Like many other keyers on the market I chose the easy way out when designing the Mini’s sidetone. To save costs I just used one pin configured for pulse-width modulation (PWM) and set the frequency. A pot controls the amplitude of the square wave fed to the headphone amplifier on the Mini+. There is no volume control on the original Mini. A small capacitor on the input to the sidetone amp provided a little bit of filtering, but the tone is still harsh. I figured that it didn’t matter too much since most radios have sidetone built-in so the Mini’s sidetone wouldn’t be used a lot. I was wrong!

If you use your Mini for code practice or operate your Flex radio remotely using network keying you’ll soon tire of the harsh sidetone. A sine wave sidetone makes it much easier to listen to the Mini’s keyer. If you do high-speed CW or have an interest in iCW operations (internet base CW using audio servers) you’ll definitely need a sine wave sidetone.

This post will explain the difference between the current sidetone in the Mini and the filtered sidetone. I’ve included audio samples of each at different speeds. The unfiltered square wave tone definitely gets harsher as the CW speed increases to the point that it’s almost unusable above 40 WPM. Finally, I’ll show you what it takes to add a filter to the Mini+. There’s not enough room in the original Mini for it but it can easily be built into an external enclosure if you want to add it.

Filter Options

There are many filter options available on the internet. The iCW group recommends a couple of different filters. A passive filter designed by W3NQN can be found here. This filter is fairly large and has problems with ringing when driven with a square wave. A much better filter is the HI-PER-MITE CW filter designed by NMØS from Four States QRP Group. This is a 4-pole active filter. Although it’s based on an design from 1994 it works remarkably well.

Unfiltered vs Filtered

I purchased a HI-PER-MITE kit to see how it would work with the Mini. To say the least, I was amazed. Here’s a few photos and audio files of the unfiltered and filtered sidetone audio.

Mini+ unfiltered sidetone output – ‘K’ character set at 700 Hz at 50 WPM. Note the ringing on the front porch of the wave and the slow release on the back porch.
Zoomed into the unfiltered dit in the middle of the ‘K’.
Close up of the front porch of the unfiltered dit. The full volume square wave at the start of the wave causes ringing in the sidetone amplifier.

Now lets look at the same character, again sent at 700 Hz at 50 WPM using the HI-PER-MITE filter to filter the square wave.

Mini+ filtered sidetone output with the HI-PER-MITE installed – ‘K’ character set at 700 Hz at 50 WPM
Zoomed in to the filtered dit in the middle of the ‘K’. No ringing and no overshoot.
Close up of the front porch of the filtered dit. Note the gradual amplitude increase at the start of the wave with no ringing.

Audio Samples

The following audio samples help you really understand the difference between unfiltered and filtered sidetone. You can hear the ‘pop’ at the start of each element in the unfiltered audio.

All were sent with the sidetone set at 700 Hz and fed into my PC’s sound card. An isolation transformer was required for the unfiltered Mini+ sidetone because it’s output is not referenced to ground.


20 WPM – without filter
20 WPM – with filter



40 WPM – without filter
40 WPM – with filter



60 WPM – without filter
60 WPM – with filter



80 WPM – without filter
80 WPM – with filter


Adding a HI-PER-MITE to the Mini+

It’s relatively easy to add a filter to your Mini+, but it will take some work. First, order the HI-PER-MITE kit from Four States QRP Group. It’s $28.65 including shipping. Unfortunately they only ship the US.

Before you build your filter you must decide on the center frequency you want to use. The kit comes with resistors for a center frequency of 700 Hz. They have a table showing the resistor values for other center frequencies. Luckily the resistors don’t have to be high precision and you don’t have to change the caps. Be forewarned that although the filter has a 3 dB bandwidth of 200 Hz it starts to overshoot on the front porch about 50 Hz above and below the center frequency. This causes noticeable distortion in the sidetone. I built mine using the supplied resistors for 700 Hz and running the sidetone at 600 Hz sounds awful. I’ll probably move the center frequency down to 580 or 640 Hz in the future.

Here’s what the output looks like when a 600 Hz sidetone is sent through a filter with a 700 Hz center frequency. Note the overshoot at the first of the envelope and the general squaring of the envelope as compared to the 700 Hz sidetone screen captures above.

600 Hz sidetone at 50 WPM through a 700 Hz filter with 200 Hz bandwidth

Built the entire kit, including the LM386 audio amp. You’ll be using this amp instead of the amp built into the Mini+ front panel primarily because this amp is referenced to ground. The output of the amp in the Mini+ is not referenced to ground so it cannot be plugged directly into a PC sound card or amplified speaker.

Wiring the Filter

Follow these steps to install the filter in the Mini+:

  1. Disassemble the Mini+ by removing the two brass standoff legs and the two screws securing the case halves. Do not remove the four screws holding the bottom board to the bottom case.
  2. Remove the bottom of the case and bottom board.
  3. Unplug the 16-conductor ribbon cable from the top board and set the bottom case aside.
  4. Remove the two jumpers from the Headphone jack, J4, to the PCB
  5. Unsolder and lift the center tab (sleeve) on J4 away from the PCB. Make sure this tab does not connect with the PCB.
  6. Remove the jumper on the center terminal of RV1 to the PCB (volume control)
  7. Remove both wires on the Speaker from the PCB pads
8. Connect the filter’s IN terminal to the center pin on RV1 as shown in the photo above
9. Connect the filter’s GND terminal (between the 9V and IN terminals) to the pin to the right of the center pin on RV1
10. Connect the 9V terminal on the filter to the +12V switch output on RV1 thru an inline 1N4001 diode with its cathode connected to the filter board as shown above. This diode eliminates power supply noise present when the Wio Terminal’s WiFi radio is enabled from reaching the filter.
NOTE: If the 12V power source connected to the Mini is greater than 12.7 volts you should replace the LM386-1 amplifier IC supplied with the filter kit (which is rated to 12 VDC) with an LM386-4 IC (which is rated to 18 VDC) .
11. Connect the filter’s OUT terminal to the top terminal on the Headphone (HP) jack J4
12. Connect the Speaker’s (+) wire to the side terminal in the Headphone (HP) jack J4
13. Connect the filter’s OUT GND terminal and the Speaker’s (-) wire to the bottom terminal on J4. Make sure this terminal does not contact the center PCB pad below it!

14. There’s just enough room at the front of the front panel to glue the filter to the back of the encoder and to the speaker. Place a small amount of silicone glue on the top of C3 and C13 of the filter. A small piece of non-conductive foam on the bottom of the board supports it when the bottom case is replaced. You can use a standoff and bolt the filter board to the top panel if you prefer.

15. Plug the 16-conductor ribbon cable from the bottom board back into front panel and reassemble the Mini+. Be careful not to pinch the wires from the filter.

The photo below shows the completed installation.

Is it worth the effort?

By now you’re probably wondering if adding a filter to the Mini+’s sidetone is worth the effort and expense. After installing it, I can say that, yes, for me it is worth it. Having a clean sine wave sidetone really reduces the fatigue from listening to practice sessions. If I decide to join the fun on iCW it will be a must have.

Only you can decide if it’s worth it to you. I am considering making this an option for future Mini+ units but it will take some major rework of the PCB and possibly a new daughter board for the filter. The components to build the filter are also on the pricey side. The poly caps alone are $12. For now, I’ll leave it as a modification that you can do if you want it.

As always, if you have any questions on this modification or anything else, please contact me. I really appreciate the input and the new ideas that help make the Mini even more useful.

73, Lynn, KU7Q