Power ratings not on RMS watts

[cheapmessiah]

I'm relatively new to this hobby, I play, or played, bass more or less with proper equipement but my last amp I bought was 10 years ago, and somehow I "forgot" about looking for RMS power ratings when looking for amps in headphones, which is the industry standard for musical instruments, or at least for good equipment companies.

I've been researching the hifiman he6/he6se and its amp matching suffering for users of this unit, and reading about amps like the 789 and the rest of the "high power" headphone amps sometimes not being enough came across someone asking if his amp would be enough, him having his amp rated for 5w@32ohm, because he didnt know if it was continuos power, and then it hit me, most companies arent disclosing the power ratings in RMS watts...

Is the ecosystem of this hobby so corporate ****** and user uninformed that noone is demanding for usable power ratings?

 

[Koeitje]

RMS power ratings are only part of it. What you need is enough power to handle the peaks and that is where most amplifiers struggle. 50 RMS with 200W peak is preferred over 75W RMS with 125W peak in a lot use cases for example. The peaks in music ask for 10-20dB more output, you aren't going to get that as RMS power. There are not many amplifiers that have 400W RMS in 4ohm for example.

 

[cheapmessiah]

I see your point, but still, if both values are necesary to understand performance, not disclosing them is not a good thing.

 

[DonH56]

Technically it is average power in watts that is the product of RMS voltage and RMS current. No such unit as "Wrms" despite its on-going proliferation in many data sheets (RMS * RMS does not yield RMS as a unit, and RMS^2 is not a valid unit for watts). Look for continuous or average power. Sometimes peak or dynamic power may also be specified for short-term peaks.

 

[pma]

Technically it is average power in watts that is the product of RMS voltage and RMS current. No such unit as "Wrms"

Exactly. Watts are watts, nothing like "rms power".

As Don has stated, P = Vrms*Irms, or Vrms^2/Rload, or Rload*Irms^2

 

[tomtoo]

RMS power ratings are only part of it. What you need is enough power to handle the peaks and that is where most amplifiers struggle. 50 RMS with 200W peak is preferred over 75W RMS with 125W peak in a lot use cases for example. The peaks in music ask for 10-20dB more output, you aren't going to get that as RMS power. There are not many amplifiers that have 400W RMS in 4ohm for example.

Whats a peak? 1ns,1ms,20ms,500ms,2s,5s ?

Peak power opens up a can of worms.

Peak power was introduced to sell cheap power supplys,

 

[DonH56]

Whats a peak? 1ns,1ms,20ms,500ms,2s,5s ?

Peak power opens up a can of worms.

The IHF peak ("dynamic") power definition uses a 20 ms tone burst.

 

[cheapmessiah]

Whats a peak? 1ns,1ms,20ms,500ms,2s,5s ?

Peak power opens up a can of worms.

and wouldnt the interval between peaks also be relevant?

 

[DonH56]

and wouldnt the interval between peaks also be relevant?

Of course, but I don't remember the dead time from the spec and am too busy/lazy to hunt for it. If you find it, post it.

I went through the multi-volume IHF set of texts (binders, actually, IIRC) and passed all the tests to become a licensed audio consultant way back around 1980-something but my career took a different path and I've forgotten much after 40 years or so. Senility...

 

[tomtoo]

The IHF peak ("dynamic") power definition uses a 20 ms tone burst.

What are 20ms on a long 30Hz sin synth bass?
Right nothing. Its sucked up in a eyeblink.

 

[DonH56]

What are 20ms on a long 30Hz sin synth bass?
Right nothing. Its sucked up in a eyeblink.

Just telling you what the spec was, don't shoot the messenger.

For the record, the lowest fundamental on a bass guitar is 42 Hz IIRC, though other instruments go significantly lower (piano to 27 Hz, organ to 16 Hz for a really big one, and of course percussives can reach single-digit Hz).

 

[DVDdoug]

For a headphone amp (which doesn't have to deliver much current) the peak & RMS are probably very-close and that's probably the RMS power.

The HIFIMAN headphones are rated in dB/mW but it's more common to see headphone amp output and headphone sensitivity in volts rather than in watts/milliwatts. That's because power depends on voltage and impedance, and headphone impedance varies quite a bit.

Most electronics is "constant voltage" which doesn't really mean constant. It means it's (mostly) independent of the load (within the specs). So your 5W headphone amp would likely put-out (about) the same voltage with 64-Ohm headphones and you'd get half the power. Into the 50-Ohm HIFIMANs you should get a bit over 3W. I calculated almost 120dB SPL at 3W.


----------------------------------------------
I have a spreadsheet for calculating wattage and another for dB...

The formula I use for Wattage is Power = Voltage squared/Impedance.
...That's derived from Power = Voltage x Current and Ohm's Law (Current = Voltage/Resistance). (And I substituted impedance for resistance.)

There are two formulas for dB.
For power it's dB =10 x log(P/Pref)
For voltage it's dB= 20 x log (V/Vref)

In your case the power reference is 1mW so 10log(3/0.001) = 34.8dB. Decibels are relative so we add that to 83.5dB SPL (from the HIFIMAN specs) to get 118dB SPL.



