Audio Transmission Line

[Ebacherville]

Audio Transmission Line

When i installed my transmiter i did it on a whim and didnt have any proper cable to run audio down so I pulled 50 foot of cat5 from the spool and ran my power and audio down it to my TX, I know this is a bad idea nd you pick up hum from running a power and audio unsheilded down the same line or close to each other.. I get a signal to my transmitter the hum is here, and when i have a very low signal or silence the compression make the hum really bad..

SO I tried some tripple sheilded cable tv coax.. problem there was the sheilding was aluminum and not copper so I could not solder it to my RCA ends properly.. it didnt work..

So I found some 50 foot RCA cables, will this sheld me enough from hom they are formed in a sheilded fashion not two wires ran parrellel to each other.. Should i try to find some copper sheilded cable tv coax? Many say this makes good rca cables.. found it on some autofile places on the net..

Or any one have any better ideas to eliminate hum in audio transmission lines.. and if you have a source for some good wire or 50 foot cables that will help me out please divulge 🙂

Jason

[radio8z]

Hi,

A frequent problem with sending signals from A to B is what is called a ground loop. If the equipment is separated by a distance, say from your studio to your antenna, and it is grounded on both ends then you have a ground loop.

The solution is to have everything grounded at a single point. Try running your system ungrounded at the antenna and see if the hum goes away. If it does, then this is the problem.

The SSTRAN ground decoupling works with RF but not at 60 Hz. The best way around this is to use what is called differential mode for the audio. To do this you need two audio transformers, one at each end of the system. I have done this running analog signals over four miles of differential cable with great results.

Hope this helps

Neil

[radio8z]

Hi,

I have another thought on the ground loop problem. One thing that is done is to connect the antenna ground to the house ground at the stake near the meter using a bare #6 wire. This provides an equipotential ground for everything and may solve the hum problem. It is also important for lightning protection.

Here is a link I found on the subject. It is rather a long read, but gives some information about this.

http://members.cox.net/pc-usa/station/ground0.htm

Things are getting complicated aren’t they?

Neil

[oldguy]

one other possible solution if it is a ground loop is from Radio Shaque. You can get a ground loop isolater for about $15.00. They are used to rid engine noise from car stereos, but work great for ground loops in A/V equipment. I found it indespensable when running audio to video gear while taping shows, theater, concerts.

Justs a thought!

[Ebacherville]

SO you think this is a ground loop and not from running my power and audio all down the same piece of cat5 cable…

As for the transformer at each end, are you talking simular to what you would use on a 70 volt PA system?

I have see these little blauns for running rca signal down twisted bair copper for long runs, i wonder if this is actually little transformers in each one.. you use one on each end of your run.. we use then at work for our PA system..

I have seen the radio shack ground loop isolators.. would this go on the TX side or on my source side… I used to have one many many many moons ago when I was “broadcasting with SPL” vs Amplitude Modulation (16 – 10″ subs in a sub compact car with a 1000wats RMS driving them, yes I was one of those annoying people, but it was fun to see heads turn blocks away)

Im going to try messing with the ground isolation jumpers in the sstran TX to see if it makes a diffrence.

Jason

[Ebacherville]

Well that did nothing at all…

I did mess with the input, and even when im only conecting the positive side of the audio feed I get the hum.. Does that tell any one anything.. so that would be only connecting the center post of the rca input to the signal comming from the studio..

So here is how im going to test this im going to hook up my portable battery powered MP# player to the line on the studio side, this will have no chance of a ground loop.. if i do this and get hum then it from my power running next to audio…

[Ebacherville]

OK so i totaly unpluged my rca from the mixer output.. so there is no way any ground is feeding back over the audio line its not connected to anything.. so no signal at all not hooedto anything at all .. hum is still there..

This means that it has to be getting picked up in the run next to the power back to the studio..

My next hack will be to hack togethe a seperate audio line and move it away from the power line this should clear it up.

Jason

[Ebacherville]

OK that didnt work either, I also tried plugging in the power transformer on a diffrent circuit that goes directly to the fuse box with no other devices on it.. no change..

My signal is totaly seperated from the power and they dont run near each other, even if i have nothing on the end of this signal line, in the studio, the hum is still there.. the only way to stop it is to unplug the audio rca jack from the TX..

DO you think my transmission line is picking up some interfierance from the actual transmitter its self?

Im stumped at this point.. the line is picking up the hum from some where..

Jason

[Ebacherville]

OK so i had a idea.. I pulled the audio line from inside to studio out of the hours and into the yard,, still connected to the TX and the further i got it away from my house the less the hum got.. and when I went closer to the house the hum got worse again…

SO its picking somthing up from inside the house.. Where do I begin to track this down.
I dont get any hum on any of my audio or video equipment other wise.

