Tracking down hum problems with the SSTran
Posted on January 12, 2008
I have invested some effort in finding out what can cause hum on in-home broadcasts to ac powered radios when using the SSTran transmitter. I hope this information will be helpful. If you disagree with my findings, that is fine– I am just reporting on what I found.
First, I will deal with the topic of hum that is present on the carrier when there is no audio input and the Gain control is turned all the way down. There have been some complaints that the included ac transformer is responsible for hum on the carrier, and I have to say that after careful testing, I cannot agree with this. Below, I describe what I did.
I set up the SSTran in a typical “home broadcaster” fashion, with the transmitter indoors and the ac transformer plugged into a wall outlet along with other appliances. The standard wire antenna was run up along the wall, and the transmitter was grounded with a short wire. I experimented with various grounds available in the living room, and eventually found that grounding to the outlet screw produced the lowest hum level.
At this point, I could not hear any hum on a good portable radio (GE Superadio III), but I confirmed that I could hear a small amount of residual hum on a couple of ac powered radios around my home, including my new Cambridge Sound Works 735i. This radio has extended low frequency response and very good sound quality. For the rest of my tests, I used it as the reference. To put the hum level in perspective, it was well below the point that you would be able to hear it with any modulation present, but with no programming it was certainly audible.
I first disconnected the ac transformer and substituted various dc supplies that I had on hand. A few produced noticeably more hum than the supplied adapter, and I confirmed that with these, I could also hear hum on the portable radio. The reason turned out to be that the SSTran requires at least 20Vdc at its input to remain in regulation (if you want to know why, just ask). However, even the dc supplies that didn’t introduce additional hum still resulted in the same level of residual hum as the supplied ac transformer.
Next, I brought home my 20+ pound lab dc supply from work. This supply produces dc power that is arguably as clean as a battery (maybe cleaner). I set it for 20V and hooked it up to the SSTran and guess what? The residual hum level was no different! While I was at it, I measured the current drain. In the absence of modulation, my unit draws 50.7 mA. So if you are thinking of running yours on batteries, plan accordingly.
At this point, I am of the opinion that there is no intrinsic source of hum within the SSTran due to the power supply. If you disagree, that is fine, but this is my conclusion. HOWEVER, please read on for some additional very important qualifications that may help you if you are having hum problems.
There are three chokes inside the SSTran with associated jumpers. Jumper S1 shorts L1, which is in series with the audio input ground. I cannot condone operating without this jumper installed. I have never yet found a situation in which you can obtain hum-free audio if you leave it open. The reason is that with L1 in the circuit, the ground return for your audio input will be primarily through the power ground, which is bound to be dirty. You are better off to operate with S1 installed and leave S2 and S3 open. The manual says that you can jumper S2 and S3 to take advantage of the ground path provided through the ac adapter and your house wiring. Now this is probably OK for casual operation, but it is really not a very good RF ground, and it is definitely going to superimpose RF on your house wiring. When this RF enters the power supply of an ac line powered radio, some of it is going to be rectified in that radio’s power supply, which will almost certainly add hum. The reason is that unlike the diodes in your SSTran, the diodes in the radio are probably not bypassed. This is the problem that carrier current stations have to fight all the time. So if you can avoid it, you should try to isolate the transmitter ground as much as possible from the ac line and ground it to a good earth ground if one is available. If not, I think it is better to ground it to the ac outlet screw, which should be attached to the conduit, than to drive the hot and neutral lines in common mode fashion by grounding to the ac transformer.
Upon further testing, I have found that a significant amount of RF energy still gets injected into the ac line through your audio equipment when you have the audio input cable connected and have jumper S1 in place (as previously recommended). If this is causing a problem, I would suggest trying a common mode choke made by winding the audio input cable around a #77 ferrite toroid for as many turns as possible (you will probably want at least a 1.4″ core). This will attenuate the RF without affecting the audio.
Next, I am going to further discuss the issue of hum that is present when the Gain control is turned up. The SSTran has a relatively high impedance input circuit that makes it vulnerable to hum pickup, particularly since it is in a plastic case. If you are driving the transmitter from a low impedance source like a computer sound card or an Ipod, you could benefit by lowering the input impedance. This can be done by bridging the audio inputs with a 1K resistor. This should not drop the audio level significantly if you have a low-Z source. If it does, you can experiment with a higher value (e.g. 5 or 10K). You should notice the difference right away if you turn up the gain with the inputs disconnected– the hum should be a lot lower. If you do have a high impedance source, you will benefit by keeping the level coming into the SSTran as high as possible, and controlling it with the input gain control. In other words, to minimize hum pickup, you want to run the input gain as low as possible, consistent with obtaining the proper modulation level.
Finally, as has been discussed elsewhere, if you are feeding audio to the transmitter over a long cable, as for example when it is mounted outside, you should get an audio transformer and drive the transmitter with a balanced audio source. Mount the transformer at the transmitter input. This will pretty much totally eliminate any hum pickup on the cable.
In the future, I will devote some time to explaining more about what I have learned about setting the Gain, Compression, and “Modulation” controls. My format is classical, and I don’t like a lot of compression, but a moderate amount can be helpful on AM. However, what particularly bothered me was the low level “noise gate”. I have found that if you drive the transmitter input hard enough, you can turn down the compression control all the way and pretty much eliminate all of the noise gating. You can still engage the “Modulation” (limiter) circuit and benefit from some top end limiting, which is helpful.