Touch control volume control. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Tone, volume controls
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The volume control with touch control is designed to work with a power amplifier having an input impedance of at least 10 kOhm and a rated input voltage in the range of 0,1 ... 0,7 V.
The main parameters:
- Rated frequency range, Hz.......20...20 000
- Volume control range, dB.......0...-50
- Transfer coefficient at maximum volume (0 dB) ....... 1
- Harmonic coefficient (with an input voltage of 500 mV), %, no more.......0,35
- Relative noise level, dB.......- 54
- Loudness depth, dB, at minimum volume at frequency, Hz 30.......30
- 18 000......30
The device is assembled on the basis of a five-channel integrated switch K190KT1.
Two of the field-effect transistors included in it are used directly to control the volume, the other two are used as keys in the V1, V2 LED circuits, indicating levels of -40 and -20 dB, and one is to compensate for the attenuation of the signal introduced by the regulator.
When the power is turned on, the voltage on the "memory" capacitor C3 is zero, so the transistor to the gate of which it is connected is closed, and LED V1 is not lit. At the same time, the transistors that control the volume are open (a negative bias voltage is applied to their gates through resistors R11, R13), the resistances of their channels do not exceed several hundred ohms, so the signal circuit is actually closed to a common wire through high-capacity capacitors C4*, C6. LED V2 does not light up, because the voltage at the gate of the transistor, in the drain circuit of which it is connected, is small.
The volume increases when touching the touch contacts El and E2, when the capacitor C3 begins to charge from the power source through the resistors R1, R3 and the resistance of the skin of the finger. As the voltage across the capacitor rises, the resistance of the transistor channel in the V1 LED circuit decreases. As a result, the voltage at the gates of the regulating transistors also decreases (circuit pins 3 and 7), and the resistance of their channels and the volume increase. At a signal level of -40 dB, LED VI starts to glow, at a level of -20 dB, LED V2.
When the touch contacts E2 and E3 are touched, the control process proceeds in the opposite direction and the volume decreases. The set volume level decreases by no more than 3 dB over a period of 2 ... 3 hours. Loudness at low volume is provided at lower frequencies by capacitors C4, C6 (the compensation depth is adjusted by selecting their capacitance within 0,5 ... 2 μF), at higher levels - with an oscillatory circuit L1C2 tuned to a frequency of 18 kHz (the depth is controlled by selecting a resistor R6).
In order for the time to maintain the set volume level to be large enough, the touch contact E2 must be well insulated or the common wire, and the capacitor C3 must be selected with a low leakage current (capacitors K76P-1, K73-11, etc. are suitable).
Setting up the device comes down to setting the operating mode of the transistor used in the cascade, which compensates for signal losses in the regulator. To do this, connect the contacts E1 and E2 with a wire jumper, apply an alternating voltage of 0,5 V with a frequency of 1 kHz to the input and achieve the maximum undistorted signal at the output with a tuning resistor R15. The transfer coefficient equal to 1 is set by selecting the resistor R17 *, after which the jumper wire connecting the contacts E1, E2 is removed.
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