Power amplifier sensitivity question....

Michael John

Junior
Jun 25, 2011
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Sydney, Australia
eclipseaudio.com
Hi,

I've been looking at various power amplifier specs and I'm wondering why amp manufacturers quote really large maximum input levels?

Here's an example using manufacturer supplied numbers for a QSC PL325:

The rated input sensitivity is 1.2 Vrms (approximately +4 dBu).
The rated amp gain is 34.5 dB
Therefore for a sine wave @ 1.2 Vrms, the output voltage will be 63.7 Vrms or 90 Vpeak.
63.7 Vrms into 8 ohms is approximately 507 Watts RMS. This matches the QSC quoted number of 500W into 8 ohms (EIA 1 kHz 1% THD).

Now here's the problem.

For a 1.2 Vrms input sensitivity, QSC states a maximum input voltage of 11 V. (I assume peak.)
Using the rated amp gain of 34.5 dB, this input peak equates to an output voltage of 584 Vpeak.
Clearly the amplifier's output cannot go this high.

This is a class H amp and I guestimate that the maximum output voltage is a little above the 90 Vpeak from above, but nowhere near 584 Vpeak.

In reality, the amp will clip with input signals only a fraction larger than the 1.2 Vrms input sensitivity. So why the maximum rated input of 11 volts?

Is amp output clipping somehow preferred over the input circuit clipping?

[These calculations are for the 1.2V input setting, but the questions still arise for the 32 dB and 26 dB settings.]

Best,
Michael
 
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Re: Power amplifier sensitivity question....

The reason is that the two specs are kinda-but not always related.

The maximum input is the point at which the INPUT stage will clip.

The output wattage is the maximum output "power" (voltage actually)

The gain is the gain of the amp.

The input voltage required is ASSUMING that the level controls on the amp are run full up. If you turn the levels down, then the overal gain of the amp (from input to output) is less-so you have to drive it harder to be able to get the full output. The maximum input is the most you can drive it with-before the input stage clips.
 
Re: Power amplifier sensitivity question....

Thanks Ivan. My point is that the amp output will clip long before the input stage ever clips, so why does the input stage need so much headroom?

(Yes, I understand that turning the amp down allows me to drive the amp harder. In my case I get far better signal to noise because my Lake Contours can drive 20 dBu or higher on output. I typically run my QSC PLX series at -10 dB on the level controls and adjust the Lake limiters accordingly.)

Back to the QSC example I mentioned. This amp has two other sensitivity or "attenuated" settings/modes. These effectively change the amp gain from 34.5 dB to 32dB and 26 dB. The maximum input level changes from 11 V to 14.6 V and 25 V respectively. (The input sensitivity changes from 1.2 Vrms to 1.6 Vrms and 3.28 Vrms.) So from all these numbers, regardless of the input sensitivity setting/mode, the amp's output will clip long before the maximum input levels are ever reached.

Am I missing something?

Best,
Michael
 
Re: Power amplifier sensitivity question....

Hi Michael:

One thing's for sure - if I lived in the capital of New South Wales - I'd figure out a way to get to know Dr Thiele. :)

---

On the amp input clipping thresholds, the ultimate goal QSC has in mind is that their amps sound good even when used by the knowledge-challenged. If you can arrange this kind of forgiveness in your product it will sell better.

The amp's built-in limiters save the end user from himself by clamping down on the input voltage once it exceeds the point where the amp would start converting the music into square waves. Thus you have a heavily compressed result, but it sure beats clipping of either the input or output of the amplifier.

Even if you "disable" the clip limiter switches on the back of that amp I bet it'll still limit clipping, but do so with more of a hard knee.
 
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Re: Power amplifier sensitivity question....

it might help to think of it in reverse.. if you put a 10V signal into the amp it will obviously clip.. but then if you start backing off the attenuators on the front of the amp there will be a point where it isn't clipping anymore right? (might be one notch above -inf but there will be a point) at this point, it will sound fine, whereas another amp with a lower max input voltage will still sound distorted even if the amp's output isn't running at full power (because the first chip in the signal chain is clipping)

As Langston said, it's to compensate for people who might have terrible gain structure.. it's not enough to say you shouldn't be pushing past +4 so the amp clips at +5 regardless of where the controls are set. so why not err on the side of caution and leave more headroom?

Jason
 
Re: Power amplifier sensitivity question....

I'd also like to mention that if the amp has any built-in DSP or crossover that this is where it is especially helpful. If the input signal is full range and the output signal is just some portion of full range, the output signal will be a much lower voltage than the input signal just by the nature of it being less of the audio spectrum.

So in many cases, DSP amplifiers have to handle much higher input levels so their outputs can still be driven to full power. I know this is a serious issue with Itechs and XTis, as in many cases the input will clip first no matter what, and makeup gain has to be applied in the DSP to compensate.
 
