double down, but limits dynamic power to those figures, where an unregulated supply won't double down, but can provide brief, short bursts exceeding their continuous rated power. All that matters is that the amp can deliver the requisite power into the load presented, which a properly chosen amp of either type could achieve. Regulated power supplies add lots of parts and cost to an amp.

 

[slipperybidness]

Yes and no. Keep in mind what Irv said regarding regulated supplies vs. less well regulated supplies. The regulated supplies allow you to double down, but limits dynamic power to those figures, where an unregulated supply won't double down, but can provide brief, short bursts exceeding their continuous rated power. All that matters is that the amp can deliver the requisite power into the load presented, which a properly chosen amp of either type could achieve. Regulated power supplies add lots of parts and cost to an amp.

A TRUE Regulated Power Supply is a poor choice for an amp power supply! An unregulated linear power supply is by far the superior choice for a POWER AMP for many technical reasons!

Since you previously referenced info from Bob Cordell, I'm pretty sure that you know this (if not, then keep reading ).

But, I don't think we are talking about a true regulated power supply here, I think he was referring to "regulating effects" of a "stiff power supply".

Personally, I tend to reside in the camp of "the stiffer the power supply the better", even if that means less ability for transients. You have to make a design tradeoff/balance, as those are mutually exclusive.

 

[killdozzer]

When good, clean power is cheap, why limit yourself to amps of only 25 watts? With power, too much is just enough.

Exactly because power is much cheaper today, I'm trying to understand why I keep seeing these low power amps from good producers and costing pretty much. I wanted to know could there be another reason that justifies their price (and it seems it could) other than brand name? I also wanted to know can they do the job if constructed properly? I'm trying to understand how indicative is the e.g. 50Wpch into 8Ohms rating, even when conservative. Is there such an amp that won't go into clipping as easily even if it appears to putout low power.

Thank you all! You did answer my question. It IS possible to have an amp with lower power even for loud listening that performs well.

I just get the feeling that it is cheaper to buy 2x150Wpch and be safe, than it would be to buy 2x50 and wonder whether it is constructed properly.

 

[ski2xblack]

Chalk it up to the marketing forces that exploit postmodern baroque amplifier irrationality, perhaps? (Loved that post in the purple prose thread!)

 

[PENG]

That formula is indeed correct, but it is only showing the real (i.e. resistive) part of the load. What you are hinting at but didn't quite state: The amp will also see the reactive load, thus we need to be looking at APPARENT POWER, which is the sqaure root of the squared sums of the real power and the reactive power.

But, that really complicates this discussion more than is necessary right now.

I did say it was not correct for a reactive load. Of course it is right for a load that is a perfect resistor.

 

[PENG]

The quick and easy answer is that you run the power rails at a higher voltage to get more power.

When you do that, you will need higher voltage caps, and bigger heatsinks, etc.

You need a stiffer power supply. Typically that means more filter capacitance, bigger transformers to avoid hysterisis and saturation, bigger heat sinks.

The power supply is one of the most critical parts of a good amp design.

Now killdozzer's question is 99% answered, finally.

 

[Irvrobinson]

A TRUE Regulated Power Supply is a poor choice for an amp power supply! An unregulated linear power supply is by far the superior choice for a POWER AMP for many technical reasons!

Since you previously referenced info from Bob Cordell, I'm pretty sure that you know this (if not, then keep reading ).

But, I don't think we are talking about a true regulated power supply here, I think he was referring to "regulating effects" of a "stiff power supply".

Personally, I tend to reside in the camp of "the stiffer the power supply the better", even if that means less ability for transients. You have to make a design tradeoff/balance, as those are mutually exclusive.

In audio amplifiers, the term "regulated" is typically used to describe output stage and power supply circuitry which keeps the voltage on the output transistor rails relatively consistent regardless of load (meaning impedance) and mains voltage from the wall. I agree with slipperybidness, the increased level of power supply regulation being discussed is not to my knowledge using a closed-loop strategy like fully-regulated DC supplies used for lab work.

 

[mtrycrafts]

Killdozer, you have to account for losses due to distance, too, which in a typical room are typically ~4 db/m. ...

That works for distance from 1m to 2m, not to 3m and on. It is all about doubling the distance for that ~4dB: 1m, 2m, 4m, 8m, etc.

 

[killdozzer]

Great! Really great!

You see, Here people often talk about the importance of having substantial power in a well designed amp. Then again, other than being overpriced, I never heard one bad word about these high-end Japs like Accuphase:

Most of people seem to agree that these are not mere objects of desire and beauty, but really well engineered units. Now, the one in the picture has 2x30Wpch into 8 Ohms. THIS is why I asked all those questions. I wanted to know is it 'much ado 'bout nothing'.

The description says this:
In the output stage, power MOS-FETs renowned for their excellent frequency response, sonic performance, and superior reliability are used in a triple parallel configuration and driven in pure class A. This is sustained by the strong power supply featuring a massive transformer and two large 47,000μF filtering capacitors. Output power in each channel is rated for an impressive 150 watts into 1 ohm, 120 watts into 2 ohms, 60 watts into 4 ohms, and 30 watts into 8 ohms. Even speakers with ultralow impedance or drastic fluctuations in impedance curve can be driven reliably by this impressive amplifier.

OK, I see there's some 'sonic performance' talk, but you gotta have a wee bit of marketing. They also say that it could run difficult speakers. I guess it could.