"Howling" when volume is above 10, but only with narrow CW filter

[thrillscience]

The speaker (internal or external) squeals and howls when the volume is above 10, but only in CW mode. In USB/LSB mode or AM, the receiver works well.

I've seen people complain about the howl, and blamed it on "speaker self resonance" but I'm not sure because this also happens with an external speaker or earphones.

So I realized I can change the filter bandwidth on CW, and when I moved it to 3 kHz, which is what my USB settings was, the howl went away.

I haven't seen anyone else talking about this. Could there be something odd with my hardware that's making the narrow filters howl? Or is this a software issue?

I have an authentic (tr)uSDX from an approved vendor, and the CPU board was pre-assembled. (I'm running a high-band board I assembled on it.)

[dl6sez]

Hello OM,

you can read about this here in the forum... use Headphones with higher impedance, for example 32Ohm
but it is normal behaviour due to the fact of we are using the output of the Atmega328 without amplifier and the Cs C25 to C28 in X7R ceramics due to little 0603 SMD so necessary and they are microphonic.
If you substitute them with large Styroflex capacitors or other non ceramic Cs and make all 100Ohm resistors and C25 to C28 as equal as possible you will improve it a little bit
But i don't think it is worth the effort

73 de Chris

[thrillscience]

Thank you!

I saw people talking about "howling" but I've never seen mention of it related to the filter width.

Frankly, I'm OK with using the "SSB" filters in "CW" mode. It's OK.

What would cause the howling to only appear with the narrow filters? I'm trying to think of why that would matter.

It's actually a decent receiver for what it is on the 3kHz filter.


Edit: I found the reference to what you're talking about, not in the forums but in the FAQ

https://dl2man.de/troubleshooting/

I think I'll try the capacitor

[dl6sez]

Hello OM,

this solution you found is for the former version the uSDX not for the (tr)uSDX because the (tr)uSDX has already the 470uF tantal electrolytic in the circuit......
The (tr)uSDX wass developped after the uSDX and all experience we learned there was added into the (tr)uSDX
So use higher impedance headphones is your only choice for getting it better without an additional amplifier for your headphones or speaker.

73 de Chris