Audio: 2 stages Transformer Audio PA

ar/21 2016

these days, i built several simple receiver with transformer Audio PA. cascade and impedance match problem is interesting.

Audio Amplifier: PA

test condition, 4:1 transformer, 450 ohm earphone

0.6v/20mV = 600/20=30 @690HZ
0.8v/20mV = 40@1.8khz
120mV output with 20 mV input, voltage gain 6 @690HZ
input impedance 1.2k

cascaded with a pre-amplifier:

4:1 transformer, use 64 ohm earphone:

Gain : 220 (raw test data: -50 dbm audio(3.3 mVpp), output 700mV)

Input impedance: 3.6k (raw test data: 600ohm 35 mVpp, after connect to amplifier 30mV)

LTspice simulation with NPN transistor

i don't have any LTspice mode same as the transistor i used, so just pick 2SC9018, a low beta but high Ft transistor for the simulation, and it's NPN.

64 ohm load @600Hz

Gain: 280 voltage gain (48dB)

Input impedance: ~ 2.3k

Single stage Gain, PA stage:

64 ohm load @600Hz

Gain: 9.5 voltage gain (19dB)

Input impedance: ~ 1.2k

Single stage Gain, pre Amplifier:

1.2k load @600Hz (PA stage input impendace)

Gain: 30 voltage gain (30dB)

Input impedance: ~ 2.3k

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