Amplification and the gain-distribution problem
One answer to this difficulty is to take the total gain and split it so there is some before and some after the volume control, so there is less gain amplifying the noise at low volume settings.
The question is – how much gain before, and how much after? This is inevitably a compromise, and it might be called the gain-distribution problem. Putting more of the total gain before the volume control reduces the headroom as there is no way to reduce the signal level, while putting more after increases the noise output at low volume settings.
If you are exclusively using sources with a predictable output, of which the 2 Vrms from a CD player will be the maximum, the overload situation is well-defined, and if we assume that the pre-volume gain stage is capable of at least 8 Vrms out, so long as the pre-volume control gain is less than four times, there will never be a clipping problem. However, phono cartridges, particularly moving coil ones, which have a very wide range of sensitivities, produce much less predictable outputs after fixed-gain preamplification, and it is a judgement call as to how much safety margin is desirable.
As an aside, it’s worth bearing in mind that even putting a unity-gain buffer before the volume control, does place a constraint on the signal levels that can be handled, albeit at rather a high level of 8 to 10 Vrms depending on the supply rails in use. The only source likely to be capable of putting out such levels is a mixing console with the group faders fully advanced. There is also the ultimate constraint that a volume control pot can only handle so much power, and the manufacturers’ ratings are surprisingly low, sometimes only 50 mW. This means that a 10 kΩ pot would be limited to 22 Vrms across it, and if you are planning to use lower resistance pots than this to reduce noise, their power rating needs to be kept very much in mind.
Whenever a compromise appears in engineering, you can bet that someone will try to find a way round it and get the best of both worlds. What can be done about the gain-distribution dilemma?
One possibility is the use of a special low-noise amplifier after the volume control, combined with a low resistance volume pot as suggested above. This could be done either by a discrete-device and opamp hybrid stage, or by using a multiple opamp array. It is doubtful if it is possible to obtain more than a 10 dB noise improvement by these means, but it would be an interesting project.
Another possible solution is the use of double gain controls. There is an input-gain control before any amplification stage which is used to set the internal level appropriately, thus avoiding overload, and after the active stages there is an output volume control, which gives the much-desired silence at zero volume. The input gain controls can be separate for each channel, so they double as a balance facility; this approach was used on the Radford HD250 amplifier, and also in one of my early preamplifier designs . This helps to offset the cost of the extra pot. However, having two gain controls is operationally rather awkward, and however attenuation and fixed amplification are arranged, there are always going to be some tradeoffs between noise and headroom. It could also be argued that this scheme does not make a lot of sense unless some means of metering the signal level after the input gain control is provided, so it can be set appropriately.
If the input and output gain controls are ganged together, to improve ease of operation at the expense of flexibility, this is sometimes called a distributed gain control.
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.