Relationship between input voltage and output power?

[Audiofire]

What is the right interpretation of input sensitivity for an amp?

This came out of a discussion in another thread between @peng and I and a subsequent PM exchange. We agreed that this was a confusing issue. Assume just unbalanced inputs for the following since the unbalanced vs balanced spec is unrelated to this fundamental question. Bringing that in will...

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[Endymion Joshi-Godrez]

Sort of, but not quite, as DonH56 said:

Max input could be where input or output clipping occurs, i.e. the voltage excursion limits of the input (or output) stage.
So "maximum output voltage" also need the test condition to be specified, such as "at 0.05% THD+N", otherwise, like Don said, "where ... clipping occurs.." would be a reasonable assumed condition.

Topping did not say that explicitly, but for "ball park" purposes, yes, that would be a reasonable assumption.

So, I think DonH56 answered your questions already, if not, feel free to ask again.

I realise that now. The THD must be the same or the input sensitivity measuments would be weighted for a given gain . So maximum output voltage could just mean that this is the hottest input the amp will allow, NOT the input that is required to give you the maximum output voltage at the highest gain. If there is enough gain, the voltage I would need to achieve the same volume could be considerably lower. Its a shame only topping show the input sensitivity of all gain settings.

 

[staticV3]

I realise that now. The THD must be the same or the input sensitivity measuments would be weighted for a given gain . So maximum output voltage could just mean that this is the hottest input the amp will allow, NOT the input that is required to give you the maximum output voltage at the highest gain. If there is enough gain, the voltage I would need to achieve the same volume could be considerably lower. Its a shame only topping show the input sensitivity of all gain settings.

Topping's way of specifying input sensitivity is somewhat useless, as you can just calculate it from maximum output power and gain.

More interesting IMO is the maximum input voltage, so the highest voltage before the input clips. Unfortunately barely any manufacturer specifies it, and barely any reviewer measures it.

 

[Joesax]

Interesting I was asking the same question in another thread. In my case

The Benchmark AHB2 amp has variable gain. At a gain of 17db the input sensitivity is 4v to drive rated power.

I like to run my DAC directly into the amp with the DAC set to its low output impedance of 150 Ohms. This nets an output voltage of 1.75v.

It sounds fine and is the best combination I’ve tried but I was curious to know exactly how much power? Watts? I’m sacrificing with this combination of sensitivity/output voltage.

So I can only drive the amp to 25% of its rated power?

 

[Elitzur–Vaidman]

So I can only drive the amp to 25% of its rated power?

Slightly less than 25%, but yes. I wouldn't worry about it unless you find yourself needing more volume. And even then a 4V input would only net you an additional ~7db of headroom

 

[Joesax]

Ok then a non issue

 

[staticV3]

Interesting I was asking the same question in another thread. In my case

The Benchmark AHB2 amp has variable gain. At a gain of 17db the input sensitivity is 4v to drive rated power.

I like to run my DAC directly into the amp with the DAC set to its low output impedance of 150 Ohms. This nets an output voltage of 1.75v.

It sounds fine and is the best combination I’ve tried but I was curious to know exactly how much power? Watts? I’m sacrificing with this combination of sensitivity/output voltage.

So I can only drive the amp to 25% of its rated power?

With the AHB2 set to High gain, 1.75V input is enough for 100W at 4Ω or 50W at 8Ω.

At Medium gain, you'll get 55W at 4Ω and 27W at 8Ω.

 

[Joesax]

I guess I can go back to the High Impedance setting on the DAC which will give me 3.57v output into the 4v amp sensitivity. I’ll be upgrading to a different amp next month that has a several gain settings: 28db, 25db, 22db and 16db. The highest gain setting has a sensitivity of 1.37v. I’ll have to try them all. So many combinations.

 

[staticV3]

I guess I can go back to the High Impedance setting on the DAC which will give me 3.57v output into the 4v amp sensitivity. I’ll be upgrading to a different amp next month that has a several gain settings: 28db, 25db, 22db and 16db. The highest gain setting has a sensitivity of 1.37v. I’ll have to try them all. So many combinations.

Usually, the highest DAC output and lowest Amp gain will give you the cleanest signal.

 

[Joesax]

Related question. If I’m driving the amp to 27w at 8 ohms I assume what happens when there is a large dynamic peak in music the amp has access to the power it needs to reproduce that peak? If so then 27w is plenty for normal listening as long as the amp has power in reserve to handle dynamic peaks while being driven to 27w.

 

[staticV3]

Related question. If I’m driving the amp to 27w at 8 ohms I assume what happens when there is a large dynamic peak in music the amp has access to the power it needs to reproduce that peak? If so then 27w is plenty for normal listening as long as the amp has power in reserve to handle dynamic peaks while being driven to 27w.

If Medium gain at 1.75V input is loud enough for you, then that means that all dynamic peaks in your music are currently below 27W in amplitude.

 

[Joesax]

Ok. Here’s another way I’m understanding it

I’m listening to the amp at mid gain (17db) and driving it with the DAC’s 1.75 v. I’m listening to a piano recording at the volume resulting from the full output voltage and the resulting 27w. My volume control is full up. Then a full dynamic orchestra crescendo happens. I would think that the amp can deliver move watts to handle that dynamic aspect of the recording since it does have 100w before clipping. But can the DAC handle it? Or does it clip?

