Microphone Preamp Low Noise Balanced Circuit DiagramThe ascribe date is configured for atomic babble and this has meant a non IC approach. There are some appropriate ICs that can be acclimated for mic pre-amps, they accommodate a ambit like this one except bogus on one chip. Examples accommodate the SSM2017 (now obsolete) or the backup INA103 or similar.
Components should all be readily accessible except for the 10 k ohm pot for the accretion control. This needs to be a about-face log abate - or abroad use a multi-position about-face with 6 dB accretion accomplish accoutrement the 60 dB ambit of the circuit. Accomplish abiding it is accomplish afore break.
Editor's Note - Alternatively, a accepted log pot can be used, but active "backwards". This will assignment accomplished if it is labelled "Attenuation" instead of "Gain". As the pot is avant-garde clockwise, the accretion is bargain (attenuation is increased). Maximum accretion will accordingly be activated back the pot is absolutely anti-clockwise. Note that this is not a botheration that is specific to this ambit - all the IC mic preamps accept absolutely the aforementioned problem.
The +/-15 Volt ability accumulation is important too, it charge be adapted and low noise. If the accepted voltage regulator ICs are acclimated I acclaim applicable a column clarify consisting of a 10 ohm resistor and a 470 uF capacitor to abolish any babble generated in the ICs (as apparent in Figure 1). Some 7815 ICs could be awash as babble generators, the adjustable voltage ones (LM317, LM337) are actual abundant quieter. A distinct regulator lath may be acclimated to ability assorted preamps, with anniversary preamp accepting its own column clarify circuits. Because of the all-encompassing clarification applied, the P05 (Rev-A) ability accumulation is recommended for this preamp.
Good affection apparatus should be acclimated with metal blur resistors in the collectors and emitters of the ascribe pairs for atomic noise. Where a resistor has cogent DC voltage imposed on it in aerial accretion circuits consistently use low babble types. Metal blur resistors are about the best alone bettered by wire anguish which is a bit impractical. Abstain cermet, metal glaze, and actual old carbon agreement types. Also abstain bean tantalum capacitors, as they go adulterated and crackle. They are aloof about the best brittle cyberbanking apparatus made. The 100nF capacitor (C6) should be army as abutting as accessible to the opamp accumulation pins - a bowl cap is recommended for best bypass achievement at aerial frequencies.
The 1000uF capacitor can be a accustomed electrolytic of 10 or 16 volts rating. There is usually no botheration with aught DC bent on avant-garde electros. All added electros should be 25V appraisement as a minimum.
Upon blockage the appear specs for the SSM2017 in commendations to noise, my branch adaptation of the preamp measures at atomic as acceptable with a 200 ohm antecedent attrition (typical of best activating microphones).
EIN = 0.27 uV rms in 20 kHz bandwidth with 200 ohm source.
= 1.9 nV per root Hz (equal to spec for SSM2017)
Noise Figure = 0.9 dB rel 200 ohms.
By Phil Allison
