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TECHNICAL NOTES

A note on setting up power amplifier levels

A bit of preamble

Let me say at the outset that I suspect some people are going to get very upset about some of the things I write in the following note. This is because these people have been setting up and operating sound systems for a long time. They have always done it their way and it has worked perfectly well using the equipment they had available. Unfortunately, a lot of them have been setting up their sound system incorrectly and, although they may have “got away with it” using the sound equipment they had available, when they encounter something new, they run into problems.

So, now that I have your undivided attention, let me also say at the outset that I am not criticising anyone and the first paragraph is ‘tongue in cheek’. This note is intended to educate. Critically, for those who have been doing things the same way for many years, it will also explain why they may encounter difficulties in setting up new equipment, encounter unwanted noise and maybe find controls over sensitive.

 

So, the background as to why I decided to write this note is that it has become apparent to me recently that there are an awful lot of people out there operating sound systems who, in all circumstances, turn the power amplifiers up full. Sub, bass, mid, top, or full range amplifiers; they all get the same treatment. The amplifier gain goes automatically to maximum. It doesn’t matter what is supplying the signal to the amplifier and it doesn’t matter what speakers the amplifier is supplying an amplified signal to; the power amplifier gain default position was always maximum. I thought this was curious for reasons that will become clear, so I canvassed opinion discretely and not so discretely with various sound system operators who work like this to understand exactly why. I was surprised at some of the answers. Some of the common ones included:

 

“you set the amps on full gain, then control levels with the preamp”.

 

“I’ve been advised by people who have done this for years that it’s the best thing to do”.

 

“I think the amps operate in a more linear region when gain is full”.

 

And finally, the classic “I just prefer it that way”.

 

I pushed my luck a bit further and asked if they understood what power amplifier input sensitivity was and, rather provocatively, why do they think amps are built with level knobs on the front in the first place (well most of them anyway, if you consider PC based gain control to be a virtual knob). I got even fewer answers back.

 

How an amplifier works

So let’s get serious now. If you turn that knob on the front full clockwise by default this is a terrible idea. There may well be occasions when it is fine and / or necessary to run such settings, but I can assure you that these occasions will be relatively few for many. And for S&P preamps I can assure you that you should absolutely never run like this. So why is this so?

 

As a precursor, it is useful to understand at a very basic level how a power amplifier works, and to appreciate that internally there are generally two important stages. Once you understand this, the further understanding of setting of gain levels should come easily.

 

There are essentially two parts to a power amplifier – the preamplifer stage and the amplifier stage. It should be no surprise that the preamplifier comes before the amplifier. We will get to the preamplifier stage in a moment and (counter intuitively) look at the second stage first. The amplifier stage is interesting; this is because a lot of people are not aware that it doesn’t matter where you turn that knob to on the front, the amplifier stage will amplify a signal by exactly the same amount. So, if the amplifier has a voltage gain factor of x100 for instance and you feed it with a 1V signal, then you will get 100V out (1V x 100 = 100V). And if you feed it with a 0.5V signal, you get 50V out (0.5V x 100 = 50V). So the amplifier stage is fixed and it should be obvious that in order for the amplifier stage to achieve maximum rated power, this stage should be fed with the correct size signal that it was designed for. It is also not a big leap to understand that if the amplifier stage was fed with a signal larger than it was designed for, then the output signal is going to become distorted (typically clipped). So, for instance, if we took our example where the rated amplifier power was designed to be achieved with a 100V output signal into some nominal impedance, then if we fed the amplifier stage with a 2V signal, then the amplifier stage would be trying to hit 2V x100 = 200V. However, the actual output voltage the amplifier can achieve is likely limited by the design of the internal power rail voltage and so it can’t swing as far as 200V and instead clips at the 100V limit. This of course sounds terrible and risks damaging your sound system.

 

What does that knob on the front of the power amplifier do?

So this brings us to the preamplifier stage (not the reggae preamp, but the power amplifier internal stage). The purpose of this stage is to precondition the input signal, which could be from a mixer or a reggae preamp, and get it to an appropriate level for: i) the power amplifier stage; and ii) the amount of power you may want to deliver to the speakers (i.e potentially less than full power due to speaker power handling capacity or maybe the size of the venue).

 

So this is what the knob on the front of traditional power amplifiers is for. It is actually to set the level of the input signal to a level suitable for the internal power amplifier stage and also for the level of power you want to achieve. It is not, as many believe, a volume knob or a fixed power level knob. It can do these things of course and, to laypeople, it appears to behave in that way when you turn it. But it is actually for matching your signal level of your upstream equipment to the power amplifier and that helps you set up the gain structure of your sound system. As you read further on in this note, I will sometimes use the words gain or sensitivity when referring to this knob.  It depends on the context of the message I’m trying to get across.  Stick with it, read on and all will become clear.

