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  • JFS
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How much power needed for stereo amplifier?
I am trying to determine how much power I need for a stereo integrative amplifier. I looked over the Internet and hear some argue for lower power like 10-50w, and some larger 100W+. I don't  have experience with a good stereo system, so am not sure what to think. I have heard that it is better to have an amp with lower power that is within clipping than a large amp running at low power. Is there any truth to this?

I am not sure what I will be getting for speakers, but I think most speakers are 85-90db sensitivity. I was thinking of small floor speakers or larger stand mounts. Will have to explore possibilities.

My listening area is about 10' away from speakers and 14'wide. The space behind me is open and goes into a small dining area and then a small kitchen. I like music at a comfortable level that sounds good.

I was thinking of around 50-100 watts depending on what I can find used. How much do I need to allow for peaks? I listen to rock with a good drum beat. Also like folk music and classical/jazz.

Would appreciate direction on what to look for.

I also want to use the headphone amp in the integrated amp. Does anyone know how much the impedance is common? I heard that it is around 100 or more ohms. Is that going to be a problem; do I need to get a headphone amp instead?

Joe

  • KozmoNaut
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Re: How much power needed for stereo amplifier?
Reply #1
How much power you need, depends on how loudly you want to play. For the vast majority of people 50w is more than plenty and 100w is enough to allow for all the headroom and peaks you could want. Remember that you're probably looking at the continuous power rating (give or take some marketing fluff) and that amps can deliver much more power than that for small bursts.

For instance, I used an NAD C320BEE for a while. Rated at 50w continuous into 8 ohms, it could also momentarily deliver 110w into 8 ohms or 210w into 2 ohms (a lot of speakers dip in impedance at low frequencies), and a lot of amps have similar performance.

So any decent 50w amp really should be more than adequate, unless you want to play stupidly loud, discotheque levels.

As for headphone sections in amplifiers, that varies wildly. Some are quite good, others are more of an afterthought. It's hard to give a rock solid rule to go by, other than to try and find the lowest output impedance. Hell, even the fact that an amp maker bothers to list the headphone output impedance and available power is a decent indicator that the headphone output is of reasonable quality.

  • knik
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  • Developer
Re: How much power needed for stereo amplifier?
Reply #2
How much power you need, depends on how loudly you want to play.
Exactly, I actually measured it once and apparently my comfortable level that sounds good is about 2x0.1 watts peak. 100W would be much more than enough.

  • mmrkaic
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Re: How much power needed for stereo amplifier?
Reply #3
How much power you need, depends on how loudly you want to play.
Exactly, I actually measured it once and apparently my comfortable level that sounds good is about 2x0.1 watts peak. 100W would be much more than enough.


Cool. How did you measure it? I mean, speakers have complex impedances and cause phase shifts— so did you integrate the instantaneous power or did you use some other approach. I have done some simplified measurements and got similarly tiny power numbers. But I wonder if you did it exactly.

  • knik
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  • Developer
Re: How much power needed for stereo amplifier?
Reply #4
You don't really need any complex measurement, it's more about estimation:
Set suitable volume then play 1kHz full scale, measure RMS voltage.
Using nominal impedance is probably accurate enough (it's measured @1kHz, isn't it).

Edit: When phase is shifted the power would be reduced so when measuring peak power why not just assume resistive load.
  • Last Edit: 05 December, 2017, 12:59:43 PM by knik

  • mmrkaic
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Re: How much power needed for stereo amplifier?
Reply #5
You don't really need any complex measurement, it's more about estimation:
Set suitable volume then play 1kHz full scale, measure RMS voltage.
Using nominal impedance is probably accurate enough (it's measured @1kHz, isn't it).


Your suggestion would give a useful estimate, but I would like to be more precise for two reasons. First, there is a phase shift between the voltage across the speaker and the current through it. So, you need to know the phase angle to estimate the power, even if you focus only on 1kHz. Second, music has a lot of spectral content below and above 1kHz. Since the phase angle and impedance are frequency dependent, using the 1kHz values would not be too precise.

  • DVDdoug
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Re: How much power needed for stereo amplifier?
Reply #6
Quote
Your suggestion would give a useful estimate, but I would like to be more precise for two reasons...
Speakers are not measured/specified that way.    If your 8-Ohm speaker is rated for 90dB at 1W and 1M, that's based on the nominal rated impedance, with an applied voltage of 2.83V RMS.    The actual power consumption may be higher or lower as the impedance changes over the frequency range.  

And, amplifiers are rated for the nominal impedance. 





