Still working on DSP version :-). However, this time let’s have a look at the very simple (but still working) version of audio compressor:
The above schematics was taken from a very well known in SP book by Andrzej Janeczek SP5AHT (famous Polish constructor), called: „Ham-radio constructions for begginers”. See also the PCB and mounting in an „old-school” nice style:
Now, let’s have look at the LTSpice electronics simulations. I decided to slightly „refresh” the circuit – using currently available and popular semiconductors. I’ve also omitted in the simulation power supply filtering resistors and capacitors as the power supply of V1=12V is „ideal” here:
Some changes had to be done to the values of the rest of R-C components in order to keep well-performing characteristics. See the frequency response of the circuit (small signal AC analysis) for the range from 100Hz to 10kHz:
The gain in the middle of SSB audio bandwidth is about 44 dB while -3dB bandwidth is limited to ca. 2,3kHz.
The compression characteristics, taken from the time domain (transient) analysis, are as follows below:
On the upper plot we see the input signal V(input) growing up from 0,4 mVp-p to 22 mVp-p (typical for electret microphones with internal JFET amplifier), while on the lower one there is a plot of V(output) – changing from 60 mVp-p to about 330 mVp-p. Thus, the initial gain of about 44 dB is reduced downto 23,5 dB. So, the experienced compression ratio is even slightly higher than 20 dB, but at the cost of THD coefficient (Total Harmonic Distortions) of more than 10% on the output of the compressor. However, it should be stressed here, that it is absolutely justified in the simple limiting circuit – like this one :-).
(to be continued soon …)