Relay/resistor volume control for Audio Analogue Maestro Anniversary and ABsolute.
“In the amplifiers of the 70’s volume control was done with a double potentiometer. This type of control had three main problems:
- The remote control had to be realized with the use of an electric motor.
- The life of the potentiometer was limited. The use of hermetic models brought costs very high.
- At low volumes the balance between the two channels was bad.
- Some luminaires then switched to structures with multipole switches and attenuators made with discrete resistors. They worked very well but still had the problem of remote control and a new noise problem on the speakers during switching (“click and “pop”).
With the release of semiconductor attenuators all the problems of the old potentiometers have been solved but new ones have come out. The resistors made on board the chips are not too linear and often one or more op-amps are inserted in the signal path. In Audio Analogue we have chosen to use semiconductor chips, obviously without op-amps, although we have always wanted to realize our own volume control with resistors and relays. This type of control can be remotely controlled, it is very precise and makes use of high quality resistors. Like the old multipolar switches, this type of control also has the problem of noise during volume changes. In fact, during the operation, there may be “clicks” and “pops” due to the random switching of the relays with respect to the input signal. In the literature there are some methods to reduce these problems. One of these is to lower the volume to the maximum and go up slowly until the desired volume is reached so that there are no jumps at high volume. Another makes use of programmable logic to adapt to the switching times of relays that can reach values of a few milliseconds. Another makes use of relays type “break before make” that is, that guarantee the opening of the contacts before closing. This allows to have an increase of the attenuation before its reduction.
Our approach to the problem was to try to understand the reason for these disorders. Surely the random and simultaneous switching of the relays can lead to loud noises on the speakers due to the random change of the attenuation before getting to the desired one but it is not the only problem. Even increasing the attenuation gradually or otherwise lowering the volume before raising it if the switching takes place when the sound signal is at its maximum (or at its minimum) will have one or two steep fronts that will cause a “click”. In semiconductor chips, to overcome this problem, a circuit is used that can operate the switching only when the signal passes through zero so as not to create discontinuities. This method is excellent but using it with relays you will come across the problem of the slowness of switching that prevents you from finding the exact crossing point. For example if we took a signal at 1khz we would have about three periods in the time when the relay moves (about 3 ms). Even if we could take the passage to the zero the restart would still have a steep signal. To solve this problem in Audio Analogue we have created a very fast “mute” that is activated when switching to zero, is kept active for 4ms (that is, the time that the relays move a little more margin) and is disabled at the next step to zero. This eliminates the steep fronts that are the main cause of disturbance during volume changes. All these operations are carried out with very precise logic circuits. Obviously, during the volume change, there will be a signal vacuum of about 4 ms which, however, in normal conditions, is not felt. Our control therefore solves the following problems:
- allows remote adjustment with remote control;
- guarantees a high purity of the signal and a service life of 10 million drives through the use of high quality signal relays and precision military resistors;
- reduces “click” and “pop” disturbances typical of relay controls;
- maintains a perfect balance between right and left channels even at low listening levels;
- does not use op amps on the signal path to maintain our “Zero Global Feedback” philosophy.
RR volume control is currently available for RR versions of Maestro Anniversary and Absolute and all standard versions of Maestro Anniversary and ABsolute can be upgraded to RR version.”