I tried turnig things off that were on and unplugging any thing that i felt could be causing a issue.. and its still there… I turned off all my auqarium pumps and lights turned off all the floresent fixtures in the house.. The only thing running would be couple computers and my WiFI router and DSL modem and again.. still there.. I could start killing those but dont feel thats the issue. Its like the hum is comming from my house.. maybee the wiring.. but the house is brand new and we have never had any electrical problems or noise on any other piece of equipment.

I could understand a ground loop but even when the wire is not connected to anything and outside of the house its still there.. Thats odd.. very odd.

I guess it may need to consider the FM wireless link that was talked about here… if i cant figure this thing out… It really bugging me now that i cant find the cause of the problem.

Jason

[radio8z]

Jason,

This sounds really wierd. You are on the right track with your tests and believe me it will work out. Keep trying.

I wonder if you are getting the proper AC at your transmitter. If the voltage is low, the 7815 regulator will drop out and allow 120 Hz. hum on the SSTRAN 15 volt bus.

Did I understand correctly that you have hum even when the audio is disconnected. This really sounds like the AC is too low in voltage at the transmitter. You might try running an extension cord out to the transmitter and plugging the transformer there and connecting it to the transmitter. This will eliminate the drop in the Cat 5 wire. I really doubt that Cat 5 would drop enough voltage to cause this problem, but this could be a rule out.

Another thought, did the transmitter work OK on your bench? If not, maybe you have a diode in wrong. Check D1, D2, D3, and D4 and make sure C4 is not in backwards.

Another old trick to test DC supplies is to connect your meter and set it on the AC scale. If you have DC only the meter will read millivolts. If there is hum on the supply, it will read volts. Connect the black lead to T2 and the red lead to the “hot” side of L8 ( the right side lead as you face the front of the PC board.

Keep trying. It WILL work.

Neil

[PhilB]

Ebacherville,

Check your COMPRESSION setting on the transmitter. If it is even just a little above zero, the audio gain of the transmitter will be very high when no signal is present. This will exagerate hum and noise on the line, especially when you have the line unplugged at its source.

If that is not the problem, try unplugging the audio from the source and shorting the RCA plug(s), center pin to outer ring. That should kill hum that is picked up by the CAT5 audio twisted pair(s) being adjacent to the pair feeding AC power to the transmitter.

Running CAT5 wire with the 18VAC on one pair and audio on the other pairs places the audio immediately adjacent to the AC feed for the entire run, allowing the possibility of cross coupling. The SSTRAN transmitter has high impedance audio inputs. If you feed from a high impedance source, the cross coupling will be high. Feeding audio from a low impedance source will kill the cross coupling and it will greatly attenuate the susceptibility of hum and noise pickup at the transmitter end of the line. You can check out the effect of a low impedance source by feeding the audio from the headphone output of a CD player (not the line out, if it has one). The headphone output is low impedance.

Here is what is probably most relavent to your situation: set your audio source to MAXIMUM output level and reduce the GAIN control on the transmitter. This will swamp out any hum pickup along the way relative to the high audio level. Set the transmitter GAIN control as follows: start at max and reduce the GAIN until you just start to hear a significant drop in audio level, then increase it a little. This is the optimum setting that will give good limitting without excessive gain.

The ground loop problem is real in some installations. It sounds like you have ruled out a ground loop as being your particular problem.

[12vman]

Just for a quick check.. 8)

I figure you cut the cable on the transformer to connect to the leads you have running to the xmtr now for power. I figure this connection is inside the house. Do you remember which wire (color) you have connected to the center connection of the power plug at the xmtr? I don’t believe this really matters but, just to be safe, note this.. (It worked for me)

Take (2)-9 volt batteries and connect them in series. This will give you a 18 v.d.c. supply. This will operate the xmtr for a while, long enough to see if this makes a difference. Again, if you know which color wire is connected to the center pin of the power connection at the xmtr, connect the positive of the battery supply to that and the negative to the other. Fire up and see what happens..

[Ebacherville]

Thanks for all the help, but here is the issue again.

with the transmitter powered up and the rca NOT connected to the TX (on the TX side) its totally silent.. no hum..

When I plug in the audio RCA at the TX side, this is now seperated from the power lne, totally seperate line, that is no where near the power transmission line.. this gets hum..

I checked this line again and its wire correctly ground to ground and positve to positive..

The hum is always there even with the compression off..

The hum only stops when i unplug the audio RCA at the TX side.. if its plugged into or not into anything in the studio makes no diffrence.

I will try the two 9volts thing.. That will eliminate power supply issues.. But the odd thing is with the audio RCA at the TX disconnected its silent.. and the other odd thing is when i do connect that audio RCA line and pull it from the house the further I get away from the house the less the hum is in volume…further away i get teh lees it is. Thats really odd.

