Coming Clean about Clean Headroom

Guitar players are often going on about clean headroom, particularly in relation to tube amps: “this amp has plenty of clean headroom”, or “there’s not enough clean headroom”, or even “there’s too much clean headroom”. We also hear it mentioned, though, in relation to effects pedals, and how plugging in a 12V power supply to a 9V pedal for example “increases the clean headroom”. What does it all mean, how can it possibly work, and not turn your fancy new boutique boost pedal into a smoking pile of exploded caps? Let’s find out.

 

Let’s start with a slightly over simplified view of how an amplifier works. this will be sufficient for our purposes in this discussion. The job of an amplifier is to take a low power signal and increase it’s power level so it can do some useful work. In the case of a guitar amp, it takes the tiny output from a guitar pick up and increases it, usually in several stages, until it becomes sufficient to move a speaker cone. The number of times the amplifier can increase the output power over the input power is referred to as the gain. A gain of two means the amp should output twice the input signal, and so on. Guitar amps and other devices might often have several gain stages, each time increasing the output power over the input power.

So how does it do it? Well, in our simplified view, the amplifier is taking two inputs and creating a single output from them. The two inputs are:

1: The input signal (from the guitar pickups in our example)
2: The power supply (wall power, battery, AC/DC adapter etc)

The important thing to note here is that the load on the output, the speaker cone in our example, is being controlled by the power supply and not the guitar pickups. The signal from the guitar pickups is modulating the power supply to provide the varying output current, but the eventual output power depends on the gain of the amplifier and the limits of the input power supply.

Let’s look at an example. Let’s say we have an amplifier with a gain of 2 and a 3V power supply. If we provide a 1V input signal, the amplifier will try to increase this at the output to 2V. This is OK because the output is 2V and our power supply can deliver 3V, so all should be well. Now let’s increase our input signal voltage to 2V. Again we’ll multiply our input signal by our gain which is now 2 x 2, or an output voltage of 4V. Now we have a problem because the amplifier is trying to increase the output voltage to 4V, but the input power supply is only 3V. In this scenario the amp will begin clipping. It delivers as much of the 4V output as it can and then stops when there is not enough power available at the power supply. It’s called clipping because if you look at at the input signal as a sine wave, the output looks like the tops have been clipped off. Our signal has taken a haircut. In our guitar amp scenario, we would hear distortion.

So what can we do about it? One thing we could do is reduce the gain. If we drop the gain from 2 to 1.5, then output for a 2V input signal is 3V, or exactly our power supply. Since we are talking theoretically, we can say our amplifier circuit is 100% efficient, then this would work. In practice, of course, we have to take into account efficiency when designing our circuits. In our guitar amp scenario, reducing the gain means turning down the gain or volume knob. Another thing we could do is reduce the input signal level; playing quieter or using a lower output pickup could work. The third thing we could do though, is increase the power supply voltage. If I double this to 6V, I now have plenty of power available to deliver 4V at the output, and my signal is no longer being clipped.

This is the basis of ‘clean headroom’. It describes what is the largest signal level we can provide at the signal input for a given gain, before the power supply is no longer sufficient and the amplifier begins clipping. It also explains why increasing the power supply voltage raises the ceiling at which the amplifier clips and thus provides more clean headroom.

Before we all run off and start plugging our boost pedals into the wall to see how much clean headroom we can squeeze out of them remember that up to now we have just been discussing theory. We need to take a look at how this theory typically works in practice, and in effects pedals in particular. Above all please remember that increasing the input voltage is only going to work with devices that are specifically designed to do this. As always with electricity, we need to be careful. You should never connect an effects pedal or other electrical device to a power supply outside the recommendations of the manufacturer. Using an unsupported power supply can cause damage to your equipment, fire and/or serious injury or death. Check out the pedal power post for more information on pedal board power supplies and how they work.

Amplifiers are not just used in music electronics. A particular type of amplifier called an operational amplifier or opamp is prevalent in a vast number of electronic devices. I’m willing to bet you are surrounded by opamps right now. In your car, your kitchen, on your desk, in your pocket. From the fuel level display in your car, to the temperature monitor in your refrigerator, through computers, phones, and test equipment, opamps are everywhere. Now in most of these cases, amplifier induced distortion is a bad, bad thing. I don’t want poor audio in my phone, or an inaccurate reading on my voltmeter because an amplifier is causing distortion, but in music, particularly electric guitar music, distortion is often what we are looking for.

So if we are designing an amplification circuit for a phone, or a multimeter or some such, we are going to do our best to make sure we have sufficient power in our circuit to avoid our amplifier ever clipping within the range of input signals we expect to get in normal use. This is not always so in guitar related devices. Often we are going to deliberately cause amplification circuits to clip in order to produce distortion. Opamps are extremely common in effects pedals, they are small, low cost, robust, and available in a huge range of different specifications to meet our needs. In particular there are variants that support fairly wide supply voltage ranges. For example an Analog Devices OPA275 which is quite a common opamp used in audio applications, has a minimum supply voltage (Vs) of 4.5v and a maximum of 22v. You can find the supply voltage range of an opamp in the manufacturers data sheet, here’s the one for the OPA275.

As designers, we need to make some decisions on if we are going to drive these amps into clipping and at what level. For example, with a distortion pedal I may want my amp to clip pretty much all the time, whereas in a buffer or boost pedal I might not want it to to clip at all. Maybe with a guitar overdrive I might want some modest clipping at first with more as I increase the signal level so the tone cleans up as I back off and gets more grind as I dig in.

We also have to bear in mind the wide variety of signal chains our device may get used in. One may be a touch sensitive blues rig with low output single coils. Another maybe a full on metal rig with huge output active EMG pickups and a bunch of dirt pedals stacked together before the signal hits our device. Then we have to take into consideration the available power sources and that a 9v battery is a preferred item in stomp box land. So when we are making design decisions, we often start with an average. We may take a typical input impedance, a median output level from a pickup, and a 9v supply from a battery, and design around that so that our users will get the type of experience that we designed for with the majority of different rigs. However, we can also design in some flexibility. Let’s take our OPA275 opamp used in the example above. This has an input supply voltage range of 4.5v to 22V  If we carefully design the rest of our circuit with components that will operate within that range, the device should function with anything from a 4.5V half used 9v battery, to a 22V external power supply. Since 22V is a slightly odd number for DC supplies and we’d like to have some safety margin anyway, we’ll document our product as useable with a 9V battery or an external power supply between 9V and 18V. Users who want to start clipping earlier or have very low output pickups can stick in a used battery or use a power supply with a low battery simulator. Those who want more clean headroom can use an external 18V supply.

 

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