Hasling Professional Microphone Amplifier

In December News I wrote about designing a professional microphone preamplifier. The first part (channel1) has been finished, with a super low noise differential instrumentation amplifier, with balanced in- and output transformers, 600 Ohm in and out, output loaded with a 600 Ohm resistor. 

There is 48 V regulated phantom power supply for condenser microphones, which of course can be switched off for dynamic microphone types,  and dual adjustable regulated power supplies with extremely low noise.

My goal was to design an amplifier with extremely low noise and distortion, and with a switchable calibrated gain. I have made this with 9 steps, each step with 4,0 dB more (or less) gain, giving up to 36 dB total gain variation.

With pad switch out, lowest gain switch pos. 1 is 7,8 x = (17,8 dB), highest gain switch pos. 9 is 470 x = (56,6 dB).
With pad switch in, lowest gain switch pos.  1 is 1,25 x = (1,8 dB), highest gain switch pos. 9 is 77 x = (37,8 dB).

This gives gain choice between 1,25 x and 470 x and calibrated.  What a choice!

At excessive sound pressure to the microphone, the amplifier can be overdriven and completely distorted (clipped). To avoid this a  pad (resistive attennuator) is used before the input to the amplifer, to prevent overload of the amp.

I have made a 2 position pad with 16 dB damping of the input signal, but will possibly make it a 3 position pad, to achieve a 30 dB choice total input dampening before the amp (drums).  Especially the condenser microphones can produce really high outputs here.

The condenser microphones give a higher output than the dynamic types, and because of the high impedance of the condenser types, there is normally an amplifier built into the microphones, also to have a low drive impedance out, differential and in the order of 100 Ohm.

The last thing I would like to mention is the CMRR factor (common mode reduction ratio).  This is the amplifiers ability to not amplify common mode signals, but only to amplify differential signals.  This is in order not to pick up unwanted hum and noise signals, when connected to other amps (read mixers) through screened unbalanced inputs here.   Values should be – 70db CMRR or better at 1000 Hz.

Distortion measurements below.

                                                               Per Hasling    March 07, 2011

111.1 Dynamic range 111 dB to noise floor - third harmonic -107 dB,
the rest lower – THD 0,00175 % - SNR (signal noise ratio) 95,227 dB.
Input 4,5 mV rms – output 44,0 mV rms.

111.2 Dynamic range 117 dB to noise floor – second harmonic -104 dB,
the rest lower – THD 0,00105 % - SNR (signal noise ratio) 99,572 dB
Input 8,8 mV rms - output 90,0 mV rms.

111.3 Dynamic range 124 dB to noise floor – second harmonic -110 dB,
the rest lower – THD 0,00061 % - SNR (signal noise ratio) 104,333 dB
Input 17,4 mV rms – output 170,0 mV rms.

112.1 Dynamic range 106 dB to noise floor - third harmonic - -96 dB,
the rest lower - THD 0,0035 % - SNR (signal noise ratio) 90,103 dB
Input 4,3 mV rms - output 28,3 mV rms.