  • mmrkaic
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Re: How much power needed for stereo amplifier?
Reply #7
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Your suggestion would give a useful estimate, but I would like to be more precise for two reasons...
Speakers are not measured/specified that way.    If your 8-Ohm speaker is rated for 90dB at 1W and 1M, that's based on the nominal rated impedance, with an applied voltage of 2.83V RMS.    The actual power consumption may be higher or lower as the impedance changes over the frequency range.  

And, amplifiers are rated for the nominal impedance. 






That is exactly what I said. To calculate the average electric power absorbed in a speaker in the general case, you have to integrate I(t)*V(t)/T over an interval [0,T]. My question was how to do that integral. (Also, current sensing is not trivial in this case.)

  • DVDdoug
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Re: How much power needed for stereo amplifier?
Reply #8
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I am trying to determine how much power I need for a stereo integrative amplifier.
Integrated amplifiers, (and preamps, and power amps) are expensive specialty items.  You'll get more for you money with a receiver.  Receivers are mass-manufactured, mass-distributed, and there is fierce price competition.

Quote
I have heard that it is better to have an amp with lower power that is within clipping than a large amp running at low power. Is there any truth to this?
No, the only downside to "too much power" is if your kids (or a drunk person) cranks-up the volume and blows your speakers!   

I am not sure what I will be getting for speakers, but I think most speakers are 85-90db sensitivity. I was thinking of small floor speakers or larger stand mounts. Will have to explore possibilities.

Quote
I also want to use the headphone amp in the integrated amp. Does anyone know how much the impedance is common? I heard that it is around 100 or more ohms. Is that going to be a problem; do I need to get a headphone amp instead?
That's also hard to answer.    You might not know until you try it with your headphones.  Even then, if you don't like the sound you may no know if it's the headphones or the amp.      Some headphones have a mid-bass bump in their impedance and with a high-impedance output that turns-into a mid-bass bump in the frequency response.  You might actually like  the way they sound with a higher-impedance amp.  ;)

Manufactures don't usually publish the source impedance...  If there's a spec at all, it's usually a (almost meaningless) headphone load recommendation...    It's the same with amplifiers - An amplifier rated at 100W at 8-Ohms doesn't have an 8-Ohm output impedance, it's rated for use with 8-Ohm speakers.

  • DVDdoug
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Re: How much power needed for stereo amplifier?
Reply #9
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That is exactly what I said. To calculate the average electric power absorbed in a speaker in the general case,
It's a speaker not a heater...  Nobody cares about the "power absorbed".    We want to know how much sound  we get out when we connect a 100W amplifier and we don't care if we're getting exactly 100W.  And, we want to know if we are going to fry the speaker with a 100W amplifier.    

  • Last Edit: 05 December, 2017, 04:36:30 PM by DVDdoug

  • mmrkaic
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Re: How much power needed for stereo amplifier?
Reply #10
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That is exactly what I said. To calculate the average electric power absorbed in a speaker in the general case,
It's a speaker not a heater...  Nobody cares about the "power absorbed".    We want to know how much sound  we get out when we connect a 100W amplifier and we don't care if we're getting exactly 100W.  And, we want to know if we are going to fry the speaker with a 100W amplifier.    



Who is we? I'm impressed, you seem to speak for multitudes.

Well, I care about the absorbed electric power, because I'm building an amplifier. The mechanical power that goes into sound is a few percent of the absorbed electric power. An interesting number, but I don't care about that.

So, yeah, thank you and all those you represent for your "constructive" help.

  • JFS
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Re: How much power needed for stereo amplifier?
Reply #11
Thanks for the replies. Looks like 50-100w will be good. I guess I'll have to try the headphone amp to see. Then maybe try a dedicated amp to see if it makes a difference. I guess the amount needed for peaks will depend on the music source.

  • JFS
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Re: How much power needed for stereo amplifier?
Reply #12
I have a question about high current amplifiers. What makes an amplifier put more power into lower impedance? Why doesn't an amp put more current into a speaker of lower impedance out of its power supply and just run within its power specification, and have less overall volume?

  • lithopsian
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Re: How much power needed for stereo amplifier?
Reply #13
Quote
I have a question about high current amplifiers. What makes an amplifier put more power into lower impedance? Why doesn't an amp put more current into a speaker of lower impedance out of its power supply and just run within its power specification, and have less overall volume?
More current needs fatter wires and other components that can dissipate or not create that much heat at high current, but (for the same power) less voltage.  Completely different design parameters than a higher-voltage lower-current device.  Some amplifiers can provide the current, others can't.  Trying to extract more current than an amplifier is designed for risks overheating and damage, effectively you're short-circuiting it.  In some cases, an amp will cope by refusing to supply the current, in other cases bad things will happen.

For the OP, you need 38.475W.