hello. i have my amt-3000 installed on my roof. the roof flat and is made of corrugated metal covered in tar paper. i dont have access to soil to sink an earth ground so i have the ground of the transmitter connected to the metal roof. i get acceptable range but when my station is quiet i can hear some other radio station coming through my receiver.
this other station IS NOT broadcasting on the same frequency. if i turn OFF the amt-3000 then i cannot hear the other station at all.
any idea what is going on and how i can fix it?
Sounds like somehow the RF of the other station is making it's way into your carrier.
Try moving around the wires from the TX all the way to whatever its connected too.
Hello dietnews. In addition to the remedies suggested by The Crow, here are my experiences which might explain what is happening.
What frequency are you on? Find out the frequency of the interfering station. If it is a nearby frequency, chances are your transmitting antenna, which is tuned to your precise frequency (the antenna Q), may be acting as a receiving antenna and picking up the other station and re-modulating it in your carrier.
The other possibility is that a nearby full power station of sufficient power is getting into your wiring and being re-transmitted by you.
I had a 5 kW station 1-mile away get into my phone line. You could listen to it during phone conversations and it wrecked my DSL internet signal.
RFBurns has given a cure in the form of an inductor which filters out the unwanted station. A filter might help you if you find where it's getting into your wires.
thank you for the advice. i wont be back at my studio until wednesday but when i go i will try to figure out what frequency the interfering station is broadcasting on.
i know that the amt-3000 has an optional jumper to isolate the RF signal from the AC wiring. i cant remember if i have that jumper in place or not but is it possible that i could fix it by isolating the ac lines like this?
There are 3 jumpers, S1, 2 & 3.
S1 decides whether the AMT3000 ground is connected directly to the audio ground, and S3 & 4, which should both be either installed or removed together, decide whether you are grounded through the AC power.
Since you have a ground system try removing all three of these jumpers and it might help kill the RFI (radio frequency interference).
It may be necessary to run balanced audio out to the TX if other methods to not remove the problem.
Radio Shack has a few inductor core forms and you can make chokes by wrapping a few turns of that audio cable around the inductor core. Best place is right at the transmitter just before the audio plugs into the transmitter, and if necessary, again at the beginning of the run of wire downstairs.
You may also have to do the choke approach with the power cable as well.
RFB
Oh, man, if you don't have balanced audio I'll double emphasize what RFB says.... you've got to have balanced audio for that kind of long wire setup, or you'll almost certainly have interference getting into the audio cable.
You might also have high frequency roll-off (loss), and more hum than you want in the audio.
all of this sounds exactly like my setup. what is the cheapest way to setup a balanced audio input? DI Boxes from the music store are really expensive.
Radio Shack has some little 600:600 ohm matching transformers very inexpensive.
The output of your audio equipment may already provide balanced lines, be sure to check.
If you describe what your audio lines are like we can instruct you to build balanced lines.
audio input to the transmitter is from a PA mixer. it has balanced XLR outputs. i am currently using unbalanced outputs from the mixer into a 40ft RCA cable going up to the transmitter on the roof. i went with the unbalanced output because the amt-3000 has unbalanced inputs.
Stage One -- You can make an immediate improvement until you have your balanced-to-unbalanced transformers wired.....
Use the balanced outputs from your mixer, 2-center conductors plus shield, all the way up to your transmitter.... at the transmitter tie the shield and negative together thus making a cheap conversion to unbalanced, and connect to the transmitter inputs.
Stage two -- the far-end (transmitter end) of both lines add transformers, neatly and safely installed in small plastic project boxes (also Radio Shack), converting balanced to unbalanced, and into your transmitter.
EDITOR'S ADDITION Frankly, why run two entire audio lines up there when it ends up being converted to mono in the transmitter? Instead, you could convert to mono right at your mixer....MAYBE YOUR MIXER HAS A MONO OUTPUT, IF IT DOES USE THAT!
If your mixer only has a stereo (2-channel output), convert the balanced lines to stereo as follows:
Tie the shields(grounds) together in the usual way all the way from end to end;
Connect the left and right + wires from left and right channels each through a 5k resistor, and combine the resistors to the same mono cable;
Connect the left and right - wires from left and right channels each through a 5k resistor, and combine the resistors to the same mono cable;
DO NOT COMBINE L + R OUTPUTS DIRECTLY WITHOUT A SPLITTER RESISTOR NETWORK!
At the transmitter end feed the un-balanced wire, after conversion by the transformer, through a normal splitter and plug into red and white audio inputs on the transmitter..
If this is hard to follow, perhaps someone will provide a diagram.
Here is a bit of info on balanced/unbalanced conversion.
Since this approach uses passive components, the configuration can work in either direction.
http://www.dplay.com/dv/balance/balance.html#cross
RFB
i went over to radioshack and bought an audio transformer. the shelf had labels for two different ones. there was this one which i bought: http://www.radioshack.com/product/index.jsp?productId=2103254 and another label that said "1:1 audio transformer" that was not in stock.
for this purpose do i want the 1:1 transformer?
i tried out the one i have on my test bench. i put a 1VRM audio frequency sinewave into the primary (unbalanced) side and attached the secondary side to the two probe inputs on my o-scope. the balanced outputs are properly out of phase but their voltage is 2.5VRMS.
if it was a 1:1 transformer, would i get the same voltage out as in? is the transformer i am using going to work fine?
The transformer I mentioned is 1:1, namely 600 ohms in and out.
You should get the same level out as you put in, but the audio level controls on the AMT3000 can be used to adjust so you have a good amount of room for differences in signal level.
thats what i was worried about. will this transformer effect the audio fidelity at all? i suppose if it runs too hot i can turn down the mixer right?
is the voltage change caused by a transformer linear or is it possible the transformer will cause distortion?