simple compressor

[PA0NVD]

For my AM transmitter I did experiment with some compressor  principles. Basic, an opamp and a J-Fet in the neg feedback to adjust the gain using a rectified sample of the output. It worked, but it was a little disappointing, Not too much dynamic range and by driving hard quite some distortion visible. So I tried a LDR resistor in the neg feedback to avoid the distortion driven by a white LED. That worked a whole lot better. A by coming advantage is that the LDR becomes quite slow with the low light used (the LED pulls only a few micro amps), so smoothing low level signals nicely and reacting much faster at peaks where the light level increases.
No visible distortion in a 1 : 100 input voltage range from 2 mV to 200 mV and an output constant within 10%. and, more simple impossible, you don't even need to smooth the rectified output, tha LDR des that for you
I glued the LDR and a SMD LED together and covered them with black tape
To complete the experiment I did add a 3 channel tone control and a low noise J-FET input to be able to connect all types of microphones

[PA0NVD]

here the 20 mV input and the 200 mV input scope pics

[WD5JKO]

Nico,

   That circuit is very simple. I wonder what the attack time is before the gain is reduced? Sometimes you need to follow a compressor with a peak limiter or clipper.

I attach two files from fellow member Georgio IN3IEX. You may find those interesting.

Jim
Wd5JKO

[PA0NVD]

Indeed interesting Jim, Thanks
I did test the compressor at the transmitter, and made a little change. The max gain was really too much, the LDR goes to infinity. When it was silent, the sound of flies and ouside birds started to modulate 50% !! .   I did put a 680 kOhm across the LDR and reduced the gain pot. I also added a 100 Ohm is series with the output, the op-amp did not like the capacitive load of the coax to the transmitter
I added a limiter just before the compressor as a neg feed-back over the output amp of the tone control with two Si diodes. That did cut occasional peaks.
You can adjust the reacion time with the resistance values of the neg. feedback, the pot and the 680 kOhm across the LDR. When you use lower value's, the LDR needs more current and becomes faster.. I am very happy with the attack time using 680 kOhm across the LDR. I use a small Chinese LDR and tested various. They all seem to perform the same.
I did test with an Electret microphone, really amazing. I consider the sound quality excellent!!. Compared to the normal mike, the average modulation and the loudness increases considerably. and it sounds really fine
I will keep it like it is and will make a case around it to use as a standard for my Philips transmitter

[PA0NVD]

By the way, when you glue a white SMD high brightness LED like used in lighting directly at the face of the small LDR, you need a whole lot less current as in the graph. I run it in the micro amp range.  The LED LDR coupler in the graph seems very inefficient to me

[PA0NVD]

Here it is . Only missing the sponge around the microphone. It carries a 9 V battery. The sound quality is really amazing. I tried several mics, also a D104 crystal  mike. But the small Electret beats them all. Tried several Electret microphones, the 6 and the 10 mm. Actually very little difference, all excellent. Only the electret microphones out of the low cost toys are sounding bad, but all I bought from China are quite amazing. Really a surprise to me, perhaps it is also the compression. The attack time is fast, no plopping or what so ever. The reason may be that there are no caps to be charged when the compressor kicks in. seems instantly

[PA0NVD]

and the complete diagram

[KD6VXI]

Below the 12k resistor on the low eq adjustment looks to be a part missing its value.

Or maybe I'm too tired to see it lol.

Great looking project...  Would love to hear it in your station....  Someday I'll have the time to set mine back up.

--Shane
KD6VXI

[PA0NVD]

Indeed Shane, sorry for that omission. that resistor (from the wiper of the low adjustment) is also 12 kOhm
I will be on 40 now, but with a very small antenna, so would be difficult. I am at 7.295 MHz

[Opcom]

How fast do the compressor circuits you are buiding work on sudden increases or pulses?

With op-amps and LEDs and so forth, it seems it should be almost instantaneous even clipping if pushed hard enough. I mean, does it respond within less than 100us? I always hate the pop or other noise that gets through and tried to clip the modulator.