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Figure 2.  Examples of the effect of gain structure on signal-noise ratio

First you must be made aware that noise is generated by all electrical electronic equipment. As resistors heat up they generate noise; opamps have a level of noise “built in” which is published by manufacturers; and there are many other sources. There is no avoiding it, so the trick is to manage it. In example A you will see at the output of the preamp, there is also now a much smaller noise signal which will be superposed on the music signal. This noise is generated by all of the internal circuitry in the preamp. Then, when this signal comprising both music and noise pass through the power amp, they are both amplified by a factor of x10. Although the Figure 2, example A is not drawn perfectly to scale, I have tried to illustrate this and you should see both amplified music signal and noise at the output of the power amplifier.

Now let us look at Figure 2, example B which is a modified gain structure intended to manage the noise component of the final output signal to a lower level. In example B, we have doubled the gain of the preamp to x2 and halved the gain of the power amp to x5. This will still achieve the overall gain as example A and provide us with a music signal amplified by a factor of x10 at the output of the power amp. But look what happens with the relative noise levels at each stage. The noise level that appears at the output of the preamp is at more or less the same level, since this is predominantly associated with things like component selection and architecture (actually this is not absolutely correct, but for the purposes of this simple illustration, you can assume it to be so). So when the music and noise pass through the power amplifier stage, they are both only amplified by a factor of x5. The end result in both cases is that the music has been amplified by an overall factor of x10, but in example A the preamp noise was amplified by a factor of x10, whilst in example B, the preamp noise was amplified by a factor of x5. The relative level of noise that you would hear from the speakers is halved in signal level.  The gain structure of example B provides a significant advantage over A in terms of noise level.

If you have got your head around that lot, it should be obvious what happens if you are foolish enough to run a high gain preamp and a high gain power amp together.  It’s a disaster.  Using our examples A and B and so applying a preamp with a gain of x2 and a power amp with a gain of x10, together they are trying to achieve an overall gain of x20.  This may be beyond the capability of the amp and so you have to run the preamp at lower level to prevent clipping.  Running like this you are limiting the range of preamp controls you can use and generating the highest level of noise. 

So hopefully you can appreciate the advantages of running high gain upstream in the system, low gain at the final point of amplification. It’s common sense really. A power amplifier on high gain amplifies everything, both good and bad.  And if you have loads of outboard equipment and multiple gain stages, there could be a lot of noise getting added up.  So minimise the quantity of bad stuff going into the final power amplifier stage and run it at as low a gain as can be managed.

How do I set power amplifier input sensitivity levels?

Now that you understand in principle what input sensitivity is, you should also appreciate that setting the power amplifier to full gain, whatever the circumstances, is a bad idea.  If you have a high output signal device, such as a S&P preamp, it’s a terrible idea. So there are two methods that I use to set up power amplifier gain or input sensitivity as it should be correctly called. In these discussions I’m assuming no intermediate speaker management for simplicity. The first way is the proper way using some simple maths. The second way is a much simpler and intuitive method with no maths, which I regularly use and it works perfectly well. Someone, somewhere is going to be angry about the second method, but since I recently discovered Crown Amplifiers have for years published the same method, then I feel a lot more comfortable. Who is right? Mr Angry or Crown Amplifiers?

The mathematical way is 2 staged.  There are potentially a lot more calculations you can do, but I’ve really simplified this as much as possible in order to give those unfamiliar with this stuff a fighting chance. The first stage is to identify the power amp input sensitivity to match the output level of the preamp. The second stage is to then modify power amp gain setting further as necessary to accommodate situations where the potential power delivery of the power amp far exceeds the power handling capability of the speakers (or the room). Some don’t bother with stage 2 because they are confident the power amp won’t destroy the speakers or they have some intermediate speaker protection. The output levels of S&P 5 way preamps can be tricky to pin down because, unlike a mixer (and with all big preamps) there are multiple ways to EQ the same frequency. So if you think about bass, you have bass tone control, an equalizer, bass master, a parametric, etc. If you turn that lot up full, you will have an enormous, unmanageable signal. So from experience, I’d advise that assuming a 14.75dBu preamp output level is a good place to start. This should represent a heavily EQ’d practical maximum, and most of the time would be expected to run much lower. But this is a good place to start because its easier to adjust a power amp sensitivity up during a dance than it is to replace a speaker cone embedded in a wall the other side of the room. Now I would love to be able to provide a worked example of the maths that you could apply on every power amp, but different manufacturers have different ways of adjusting sensitivity. Some just have a knob on the front, some have internal setting switches plus knobs, some are digitally controlled with multiple approaches to achieving the same result. But I’ll provide one example to give you a steer using MC2 E90 (I’ve got some so it’s easy for me).

