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MKH 20...70
04/2005
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 150
Ω
. 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.
Summary of Contents for MKH 20
Page 16: ...MKH 20 P 48 16 53 04 2005 MKH 20 circuit diagram ...
Page 22: ...MKH 30 P 48 22 53 04 2005 MKH 30 circuit diagram ...
Page 28: ...MKH 40 P 48 28 53 04 2005 MKH 40 circuit diagram ...
Page 31: ...MKH 50 P 48 04 2005 31 53 Frequency response Equalization Polar diagram ...
Page 34: ...MKH 50 P 48 34 53 04 2005 MKH 50 circuit diagram ...
Page 37: ...MKH 60 1 P 48 04 2005 37 53 Frequency response Equalization Polar diagram ...
Page 41: ...MKH 60 1 P 48 04 2005 41 53 MKH 60 circuit diagram ...
Page 44: ...MKH 60 1 P 48 44 53 04 2005 ...
Page 47: ...MKH 70 1 P 48 04 2005 47 53 Frequency response Equalization Polar diagram ...