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

[Vasr]

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 only obfuscate.

How do you understand it with respect to a specific amps driving 8ohms or 4ohms speakers?

Informally, the input sensitivity is understood as the voltage required at the input to drive the amp to max stated power. So if 0db on your pre-amp is matched in output voltage to this, you get max power out of the amp at 0db volume setting. It is never that ideal in practice (unless you have gain control on the amp) but you try to get it somewhat in the ball park. Is this accurate?

But power at what load? Typically, the specified input sensitivity seems to work out in calculations to max power spec at 8ohms for the published gain. But never for 4ohms (unless the power is double for 4ohms not usually the case).

Power, gain and input sensitivity are related in a fixed equation. So, if max power depends on the load, one of gain or input sensitivity or both must also depend on the load for that equation to hold.

If it is input sensitivity (as understood earlier) that varies depending on load, then its use if you happen to have 4ohm speakers is limited unless you do your own calculations using the stated gain and max power at 4ohms. If so, why isn't the input sensitivity specified as for 8ohms in particular.

Can someone with the correct fundamentals to unravel these technical measures, what they mean and how it is to be used depending on whether you have 8ohms speakers or 4ohm speakers? Thanks.

 

[AnalogSteph]

You basically have this correct.

Input sensitivity is generally stated relative to rated (FTC) output into 8 ohms... because convention, I guess.

There isn't much more to it, really... as you say, you only ever need any two out of rated output, voltage gain and input sensitivity.

 

[waynel]

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 only obfuscate.

How do you understand it with respect to a specific amps driving 8ohms or 4ohms speakers?

Informally, the input sensitivity is understood as the voltage required at the input to drive the amp to max stated power. So if 0db on your pre-amp is matched in output voltage to this, you get max power out of the amp at 0db volume setting. It is never that ideal in practice (unless you have gain control on the amp) but you try to get it somewhat in the ball park. Is this accurate?

But power at what load? Typically, the specified input sensitivity seems to work out in calculations to max power spec at 8ohms for the published gain. But never for 4ohms (unless the power is double for 4ohms not usually the case).

Power, gain and input sensitivity are related in a fixed equation. So, if max power depends on the load, one of gain or input sensitivity or both must also depend on the load for that equation to hold.

If it is input sensitivity (as understood earlier) that varies depending on load, then its use if you happen to have 4ohm speakers is limited unless you do your own calculations using the stated gain and max power at 4ohms. If so, why isn't the input sensitivity specified as for 8ohms in particular.

Can someone with the correct fundamentals to unravel these technical measures, what they mean and how it is to be used depending on whether you have 8ohms speakers or 4ohm speakers? Thanks.

I think you are overcomplicating this.
Sensitivity : Voltage input needed to drive to maximum rated voltage output(which is used to rate the power )

 

[sergeauckland]

Input sensitivity is indeed by convention related to rated output in watts into 8 ohms. However, what this means is that it's related to the number of volts out, nothing to do with power. For example, 100 watts into 8 ohms is 28.3v. A good amplifier will give that many volts out also into a 4 ohm load, but then the power output is 200 watts. Sensitivity is the same, X volts in for Y volts out.

In practice, most amps give a little less than their 8 ohm volts into 4 ohms, but in good amps, this 'little less' should be perhaps 10% less.

Another way of thinking of this, is that as sensitivity is X volts in for Y volts out, that also defines the gain. Voltage gain should be the same whether for 8 ohms or 4 ohms (in practice a little less).

S.

 

[Mnyb]

Hmm , at specifi

I think you are overcomplicating this.
Sensitivity : Voltage input needed to drive to maximum rated voltage output

this !
Input voltage creates output voltage.
Load pulls current from said voltage according to its impedances.

 

[Mnyb]

Amp has two “modes”of overload , voltage clipping, it can’t produce the voltage needed to follow the input signal. And clips due to that .
Or it can try to produce the voltage but load is drawing so much current that you hit a current limit or the supply sags to much .

 

[waynel]

Maybe this will help your understanding:

Audio Power amplifiers are Voltage amplifiers (until they run out of current)

 

[Mnyb]

Power=volt*amps slightly simplified

 

[Vasr]

Thanks for the inputs.

I haven't seen this "industry secret" (!) convention considered or understood correctly in discussions here before. Why don't they qualify the input sensitivity as specified for 8ohms? 4ohms speakers are not that uncommon. If there is a reason to do this separately for power output...

I think understanding this correctly has a lot of implications not only on the amp discussions to match pre-amps with amps but also in the discussion of output capability of DACs, pre-amps or anything that is directly connected to amps these days.

