2MLDPA RX Hum

Butch

Question for those using the 2MLDPA. When receiving 2M FM signals I hear a low frequency hum. I've had this low frequency hum since the 1st day I installed my 2MLDPA. Initially I thought the hum might be caused by bad power supply, so I changed them, but no difference. Currently I'm using different linear power supplies for my Flex-6700 and 2MLDPA (Astron VS-50 for my Flex, and Astron VS-35 for the 2MLDPA). Both power supplies are in excellent condition. I don't hear any hum when I open the 2M squelch, only when I'm receiving a FM signals. And I get no hum when monitoring the other bands. My work around has been to just adjust the RX EQ to reduce the 2M FM low frequency hum, but I'm curious if anyone else has notice this with their 2MLDPA, and what might be causing this?

Butch, KF4HR

Geoff AB6BT

Is it on all signals ? Perhaps the transmittimg station has CTCSS enabled?

Butch

Geoff, yes the low frequency hum is heard on all 2M FM signals. The hum doesn't override the incoming 2M audio, but it's there.

Butch, KF4HR

Geoff AB6BT

OK, well it was worth a shot. Is it always the same frequency?

Mike-VA3MW

It is the repeater sending a CTCSS (Private Line) sub audible tone.

Most repeaters send them (there are many different ones) and most radios will cut off the lower 250Hz of the FM signals.

Currently, your FlexRadio does not have a High Pass Filter in place to pass just the audio above 250Hz.

Butch

Hi Mike - I think you might be right. I found a few repeaters in my area which don't product that low hum on my 6700/2MLDPA setup. Seems the hum is coming from the repeaters which have the capability of being linked. These repeaters use a 131.8hz input CTCSS tone, but perhaps they are also transmitting a 2nd tone. Anyway thanks for your input, and letting me know the 6700 doesn't have HPF, that explains why my other 2M rigs don't hear the hum. I'd venture a guess that adding a HPF to the 6700 isn't high on FRS's priority list so perhaps I need to get busy and design one.

Butch, KF4HR

Martin_G4ENZ

Butch,

The RX Equalizer does a good job of filtering CTCSS out on FM.

73 Martin G4ENZ

Butch

Hi Martin, yes that's what I've been doing. I normally list to monitor HF and VHF at the same time and unfortunately cutting the low EQ RX sliders to reduce the low frequency hum on VHF, doesn't make the HF slice audio sound very good. But it's a minor issues. Such is life with a Swiss Army Knife! :)

Butch, KF4HR

Stan VA7NF

I have had that feature (problem?) for years on my 6700 which is based entirely on its exceptional audio quality. It shows especially when SmartSDR feeds my computer's theatre sound system.

If you don't want to hear the low bass then back off the RX EQ bass portion.