Glenn VK3PE has built up one of the 9 component anti thump circuits described by ND6T (but it is in fact a design by VA7AT ).  He has yet to actually try it in his rig.

The PCB is about 26 x 10mm in size. Only difference to ND6T’s version is he placed the 10uF’s on the PCB also. ie remove from uBITX board and fit to this board. Otherwise it follows ND6T’s web page for installation. It is made from SMD 0805 parts.


Research into the audio pop problem

John KK5VH has been working on understanding the audio pop problem for a while. He doesn’t have a fix yet, but he has identified that:

  1. The relays have a max actuation time of 7ms.
  2. K1 drives K3 via the TX voltage –  that means you have a 7ms (max) time from when the TX turns on and K3 disconnects the audio.
  3. Changing the power (turning RX/TX  on and off) on these one transistor amplifiers will cause a large spike to be coupled via C63 and C51 to the audio IC before K3 can cut them off.
  4. Both the amplifiers draw tiny amounts of current, Q6 about 1.3ma and Q70 about 2.2ma.  They do not have enough current draw to bring their respective power sources down quickly. Secondly, they have 47µF capacitors that hold the power up within that circuit.

John has been simulating the circuit via LTspice with some results.

He increased C64 to 517µF by paralleling a 470 µF cap across it and changed C52 from 47µF to 0.1 µF.  That made a timing difference that cured the turn on pop but left a gigantic pop on turning off the rig or moving from TX back to RX.

After all of this playing around he still don’t have a good hardware answer to the problem!

However, John suggests that if the Arduino Nano controlled K3, this could solve the problem using timing delays. A simple sequence would in moving from RX to TX, first turn on K3 (disconnecting the audio chain) then switch on K1 into TX mode.  When finished with TX mode, hold K3 on for a number of milliseconds to all the RX circuits to stabilize before switching back to RX on K1. Hopefully, this would solve the problem.  John welcomes comments!

Audio Pop mod from QCX submitted by Greg N3NW

This audio pop mod from Greg N3NW appeared on the BITX20 lO Group list before this website went live in January.   The implementation may not look that elegant (sorry Greg), but this was about an experiment to reduce pops not an entry in a beauty contest!

Greg “borrowed” a rather elegant circuit snippet from the QCX transceiver that takes care of the pop completely.  This is effectively the same mod as that submitted byJoe VE1BWV (and no doubt taken from the same source) who confirmed that it works.  The mod has now been confirmed independently by both Pete W8KS  and John AD0RW.    So this mod, is known to work, however, it may come with one downside, for which John AD0RW has a work around.

The circuit diagram for the popular QCX transceiver (developed by QRP Labs) can be found near the end of the very well documented assembly guide for this single-band CW rig.  An excerpt that contains the TX Mute circuit installed by Greg can be seen below:

The TX mute switch in the QCX involves just 4 parts – a BS170 (Q7 above), a 0.1µF capacitor (C52 above), a 1N4148 diode (D5 above), and a 120K resistor (R60 above).

Greg used a 300K resistor for R60,  and a 440nf capacitor for C52 to adjust the timing, which keeps BS170 open just long enough when switched back to RX to remove the pop.  The CW sidetone gets weaker with this, however in my uBITX board 10K R253 was replaced by a 1K resistor to make the CW tone just loud enough with this mod.

Pete W8KS has, however, found that the CW sidetone (even after replacement of R253 with a 1K resistor) to be insufficient and asked for suggestions to address this mod’s shortcoming.

John AD0RW found a much better place to connect the drain of the MOSFET. First he replaced R70 (in µBITx circuit diagram (see circuit excerpt above) with a 510 ohm resistor.  Then he connected the MOSFET drain to the relay side of that resistor, i.e. to the point M2 on the schematic above.

This setup still shorts the incoming audio to ground and kills the pops, but leaves a voltage divider of R253 and R70 connecting the sidetone to the volume control, increasing the amount of signal injected.  Depending on your value of R253, this may be perfect as is. The value of R253 varies (some early production kits had a 1K resistor and more recent kits get a 220K resistor).  John’s was 220 k and he needed more volume, so he put a 50 k trimpot across R253. It is just about right at maximum resistance, but can be adjusted downwards if more volume is required.  If your R253 is a smaller value and the sidetone is too loud, just replace it with something of higher resistance.  The volume control now has final control over the sidetone level and can be tweaked in operation if needed.

Reference 1
Reference 2

Audio Pop Fix

Don Cantrell ND6T saw a suggestion from Wayne Chang, VA7AT for the audio pop problem in going between RX and TX (and TX and RX).  This was the first solution that seemed sensible to Don. He suggested rewiring the preamplifiers to be permanently powered and inserting serial gating in each of the inputs, controlled directly from the TX and RX power busses.

This modification, although the most complex of those that Don had tried, actually works!

It only requires 9 components, all quite common, and costs less than 25 cents. It requires 2 traces to be cut and 2 small jumpers to be placed across traces. The new circuit is then connected using 6 jumpers from the new circuit board; The connection to the T7, 2 control inputs from the TX and RX busses, 2 outputs (one for the microphone preamp, one for the receive preamp), and a jumper to a common ground point.

For full documentation of this fix see his blog posting here.

Don’t be too scared off with the surface mount technology and the miniature circuit board.  This circuit can be built with regular hole through components to a much larger scale!

RX-TX Audio Pop Fix

Erhard Haertel DF3FY comments, “There are a lot of proposals for addressing the RX-TX audio pop with the uBITX, but none of them really works”.

Erhard has come up with his own pop-fix that involves muting the TDA amplifier during TX, turning the IC on with the 12V RX line (to prevent SSB noise in headphones during transmit) and additional decoupling for the 12V line.

Erhard says, “This fix cures all the issues”.  Write and tell us if you have tried this fix.

Another audio pop mod

Joe VE1BWV provides another audio pop mod:

You probably will want to install a pop click fix to eliminate a loud pop whenever TX is turned on and off.   This circuit uses a 1N4148 diode, a 470k  or 300k resistor, a .22 or .44uF cap and a 2n7000 MOSFET.  Joe says this works amazingly well and mounts on the rear of the volume pot as indicated below (ignore the 0.1 uF capacitor).

Removing the audio pop on T/R

A mod developed by Gary N3GO (and required for Full QSK) also mitigates the Tx and Rx pop/thump noises in SSB mode.   This mod should work fine with the existing relays and with or without other mods, but has yet to be verified by others.