Sensitivity, Gain, Power, Volume - Different Words For The Same Thing?

Sensitivity, Gain, Power, Volume - Different Words For The Same Thing?

Lookout, its soap box time again!  In the last issue of Production HUB I vented my frustration on the humble Y-combiner lead. A harmless looking device, regularly found in the audio guy’s bag-of-tricks, yet capable of causing serious problems if used incorrectly. This month I wish to tackle a topic with far greater destructive potential: Amplifier gain!

You may recall Bob our mythical contractor from the Y-combiner article. Well Bob has once again provided an excellent example of how NOT to setup a sound system, and the source material for this article. Thanks Bob!

Bob had a gig as house tech at a night club with a sizable sound system. The sound system was installed and commissioned by a contractor who, like all good contractors, set and locked down the system controllers to minimise the possibility of ‘fiddling’ by people or persons unknown. Bob believed that the system was not loud enough and convinced that the contractor must have set it up incorrectly, searched for ways to coax a couple of additional dB out of the system.

One night he noticed that all of the sensitivity switches on the back of the amplifiers were set to something called “26dB”. The sensitivity switch was a three position device labelled “0.775V-26dB-1.4V”. Bob didn’t fully understand the function of this switch, but couldn’t resist the temptation to play with it anyway. He quickly discovered that the amplifier produced a much louder signal when switched to the “0.775v” position. Eureka! He had finally found the reason the system was not loud enough. That stupid contractor must have set all of the sensitivity switches to the wrong position. He quickly changed the sensitivity switch on all of the amplifiers to “0.775v” and looked forward to hearing glowing reports of the system’s performance tomorrow night. 

But alas all did not go as expected the following night. Bob was not greeted with smiles and showered with praise. Nor did the club owner offer him a big pay rise. It seems the gig was a total disaster. The system sounded terrible and several drivers seem to have failed. So what went wrong?

In a nut shell, Bob stuffed up. He made changes without fully understanding the ramifications of those changes. He didn’t understand the difference between sensitivity, gain, power and volume, and simply assumed that louder must have been better. Let’s spend a couple of minutes to examine the differences and hopefully we’ll not end up in a similar situation to Bob.

Before I can properly explain the meaning of these terms I need to explain the basics of how an amplifier works. “An amplifier takes a small electrical signal and makes it bigger” – there, that was easy! Obviously it’s more complex than that. One of the most important specifications of an amplifier is the amount of “amplification” it provides. How much “bigger” it makes the input signal. This is called the amplifier GAIN and is usually expressed as a ratio. E.g. if an amplifier is designed to take an input signal of 1 volt and amplify it to an output signal of 50 volts, it would have a gain of 50. In other words the output signal (in volts) would be 50 times larger than the input signal. Amplifier gain is sometimes expressed in dB, in the example above the amplifier would have a gain of 34dB (dB=20xLog50).

But amplifier specification sheets and advertisements rave on about amplifier POWER ratings, measures in watts. How does this relate to GAIN?

Amplifiers are (generally) fixed gain devices. They always exhibit the same voltage gain. From this you can calculate the power the amplifier is capable of dissipating into a defined load. In our example above we said the amplifier had a voltage gain of 50. It produced an output signal of 50V from a 1V input. We can calculate the output power for various loads using ohms law (V2/R), thus 50V across an 8 ohm load = 502/8 = 312.5 watts, or 625W into a 4 ohm load.

Amplifier manufacturers usually back engineer the amplifier voltage gain such that it will achieve full output when driven by a “standard” level input signal. Common standards are 0.775V (0dBu), a common standard for mixing consoles and signal processors, and 1.4V (0VU) more common reference in professional broadcast systems. This is referred to on specification sheets as the SENSITIVITY of the amplifier. Obviously 0.77V is a lower input signal level than 1.4V and thus an amplifier designed with an input sensitivity of 0.77V will require greater voltage gain to achieve the same output as an amplifier with an input sensitivity of 1.4V. This does not mean that one has any more or less output power capacity than the other. It simply means they need to be driven with different input signal levels to achieve full output. 

It’s probably about time I used the obligatory car or water analogy to illustrate this concept. Imagine two identical cars. Same engine power, same top speed. If you were to adjust the throttle linkage on car B such that full throttle required very small movement of the accelerator pedal it would require less movement of your foot (input signal) to achieve full power. Car B could now be described as having more accelerator gain or greater sensitivity than Car A. However you have not changed the power capacity of Car B. It still has exactly the same power and top speed as car A.