For example, 2V/4V out is considered a necessary sign of virtue in anything that is to be connected to amps. But it is seldom mentioned that this is required only for 8ohm speakers (if at all and for many amps much less than that). If you have 4 ohm speakers, it will clip before it (and sometimes much before it). Very few amps if any are able to output twice the power into 4ohms compared to 8 ohms. So their input sensitivity for 4ohms will be less. This has an implication on the volume controls of what you attach to it and what it outputs, so you don't clip and so to usability and selection of matching pre-amps/DACs.

As a theoretical standard to have I can understand but from a practical perspective, the case for dinging equipment that does not develop the full 2V/4V seems not only wrong but misleads people into thinking that it is necessarily a good thing for an amp that they have regardless of speakers, even those that need the 2V/4V for 8ohm speakers.

My view is that one purchasing a separate amp for the long haul should prefer those that have a variable gain control (not many of them) and so can be calibrated to the 0db on the input device to develop full power so that even by accident you don't turn up the volume to beyond clipping point depending on what you have connected to the amp. Perhaps separate amps should be dinged for not providing that gain control instead.

 

[waynel]

Thanks for the inputs.

I haven't seen this "industry secret" (!) convention considered or understood correctly in discussions here before. Why don't they qualify the input sensitivity as specified for 8ohms? 4ohms speakers are not that uncommon. If there is a reason to do this separately for power output...

I think understanding this correctly has a lot of implications not only on the amp discussions to match pre-amps with amps but also in the discussion of output capability of DACs, pre-amps or anything that is directly connected to amps these days.

For example, 2V/4V out is considered a necessary sign of virtue in anything that is to be connected to amps. But it is seldom mentioned that this is required only for 8ohm speakers (if at all and for many amps much less than that). If you have 4 ohm speakers, it will clip before it (and sometimes much before it). Very few amps if any are able to output twice the power into 4ohms compared to 8 ohms. So their input sensitivity for 4ohms will be less. This has an implication on the volume controls of what you attach to it and what it outputs, so you don't clip and so to usability and selection of matching pre-amps/DACs.

As a theoretical standard to have I can understand but from a practical perspective, the case for dinging equipment that does not develop the full 2V/4V seems not only wrong but misleads people into thinking that it is necessarily a good thing for an amp that they have regardless of speakers, even those that need the 2V/4V for 8ohm speakers.

My view is that one purchasing a separate amp for the long haul should prefer those that have a variable gain control (not many of them) and so can be calibrated to the 0db on the input device to develop full power so that even by accident you don't turn up the volume to beyond clipping point depending on what you have connected to the amp. Perhaps separate amps should be dinged for not providing that gain control instead.

This doesn’t require 5 paragraphs of discussion/rationalization.
Sensitivity : Voltage input needed to drive to maximum rated voltage output(which is used to rate the power )

 

[Blumlein 88]

All of this is why looking at amplifier gain in db is much simpler and less confusing in the end.

If an amplifier has 26 db of gain then input voltage x 20 gives output voltage.

If you know the load, and your amps power output into that load, the rest is easy. Or if not easy at least not confusing.

Most modern consumer specs are by convention at 8 ohms unless stated otherwise.

 

[Zedly]

One thing that makes it more confusing is that there is no easy way to know the voltage that is being output from the pre-amp. For example, on my Denon receiver, the volume displays as either 0-98 or in dB as -79.5 - 18.0. Plus, I adjust the levels of the individual channels, which will also affect the voltages. So I have no idea what the output voltages are for any specific volume setting.

 

[waynel]

One thing that makes it more confusing is that there is no easy way to know the voltage that is being output from the pre-amp. For example, on my Denon receiver, the volume displays as either 0-98 or in dB as -79.5 - 18.0. Plus, I adjust the levels of the individual channels, which will also affect the voltages. So I have no idea what the output voltages are for any specific volume setting.

You don't have to know the voltage at each volume setting just make sure:
Maximum clean output voltage from preamp >= Amplifier input sensitivity and you are set.

 

[Vasr]

All of this is why looking at amplifier gain in db is much simpler and less confusing in the end.

If an amplifier has 26 db of gain then input voltage x 20 gives output voltage.

If you know the load, and your amps power output into that load, the rest is easy. Or if not easy at least not confusing.

Most modern consumer specs are by convention at 8 ohms unless stated otherwise.

That is an engineer nerd way of looking at it and so not confusing.

