Symetric-non symetric audio question

[Othello]

I am struggeling with mixing two types of equipment, some home hifi stuff and some broadcast equipment.

What should I expect on a symetrical line, as viewed on a scope, two equal but opposed signals?? One of mine is weaker than the other, or so it seems.

Also, I seem to remember that there is a way to stick a symetrical line into a power amp input with non symetrical cinch plugs, but somehow I must have gotten it wrong, awful hum. Do I have to use a transformer??

Uwe

 

[audioguru]

The word is balanced, not "symmetrical". Broadcast equipment has balanced inputs and outputs so that both wires have signals with opposite polarities.
Hi-fi equipment uses unbalanced inputs and outputs so that only one wire has signal and the other wire is its shield and is connected to ground.

A transformer or a differential amplifier circuit can convert balanced to unbalanced. Taking one signal wire of the balanced connector and using the source's ground as its shield also works.

 

[Othello]

Ok, balanced line!

Well, I have no hum whatsoever throughout my audio board where all the other devices (tape machine, tunes etc.) are connected. That whole side of my equipment uses balanced lines and works perfectly.

I use two home made amps which by themselves are quiet too.

If now I plug the signal from the board (using the technique audioguru suggested) into my amps there is hum. If I plug in only one channel everything is quiet. Only the other channel is quiet as well but as soon as both go in the hum starts.

The amps are connected to a toroidal transformer and are not grounded other than thru the ground of the input line.

Where is my mistake??

Uwe

 

[mneary]

The 'ground' input of an audio board is sometimes just a reference for the amplifiers and can't tolerate very much current. Try a three-wire ground on your home made amps, or a wire straight from the board to the amplifier chassis.

A ground on your home made amp is normally a good idea anyway.

 

[Othello]

I would like to understand this fully before I solder wires.

Right now my homemade amps are just floating and if I understand you right you propose to connect the amp "ground" of the circuit board to some ground reference outside of the amp.
Currently that happens the moment I connect it to my board via the input wire (where the shield and one of the balanced wires are connected and go to the ground of the amp circuit board).

That is not enough you say??

Thanks for your help

Uwe

 

[mneary]

That is not enough you say??

If it were enough, you would have no hum.

The screen of an audio cable is not intended to carry appreciable current; depending on your specific arrangement you might be able to reduce the current that's flowing as I suggested by:

1) Provide an Earth ground for the amplifier chassis(3-wire plug). Unless your amplifier was carefully designed to be safe without an earth ground, this is a safety issue that you should already be concerned about.

or
2) Provide a direct connection between the chassis of the two pieces of equipment.

If I understand your setup, this should eliminate your problem without any changes to your audio wiring.

 

[Othello]

Well, maybe this is the issue then, there is no chassis, (yet!).

The amps are still on the bench.

No hum though, even with a cable attached. Only when I plug the cable in does the hum start. That's why I didn't think that I would need a chassis urgently to eliminate the hum, since there is none unless BOTH channels are plugged in.

Uwe

 

[Speakerguy]

This is a serious safety hazard. You need to ground your amps.

The way most audiophile guys do this is to take two 1N5401 diodes and a 2W 5ohm resistor, put all 3 in parallel (diodes facing opposite directions). Take the ground from your wall outlet, connect it to one side of the parallel combination, and also connect it to your power amp chassis. This is for safety. Now take the other side of the diode/resistor parallel combo and connect it to your ckt board ground.

Basically this ensures that no noise on your house ground <0.7V gets through to your hifi signal ground. But if you have a fault the diodes are there to let the fault current pass. Make sure you have fuses on your supply rails that will blow before the diodes.

Here is an excellent article:

**broken link removed**

 

[Othello]

Thanks for all the advice and the link to the article.

In the end the solution to the hum was a surprise to me.

As I said I have two monoblock amps on my bench which are connected via shielded cables to the console. And there was hum.

A simple wire connecting the two monoblock grounds with each other solved the problem completely, but I am still trying to wrap my mind around why this is a solution.

But at least the noise is gone.

Uwe

 

[Nigel Goodwin]

Something probably wasn't earthed?. Earth loops are a BIG problem, and are dealt with in various different ways, there might have been no earth to one of the amps?.

 

[Othello]

Nigel, both amps, which are separate units, one for each channel, were earthed thru the balanced input lines and since this was considered possibly not adequate I also tried a separate ground connection at one point, but the hum stayed.

The hum went away after I ran a wire from one amps ground (on the circuit board) to the other amps ground. So now we have the situation where there is a common ground in the board, two separate shielded input lines to the amps (with the shield being connected to the boards ground) and then a 4" straight wire ground connection between these amps.

I did not expect this result, in fact I expected this arrangement to create hum, but magically it made it disappear.

Uwe