TX audio chain

It started as a simple project to construct a switch box so I could use a single microphone with multiple radios in my home shack. It’s ended (although i’m not sure it will ever end!) with more knobs, buttons, switches and flashing lights that the flight deck of a Dreamliner.

I’m still experimenting and will be posting a walk through soon so stay tuned but for now, here’s a little picture…

M0LMK Audio Chain

3 Replies to “TX audio chain”

  1. Hello Matt,
    Two items on your site interest me in particular.
    As a diehard old school top band A.M. enthusiast your TX audio chain
    looks interesting.My home-brew transmitter for that band is a class e mosfet PA with linear modulator fed from a microphone and amplifier tailored to suit my voice.
    The top band 240ft doublet is fed via balanced matching unit with plug in link coupled coils for each band.Remote switching converts it to Marconi ‘T’ fed against extensive ground mat via ‘L’ match.
    Third option is like your version using SGC230 and home-brew 1:1 line isolator to use doublet on all bands.
    Current experiments are with pulse width modulation but I think I have wound filters incorrectly as still have artifacts (spikes) on scope!
    I keep an ear on your digital exploits with the repeater but my prime interest is the the challenge of ‘proper’ AM on top band and the efficient transfer or power between PA and aerial!

    I hope the light hearted banter on GB3EK in the mornings is not too over the top,i like the technical chat but that might scare away newcomers so we try to strike a balance!

    Regards, Nigel (G0IFS)

  2. Hi Nigel.

    I’m glad you found something on my tiny corner of the internet that interests you! I’m not all about DMR and repeaters you know! I enjoy playing around with SSB audio (not that I am any good at it) but I’m not an ESSB fan, I just like to see what I can get out of 2.7Khz and find it much tidier if I can switch between headset or desk microphone without having to unplug anything and have lots of wires running around the desk. Currently, I have 2 microphone inputs (a boom arm mounted Behringer XM 8500 and a Heil Pro Set Elite headset) which then run through the processing gear and into a small rackmount mixer. I can change audio profiles with just a few button presses on the audio processor and select which microphone to send to the radio selector switch. I can then use the selector switch to send the mic input to whatever radio I like in the shack (TS-590, FT-857 or a TM-V71).

    My doublet works great on 40M and up and does an OK job on 60M but is far to short for effective use on 80M or topband. I have played around with remote switching to turn it into a Marconi ‘T’ with mixed results as I have limited space for ground radials. I think I may have another go when the weather is a little warmer and try to lay as many radials as I can under our small patch of grass.

    I’m very interested in your homebrew AM set. I don’t venture onto top band much but have recently built an active magnetic loop for RX only that works great so I’ll keep and ear out for you.


  3. Thanks for the detail, I have tried injecting audio into the ACC socket on back of 756pro3 bypassing roll off in mic amp but as you say you are limited by internal filtering.Injecting a negative ALC at about 1.36 volts (with power at max) produces approx 8 watt carrier on AM mode (with headroom) thus x4 = 32 watts on voice peaks, the maximum allowed at top end of band, (assuming 100% mod which you never get). That is about the best you can do with commercial products from Japan.
    Would love to run ‘Big Iron’ like they do in the states but probably wouldn’t be popular with the band police!

    The Flex radios, the Anan, SDR play RSP2 give a lot more options for audio tailoring, I guess that is the direction to go in for flexibility.
    I am fortunate to have the space to experiment with bits of wire for the lower bands and various TX/RX/scanning aerials for higher up. When i get bored with all that I go back to home cinema/hi-fi stuff!
    Keep on soldering………regards Nigel.

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