The quest for a better volume control

Potentiometers

Many, even high end preamplifiers are equipped with potentiometers as volume controls. Some are motor controlled so they can be controlled by a remote but still they are potentiometers.

What is the problem with that?

All potentiometers, even so called high quality ones have bad channel balance!

This means that the volume setting will be different between the channels, and even worse, this unbalance varies with the volume setting. It can be skewed to the right for low settings and change to be skewed to the left for higher volume or vice versa. What is common and can be very annoying is if there is a small signal left in one of the channels when the potentiometer is set to its lowest setting.

So how bad it is?

Lets compare some common "high quality" audio potentiometers

• First choice: ALPS "blue" real part number series RK27, a very popular audio potentiometer that is used in many high end pre-amplifiers, it can also be provided with an electric geared motor that make it possible to remotely control it. According to the data-sheet the channel unbalance for this high-end pot is 3dB! or as much as 5dB! at low setting. Quote from the data-sheet: "−60dB to 0dB within 3dB −80dB to −60dB within 5dB possible depending on total resistance and taper"
• 2nd choice: TKD CP2500, this potentiometer is often mentioned as an upgrade from the ALPS ”blue” but in reality the specification is not much better. According to the data sheet the channel matching accuracy is within 3dB. Potentiometers from other manufacturers have similar performance.

Does it matter, does it influence your listening experience?

It is often said that the smallest difference in sound level that can be recognised is 3dB, but that is only true when comparing sound level from 2 sources at different points in time.

When changing volume setting playing music it is possible to recognize much smaller difference in level, as small as 0.1dB. It goes without saying that a potentiometer that change balance between the channels as much as 3dB when it is adjusted can have a very bad effect on your listening experience.

A better volume control

The solution is simple and elegant, a stepped attenuator with fixed resistors.

An attenuator with standard 1% tolerance resistors will have a maximum channel imbalance of ~0.2dB, a major improvement over even high quality potentiometers, using 0.1% tolerance resistors this can be improved further to 0.02dB channel imbalance or even better*.

In a attenuator with resistors they need to be switched, i.e. different resistors should be switched in or out of the audio circuit to adjust the attenuation. This switching can either be done with a mechanical rotary attenuator or with the help of relays. One issue when using a rotary attenuator is that they tend to have a limited number of steps like 24 or in best case 48, a stepped attenuator with 24 steps therefore have quite large steps which could be a problem as volume will change too much for each step. It is also a problem to arrange the possibility of remote control with a rotary attenuator.

The best solution for a stepped attenuator is instead to switch resistors using relays, high quality hemetically sealed relays are available with different kind of contact material where precious metal contacts plated with pure Gold and/or Rhodium are the best for audio signals.

Often stepped attenuators with rotary switches consist of a series resistor chain where the signal is taken from a connection between 2 of the resistors. Using this scheme you would need one relay for each step and therefore a 64 step attenuator would need 64 relays!

A much better method is to switch relays in a binary fashion using attenuators of 1, 2, 4, 8, 16 and 32dB, then it is possible to build a 64 step attenuator with 1 dB steps and only 6 relays.

The solution from Q-tron Audio

Q-tron Audio very early recognised the problems that arise while using ordinary potentiometers as volume controls. We therefore have developed our own 64 step attenuator with 1 dB steps using high class relays switching high precision resistors. Many of our customers have recognised the vaste improvement over potentiometers, this is something we encourage you should try also, you will not be disappointed!

*Note! these are maximum values of channel imbalance, in reality the tolerance of resistors from the same production batch usually have much less spread between the value of 2 resistors even though the value itself can be close to the stated tolerance.

Example: 2 1% resistors from the same production batch can have values of 10.1kohm and 10.09kohm, both are within 1% tolerance but they are only 0.1% apart so when using these resistors in each channel the channel balance would be excellent.