Hum after inserting external speech proccessor

[S57W]

I had used Heil Proset with HC4 mic at least 15 years with my IC-781. At the beginning there was some noise (hum), so i wrapped mic/headphone cable over ferrite rod (4 inches long).(I didn´t checked ferrite material, but the thing helped.) The hum dissapeared, my audio was excellent..
In the May this year I heard one U.S. ham using TEN TEC 715 RF Speech Processor. I was positively surprised with audio, I had heard. I bought the new toy, just inserted it between mic and transceiver. The hum again appeared.. Any ideas? Thanks..

Igor, S57W

[W9GB]

The Pro Set is available in four versions: the Pro Set 4 (PS 4) is supplied with the HC-4 "DX Dream Machine" element, with an aggressive low-frequency cut-off and high emphasis at 2000 Hz, for excellent pile-up "punch."

I use a Heil Proset with HC4 element. At the beginning there was some noise (hum), so i wrapped mic/headphone cable over ferrite rod (4 inches long). The hum dissapeared, my audio was excellent.
In May, I bought and installed the TEN TEC 715 RF Speech Processor between Heil headset and Icom IC-781 transceiver. The hum again appeared.. Any ideas? Thanks.

You are using a dynamic microphone element (HC-4) with an Icom transceiver -- that places DC voltage on its microphone (+) line!
http://heilsound.com/amateur/dspsettings/all_icom.php

Hopefully your Heil headset has the DC blocking capacitor installed (see Heil hyperlink above).

You may also have a GROUND LOOP (common in audio interconnections).
Are you using the Heil Adapter cable? OR Did you build your own adapter cables?

You should NOT tie the Audio GND .... to the PTT GND.

[KC8VWM]

Is the placement of the speech processor box in proximity to a power supply or a similar transformer? EM fields have a funny way of getting into the audio chain.

[S57W]

Thank you very much for your answer.

The hum appeared after inserting external RF Speech Processor..

I will check if there is the DC blocking capacitor installed (thanks for Heil Hyperlink).
(A am not using original Heil Adapter cable).
I will check if a have a GROUND LOOP, and check if there is no connection Audio GND ---> to the PTT GND.

Thanks again, Igor, S57W

[S57W]

Gregory (W9GB), thanks again for reminding me about possible GROUND LOOP..

I`ve just read an excellent article about Ground Loops in NCJ July/August issue. (The name of the article is RTTY Contesting, on page 18, author is Don Hill, AA5AU).

Igor, S57W