RaDAR – QCX Audio

I think I have sorted out all the QCX alignments which are quite simple to do and appear to be correct. I feel I did a good job during the build process and have used the little rig a few times trying to get the feel of it.

I’ve made peace with the fact that it is a well designed modern receiver that is filtered to the maximum (200 Hz). It does receive signals but the purpose of this Blog is to share my experience of the audio. My QSO with Z21GC last night was a good example.

I have tried various headphones but the audio is not sufficient to drive them to a comfortable level. I’ve used my cell phone earphone with limited success. I also used a PC’s shoX powered speaker and that is what I used to complete my QSO with Brian last night. I held the speaker close to my ear, constantly adjusting the volume control as the signal had deep QSB. The audio amp can be easily over driven and the distortion can be heard almost sounding like a 555 oscillator. If you cut back on the audio it sounds more pure but then it’s a battle to hear properly.

It appears there is no AGC (Automatic Gain Control) so there is a constant fiddling with the volume control to prevent overdrive. Certainly a lot went into the design of the QCX. It’s a modern style rig and the TX is really awesome …. Oh that reminds me …. I noticed, on key down, the sidetone audio level gradually gets less, the signal indication gets less which appears to be essentially an audio level indicator in it’s standard configuration.

I checked my supply voltage and it’s reasonably constant. I put my home brew dummy load / QRP Power meter on the output and it stays constant but as explained above, the sidetone audio levels gradually get lower. You don’t notice it while sending CW only on key down for a few seconds.

A lot has gone into the design of this rig but the audio falls short of my expectations which builds up frustration. But, I’m a “die hard” I’ll be back hoping there is something I can do about it.

I have a spare 49er board which I may use for the audio section only and hook that up to the QCX.

                                                                   The QCX’s Audio output stage


It will be a simple task to wire the 49ers LM386 audio amp via the volume control pot. The 49er doesn’t have a volume control either! Then wire the power supply in parallel with the QCX’s


           The 49er’s Audio output stage


Update 2017-10-27

I built the audio and power supply sections on the 49er board and did the tests.

The LM386 isn’t the quietest audio amplifier you could ever use which probably explains why Hans didn’t use it in his design but I can hear what I need to hear now even if it’s mixed with the LM386’s “Hiss”. The headphones I use now have sufficient drive for comfortable listening and I simply still use the QCX’s volume control for comfortable listening level.

Without wanting to be invasive I used the standard QCX audio output and fed that into the LM386’s input. No RFI …. there is sufficient filtering on the 49er circuit.

So, I’ll make this a permanent solution to my QCX problem for the time being.

Update 2017-10-31

Last night I wired the 49er audio section to the volume control of the QCX which is just before the QCX’s audio section. The audio is lively!!! More than enough. So it appears that there may be a fault in the region of IC10 on the QCX board as reports worldwide say that the QCX has quite sufficient audio to drive headsets or whatever you use to play the sound.

It wasn’t a waste of time doing this for once I do manage to find and fix the fault I have an alternate audio port for recording or even for a public address facility!

I am still concerned about T1 and the band pass filter peaking. I get a peak but don’t see the peak drop while turning C1 before it’s unmeshed again. I may have to further reduce the turns on the toroid one turn at a time. Maybe I’m close to resonance but not exactly there. I doubt it though that that be the cause of the audio levels of the QCX. The 49er addon proved that I think???

73 de Eddie ZS6BNE

3 thoughts on “RaDAR – QCX Audio

  1. Hi Eddie

    I read your blog entry with interest. I believe there must be a fault somewhere in your radio. The QCX receiver was designed with PLENTY of gain. I spent a lot of time studying other similar receiver architectures during the design process. One important consideration was, what total gain to provide. I always like to provide a little extra gain, because you never know what different people are using for antennas, and there is a huge range of sensitivity between different earphones, etc. So I configured the QCX design with 5-10dB more gain than other similar designs. I also felt justified in doing this because the very low noise HiFi op-amp LM4562 is used throughout – so providing additional gain which is later attenuated by the gain control does not damage overall sensitivity.

    These design considerations have also been confirmed by all feedback received so far (until yours, hi hi). I have received several comments from people who are operating the radio with the audio gain control barely off its minimum. Others have measured and confirmed the high sensitivity.

    So, according to the design and the results people are having – it seems to me that your experience is atypical and therefore suggests a fault somewhere. You should NOT need to add an LM386 to get sufficient gain the provided gain should be more than enough. Adding an LM386 masks the fault but does not address the underlying fault causing the low audio.

    I think that it would be worth re-checking T1, have you got all the windings correctly phased and is there DC continuity everywhere with good joints? Since you have an RF signal generator it would also be a good idea to trace the signal through the LPF and through the RxTx switch to make sure that there is not a fault there in Q5. I’m not sure what else to suggest but this should be a start. If you sent it to me I’d be happy to take a look at it, but postal times are likely rather slow.

    73 Hans G0UPL

    • Thank you for your comments Hans.

      Of course the first prize would be to determine why my audio is so low. The alignments appear to be correct and I do get a very strong signal when applying 1mV (Lowest it can go) from my signal generator. It’s just the audio which is low or at least in the final stage? I wish I knew why? I’m starting by comparing other’s experiences but seems theirs is good.

      I think T1 is correct (Even though I “needed” to remove the five turns?).

      Back to the drawing board for me 🙂

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