Audio Preamp
Circuit : Andy Collinson
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Description
A 2 stage audio pre-amplifier with very good performance.

Specification
Power 12 Volts nominal 1.6mA current drain
Gain 27dB
Bandwidth 9Hz to 127kHz -3dB
Input Impedance 99.3Kohm @ 1Khz
Output Impedance 127ohm @ 1kHz
Noise 760uV input referred noise from 1Hz to 10MHz
THD 0.11% @ 6V pk-pk 1KHz
SNR 107dB @ 1KHz

Notes
This preamplifier has two direct-coupled common emitter stages and uses series-voltage feedback. The circuit has high input impedance,low output impedance, voltage gain can be set independently and has low noise levels and good signal noise ratio.

For a moment, consider the circuit without R3 and C3. Q1 operates in common emitter, its collector voltage directly-coupled to Q2 and sets the bias for Q2. Output is taken from Load resistor R5 and the emitter voltage of Q2 is developed across R6. The emitter voltage is used to bias Q1 base fed via R4. This feedback provides DC stabilisation of the circuit. Q1 is operated at a low collector current of 215uA for has low noise. If R5 did not exist and output was taken from the emitter of Q2 the circuit would be identical to the ECM Mic-Preamp circuit. The combined gain of both stages is high, about 60dB.

The inclusion of R3 and C3 sets an additional feedback loop from Q2 collector to Q1 emitter. R3 takes the collector voltage of Q2 and feeds it back to Q1 in series. This is series-voltage feedback and sets the overall gain of the circuit to:

AV = 1 + R3
R2

With R1=1k and R2=22 this is a voltage gain of 23x or 27.2dB This figure can be seen in the bode plot below. All plots and analysis are made with LTspice. C3 rolls off the high frequency response and sets the high frequency cut-off point.

The BC549C is a plastic package version of the BC109C. Alternative low-noise transistors could be used like BC650C for even better noise figures. The noise figures calculated by Spice assume a perfect power supply. In the real world, any noise on the power supply is likely to be higher than the preamplifier. A 7812 regulator is quoted as having 76uV of noise on its supply rails, this is 1000x greater than the preamp.

Bode Plot
As mentioned previously, gain is set by the ratio 1 + R3/R2. High frequency roll-off is set by C3 and low frequency cut-off point by C1. Although the human ear cannot detect frequencies higher than 20kHz, some audio enthusiasts believe that harmonics contain high frequency detail. I am sure this is true but you would have to have very expensive hi-fi equipment to prove this theory.

Input Impedance
Series voltage feedback increases the input impedance. The input impedance is 99.3k for most of the audio spectrum.

Output Impedance
Series voltage feedback decreases output impedance. As shown below the output impedance is quite low at 127 ohm. The white vertical line is the cursor..

Transient Response
The output waveform is unclipped at 5V peak to peak. At 5V pk-pk there is just 0.11% THD distortion. The input signal level was 120mV. Inputs higher than t120mV will increase the THD.

Harmonic Distortion
As calculated by LTspice, the first nine harmonics are shown below, measured across a 10k load resistor.
Fourier components of V(out)
DC component:-0.0350794

Harmonic	Frequency	 Fourier 	Normalized	 Phase  	Normalized
 Number 	  [Hz]   	Component	 Component	[degree]	Phase [deg]
    1   	1.000e+03	2.689e+00	1.000e+00	    0.31°	    0.00°
    2   	2.000e+03	1.976e-03	7.350e-04	   91.94°	   91.63°
    3   	3.000e+03	1.980e-03	7.363e-04	    0.24°	   -0.07°
    4   	4.000e+03	4.749e-04	1.766e-04	  -89.84°	  -90.16°
    5   	5.000e+03	2.891e-04	1.075e-04	   -1.50°	   -1.82°
    6   	6.000e+03	4.255e-04	1.583e-04	  146.14°	  145.83°
    7   	7.000e+03	2.219e-04	8.252e-05	 -149.83°	 -150.15°
    8   	8.000e+03	4.461e-04	1.659e-04	 -174.30°	 -174.61°
    9   	9.000e+03	8.429e-04	3.135e-04	 -162.90°	 -163.22°
Total Harmonic Distortion: 0.113260%

Noise
The noise spectrum is shown below. The noise is referred to the input and peaks at 0.75uV. There are two main sources of noise in transistors, shot noise and thermal noise. They are present in every transistor.

Signal to Noise Ratio
With use of the .measure command it is possible to produce a graph of Signal-to-Noise ratio versus frequency. This clever technique was first demonstrated perfected by Helmut Sennewald, on the Yahoo LTspice forum1.

Please remember this circuit is mono, so build two for a stero preamp or four for a quad preamp. I will show details of the simulations used later in a ltspice tutorial and an analysis of this circuit using the hybrid pi model in a later update on Circuit Exchange.


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