Isolation transformer

[ubxf]

Hello,
I'm trying to help a church with a hum in their PA system. The main church has 8 speakers . The trouble is that they send a speaker cable to and adjacent building 20 meters away and feed that to an amp that in turn powers 12 outdoor speakers > That not surprisingly is creating hum in every speaker . I was thinking to send a line level feed from the mixer in the church to the second building amp and also add an audio isolation transformer . Would a 600:600 be good or a 10K:10K ? Another twist is that the system was originally designed as a 70V but over the years the amp got replaced by regular amps .

 

[ccaudle]

send a speaker cable to and adjacent building 20 meters away and feed that to an amp

So the feed to the amp is two-conductor unshielded cable? Line level signal sent on speaker wire, or a high level signal from the output of an amplifier sent, which is then attenuated at the input of the amplifier and used as input signal?
Is the amp an unbalanced input, or balanced input amplifier? Two-wire or three-wire power entry on the amplifier?

creating hum in every speaker

Did you mean literally every speaker, or every outdoor speaker? Are the indoor speakers also affected?

adjacent building 20 meters away

Above ground or underground feed? When sending signals between buildings you really need to consider power faults and lightning induced ground differences.
Do the buildings have separate power entrances with separate protective earth rods at the power entry?

@MisterCMRR are you familiar with any isolation transformers which are made for inter-building use, e.g. have GDT overvoltage protection or similar heavy duty protection capabilities to account for power faults or earth potential differences during thunderstorms?

 

[ubxf]

Thank you for your response, I have to go back and investigate properly to be 100%sure but from the quick evaluation
feed to the amp is two-conductor unshielded cable? yes
high level signal from the output of an amplifier sent, which is then attenuated at the input of the amplifier and used as input signal? yes
Is the amp an unbalanced input, or balanced input amplifier? Two-wire or three-wire power entry on the amplifier? I need to check but I think balanced
Did you mean literally every speaker, or every outdoor speaker? Are the indoor speakers also affected? yes
Above ground or underground feed? above ground
Do the buildings have separate power entrances with separate protective earth rods at the power entry? I don't think so but I have to check.

 

[ubxf]

I read this application paper and I will follow recommendations
https://www.jensen-transformers.com/wp-content/uploads/2014/08/an004.pdf
Maybe use this transformer
https://www.jensen-transformers.com/product/pi-xx/

 

[JohnRoberts]

They are sending a speaker cable to feed audio into another amplifier? Most amplifier signal equipment inputs are not designed to accept speaker level signals. When using active powered equipment in different buildings, each with their own utility service, it is not unusual to have significant potential differences between safety grounds.

#1 If you pad down the speaker level signal with a resistive pad that will not only reduce the signal level but attenuate the ground potential differences.

#2 A small audio transformer can generally isolate the two safety grounds.

JR

 

[sodderboy]

600:600. It's not a science project.
Top shelf Jensen Inside™ solution
Mid shelf solution
Bottom shelf is the same on Amazon, mostly for cars.
The Radio Shack Ground Loop Isolator has a proud history on this site, and I was using it long before in all sorts of applications. For a stereo feed between church systems it is great! Wire it balanced with hot on 2 (tip) and shield on 3 (ring) no pin 1 (sleeve) connection.
If the amp and speakers work quietly with a local source, then isolate your external feed and enjoy the praise.
Mike

 

[micolas]

@ubxf
Have you thought about a wireless system, very affordable and cheap these days?

 

[Bonnie1]

Hello,
I'm trying to help a church with a hum in their PA system. The main church has 8 speakers . The trouble is that they send a speaker cable to and adjacent building 20 meters away and feed that to an amp that in turn powers 12 outdoor speakers > That not surprisingly is creating hum in every speaker . I was thinking to send a line level feed from the mixer in the church to the second building amp and also add an audio isolation transformer . Would a 600:600 be good or a 10K:10K ? Another twist is that the system was originally designed as a 70V but over the years the amp got replaced by regular amps .

Hello ubxf,
No transformer needed, (it's not a proper solution, at best a band-aid)
Send line level signal from console/ drive rack to amplifier driving outdoor speakers. Along with the balanced and shielded line level cable you want to run power from the same power your indoor amps and console is connected to.
-Another option is to use a spare wireless transmitter and receiver, if you have one available. (Your outdoor amplifier will not need to be on the same power source as the indoor system with this method)

If you disconnect the outdoor amp completely from the indoor system, does the hum go away? If not, you have a fundamental problem with how the indoor system was installed.

Cheers, -dmax

 

[ccaudle]

No transformer needed, (it's not a proper solution, at best a band-aid)

It is an appropriate solution for the problem of different systems running from different power feeds. High CMRR galvanic isolation is exactly what is needed.
Transformers are not cheap, but neither is permanently running a new power feed from one building to a different (which may not even be allowed by building code if the second building already has a separate power feed).

above ground

Then you really need to consider lightning protection. Standard building electrical code might require that any wire entering a building have a bond to the building safety earth system. It is a little more straight forward for an antenna which would be bonded only at the building entrance, less clear how that applies to a connection which is potentially already bonded to building safety earth at the next building over.
I have never worked on multi-building systems, so perhaps consult with a local electrician to see which local building codes might apply to a low voltage connection run between buildings.

 

[DaveDC]

I worked for a major DAW manufacturer. In the mid 90’s, we had a large post production customer in Burbank who kept returning audio interfaces for blown line-level output drivers. That product used the SSM2142 balanced driver IC. It turned out that this customer had a multi-building site and had run underground tie lines between buildings. One of the buildings also housed a welding shop. I guess you know where this is going… I ended up revising the interface to add clamping diodes between the output signal lines and the drivers’ power rails. I also had to add Zener clamps to the power rails to absorb the voltage overages. So environment-induced damage is a real thing. Go for the transformer isolation and you will not regret it.