I don't think consumers should be doing math to see if their input DAC/pre-amp is fine with their amp. The correct general formula

Voltage Gain = 20* log(Vout/Vin)

is not very friendly for consumers.

Manufacture specs provide power at 8ohms and 4ohms. Why not do so for input sensitivity as well? Same reason should hold for both. Unless manufacturers give the 4ohm numbers just to put up a bigger number!

Problem for a consumer is that since the Power ratio of 8 ohms to 4 ohms is very amp dependent, blindly ensuring that their pre-amp can put out more than 8 ohm input sensitivity doesn't tell them that they cannot move their volume control beyond some unknown number and worse if they have 4 ohm speakers. And by selecting for sources that provide maximum voltage (as encouraged on this site), they would suffer more in terms of volume control limits.

In a way it is safer for consumer to undershoot than overshoot on output of the source as long as it is not too way-off and their use does not require every Watt of power from the amp. The full range of the volume control will be safe for sacrificing something that they would likely never really need in SPL.

This is one of the pain-points of buying separate DAC/Pre-amp and power amp that nobody wants to address (other than those that provide variable gain for their amps but that still puts a bit of burden on the consumer).

 

[waynel]

Manufacture specs provide power at 8ohms and 4ohms. Why not do so for input sensitivity as well? Same reason should hold for both. Unless manufacturers give the 4ohm numbers just to put up a bigger number!.

They do, The voltage input required to clip an amp into 4 ohms is the same OR LESS than the 8 ohm value, so a voltage sensitivity spec for 8 ohms is fine for 4 ohm speakers.

In a way it is safer for consumer to undershoot than overshoot on output of the source as long as it is not too way-off and their use does not require every Watt of power from the amp. The full range of the volume control will be safe for sacrificing something that they would likely never really need in SPL.

Many sources have user settable volume limits, that's what you want if you want to prevent someone from turning the system too loud and clipping the amp. Honestly, you are making problems out of non-problems here.

 

[Vasr]

This is what happens when you put engineers as product managers/marketeers.

 

[waynel]

This is what happens when you put engineers as product managers/marketeers.

You continue to expose your ignorance. Some advice for you: focus on increasing input and decreasing output.

 

[Vasr]

Once the current hardware-only dinosaurs in the electronics industry who are petrified of software become extinct as expected sooner or later, a smarter set of devices would be self-configuring:

Power amps self-configure to be aware of nominal impedance of their speaker connections. They attenuate the input automatically to be below the clipping voltage for the load if the source upstream is not smart-enabled.

If the source connected to the amp is smart-enabled, there would be a handshake between the amp and the source on connection to exchange parameters. The source unit would calibrate its output to be limited by the input sensitivity reported by the amp and calibrate its volume display within that range so 70% is always 70% of maximum output possible and there is no danger of clipping ever.

But it may require a Steve Jobs of audio to drag the audio engineers kicking and screaming like he did at Apple to build smarter designs or fire them.

 

[March Audio]

Once the current hardware-only dinosaurs in the electronics industry who are petrified of software become extinct as expected sooner or later, a smarter set of devices would be self-configuring:

Power amps self-configure to be aware of nominal impedance of their speaker connections. They attenuate the input automatically to be below the clipping voltage for the load if the source upstream is not smart-enabled.

If the source connected to the amp is smart-enabled, there would be a handshake between the amp and the source on connection to exchange parameters. The source unit would calibrate its output to be limited by the input sensitivity reported by the amp and calibrate its volume display within that range so 70% is always 70% of maximum output possible and there is no danger of clipping ever.

But it may require a Steve Jobs of audio to drag the audio engineers kicking and screaming like he did at Apple to build smarter designs or fire them.

Good way to make products far more complex and more costly than the need to be.

You are worrying about things that don't need to be worried about. When was the last time you came across an amp that was totally incompatible with a pre due to widely disperate outputs, and sensitivities?

Trust me, the engineers and designers have given thought to this.

 

[Vasr]

Good way to make products far more complex and more costly than the need to be.

You are worrying about things that don't need to be worried about. When was the last time you came across an amp that was totally incompatible with a pre due to widely disperate outputs, and sensitivities?

Trust me, the engineers and designers have given thought to this.

People using flip phones said the same thing about smartphones. Very few using flip phones now.

You are arguing by extremes. They don't need to be totally incompatible to be a problem. Just make it so that people don't have to think about it. The number of questions that keep propping up on the input sensitivity matching here and elsewhere indicate that it is not a non-issue for consumers. Otherwise, why even provide that number in the specs. As audio porn?

I will stop before you get into your self-confessed Doc Martin mode.