Fix: Audio Pop

UBITX.NET RECOMMENDS YOU PREVENT AUDIO POPS

The transitions from RX to TX and from TX to RX result in annoyingly loud pops in your speaker or earphones on the µBitx.   These pops are most annoying on earphones.

There are potentially several ways that audio pops can be removed from your µBitx.

Option 1 – TX to RX pop reduction (Ashhar Farhan VU2ESE)

Add a 470uf (or a 220uf) capacitor in parallel to C50. The C50 is the 1uf capacitor that feeds the detector signal to the audio pre-amp. Increasing this to 470uf, prolongs the ‘switch on’ time of the audio preamp just enough to not hear the TX-to-RX thump.

The positive lead of the capacitor should point towards the trace that leads to R50.

uBITx. net Comment

This fix only works in part, reducing the length of the pop in going from TX to RX (not RX to TX).

Reference

Option 2 – Four part fix – TX/RX and RX/TX purging switches (Gary N3GO)

This mod uses transistor purging switches and is required for the Full QSK mod.    However, the Full QSK mod is not required to mitigate the Tx and Rx pop/thump noises observed 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.

Instructions for effecting the pop, including a picture of a typical layout on a small piece of perf board are still to come.   This page also needs an indication where to tap into the RX and TX lines on the main board.

uBITx.net comment

This fix has yet to be verified by other constructors. Write and tell us if you have tried this fix and you have found it to work.

Option 3 – Four part fix (Joe VE1BWV)

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).  Note that pin 7 of U1 (the TDA2822) is connected to the wiper (centre connector) on the volume control.  The photo below should clarify how Joe built this circuit on the rear of the volume control.

Ubitx.net comment
This fix has been tested by other constructors.  It reduces the scale of the pop, but not does not fully eliminate the pop (suggesting some experimentation with the capacitor value may be required).  It also tends to eliminate the sidetone!  Write and tell us if you have tried this fix and you have found it to work.
REFERENCE

Option 4 –  Nine part fix (Erhard DF3FY)

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”.

Ubitx.net comment
This fix has yet to be verified by other constructors.  Write and tell us if you have tried this fix and you have found it to work.

Option 5 –   Verified fifteen or nine component fix (Wayne VA7AT)

uBITx.net recommended fix

The original nine component fix by Wayne VA7AT has been verified to work and documented by another constructor (Don ND6T).  The expanded fix addresses one further remaining issue, and has been verified to work by Ricardo LU7CRA.

Both variants are, therefore, recommended by ubitx.net for use by other constructors.   However, it would be good to know if other fixes actually work!  Let us know if you have tried any fix.  Fixes that don’t work will be removed from this article, and fixes that work will also become recommended fixes.

Fifteen component fix

This circuit rewires the preamplifiers to be permanently powered and inserts serial gating in each of the inputs, controlled directly from the TX and RX power busses.

Wayne VA7AT has updated his circuit slightly from that described by Don Cantrell ND6T in his article (see the news item here).

This 15 component audio pop fix from VA7AT has been built by Ricardo,  LU7CRA.  Ricardo has found it to work perfectly removing all popping noises in both transitions.  He also observes that this solution is the most “healthy” for the PCB as no cuts are required.   Simply remove C50, C63, R52 and R66 on the main µBITx board.   Mount resistors in the circuit above vertically and tie the free terminals above the board to a convenient 12V point.  Audio in-outs and Rx-Tx connections can be wired directly to the PCB.

Nine component 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 Don’s 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!

Details of the updated circuit from Wayne can be downloaded from the BITX20 file repository.

Reference 1
Reference 2