The E90 has an external sensitivity knob and some internal switches, providing 32, 36 or 39 dB of gain. You set these, then trim with the external knob. Some of this info is published, some you have to work out for yourself. You need to understand dBu, dB, some power relationships, etc and be comfortable working with the numbers. We’re going to assume were driving 2x 8 ohm speakers in parallel with each channel of the E90. Published power into 4 ohm is 4.8kW. So, using power=V^2/R and output Vout=10^(db/20)*Vin, we can calculate that a 14.75dBu (4.24V rms) input signal would require a gain of 30.3dB to deliver 4.8kW into 4 ohm.  This 30.3dB gain requirement sits just under the 32dB setting option, so the switches are set to 32db and then the external gain is trimmed by minus 1.7dB. This will deliver 4.8kW into a 4 ohm load with an input signal level of 14.75dBu and which is well under a 20dBu internal limiter trigger level. Onto the next stage.

Lots of people stop there because their speakers are well matched to the amps or they have other protection methods. Let us say ours are not and we need to back things off a bit more. Let us say I want to limit the power going into our 4 ohm load to 3kW. This is nice and easy to calculate. From power = V^2/R, you need 109.5V rms going into your speakers. With a 14.75dBu input signal going into the amp, this requires the E90 to be set to 28.35dB gain. So, once again we are on the 32dB setting, trimmed back by 3.65dB on the front knob.

OK, I know most of you are not going to do all of that. So what is the easy method?  Well it’s really easy and I’m going to illustrate with one of my 5 way preamps.  This gets you in the right ballpark.  It is nothing to do with EQing a room etc; that is all fine tuning after you have done this.  The process is just to get your power amps at approximately the right level for the preamp, the room, and is complete common sense.

1. Turn all music, mic and aux channels to zero gain and mid EQ.

2. Mid EQ the 16 band EQ and turn music parametrics to zero gain (full anti-clockwise)

3. Turn the 5 ways master gains to zero.

4. Turn power amplifiers to zero - you are going to set up initially without actually listening to music and just observing the preamp VUs.

5. Play music.

6. Turn input music channel gain to about 60-70%. This is important. Do not set lower. This gain level also influences the send signal. Set at this level initially to maintain a strong send signal.

7. Turn up each of the 5 way gains until your VU is visually operating where you like it. Depending on music source, you may only have to turn them up a little way. That is absolutely fine.

8. Now slowly turn up your power amps one at a time, starting with tops until the sound level is good for the room you are in. Depending on the venue, you may only have to turn the power amps on a little way. This is fine.

By applying this method you run the preamp in the sweet spot and you are setting the power amps at what is likely to be very close to an input sensitivity level you would have arrived at by calculation.  The only thing to add would be to those who use digital power amps, controlled from apps.  In these cases, set the input sensitivity at the lowest setting.  So using the previous example of the Powersoft K20, set it to 26dB input sensitivity.  You can always increase if need be.  Better to work upwards than downwards and risk damage etc.

For those who think this set up method is rubbish, be aware that Crown amplifiers recommend pretty much the same approach and publish in their manuals, as do other manufacturers.

A final note on making noise comparisons.

So if you have understood everything above, it should be pretty obvious that if you want to compare the noise levels of different preamps, just connecting them to a power amp on the same setting is a crappy idea and tells you nothing.  Power amps have to be on the correct sensitivity for the preamp or you can get a complete reversal of results – not helpful and illustrated in Figure 3.  This shows that preamp A, which has lower noise floor than preamp B, when tested correctly vs. preamp B gives a correct result.  But if preamp B is tested on a lower gain level than A, and you just listen subjectively to noise, you reach the mistaken conclusion that B is quieter, even though B still has the same poor signal-noise ratio. This is exactly what happens if you compare high and low gain preamps on the same power amp settings.  Time and time again, I hear about people turning power amp gain up full and comparing equipment.  It is WRONG WRONG WRONG, and if you’ve read and understood this note, you now know why.  Unless you have the proper equipment to make noise measurements, there is really only one way subjectively test them.  The preamps have to be connected to power amps with the correct sensitivity settings and then the preamps, playing music, should be brought up to achieve the same overall sound levels as each other.  Then pause the music and listen.  If you don’t do this, you are wasting your time and just demonstrating that you don’t understand sound system gain structures.