Thank you for your input John. Indeed I’m already convinced by the filters, on my sota rig I have them electronically, I can tune them on the go as I want them to be. It’s fantastic !
I will check the second hand market for a few weeks and see if good opportunities show up locally !
I learnt my trade using a home brew RockMite so I feel that I’m pretty decent at filtering signals using my ears. I got a 300Hz CW filter secondhand for my FT-817 and I really like it. The filter is Yaesu brand and supposedly they’re more like 500Hz anyway due to the skirts.
I tried a NEQRP NESCAF kit with my RockMites and the results were impressive to be honest, better than I expected.
I built myself a SCAF unit on a tuna can for my QRPp rigs and it works well.
An IF filter is the best option (beyond training the ears) but an AF filter can definitely help.
73, Colin
Wow I love it !
As I tried to explain before, my problem is not with isolating the signal in my head. The problem is that I cannot stand the QRM. When someone is sending through a very busy band and as chaser I don’t make contact in the first minute or so, I just quit. All the high pitched beeps from the stations above are killing me.
Using lower fidelity headphones helps with the high frequency stations.
While I like crystal filters in my rigs, I find that adding some audio filtering does make things much more tolerable, as it also removes noise that gets added after the detector stage.
And for very weak signals, my old analog(ue) radios with crystal IF and op-amp audio filters are still my preference: they are devoid of the digital “crud” that I hear in the digital equipment.
Is an external audio filter perfect? No, especially not when strong signals outside the passband still affect the AGC. But they can help a lot with the hearing fatigue.
Perhaps you could try a resonant speaker, like a large piezo disk in a cavity?
Small ones are a fairly high resonant frequency, but this one is 1Khz. Perhaps that is OK, or perhaps lengthening the helmholz resonator would lower it.
Perhaps start with a large disc and just make the resonant cavities.
If you have two at different frequencies (e.g. 600Hz left, 800Hz right), do you get a sound stage effect that also helps?
Has anyone got experience with used a stereo phasing sound stage effect where the apparent position moves with pitch?
I wonder if any of the old reed/tuning fork selcall / squelch filters would be useful if they were coupled and damped?
These are a steel reed with a piezo transducer about 20mm long. I think they responded to a steam whistle.
I have 592Hz,622Hx,727Hz in the drawer. They are tiny, light and zero power.
I have to find a use for them some day…
I have seen a couple designs, but the website for the one I tried appears to have disappeared. It uses a regular small (50mm?) speaker in the base of a resonator built using plastic water pipe. Unfortunately, I expect that the differences in pipe connector shapes from one manufacturer to the next has caused the resonant frequency to shift out of the available tuning range.
With a bit of reconstruction I think I can still get it to work.
It does provide audio filtering even using headphones connected in parallel with the resonant speaker, but that requires the speaker to be operational. More often, I’d rather not make more external noise then necessary, so something that can drive headphones alone seems like a better choice.
