mlr,
It can be frustrating can't it. I had hum with both my FM and AM units but by trying things one at a time they are both hum free. My biggest problem with the SSTRAN was due to a defective audio cable which was new out of the box. Solder all your audio connectors and check that you haven't reversed the signal pin with the ground along the way.
Could it be you are getting noise from a light dimmer? Many of these put out junk even if the lamp is off. This would sound more like a buzz than a hum.
Try the jumpers. I removed all three and it improved things. Also, does the hum go away if you disconnect the audio cable? If so then it is probably a ground loop, especially if you are using a ground stake which is not connected to the electrical service drop stake. It is not unusual to have an AC potential difference between two ground stakes. If this is the case, the the ground loop isolator would probably take care of the hum. Also, one of the posters above mentioned the front panel settings. If set too high any hum in the system will get worse.
Twisted pair cable is intended to work with differential signals and loses the hum rejection ability with single ended signals. Don't rush out and buy shielded cable yet since that won't help if you have a ground loop.
Regarding the power supply, that's the last thing I would try in case nothing else works. If you are going to do this you will need to supply the unit at least 18VDC and maybe more (and I don't know the upper limit). Once more, if you get a clean carrier with the audio unplugged, the problem is not the power supply.
You have probably thought of most of these suggestions but I went pretty long here in case there is one you missed.
Neil
A lot of these things I have thought of - sometimes, I'll ask questions I already know the answer to just so that there is a record of it here for any newcomers. Although, most of the time, I get answers that are totally different than my thoughts, so it really helps me too 😉
For a nice, compact switching supply, look for a laptop supply. I found two Compaq laptop supplies that put out just over 18VDC at the local surplus store. They work perfectly with my SSTRAN.
According to the LM7815 datasheet, the input voltage needs to be between 17.7V and 23V, so 18V is close to the lower end, but as long as it is stable and doesn't drop lower, it should be fine. I have been running mine 24/7 for three months with no trouble.
If you want to compute the voltage drop for a given length of Cat5 cable, use this calculator: http://www.demarctech.com/techsupport/poecalculate.htm. If you are using a different type of cable, the loss will be different. I measured the current consumption of my SSTRAN transmitter at 53mA. I have less than 60ft of Cat5, so the voltage drop is neglible.
Don't forget the voltage drop across the two diodes in the full wave bridge. This will up the ante by 1.4 volts above the voltage difference required above the the minimum difference between the input and output of the 7815. For example, if the 7815 requires 17 volts in, add the 1.4 drop and it becomes 18.4 VDC minimum into the power connector.
Can't argue with success, but I would check that your 7815 output is really regulating. It could well be that it is but with 18VDC in it might not be. As you said, if it is stable things should be OK but if the regulator is not regulating, it can crack into oscillation. My point is that the stability could be due to the switcher and not the on board regulator.
The DC voltage into the regulator in my unit measures 25.8 volts DC using the SSTRAN supplied wall wart. This would suggest that about 24 VDC in would work OK without overheating and would give the 7815 plenty to work with to regulate to 15V.
Neil
Yes, I did forget that (oops). I just took my spare SSTRAN and hooked it up to that same supply. Measured voltage into the 7815 is 16.75V. Measured output from the regulator is 14.75V. Although the output is still within the specs (14.4-15.6), it is close to tolerance.
Although this has been working fine for months, I guess I will search for a 19VDC laptop supply, for some extra margin. Laptop supplies are available in a range of voltages. I just didn't find any higher than 18VDC the last time I looked at the surplus store. I could order one, but new ones get expensive.
Using the Cat5 voltage drop calculator, with 60 ft of cable (using two pair), I'm still losing well under 0.1V, so this doesn't noticeably affect things. BTW, that calculator doesn't seem to take less than 0.1A, despite saying it will accept 0.01A. I'd get out my calculator and do the math myself, but it is late and I'm feeling lazy.
A quick check with the chairman of my HOA design review committee affirms that 24 volts DC is an acceptable choice for powering a remote mounted SSTRAN. Now we are back on topic!
Seriously, though the thread has drifted a bit from the original topic I hope there was some useful information presented that won't get lost.
Neil
They need to pass laws restricting what HOA's can mandate on home owners. one should be that the laws and convenants they pass can't be any more restrictive than the munipality in which the home(s) reside.
if the town allows a 60ft tower than the HOA should not be allowed to make restrictions that are more stringent. (such as no towers.)
HOA's should stick to pools, streets and parks. stay out of the homeowners personal property. that should be the sole responsibility of the town.
Thank You,
Rev. Robert P. Chrysafis
Universal Life Ministries
http://www.ulc.org
Moderator Hunterdonfree
http://groups.yahoo.com/group/hunterdonfree
The problem is there too many laws. One law leads to another.
WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager
Rev. Robert,
As you say "thank you" for your comments.
As I have explained here I have a bit of experience with HOA's and it comes down to this in my opinion: The convenets and restrictions are comprised of limitations on property use that a group of people believe will result in a "quality", by their definition, neighborhood. I am a firm believer in private property rights, and with that being said, when I bought property in my neighborhood subject to the CC&R's I not only knew and understood them, I thought and agreed that I could live with them. After 20 years here I have not had any problems with this.
Like them or not, CC&Rs exist and at least here they're not imposed retroactively. If that happens, then I can see a problem.
Neil
part of the problem is certain states and municipalities won't allow new projects with out a HOA. illinois is one that comes to mind. i'm sure there's others. while now, yes, you have options, but this is changing rapidly. I recommend that anyone considering living under a HOA watch the movie "the colony" with john ritter.
See this for movie info...
http://www.imdb.com/title/tt0112703/
Thank You,
Rev. Robert P. Chrysafis
Universal Life Ministries
http://www.ulc.org
Moderator Hunterdonfree
http://groups.yahoo.com/group/hunterdonfree