Increasing volume on Clipsal Doorstations

I have one audio doorstation, one video doorstation, two video intercoms and two audio intercoms connected in star configuration for every one of the four connections (14V+, Com, Audio, Video {NC on Audio only devices}).
Every intercom has its own power supply.

All the intercoms were connected inside the house with Cat5 STP cable, linked to the outside doorstations with an unshielded 30 Core cable. Apparently this was installed because the conduit was too small for running the two doorstations? connections and the cabling for the three striker locks (which are individually controlled by the intercoms).

The problems are twofold: firstly, the video image kept rolling (like on old TV set where you had to set the Vsync pot) and secondly the sound on the doorstations was very soft, also given that the house is on a very busy street.

I tripled up the connections on the 30-core cable to the Cat5 STP cables. The result was better audio volume (only slightly) and the video seemed to roll once or twice and then stabilise into a perfect image. Opening up the doorstations revealed no trimpots and I?m not to keen on soldering out the feedback resistor on the opamp and replacing it with a bigger one (assuming that an opamp is in use ? I didn?t check). The strange thing is that the audio on the intercoms? side inside the house is perfect. I suspect that the main problem is with the noisy background ? any idea how I might increase the volume. And maybe why the video rolls?

 

Potemkin,
First question: What lengths are the runs of comms cable to the doorstations?
The limit is normally about 50m on a single run. The fact that you are not using a CAT5 STP to the doorstations may also add to your problem. You are running your supply voltage with your AV signal which is less than ideal, especially on the cable you're using. Try to seperate it if possible.
Is it possible to replace the comms cable with STP?
Secondly: Did you common all the negatives of your different power supplies? This should be done. You also talk about 14V. The intercoms you're using use 18Vdc supplies, thus make sure your voltage drop is not too excessive.
Hope this helps

 

HeinP,
The lengths are shorter than 50m, but I see what you mean by not using STP - I will see if it can be replaced.
I did not common out the negatives of the power supplies, but is commoning the Earth connection on the Audio/ Video connector not to the same effect? The supplies are 18V supplies, but the 14V is the supply for the doorstations, internally generated by the intercoms.
I will try and replace the STP and post the results.
Thanks for the help.

 

Just a point

I apreciate that you are not using coax but the rules are similar.
Using the signal path earth?s for power supply earth?s is bad practice. Yes it is ?electrically? correct that you have earthed them through the sockets. However, the outer braid of the cable is supposed to prevent unwanted signals from being induced in the center core.
Having current that flows through the outer braid will induce unwanted signals in the inner core, causing noise distortion etc. The noise will be typically a copy of the amplified signal.
For example the sync signal will be induced in to the cable of the incoming signal and will cause syncing problems. This can also feed back on itself and become self oscillating
When designing a system try to have the ?Current circuit earth?s? and the ?Signal circuits earth?s ? kept separate. They should be wired back to one common point ?Star wired? and that common point should be as close to the power source earth as much as possible.

Colin Smith

 

Split the system into two networks and works just fine now.
The moment I take one of the doorstations off, the rest off the system works much better.
Thanks for the help gentlemen.