New BITx Mic PreAmp - for CW !

My new rig (that is to say, the rig that was new in November last year and that has been the subject of continual development) has been sprawled all over the bench for too long, making a general nuisance of itself. Well - finally, I've found a suitable enclosure - thanks Paul!

Unfortunately, before it can be neatly sealed away inside a box, there's one issue I need to sort out...

Although this is a sideband rig, intended primarily for voice, I've been using it to send CW - as readers of the G-QRP club's SPRAT magazine will have seen in the current issue.

My article "Working CW from an SSB Phone Rig" explains how I'm exploiting the flexibility of my coupled, digitally controlled VFO and BFO to use the transceiver's IF filter to turn an audio frequency square-wave (generated by the Arduino) into pure CW at RF, without disturbing any of the rest of the system's functionality.

The article also explains that I'm "jury-rigging" the phone transceiver to work CW. That currently entails hooking a mini crocodile clip onto the microphone preamp whenever I want to work CW - hardly practical if the whole thing is inside a box!

You can see the clip in the middle of the photo above (the line labelled "CW Audio"). The other white croc clip (by which the Arduino can switch the rig into transmit mode if I'm sending CW through one of the several logical "OR-ed" electronic "PTT" lines I incorporated in my build) and the yellow sidetone wire can stay in place - so they are no threat to boxing up the system.

The trouble is, hooking up the CW Audio connection loads the microphone signal path, such that the rig doesn't work as a phone rig with the croc clip in place. It is easy to see why with reference to the schematic...

Here's my current microphone pre-amp stage - the first stage is a general improvisation on the contents of the junk box to achieve an electret microphone pre-amp with around 40dB of gain, whilst the second stage (within the dashed red line) is Farhan's original input stage...

You can see where the croc clip goes from the label in the schematic (actually the label lies - it is easier to clip onto the collector of the first stage - the other side of the capacitor from the point shown in the schematic - that's where it is hooked to in the photo above). The problem is that the impedance presented by the clip (and the Arduino output pin at the other end of the wire) is rather low - such that any signal from the first stage is shunted by the clip and the overall operation of the microphone preamp is destroyed by the presence of the clip..

I decided to build a new microphone stage which would allow continuous connection to my CW signal source without any loading effects.

Here's my new schematic - it uses an op-amp for the first stage to make an inverting summing amplifier - a "mixer" (as the audio frequency community would call it)...

and here's the new (right) and the old (left) microphone preamp stages...

The new microphone stage - with its additional "line audio" input - is now installed on the main board, where it works perfectly. I even found an old 741 op-amp to run in it, just for "nostalgia's" sake...

While I was about it, I also took the frequency divider circuit (which forms Figure 5 of my SPRAT article)...

from its rather temporary incarnation on a little solder-less breadboard...

to something a little more permanent...

Now all the connections can stay in place all the time.

Working CW is just a matter of pressing on the key or the paddle and working voice is just a matter of pushing the PTT button on the fist mic. Just as it should be.

We're ready to box it all up.

I wonder if a quart will fit into a pint pot.

 ...-.- de m0xpd