 

[IAtaman]

Ok. Here’s another way I’m understanding it

I’m listening to the amp at mid gain (17db) and driving it with the DAC’s 1.75 v. I’m listening to a piano recording at the volume resulting from the full output voltage and the resulting 27w. My volume control is full up. Then a full dynamic orchestra crescendo happens. I would think that the amp can deliver move watts to handle that dynamic aspect of the recording since it does have 100w before clipping. But can the DAC handle it? Or does it clip?

No. You are confusing the max with average, that is what staticV3 was trying to explain. If 1.75V is the input required for your amp to be able to deliver the peak dynamic range of the crescendo, the output of your DAC at relatively quiet piano sections would be much less than 1.75V.

 

[Joesax]

Ok thanks. Got it.

 

[wwjokerk]

I am still confused with all that numbers and formulas. It is always mentioned to match the gain of DAC/preamp with the gain of the amplifier.

Here we talk about output and input voltage. Isn`t the output voltage of the preamp the input voltage fed to the amplifier?

Can someone summarize in easy words and clear formulas, what the relationship is between output/input voltage, gain of amplifier, power of the amplifier and how that translates to volume levels reachable with a given speaker and its sensitivity?

Thanks
Kurt

 

[audio_tony]

I am still confused with all that numbers and formulas. It is always mentioned to match the gain of DAC/preamp with the gain of the amplifier.

Here we talk about output and input voltage. Isn`t the output voltage of the preamp the input voltage fed to the amplifier?

Can someone summarize in easy words and clear formulas, what the relationship is between output/input voltage, gain of amplifier, power of the amplifier and how that translates to volume levels reachable with a given speaker and its sensitivity?

Thanks
Kurt

No need for any formulas.

Put simply... A DAC will output 2 volts RMS - this is then fed to a preamp which has variable gain (volume control).

The preamp will then output a level determined by the position of the volume control - so it could be 2 volts in and 100 millivolts out (because the level has been attenuated by the volume control).

The power amp then 'sees' this 100 millivolts (mV) at it's input, and applies gain (both voltage and importantly current gain).

The power amp might have an input sensitivity of 1 volt for full output power, therefore 100 millivolts will provide a much reduced output level suitable for quiet listening.

Also note that the DAC will only produce it's maximum output of 2 volts RMS with a signal that represents maximum amplitude (0dBFS) - so the output voltage will vary according to the amplitude of the music.

You can visualise it as a chain held in your hand. The more you shake the chain (the input) the more the chain will jump around (output).

I've tried to explain this in the most simple way possible, without getting into dB / voltage gain calculations etc.

 

[wwjokerk]

Thanks for your explanations. I would really be interested in understanding the maths behind it.

If I set the DAC to pass through (similar to volume 100%) the real output voltage will be the maximum output voltage of the DAC right? Thus the preamp will attenuate this voltage to a smaller voltage fed to the amp. Depending on that smaller voltage the amp will generate a certain power depending on its gain?

I don't understand how voltage then transfers to watts by adding dB as gain

Thanks

 

[staticV3]

Isn`t the output voltage of the preamp the input voltage fed to the amplifier?

Correct.

Can someone summarize in easy words and clear formulas, what the relationship is between output/input voltage, gain of amplifier, power of the amplifier and how that translates to volume levels reachable with a given speaker and its sensitivity?

Vout=Vin*10^(Gain in dB/20)
Pout=Vout^2/Speaker impedance

If your speaker has a dB SPL (1m/2.38V) spec, then it's best to work purely with voltage.

First, find out how much voltage your Amp can output at the impedance of your speakers. Amir's power cube graphs are useful here:


You can convert W numbers back to V using
Vout=sqrt(Pout*speaker impedance)

Then, check if this voltage can even be reached with the Amp's gain and the input voltage that you can provide to it. Use the Vout formula from the start.

If you're unsure how much voltage your Preamp/TV/CD Player/etc can provide, you can use a basic multimeter and a 60Hz test tone to measure it.

The same can be used to measure your Amp's gain.

Gain in dB=20*log10(Vout/Vin)

If you have your output voltage and speaker sensitivity,
Max SPL=Sensitivity in dB (1m/2.38V)+20*log10(Amp output voltage/2.38)

That number is for a single speaker in an anechoic environment at 1m.

You gain 3dB on top from a second speaker (Stereo pair)

Another 3dB with the speakers placed near a wall, or 6dB with them placed in the corners of your room.

Finally, you lose SPL if your listening position is >1m from the speakers.

SPL loss in dB=6*log2(distance in m)

If your loudspeaker is specced with dB SPL (1m/1W) instead of 2.38V, then convert your Amp output voltage number from above into W using the Pout formula.

Max SPL (1m/1 speaker/anechoic)=Sensitivity in dB (1m/1W)+10*log10(Amp output power)

Then go through the modifiers again to account for listening distance, speaker pair etc.

Useful calculators:
http://www.hometheaterengineering.com/splcalculator.html
https://sengpielaudio.com/calculator-levelchange.htm

 

[wwjokerk]

Thanks a lot, now I have to go through your excellent explanation step by step - wonderful. THANKS