The present invention relates to a stepwise adjustable attenuator especially for use in volume control of audio- signals in amplifier systems and generally for attenuation of electrical signals in other types of electric appliances.

The attenuator is provided with a fixing nut for mechanical suspension in a front panel or similar. The electrical connections are established by tin-lead soldering or by means of a terminal connector. The attenuator is a stepwise adjustable type, which consists of a rotary switch with end-stop. The electrical function consists of two identical circuits tracking and corresponding to the same setting to fulfil the demand for equal treatment of the two channels for stereo-signals.

Furthermore, the attenuator includes two printed-circuit boards on which the necessary electric resistors have been mounted with such resistance values and in such an order that attenuation elapses smoothly and often equidistantly with respect to the sound pressure measured in decibels.

Until now attenuators have been built as stepwise adjustable types with a rotary switch fitted with two equal sections consisting of a wafer with a printed-circuit board on which electric resistors are mounted.

The disadvantage of such devices has been their relatively high demand for space. From an electrical point of view these devices have furthermore a number of disadvantageous characteristics. Some of these characteristics are moreover especially disadvantageous. For instance the total conducting length of the printed-circuit board traces and resistor elements is long i.e. normally up to one meter which makes the resulting series-inductance and stray capacitance undesirable for the optimal performance.

Furthermore, the disadvantages of the known systems have been that the electric fields from noise sources are relatively easy radiated into the electric circuit due to loops in the signal-path in which the electric fields induce currents.

Further disadvantages are that the electric fields from the signal sources are relatively easy radiated out of the electric circuits due to loops in the signal-path and the total length of the long signal-path.

Altogether the above-mentioned disadvantages cause a decrease of the bandwidth for the electric signal frequencies. Furthermore, conventional electric resistors designed with axial leads often cocsist of a resistor element mechanically terminated with metal end-caps. Like this the different materials in principle always form unreliable connections which cause non-linear electric resistances. The resulting distortion is a great disadvantage that affects the suitability for treating very low level signal frequencies and audio-signals for reference use in research and development of high fidelity signals.

The primary objective of the present invention is the provision of an improved attenuator of the nature referred to consisting of a rotary switch fitted with sections of printed-circuit boards including electric resistors. The present invention provides a shorter signal-path accomplished with a minimum of series-inductance, stray capacitance and distortion. The positive result of this is the advantageous change of the conditions for the afore-mentioned radiation conditions.

The new solution provided according to the present invention consists of the attenuator's electric resistors being so small that they can be mounted with the shortest allowable printed-circuit board traces to the wafer

terminals. And as an example these electric resistors can ideally be surface-mounted devices (SMD resistors) which have very low series-inductance and very low stray capacitance, and they can ideally be end terminated by means of a chemical process.

When applying the above-mentioned technique for the present invention, the bandwidth increases for the electric signal frequencies and the series-inductance and the stray capacitance decrease along with the signal distortion. The conditions for radiation of the electric fields change advantageously and the compact design eases the installation. The latter is specially important for upgrading existing appliances.

The present invention and preferred forms thereof are hereinafter described in details in the following with reference to the accompanying drawings in which:

Figure 1 illustrates an attenuator comprising SMD resistors according to the invention, and

Figure 2 illustrates an example of a conductive trace pattern for the printed-circuit board according to the invention.

Figure 1 illustrates an attenuator comprising SMD resistors according to the invention and its circuit diagram and symbol for one section.

The new solution shows the attenuator with the small SMD resistors mounted close to the wafer terminals according to the invention.

As shown in a particularly preferred form the SMD resistors (1), here 23 pcs. are placed very close to the wafer terminals (2), here 25 pcs. where the available area for mounting is utilized in an outer circle and an inner circle

near the wafer terminals by which the area surrounded by the loop formed by the signal-path is decreased. This configuration is optimal for the practically available components with regard to electrical conditions and mounting.

Figure 2 illustrates an example of a conductive trace pattern for the printed-circuit board on which the SMD resistors are mounted according to the invention.

As shown in a particularly preferred form the conductive trace pattern of the printed-circuit board has a dot-marking to indicate the placement position of the SMD resistors in order to avoid adjacent positions ("footprints") to be mistaken for mutual terminals ("solder lands ¹ *).