Re: Power amplifier sensitivity question....

Hi,

I've been looking at various power amplifier specs and I'm wondering why amp manufacturers quote really large maximum input levels?
To be compatible with typical sound sources.
Here's an example using manufacturer supplied numbers for a QSC PL325:

The rated input sensitivity is 1.2 Vrms (approximately +4 dBu).
The rated amp gain is 34.5 dB
Therefore for a sine wave @ 1.2 Vrms, the output voltage will be 63.7 Vrms or 90 Vpeak.
63.7 Vrms into 8 ohms is approximately 507 Watts RMS. This matches the QSC quoted number of 500W into 8 ohms (EIA 1 kHz 1% THD).

Now here's the problem.

For a 1.2 Vrms input sensitivity, QSC states a maximum input voltage of 11 V. (I assume peak.)
Using the rated amp gain of 34.5 dB, this input peak equates to an output voltage of 584 Vpeak.
Clearly the amplifier's output cannot go this high.
true
This is a class H amp and I guestimate that the maximum output voltage is a little above the 90 Vpeak from above, but nowhere near 584 Vpeak.

In reality, the amp will clip with input signals only a fraction larger than the 1.2 Vrms input sensitivity. So why the maximum rated input of 11 volts?
So the controls on the front of the amp can do something.
Is amp output clipping somehow preferred over the input circuit clipping?
In fact yes.

Amplifiers generally provide internal clip limiters that gracefully reduce gain when clipping is detected.

If you clip before you reach the amp, you can get both the clipping distortion and not realize full output power.
[These calculations are for the 1.2V input setting, but the questions still arise for the 32 dB and 26 dB settings.]

Best,
Michael

Amplifier configuration has evolved this way because it is what many customers want. Variable level/gain controls on power amps cause as many problems as they solve (IMO), but the customer is always right, so they get what they want.

JR
 
Re: Power amplifier sensitivity question....

Hi Langston,

Your comment about built in limiters got me thinking and I looked up schematics for QSC and other amps. I think I've now have an answer.

On the schematics I've seen, the balanced input circuitry - typically a couple of op-amps - comes BEFORE the level control (or pot/attenuation). And so the input circuitry needs to have a fairly wide input range, in order that it doesn't clip when the user has the level control significantly below 0dB (to optimise gain structure and/or SNR) and is driving the input harder. (Some schematics show the thermal and clip limiting feedback to this input circuitry also.)

I had mistakenly assumed the level controls came first, (like on many consumer products where there's just a pot acting as a voltage divider on the input signal).

Thanks!
 
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Re: Power amplifier sensitivity question....

Looking then at how to set peak limiters in the speaker processor....

There's effectively two sources of amp clipping - the input circuitry and the output stage. The maximum input level is often given in the amp specs, but the maximum output voltage isn't. It often has to be inferred from the rated maximum power output numbers and then factored back through the amp gain and attenuation level setting to get an input voltage that will clip the output.

(The peak limiter then can be set as the minimum of the maximum input level and the input voltage that will clip the output. And it may need to be reduced further taking into account the driver excursion limit.)

It would great if amp manufacturers simply quoted maximum output voltages. (Lab Gruppen do - the PLM 10k and 20k state 153 volts.)

Best,
 
Re: Power amplifier sensitivity question....

You can generally ignore input stage clipping in power amps as a potential problem since competent amp designs accommodate typical line level signals.

Back in the day, there were amps with the attenuator in the very front end before any active electronics, that could literally accept speaker level signals as an input source and pad them down to use without any distortion, but that topology was unbalanced so dropped in favor of modern low cost active electronic balanced inputs.

I'm not sure we need even more ways to specify power amplifiers while they literally are voltage amplifiers so specifying output voltage is not that crazy... probably less crazy than input sensitivity, while changing that now would just irritate all the people who worked to figure out how to use the existing specs. :)

JR
 
Just curious, As i am just starting to look at a new power amp., to join my Benchmark LA-4 line amp. I read what I thought was a very intersting article from an electrical engineer. He stated a great many points, many of which were abve my pay grade. However, he finished with offering a few important points when buying an a power amp. First was relaibility, second, buy as much power as your speakers can handle, third, look for an amp with a low sensitivity rating, preferably .7 to 1v, and never exceed 1.4 or 1.5v. Comments please. Robert TN
 
Just curious, As i am just starting to look at a new power amp., to join my Benchmark LA-4 line amp. I read what I thought was a very intersting article from an electrical engineer. He stated a great many points, many of which were abve my pay grade. However, he finished with offering a few important points when buying an a power amp. First was relaibility, second, buy as much power as your speakers can handle, third, look for an amp with a low sensitivity rating, preferably .7 to 1v, and never exceed 1.4 or 1.5v. Comments please. Robert TN
You were lied to, basically. What kind of bullshit article was this, anyway?