Balance Controls – The ideal balance law
The job of a balance control on a preamplifier is to adjust the relative gain of the left and right channels, and so alter the position of the stereo image to the left or right, and do nothing else at all.
As you would expect, increasing the left gain moves things to the left, and vice versa. The ideal law is therefore that of a stereo panpot in a mixing console. Two uncorrelated sound sources (and any signal will be uncorrelated once it has left the loudspeakers and bounced around the room a bit) add just like white noise sources, and so give a combined signal that is 3 dB higher, and not 6 dB higher. The sine/cosine panpot law, as seen below, is therefore appropriate for the balance control of a stereo signal, because the level of the combined signals will not alter as the image is moved across the stereo stage from one side to the other. It is a constant-volume balance control.
I am aware I said, in the First Edition of this book, that the panpot law was not the ideal balance law, so let me clarify that. The sine/cosine law is functionally ideal in terms of its operation, but is not optimal in terms of electrical performance. Since the gain is – 3 dB at the central position, there will be some compromise. If the 3 dB loss is made up by gain before the balance control, the headroom is reduced by 3 dB. If the loss is made up with 3 dB of gain after the balance control, the noise performance is likely to suffer. The only way to avoid this is to use an active balance control, which alters the gain of a stage rather than passively introducing attenuation.
A panpot in a mixing console reduces one of the signals to zero at each extreme of the pot rotation. There is no need to do this in a preamplifier. A balance control does not have to make radical changes to signal level to do its job. Introducing a channel gain imbalance of 10 dB is quite enough to shift the sound image completely to one side, so it appears to be coming from one loudspeaker only, and there is nothing at all to be gained by having the ability to fade out one channel completely.
This means that the ideal balance control is not a mixer panpot as such, but a panpot effectively limited in its rotation so that neither endstop is reached and the signal is never reduced to zero. This might be called a truncated sine/cosine law. An example is shown in below, where the 20% of the control travel at each end is not used, and the central 60% spread out to give more precise control. As a result the maximum attenuation, with the balance control hard over, is — 10.4 dB, while the minimum attenuation is — 0.41 dB. The attenuation at the centre is unchanged at – 3.0 dB; a central detent on a balance control pot is highly desirable.
In the case of a switched balance control, this is equivalent to using a 23-way switch (there must be an odd number to give a central position) and treating it as a 37-way switch, with the extreme seven positions at each end inaccessible. This gives a greater number of steps over the range in which the control is actually used.
Since the balance control is usually a set-and-forget function that does not require readjustment unless the listening room is rearranged, it is not often considered when controls are being motorised. In fact, getting the balance exactly right by leaping up and down between sofa and preamplifier is rather more tiresome than manual adjustment of volume.
If we look at the issue purely from the point of electrical performance rather than functionality, and ignore the desirability of a constant-volume balance control, the ideal balance law would have no attenuation when set centrally, and when moved to left or right will attenuate only one channel without affecting the gain of the other. This can be done with special balance pots that are available from several manufacturers. If there is attenuation at the central position then it needs to be made up by extra amplification either before or after the balance control, and this means that either the overload margin or the noise performance will be compromised to some degree. As noted earlier, the only way to avoid this is to use an active balance control, which alters gain rather than attenuation.
Excerpt from Small Signal Audio Design by Douglas Self © 2014 Taylor & Francis Group. All Rights Reserved.
About the Author
Douglas Self studied engineering at Cambridge University, then psychoacoustics at Sussex University. He has spent many years working at the top level of design in both the professional audio and hifi industries, and has taken out a number of patents in the field of audio technology. He currently acts as a consultant engineer in the field of audio design.