Tracking down audio distortion with the SSTran

If the SSTran is working properly, there should be *no* audible distortion due to the transmitter. But I have found that under some circumstances, problems can occur. Here is what I learned.

If the SSTran is working properly, there should be *no* audible distortion due to the transmitter. But I have found that under some circumstances, problems can occur. Here is what I learned.

In the manual, there are two components that are affected, depending on your operating frequency and whether you are using the supplied wire antenna or a base-loaded antenna. The first is R18. The manual says that you should leave this out if you are operating above 1,000 kHz or using the base-loaded vertical antenna. The second part is C23. The manual says that C23 should be 820 pF if you are using the supplied wire antenna, but it should be 560 pF if you are using the base-loaded antenna.

I was planning to mount my transmitter outside and use the base-loaded vertical, so I left out R18 and used 560 pF for C23. I thought I could get by with this and operate temporarily using the indoor wire antenna. WRONG! Depending on the setting of C5, the output loading trimmer, I noticed distorted audio and low modulation level. I think the reason for this might be that the collector breakdown voltage of the 2N2222 output transistor is fairly low, and when this is exceeded it can cause distortion. It is also possible that the output may be going unstable; I have not looked into that.

Experimentally, I decided to tack solder an 820 Ohm resistor in for R18 and see what happens. The first thing I noticed was that the maximum DC voltage measured at T1 and T2 dropped almost in half, but the audio cleaned up completely. So the resistor is taking a little power, but it helps stabilize the output stage loading considerably. For best efficiency, it would probably be better to leave the resistor out and change out C23 for the correct value. But in my case, I was only broadcasting around the house and didn’t need the extra signal.

Bottom line: if you are planning to go to the base-loaded antenna eventually, but want to get on the air indoors using the wire antenna in the mean time, you’ll have to install the appropriate parts or you probably won’t be happy with the results. Don’t forget to make the necessary changes when you do change to the base-loaded antenna later on.