P.S.
RMS power ratings are supposed to be more "honest" and there are measurement standards but manufacturers still sometimes cheat!

 

[tomtoo]

Just telling you what the spec was, don't shoot the messenger.

For the record, the lowest fundamental on a bass guitar is 42 Hz IIRC, though other instruments go significantly lower (piano to 27 Hz, organ to 16 Hz for a really big one, and of course percussives can reach single-digit Hz).

Oh, iam sorry. My intention was not to shot the messanger. Iam happy with that information about 20ms. Just like to show the problems with peak powers. Easy to see that iam not a big fan of them.

 

[cheapmessiah]

For a headphone amp (which doesn't have to deliver much current) the peak & RMS are probably very-close and that's probably the RMS power.

The HIFIMAN headphones are rated in dB/mW but it's more common to see headphone amp output and headphone sensitivity in volts rather than in watts/milliwatts. That's because power depends on voltage and impedance, and headphone impedance varies quite a bit.

Most electronics is "constant voltage" which doesn't really mean constant. It means it's (mostly) independent of the load (within the specs). So your 5W headphone amp would likely put-out (about) the same voltage with 64-Ohm headphones and you'd get half the power. Into the 50-Ohm HIFIMANs you should get a bit over 3W. I calculated almost 120dB SPL at 3W.


----------------------------------------------
I have a spreadsheet for calculating wattage and another for dB...

The formula I use for Wattage is Power = Voltage squared/Impedance.
...That's derived from Power = Voltage x Current and Ohm's Law (Current = Voltage/Resistance). (And I substituted impedance for resistance.)

There are two formulas for dB.
For power it's dB =10 x log(P/Pref)
For voltage it's dB= 20 x log (V/Vref)

In your case the power reference is 1mW so 10log(3/0.001) = 34.8dB. Decibels are relative so we add that to 83.5dB SPL (from the HIFIMAN specs) to get 118dB SPL.



P.S.
RMS power ratings are supposed to be more "honest" and there are measurement standards but manufacturers still sometimes cheat!

Arent ortho-dynamic transducers constant impedance regardless of frequency? I've read that but havent fact checked it.

 

[Koeitje]

Whats a peak? 1ns,1ms,20ms,500ms,2s,5s ?

Peak power opens up a can of worms.

Peak power was introduced to sell cheap power supplys,

Depends on the amplifier design. For Hypex and Purifi it is a couple of minutes. Because those are thermally limited in their output, while others might be limited by distortion or power supply.

If peak power specifications are there to sell cheap power supplies, then RMS was introduced to sell amplifiers without headroom. Nobody generates 30 minutes of a constant sine tone with their amplifier. RMS alone is simply not representative of an amplifiers power output.

 

[somebodyelse]

Arent ortho-dynamic transducers constant impedance regardless of frequency? I've read that but havent fact checked it.

Usually, but there have been exceptions so best to check.

 

[mhardy6647]

Exactly. Watts are watts, nothing like "rms power".

As Don has stated, P = Vrms*Irms, or Vrms^2/Rload, or Rload*Irms^2

I think we can blame the US Federal Trade Commission (FTC) and its regulations (originally promulgated in 1974) for the "RMS Watt"

 

[mhardy6647]

Arent ortho-dynamic transducers constant impedance regardless of frequency? I've read that but havent fact checked it.

some planar transducers are essentially resistive loads -- the problem comes, as I understand it, when a crossover's also involved (e.g., some Magnepans/Magneplanars).

source: https://www.stereophile.com/content/magnepan-magneplanar-mg16qr-loudspeaker-measurements

 

[pma]

I think we can blame the US Federal Trade Commission (FTC) and its regulations (originally promulgated in 1974) for the "RMS Watt"

If they wrote "power for sine wave" it would be OK. RMS Watts is a nonsense, you cannot make RMS on a variable that is already in RMS.

Hi Fi Writer - Meaningless RMS power

 

[peniku8]

If they wrote "power for sine wave" it would be OK. RMS Watts is a nonsense, you cannot make RMS on a variable that is already in RMS.

Hi Fi Writer - Meaningless RMS power

"Power derived from a signal's RMS voltage on a given resistance" add "signal with a crest factor of 3db" if you like

Just to add my personal thought about this topic: most power in music reproduction is needed for bass. Bass drivers (Woofers, Subwoofers) can swallow extreme powers spikes without exploding (modern high performance PA drivers can be hit with 5KW for a short burst with no damage to the woofer), but will only be able to safely take around a tenth of that power on the long term. So why bother with an amp that does the same burst power as it does sustained power? That amp is just not optimized for the task imo.
But I agree, power ratings have been abused and I agree that something like 5ms burst power 4x as high as its sustained power is hardly useful for fullrange speakers. Use it on tweeters in an active crossover setup and it'll do just fine, but for bass I'd like to have at least a few cycles of the lowest frequency I'm expecting to send to the system at full scale (100ms and more is nice).

Here is a random example of one of my (PA) amps which I've tested, which exhibits nice burst to long term power behaviour:

Please note that the drop-off times will halve when the amp is driving two channels, plus the sustained power will also halve (so it'll likely do 7KW per channel for 130ms and then drop to 2KW at 400ms).