Preamplifier with active volume and tone controls. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Preamplifier with active volume and tone controls. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Preamplifiers

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The preamplifier with active volume and tone controls is designed to work in a high-quality stereo power amplifier with a sensitivity of 0,75 ... 1 V.

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The main parameters:

Rated frequency range, Hz.......20...20 000
Rated input voltage (with an output voltage of 1 V), mV.......250
Input impedance, kOhm.......140
Load resistance, kOhm, not less than ....... 10
Volume control range (at a frequency of 2,5 kHz), dB.......56
Tone control limits (at frequencies of 30 and 20 Hz relative to a frequency of 000 Hz), dB.......±1000
Harmonic coefficient, %, no more (volume and tone controls in positions corresponding to maximum volume and maximum frequency response rise) ...... 0,03
Signal-to-noise ratio (unweighted) dB, not less than (regulators in the positions of maximum volume and linear frequency response) ...... 80

Each of the preamplifier channels (the figure shows a diagram of one of them) consists of a source follower on a field-effect transistor V1 and active volume and tone controls, made respectively on transistors V2, V3 and V4, V5.

A source follower is required when using the amplifier with signal sources with high output impedance. If the device is used in a tape recorder or player, the corrective amplifiers of which, as a rule, have a low output impedance, the repeater can be excluded. In this case, the signal is applied to the capacitor C2 (by changing the polarity of its inclusion to the reverse) through a divider made up of resistors with a resistance of 8,2 kOhm and 910 Ohm.

Amplifiers of active volume and tone controls are identical in scheme and differ from each other only in the ratings of some elements. The ratios between the ratings of the elements C2, R6, C3, R8, R12, C4 of the active volume control are chosen so that for a given output impedance of the source follower (600-1000 Ohms) to provide the best loudness over the entire control range. The gain boost required at low volume at lower and higher frequencies is created by capacitors C4 and C3, respectively. The capacitance of the capacitor C2 is selected from the condition that there is no rise in the frequency response at maximum volume. The volume is regulated by a variable resistor R8, stereo balance - by a variable resistor R29.

The elements of the tone control are included in the OOS circuit, covering the amplifier on transistors V4, V5. At lower frequencies, the timbre is regulated by a variable resistor R20, at higher frequencies - by a resistor R22. When using the tone control in other paths, it should be borne in mind that the output impedance of the previous stage should be no more than 700 ohms.

The voltage regulator on transistors V6, V8 allows you to use any unstabilized source to power the amplifier.

The amplifier can use any high-frequency silicon transistors of the appropriate structure with a static current transfer coefficient of at least 400 (V2), 300 (V4) and 100 (V3, V5). It is desirable to select capacitor C2 with an accuracy of at least 10%. If this is not possible, select capacitor C4, the capacity of which should be 0,05 of the capacity of capacitor C2. The V8 transistor must be provided with a heat sink in the form of a flag bent from sheet copper or brass with a thickness of 0,5 ... 0,6 mm.

The adjustment of the amplifier is reduced to the selection of the resistor R4 until a voltage of 1 V is obtained at the source of the transistor V6. The operating modes of the remaining transistors are set automatically.

See other articles Section Preamplifiers.

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