OK, back to amplifiers. Some amplifiers incorporate sensitivity switches facilitating selection of the appropriate voltage gain required to match the desired input sensitivity. These are usually labelled similar to (you guessed it) “0.775V – 1.4V”. This hopefully explains two of the three positions on the sensitivity switch, but what about that “26dB” position? What’s that all about?

So far everything we have discussed could be described as FIXED SENSITIVITY. All amplifiers, regardless of output power are designed to operate with a set sensitivity. This means that amplifiers with higher power ratings require higher voltage gain. For example, if we use some amplifiers from the Crown Macro-Tech series as an example we would see that the higher powered MA5002VZ requires higher voltage gain (42dB) than the lower powered MA2402 (38dB) when operating in 0.775V sensitivity mode.

Amplifier Model Gain @ 0.775V Gain @ 1.4V Output Power, 8 ohms
Crown MA602 35dB 30dB 225W
Crown MA2402 38dB 33dB 520W
Crown MA5002VZ 42dB 37dB 1300W
Table 1

Depending on the make and model of the amplifier, this may be the only choice provided.  However fixed sensitivity operation does have drawbacks in some situations.

Imagine you had a two way PA system with the high frequency elements driven via an MA2402 and the low frequency drivers via an MA5002VZ. Now what would happen if, for whatever reason, you had to swap out the MA2402 driving the HF for another MA5002VZ? The MA5002VZ has 4dB more gain than the MA2402. Which means your HF elements will be louder, effecting the overall tuning of the system. Potentially more seriously, if you were using a system processor for limiting ahead of the amplifiers, you just added 4dB of gain into the HF section AFTER the limiters. You are now in danger of permanently damaging the HF elements if the system is driven to the point where it should start to limit. In this case it is the gain difference, not the additional power available in the MA5002VZ that is likely to cause damage.

From table 1 above it can be seen that the MA5002VZ has a voltage gain of 37dB when switched to 1.4V sensitivity, which is actually 1dB LOWER than the MA2402 in 0.775V mode. So a quick fix for the above scenario would be to run the replacement MA5002VZ now powering the HF elements in 1.4V sensitivity which would give a much closer match to the original MA2402 and ensure that the system limiters are still effective.

So what about the 26dB position??   OK, OK, I’m getting to it…

The third position is a FIXED GAIN position.  In this position all amplifiers, regardless of rated output power, operate with the same voltage gain (26dB). Thus the input level required to dive the amplifier to full power varies with amplifier power rating. Table 2 below illustrates this using our same example amplifiers.

Amplifier Model Gain @ 26dB Input Sensitivity Output Power, 8 ohms
Crown MA602 26dB 2.1V 225W
Crown MA2402 26dB 3.2V 520W
Crown MA5002VZ 26dB 5.1V 1300W
Table 2

Fixed gain operation is favoured by many professional audio loudspeaker manufacturers such as JBL Pro and L-Acoustics as it allows them to publish system tunning data that is independent of amplifier selection. The system operator is free to select any amplifier power rating (provided it meets minimum required power) they wish and as long as they operate in fixed gain mode the tunings will be valid and the limiter settings correct because ALL amplifier channels will be operating with the same, known gain.

Hopefully we’ve now adequately explained the concepts of amplifier gain and sensitivity. Time to revisit Bob and work out what happened to his system.

Let’s combine tables 1 and 2 above in order to see the range of amplifier gain settings available via the sensitivity switch.

Amplifier Model Gain @ 0.775V Gain @ 1.4V Gain @ 26dB Output Power, 8 ohms
Crown MA602 35dB 30dB 26dB 225W
Crown MA2402 38dB 33dB 26dB 520W
Crown MA5002VZ 42dB 37dB 26dB 1300W
Table 3

It can be seen from Table 3 above that in the case of the MA5002VZ, switching from the 26dB fixed gain position to the 0.775V position increases amplifier gain by 16dB. The potential for loudspeaker damage that could be caused by someone switching in an additional 16dB of gain after the system limiters is enormous. But don’t just take my word for it, ask Bob! …Although don’t try to catch him at the club, I’m told he doesn’t work there anymore.

Jeff MacKenzie
Manager, Technical Resource Group – Jands Pty Ltd

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