ECLECTIC AETHER - Adventures with Amateur Radio

Speech Intelligibility

Articles published by the Radio Society of Great Britain in the February and March 2009 issues of Radcom

Here are some emails from happy constructors - I've snipped some parts out which didn't refer to this design.

?Hi Martin,


One word - Wow!!!


I built your equaliser design on veroboard from junk box parts, hooked it up to the PC sound card and used Audacity to compare the sound from my MFJ-393 headset/boom mic with and without the equaliser. There is just no comparison!


Incredible difference. I'm working hard on WAS SSB this coming summer, have it nailed on Digi and CW, just need about 12 states on SSB to finish the triple play award. Good punchy audio will certainly help!


I also bought the chips for your compressor circuit as I think the audio/processor stages in the 440 are getting a bit "soft".


73 and thanks for the simple, effective circuit.


Geoff ZL3GA?




? Dear Martin


A friend of mine gave me the copy of your article ( Radcom, March 2009 ) " Improving the intelligibility of SSB transmissions ". Mainly on HF I have a lot of problems with my voice ( a lot of bass and few high ) but making and using your circuit my modulation had a dramatic improvement in quality.


Now I?m using YOUR microphone on my station rig Yaesu 450, more on the small Yaesu 817 for portable and QRP and on Kenwood TS50 I've installed in my car for mobile operations : no more critics now, every time only compliments for my modulations ! 


So my complements to you, and I hope you will enjoy pictures of my realisation, an old microphone case with enough space inside. In both cases the voltage is supplied by RTX  mic connectors : 5 V with last Yaesu models and 8 V from Kenwood.  I'm very satisfied, thanks to you.


CIAO ! Marco IK5BHN? 



Marco experienced a few problems with RF feedback when he used it for portable operation, so he built another version inside a small TEKO 2A screened metal box.



He also noticed that although the circuit worked well with Yaesu radios, he had a problem with a Kenwood TS50 and some other Kenwood models where there was occasionally some low frequency motor-boating occurring. After a lot of tests, he cured the problem by removing C8 and replacing R6 with a trimmer, either the same value or a max of 4,7 Kohm, with the microphone output taken from the slider of the trimmer. My thanks to Marco for this information.

Picture of another version built by David, G4RQI

PDF copies of the Racom article can be found here:-

Part 1 details of basic principles

Part 2 construction of a simple equaliser circuit

Article reprinted in:-

Danish Amateur Radio Society magazine OZ August 2009 (warning 5.3Meg file)

Italian translation  by Nico Michelini, IV3ALA

 Sound samples of equaliser circuit with different preset pot resistance values

equaliser sounds vs pot settings
Sound sample of equaliser circuit via ssb transmitter with different preset pot resistance values
equaliser via ssb transmitter vs pot settings

Useful free software:-

Voice Shaper allows you to simulate different equaliser and compressor settings using a PC and sound card

Audacity PC based sound recording and editing software

Frequency Response Plotter Designed for testing loudspeakers, but great for looking at microphone equalisation