As for my settings, Im feeding the TX from my studio mixer, Its peeking right at zero DB.. The signal its self hooked to anything else sounds perfect.. absolutly no hum or interfierance. The signal in to the mixer is clean, the signal on headphones from the mixer is clean, the signal after the mixer is clean, the signal going into the encoding pc is clean, the signal comming from the encoding PC is clean on the studio monitors, and these monitors are ran off of a 12volt transformer that was on the same ccircuit as the tx power was on, before I moved it to a diffrent circuit..

I have two outputs on my mixer one that goes to the encoding PC for my internet broadcast, and the other goes to the TX.. both outputs are the same. My signal is compressed and limited by my automation system.. so I dont use any if a very small amount of compression at the TX, but even with it off the hum is still there, and not just slight its very notacable even when there is a loud piece of music or programming its always there. aleways constant.

The run is only about 30 foot long, right now the line runs 10 foot from my mixer to the window where it goes outside, down to ground leven and right now is just laying on the ground over to the TX,(abourt 20 feet) the line goes into the weather proof box, and into the TX audio jack, the joints that the RCAs are connected with are soldered and sealed with electrical tape.

My tx is set up according to the manual, like stated above, my actual settings are.. about 1/5th on the gain control, half on the modulation control, and the compression is off..

I have the trebble boost turned on on the TX, I messed with this, it didnt help.. I also messed witht he ground isolation jumers seeing if that made a diffrence.. no help there either.

Tomarrow I will pick up a ground loop isolator even though I dont think thats it.. Cant hurt I guess.

Jason

[12vman]

Try this..

Turn the gain control (the center one) completely off. (counterclockwise) This guy gave me some fits early in the game. If there’s any noise on the input, the gain will bring it out.. big time..

I don’t use much gain at all. I may have it cracked on and that’s about it. It needs to be on just so slightly for the compression control to work. If you have your input processed already, all you need to worry about is the level. I run my input level control on the xmtr at half way and adjust from the audio source. It’s not hard to modulate it to 100% with a mixer board.

On the mix..

Don’t attempt to get “big bass”. The compression on the xmtr will kick in and cover a bunch of mid/high response. I found using a very flat input works the best. The highs can be boosted to make it sound more crisp (with the jumper “in” on the xmtr) but again, don’t over do that either. Adjust to where you can hear some cymballs, “s” and “ch” sounds (around 10-15k) and let it go at that. It will never sound as clean as fm but it can sound really good..

Try to imagine this. My input audio is coming from a cheap pocket radio. (fm) It only has a hi/lo tone switch on it. I operate it with the switch on “hi” and the audio, if you listen to it through the speaker, suxs! All highs and “tinny”. But after it’s broadcasted through the xmtr, it really equals out. I was impressed on the bass response! No way could you hear it on the pocket radio but the xmtr picked up on it. It’s gonna broadcast what it hears in the low level audio. The xmtr changes the tone of everything..

You may always have some background noise of some sort. The game is to make your input higher than the noise itself and not to have the xmtr so sensitive to acknowledge the background noise much. My suggestion is to run the input at the xmtr at around 1/2-3/4 for starters so you have some headroom to play with. If this setting is too high as not to match the metering at your mixer, turn it down untill you find the happy medium. The gain control is very important! If you can get a good audio level to the xmtr, you don’t need much, if any, gain. This is the control that will bring any line noise up front and in your face! I suggest turning it completely down (counterclockwise) and check for your noise issue. I’ll figure most of it is gone. (At least that’s how it worked for me) It seems that the gain/compression controls work togather. What I did was just “crack” the gain on.. not even a 1/16th of a turn. I monitored the xmtr with a battery operated radio at the xmtr site, everything connected inside and outside, to find where the noise started to become a problem in the background. (I had no information on the line at the time.. just dead carrier) I don’t have any processing at my board but felt I needed just a little gain/compression. If you have processing, leave both of these off. Like I said earlier, it seems that the compression control doesn’t do anything unless some of the gain is used. The compression can “mud” things up also. It attacks the lows and will bring down the highs big time so be careful there too..

Tuning the xmtr can give you some issues also but not background as far as I have noticed. If not tuned properly, you will lose some “high” response but that’s a different day..lol

[Ebacherville]

WAHOOO!!!

It was a ground loop of some sort it doesnt make sence why it would hum when it want connected to anything in the studio…

But I picked up a ground loop isolator and that took care od 95% of the hum.. its still there but very very very faint and not detracting from the programming…

IM so fricken happy that this is finally taken care of…

SO for future people out there… Even if your line isnt connected to a device, it can some how pick up a ground loop… A isolator is probably a need in any situatoin.. Bust 16 dollars ive spent at rat shack in a while…

Now to make some fine adjustments, and tune my antenna more… Then i will beging construcing and coil with taps so i can really fine tune the system.

Thanks for everyones help on this odd problem.. All the seggestions helped me come to conclusions ans lack there of 🙂

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