Early today Dave Benson K1SWL, the designer of the Hilltopper radios being sold by 4 States QRP Group, posted an email to the group listing the following mods to improve audio performance of the rig. Not that it really needed much.
Side Tone Too Loud
Some ops have found the sidetone to be a bit loud, other ops have found it to be just right. If you find the sidetone to be louder than you would like it to be you can do the following mod. Change out R8 (22K) with a 47K to 100K resistor. I have not done this mod, as I find the sidetone to be just the way I like it. However I wonder if you could put in a little 100K pot instead and adjust it to your favorite sidetone level. Not sure a small pot would fit in there, but may be worth a look.
Low Audio I
The values of R5 (150K) and C22 (470 pF) may be changed to 330K and 220 pF respectively. This yields a gain increase of 6 dB without affecting that stage's rolloff characteristic.
These two components are in 'tight quarters'- between U3 and the pushbutton switch. Cutting one or both component leads and tacking the new components on the underside of the board makes the change much easier.
NOTE: I cut the components off just above the board, and used solder wick and a solder sucker to clear out the solder so I could use needle nose pliers to tug out the "stems" while heating the backside of the board. Worked well but I had to take my time.
Low Audio II
Straight from Dave, "As some of you have noted, there's a 'screech' on loud CW signals- it necessitates riding the gain control. I evaluated samples of a more robust op-amp- the NJM4556AD."
Replacing the NE5532P with that new device eliminates the screech. You'll hear some distortion when limiting occurs, but it's much less objectionable than the NE5532's rude noises. Its 'limiting' threshold is also about 6 dB higher than the present device. The net result of this change, combined with above mod, is 6 dB more audio without the rudities (that's a word!).
Note that if you increase the gain without replacing the IC, you'll find yourself riding the gain control more often
I have an OpAmp on the way, it appears to be a pin for pin swap out. The OpAmp in the Hilltopper rigs is socketed, so this swap out should take but a few minutes to perform.
73 de KB9BVN