Volume controls

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Well, this is a fine hornet's nest I've gotten into. Half of you seem to think I live in a cave and haven't any clue what high-end audio is all about and the other half think I have it right - and then yet another half think I am opening Pandora's box and letting all the digital nasties out to seek and destroy all of analog. Wait a minute, how many halves are there? For the record (and not to make a pun) I love analog, have been designing analog circuits for over 40 years, continue to do so and near the end of this series will be advocating a hybrid approach as part of the new paradigm I am advocating that plays nice with both analog and digital. So, deep breath my friends, I am not out to kick your dog or put a damper on your analog dreams and I hope we can simply enjoy these posts in the spirit they are written, fun and informative - forward looking. I really am on your side and the side of great music and audio. Really. :) One of the core elements of any preamplifier is the volume control - without it you cannot adjust the level of what you're playing and the system becomes rather useless. Let's talk analog volume controls today. There are 50 ways to Sunday to build an analog volume control and I think I may have tried and listened to every single one. In its simplest classic form most analog preamplifier volume controls are but a simple variable resistor or potentiometer (called a "pot" for short). How do these work? They are simple mechanical devices that have a long strip of resistive material with a connection on each end of the strip. One of these two connections (A) is attached to the audio source and the other side (B) is connected to ground (zero audio). A third contact (W) is pressed against this strip and connected to a knob or a lever so that the user can move the contact up and down the length of the resistive strip. This contact becomes the output of the volume control. By placing the contact on a different area of the strip you get different volume levels out - louder when you move towards A and softer as you move towards B. Put two strips together with two contacts moving together and you have a dual potentiometer or - a stereo pot. Move either one of the two contacts differently and you adjust the left and right channels differently - thus making a balance control. Remembering that all types of resistors sound different when used in the signal path, it should be no surprise that the quality of the music running through one of these pots is very much dependent on the quality of the resistive element, the contacts and the way it's implemented. In fact, there is no such thing as a neutral or transparent sounding pot or volume control. The fact that any form of volume control has a negative effect on the sound is a really important concept to grasp. Switched attenuators, Gain Cells, pots, electronic attenuators, CMOS switches, photo cells and every scheme imaginable to change the volume has a negative affect on the sound. The challenge is to figure out how to do as little damage to the sound as possible and there are many schemes out there - ranging from the absurd to the lame and cheap and everything in between. The point I want to make in today's post is that the volume and balance controls in an analog preamp all suck to some degree. We have to accept that we cannot change the volume without affecting the sound. Lastly I want to give you a piece of advice when it comes to volume controls. Less is more. By that I mean the higher the volume control level the better it sounds because less of the resistor is in the signal path. Look at the drawing I included - if the W connection is tied right to the audio input A (full volume), then essentially there is nothing in the signal path to damage the music. The farther away W gets from A (turning the level down), the more sonic damage is done. Less is more. I wrote about this nearly a year ago in a post labeled Step on the gasand I think it's not only worth repeating but it also leads us to tomorrow's post about digital volume controls. Interesting how in both cases, less is more when it comes to volume controls.
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Paul McGowan

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