What are the advantages and disadvantages of the passive volume control?
A CD-player has an output signal of 2 volt or even less, and a power amplifier needs an input signal less than 1 volt to achieve the total level control. The data should be like this when a manufacturer of a device wants it to be called hi-fi.
What can be hold against going without an active gain of the signal to control this only in the volume?
To understand the problem, we should look closely at the different concepts with which such volume control will be carried out.
1. The Potentiometer
The music signal will be darkened by a stereo potentiometer. This is the simplest way. Many devices are built like this. However, it is most important that both stereo channels are synchronous. If they are not synchronous by 100 per cent then we don't have a total detachment of the music from the loudspeakers.
An ALPS RK27112A has, for example, a tolerance of +/- 3dB. The ALPS RK40312A is a little bit better with a tolerance of +/-2dB.
This is the end of the flagpole and even the carbon potis are not as good as stereo potentiometer.
2. Activated Resistors
The person who likes to experiment with the sound quality of his or her device should build an activated resistor network. On the one hand, it is possible to select the resistors up to a certain level. On the other hand, the resistors can be chosen in favour to your tonal preferences. In the end it is a question of taste if you prefer a MIL-grade Vishay or the older carbon ground types.
Resistor networks or potentiometers are put in a line to the source, which is in this case the CD-player. Therefore, the amplifier recognises the complete impedance when the poti is untwisted, and the reduced impedance when the poti is turned down. That is okay for the amp.
The conductor between the CD-player and the amplifier is a R-C link. Whereas the R stands for the aforementioned impedance and the C stands for the capacitance of the conductor. When we turn down the volume the sound will be darkend because of the R-C conductor. The worst case is that this effect is audible for the user; and, therefore, it stands in contrast to the demand for a higher transfer quality for SACD.
3. Activated Transformer
Transformers have some advantages compared to the aforementioned resistor networks.
The signal will be sent through a transformer with primary und secondary coil. Hence, the source and the load are always galvanically isolated in a symmetric mode. As a result, a humming sound as well as the smallest signal in the conductors are oppressed.
Furthermore, the music signal is gripped in several level. We have various output voltages like in a transformer, but passed on by a switch. In a resistor network the output will be wasted by developing a certain heat. But here the signal is available with a low voltage when in low volume and with higher power. Most important, the signal has a lower impedance. As a result, we do not have a decline to high frequencies.
This is our solution:
Optimisation of the Impedance
We look again on the important aspect of the impedance. When two electrical devices are connected the source's impedance should be less that the one of the load.
We set our transformer's input impedance at 10 kilo-ohm. Usally CD-players, tuners, phono preamps, et cetera have an impedance of around 600 Ohm. Sometimes they even have lower impedances as that, that's why we are on the safe side.
The output impedance of the transformer is almost as high as the one's of the source when on maximum volume. However, we even achieve better data for the impedance when we turn down the volume. As a result, there is no problem with long conductors or exotic capacities. For sure, this fact will please the users of separated mono preamps or active boxes.
All in all, our "silvercore stepper transformer" transmits the music even in low volume very vividly and with a total dynamic range.
Optimisation of the Frequency Response
It is a fact; high impedance requires a greater number of windings in a transformer. These windings increase the capacity of the coil. Thus, this increased capacity caps the frequency response. This is simply physics. However, we just outsmarted physics, and that is the most important here.
The frequency response is in a steady manner (3dB) between 6 Hz and 240 kHz. When we have a frequency response of 40 kHz, then the excessive increase is only 0.1dB. The reason is, that the resonance of the transformer stays between 100 k and 240 k, depending on the terminal resistance. This resonance shows only a little change, almost invisible in the chart of 3dB. Later the frequency response declines. Here we receive excellent data in every switch setting. We underline that, the measurements were done with common 50 k-ohms, and not with palliating short circuit impedances. Furthermore, the stepper works also as ideal as possible when having a phase deviation.
You will see in the intermodulation noise measurement the excitation peaks’ resonance of -136 dB. This is the moment we have shortly after the ambient noise level in the DAAS system. There is no better option; no active amplifier stage could achieve this data.
Here we would like to thank Dr. Ultee from the DAAS enterprise and professor Dr. Steinke from the technical college in Münster.
We have a solid mechanical construction with an electrostatic shield of 100 per cent against electric fields.
Our transformer contains 12 secondary tappings. It cannot be denied, that open secondary coils with their free ends of wire sticking out of the case are perfect for radio conditions and every kind of intersperse. Do avoid this intersperse right at the beginning we did two things: We attached the transformer and commutator directly to a printed circuit board and then we moulded it in a case of MU-metal. The solid MU-metal perfectly shields any electrostatic fields.
We can assure you, that our entire transformers are winded and built identically. The highest priority for us is the quality of the manufacturing process. Thus, a total homogenous stereo sound can be reproduced through both conduits and in all switch settings.
Silver wire
For our best model we use 990 pure silver wire for the coil. The silver wire is produced and insulated exclusively for us. Using silver is not reasoned in a wish to be different or even more exotic - no. Silver wire is like icing on a cake: it adds to the plain sound of the amorphous transformers. Hence, it represents the demand for a clear and pure presence of the source.
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