Sennheiser MKH 40-P48 User Manual download pdf (Page 7)

MKH 20...70

04/2005 7/53

AF amplifier MKH20...50

Since the AF circuits of the microphones MKH20...50 and MKH60...70 differ

considerably, they are described separately in the following sections.

The amplifier in the MKH20...50 operates with the complementary transistors T3

(or Q3) and T4 (or Q4) in push-pull Class A mode. On the DC side, the transistors are

connected in series, on the AC side they are connected in parallel via C12 and C14.

This increases the available AC output current, enhances the linearity, and reduces

the background noise. The base voltage divider R17, R18, R19, D5 determines the

operating voltage of the amplifier and stabilizes it against temperature influences.

The AF signal of the demodulator is supplied to the amplifier input via the high pass

C19, R16, and C11. The amplifier output is connected to the microphone output via

C15, C16, R22, R23, and the RF barrier L6, L7, and C17 to C20. The output impedance

is mainly determined by R22, R23, and the resistive portions of L6 and L7, and is

about 150Ω. The Z diodes D6 to D8 protect C14 to C16 from overvoltages.

For each microphone type, the frequency response of the capsule is equalized by

frequency-dependent negative-feedback circuits described for the individual types

in the following sections.

Equalization for MKH20

The series resonance circuit L5, C7, R7 in connection with R9 and R14 causes a treble

emphasis with a maximum at 20kHz. In the S2 standard position, R9 is

short-circuited and only R14 is effective. If S2 is opened (diffuse-field correction),

R9 is in series with R14 and causes a higher treble emphasis. R13 and C8 correct the

frequency response in the center frequency range.

Equalization for MKH30...50

The series resonance circuit L5, C7, R7 in connection with R9 causes a treble

emphasis with a maximum at 20kHz. In addition, C9, R11, and R12 cause a bass

emphasis. In the S2 standard position, R11 and R12 are connected in parallel. If S2

is opened (bass roll-off), only R12 is effective and the bass emphasis is reduced.

R13 and C8 correct the frequency response in the center frequency range.

AF amplifier MKH60...70

The amplifier integrated in both the MKH60 and the MKH70 is a two-stage

amplifier. The pre-amplifier with T3 (or Q3) equalizes the capsule frequency

response. The output stage operates as a high-pass filter at low frequencies.

The AF output signal of the demodulator is supplied to the base of T3 (or Q3) via

the high-pass of C6, C7 in connection with the base voltage divider R10, R11. S2 can

be used to switch the operating frequency of the bass roll-off.

Frequency response equalization for high frequencies is realized by the

negative-feedback network R9, R12, R13, C8, and L4. S3 can be used to switch the

operating frequency of the treble emphasis, so that the result is either a neutral

frequency response or a frequency response with treble emphasis.

The collector resistance of T3 (or Q3) results from the dynamic internal resistance

of the oscillator circuit and is very high in the AF range due to the current

stabilization.

The output signal of the pre-amplifier is supplied to the output stage via C12 and

C13. The output stage operates as an active high-pass with voltage amplification 1.

The quality is determined by a positive feedback via R14 in interaction with R15

and R17. T4 (or Q4) and T5 (or Q5) operate in complementary push-pull Class A

mode. On the DC side, they are connected in series, on the AC side, they are

connected in parallel via C14 and C16. This increases the available AC output

current, enhances the linearity, and reduces the background noise. The base

voltage divider R15, R16, R17, D3 determines the operating voltage and stabilizes

it against temperature influences.

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Sennheiser MKH 40-P48 User Manual Page 7

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