How Do You Know if Your Amp Input Is Too High

True Line Level Signals Can Clip the Power Amplifier Input Phase

Sound is a mature engineering, and older than about. Its roots go all the mode back to the telegraph. This would lead one to believe that the field would be highly standardized and essentially plug-and-play past now. Unfortunately, this is not e'er the case. Here is an example.

Line Level Sound

Ask virtually audio practitioners for the definition of "line level" audio and they will say "+4 dBu" which is ane.23 Vrms. That is a defensible answer and it is why many mixers and DSPs use this (or a shut value) as their "nominal" input and/or output level. It's besides why most professional power amplifiers use one.2 Vrms (or a close value) as their input sensitivity, equally it volition drive the amplifier to its full output voltage. Feed it a higher voltage and the amplifier clips.

Signal Peaks

Unfortunately, in that location is a xx dB discrepancy in this assumption. The +four dBu audio signal from the mixer and DSP has signal peaks that exceed the nominal level by twenty dB or more. This is why the mixer and other professional person line level components have maximum output levels at or nearly +24 dBu when signal peaks are considered.

In a typical audio arrangement the mixer is operated at +4 dBu, with xx dB of "peak room" making the maximum output level +24 dBu. Nigh DSPs can handle this level without clipping. Virtually default to "unity gain" which means that the output level is the same as the input level. A problem shows up when we get to the power amplifier.  If its input sensitivity is +4 dBu, then the signal level from the mixer (or DSP) is xx dB higher than what is required to drive the amplifier to its full output voltage. "No problem" you say, because the amplifier'south input tin can be desensitized by reducing the input attenuator by twenty dB – a "standard" gain construction practise. Now +24 dBu input (the level of the signal peaks) will cause the amplifier to attain clipping, and all is skilful. Or non.

Most assume that reducing the amplifier's sensitivity past 20 dB allows the amplifier to handle 20 dB more input voltage. That'southward how information technology should work, and information technology used to exist a common do. This is no longer necessarily the instance, and information technology can lead to serious gain structure issues in modern systems. An increasing number of ability amplifiers have input stages that clip at or near +iv dBu, regardless of the sensitivity control setting. And so, even if you lot plow it downward to avoid output stage clipping, the input stage is overdriven by a true line level signal, 1 whose signal peaks approach +24 dBu.

Remedies

There are several possible remedies. I'll start with the best solution and work downwardly.

• I'll give a shout out to Bose for designing amplifiers that accept switchable input sensitivity values of 0, four, 12, and 24 dBu, with a max input voltage of +24 dBu. The PM serial is one of the few ability amps I have tested whose input circuitry tin handle a full professional line level betoken.

Figure iv – The control software for the Bose PM Series provides user-selectable input sensitivity.

Figure 5 – The QSC PL Series provides detented input sensitivity controls. When reduced, the amplifier tin can handle college drive voltages.

• Passive attenuators tin can reduce both the indicate and noise at the output of a mixer or DSP, and let the amplifier to be operated at its maximum sensitivity setting. This is a good option from a gain construction point of view, only passive attenuators are a kludgy solution for large systems, and they tin can degrade the mutual mode rejection of the amplifier. None the less, I detect them quite useful for small systems and for powered loudspeakers.

Figure 6 – A passive attenuator can be used to reduce the drive level to the power amplifier.

• Since DSPs accept adjustable output levels, the required level reduction can be dialed in there. This has become common practice and it is perchance the best compromise. The downside is that the DSP'south noise floor is typically not reduced by the output level control, then you are compromising the signal-to-noise ratio (SNR) by reducing the output signal level. In a day where everyone is enamored by high dynamic range information technology seems strange to me to have such a hit on system performance. The compromise may not be noticed in many systems since the racket flooring of the signal is significantly higher than the dissonance floor of the DSP. In that case, the DSP output level can be adulterate without sacrificing the the SNR of the finished system. In effect, less than ideal SNR at the arrangement input is masking a problem that occurs subsequently in the point chain. Just, information technology remains a trouble for systems in quiet environments, such equally padded auditoriums, studios, and home theaters.

Figure vii – The Symetrix Prism has precisely adjustable output levels, assuasive the output voltage to be reduced to avert clipping the ability amp input. Note that reducing the output point level may besides reduce the signal-to-noise ratio.

• It is fairly mutual to find power amp input phase clipping voltages of 4 Vrms (+14 dBu). And so, if the input sensitivity is +4 dBu, the sensitivity control tin can be set at -x dB re. maximum sensitivity to make the amplifier clip when driven at +fourteen dBu. That'south a stride in the right direction, but it will even so be overdriven by x dB by a +24 dBu drive betoken. Then in that location's the problem that many contemporary amplifiers have "trim pot" sensitivity controls, so there is no visual reference for setting information technology to -10 dB. It can be set with a voltmeter or scope, just information technology will merely exist an approximation. And what if you lot have a rack full of these amplifiers? In many modern systems "maxing out" the amplifier's sensitivity controls is the only practical solution.

Figure eight – Trim pots can command the gain of the power amp, merely it may still be possible to overdrive the input circuit. Achieving the same trim setting for multiple amplifiers is tedious.

• Some audio systems practice not have a DSP between the mixer and amplifiers. Increasingly the house EQ is in the digital mixer. I just did training at a major theme park where this is their standard practice. Now nosotros have to throttle back the mixer to avoid clipping the amplifier inputs. Yes, it "works" but the SNR compromise is now in the mixer. Also, nosotros lose operating range in the mixer's output faders and meters and the system noise floor is audible to the audition.

Figure nine – The master faders of this Digico mixer must be reduced to avoid overdriving the power amps. Note that the output level must exist kept below -xx dBFS.

And finally we come to the "chief street" solution to the trouble. Run the DSP at "unity," max out the amplifiers, and gear up the firm SPL with the mixer's output fader. Yes, nosotros get audio, but this sacrifices SNR in both the mixer and the DSP, and applies maximum amplifier proceeds to a compromised signal.

Conclusion

Some volition think I'g being nerdy in pointing out this problem, and some practice not even come across it equally beingness a problem. We've gotten then used to non-optimal gain construction in sound systems that we not only live with it, nosotros expect it. That's fine. Everyone still gets paid. Merely anyone paying attending should be bothered past a trouble like this existing in a modern technical system. We have sound components that are intended to form a system with incompatible signal levels – mixers that output +24 dBu (or higher), and amplifiers that can't handle information technology. The fix is a laundry list of work-arounds that are needed to produce functioning sound systems.

It'southward no wonder that IT people observe audio confusing. pb

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Source: https://www.prosoundtraining.com/2018/08/09/audio-power-amplifier-maximum-input-voltage/

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