 

Now some modern touring amps don’t have any knobs on them; all the settings can be adjusted remotely by an app, etc. But there is still a virtual knob available, and this is typically a setting to adjust called input sensitivity.

What the hell is input sensitivity?

OK, so now we have the basic stuff out of the way, we can get into a bit more detail. When you adjust that knob on the front of a power amplifier, you are actually adjusting an amplifier characteristic called input sensitivity. Manufacturers go to the trouble to put that knob on there for a reason and they also go to the trouble of publishing input sensitivity characteristics in the manuals. Below I have shown the sensitivity characteristics of the Powersoft K series amplifiers. This is straight from their manual, which is in the public domain. Of course, these Powersoft K series amps don’t have a knob, it’s a virtual setting on a computer. But it’s still for the user to set and so it is functionally exactly the same. I intentionally include Powersoft characteristics in this note, precisely to make this point.   

Figure 1.  Powersoft published settings for K series amplifiers

So what does this stuff mean? Well, what it’s telling you are the gain settings that you need, in order to hit full power when you have a certain sized input signal. So for example, consider a preamp, mixer or DSM feeding a K20 amplifier (read the far left and far right columns). If the preamp (or whatever) has an output signal level of 2.62V rms, then if I set the gain to 35db, the K20 can achieve full power (max signal immediately prior to clipping). But if my signal was bigger, say 5.22V rms, then I would have to set the input sensitivity to a lower level of 29dB. The amplifier can still achieve identical max power in both cases. The signal inside the power amplifier is simply being trimmed to an optimum level as it enters the main amplification stage. The largest voltage the K20 can handle is 7.37V rms. To get the peak to peak voltage, multiply by 2x2^(0.5) , so that is 20.84V. Trust me – that’s a big signal and so if anyone says to you, the signal output from my preamp is too high, point them towards Powersoft’s published data and ask them “well why would Powersoft build amps to run with that size signal if it wasn’t ok?”.

 

The K20 has lots of options for setting input sensitivity via an app, but simpler preamps may only have a knob on the front and there are no markings indicating input sensitivity.  Generally they show full clockwise 0dB and incrementing values of minus dB as you go anticlockwise.  So what does that mean?  We will get to that later and how you calculate and adjust input sensitivity using manufacturers published data and adjusting that knob.

 

You should also now start getting an appreciation of the problems you will face if you insist on setting the power amp to full gain/maximum sensitivity, but are feeding it with high level input signal. The power amp can go into clip when the input device (mixer, preamp or whatever) has its own gain at a relatively modest level.  Sound will be difficult to control because you can’t use the full range of preamp control.  You may also have terrible noise problems (more on that later).  Obviously what you want to achieve is to be able to use the full range of the preamp and ideally only put the power amp into clip when the preamp is at its maximum gain. SO THIS IS WHY POWER AMPLIFIER INPUT SENSITIVITY IS SO IMPORTANT.

 

You may be interested to know that when I canvassed a few non-professional, but large and well-established, sound systems, on this subject. Only one of them knew what input sensitivity was and how to calculate settings for it. So if some of this stuff is lost on you, don’t worry – you are not alone. But trust me – if you take the time understand the concept of what is going on here, it will transform your understanding of sound systems and your ability to set up gain stages for all equipment.

 

So why this variation in input signal size? What are the advantages / disadvantages?

A reasonable question might be “why don’t manufacturers standardise to a certain level of input signal?” The reasons for this are many and varied. Some are historic, some are technical. Many years ago, when power amplifiers were valve powered, inputs tended to be very sensitive. It’s another essay explaining why they were, but they were. So a very small input voltage level would send the valve amp to full power. Because all power amps were pretty much universally built like this, then the equipment that fed into them was also universally of comparable signal level.

 

So a reasonable question is why not just build everything to operate with a similar small signal level? Well smart engineers realised that there are a lot of advantages in having a much less sensitive power amplifier that can handle a big input signal. The advent of transistor amplifiers enabled this. So why would that be an advantage? Well consider a simple sound system with a music source, a preamp, a power amplifier and some speakers.

 

If you look at Figure 2, we are going to concentrate on two parts of this sound system – the preamp and the amplifier. Now let’s keep it simple just say that the objective of this sound system is to amplify an input signal by a factor of 10, from the input of the preamp to the output of the power amplifier. Well that sounds simple enough and example A in Figure 2 illustrates a way you may achieve this. The signal (shown in red) is input into a preamp which has a unity gain level (gain = x1) and so the signal that comes out of the preamp is the same level as that which entered it. That signal is then fed into the power amp which amplifies by a factor of x10 and that is what comes out of the power amp. Great – we have amplified our signal by a factor of x10, so what is wrong with that? Well now let’s take a look at the noise (shown in green) as this signal travels through these devices.

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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.

Figure 3.  Problems faced with subjective noise testing

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