This is a circuit for microphone pre-amps. This is very low noise design circuit, close to the theoretical minimum, high hum rejection and variable gain with a single rotary pot. This circuit is configured for least noise and this has meant a non IC approach. This is the figure of the circuit.
The +/-15 Volt power supply is important too, it must be regulated and low noise. If the usual voltage regulator ICs are used I recommend fitting a post filter consisting of a 10 ohm resistor and a 470 uF capacitor to remove any noise generated in the ICs (as shown in Figure 1). Some 7815 ICs could be sold as noise generators, the adjustable voltage ones (LM317, LM337) are very much quieter. A single regulator board may be used to power multiple preamps, with each preamp having its own post filter circuits. Because of the extensive filtering applied, the P05 (Rev-A) power supply is recommended for this preamp. Good quality components should be used with metal film resistors in the collectors and emitters of the input pairs for least noise. Where a resistor has significant DC voltage imposed on it in high gain circuits always use low noise types. [Circuit Source: Phil Allison].
The +/-15 Volt power supply is important too, it must be regulated and low noise. If the usual voltage regulator ICs are used I recommend fitting a post filter consisting of a 10 ohm resistor and a 470 uF capacitor to remove any noise generated in the ICs (as shown in Figure 1). Some 7815 ICs could be sold as noise generators, the adjustable voltage ones (LM317, LM337) are very much quieter. A single regulator board may be used to power multiple preamps, with each preamp having its own post filter circuits. Because of the extensive filtering applied, the P05 (Rev-A) power supply is recommended for this preamp. Good quality components should be used with metal film resistors in the collectors and emitters of the input pairs for least noise. Where a resistor has significant DC voltage imposed on it in high gain circuits always use low noise types. [Circuit Source: Phil Allison].
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