Paul McGowan writes: We started our series on Class D amps and how they work. We went through the all important input stage but before we get moving, however, quite a few of you wrote me and asked for a better idea of what common mode rejection was and what an amp designer might do to make it better or worse. So, today’s post will cover that and then we can move on.
Common mode rejection (CMR) is the real benefit of a balanced input – you know, the one with an XLR connector – the input some of us use but many do not understand its purpose. This balanced input is supposed to get rid of any noise or distortion that is common inside the connecting cable or output of a source or gain stage.
So, let me explain. Imagine a simple RCA cable; inside you have two wires, a hot wire where the signal travels and an outer ground wire sometimes known as the noise shield. The music travels down the hot wire and the return path is the ground wire. Now imagine placing that cable near a noise source such as a humming transformer or a radio, cell phone, electrical motor, etc. The radiation from these devices will get into the two wires on the RCA cable because to the noise, these two wires look like an antenna.
Our connected device will amplify the music signal on the RCA connector, but it will also amplify whatever noise has been radiated into those two wires at the same time – thus we get hum or noise – and this is obviously something we don’t want.
Now let’s take an XLR balanced cable instead. This cable is nearly identical to the RCA cable with one exception: it has another signal wire inside. This three wire setup is cool because you have the outer ground shield just like on the RCA cable, but inside there are these two signal cables. Each cable carries the musical signal, but one of the two cables is flipped over electrically – meaning it is out of phase with the other. Were you to tie these two wires together, they would cancel each other out and produce nothing. But! If you put them into a difference amplifier, which is what we call the input of a balanced amp, you get a very different result indeed.
Picture the same setup now, where we placed the cable next to a noise source such as a humming transformer. The same thing happens as did on the RCA cable – all three wires pickup hum. But unlike the RCA cable, when the two signal wires of the XLR cable are put into our balanced amp, anything that is common (the same) to those two wires is rejected – and we get a noise free output from the amp. So anything “in common” to the two wires (the hum we picked up on both wires is what is in common) is ignored and we get wonderful music out. This is because the music was not “common” or the same, it was out of phase – the noise WAS common and so it’s now gone.
That’s how it’s supposed to work – but not every balanced input is engineered properly and even those that are vary in their performance. We know that nothing’s perfect and this also applies to a balanced input. As engineers we rate the rejection amount of an input in dB – a measurement of loudness. We say an input might have a CMR ratio (CMRR) of -60dB or -70dB – and this shows us how much noise can be rejected. The higher this number is the better the CMRR is and the lower the noise and distortion we get on the output of our amp.
It’s not entirely easy to design an amp with a very high CMRR but that’s the challenge at hand when you build a good amp or preamp. Our products typically have a CMRR of around -70dB which is pretty good – while many others are far worse – and then there are those instrument grade amps that are much better. If you can get the CMRR to be -70dB, you’re in good hands with your designer.
If you’re thinking of an amp purchase and plan to use balanced cables, simply ask what the CMRR is – it’s a valuable spec to know.