Newsgroups: rec.audio.tech
Subject: Re: Burned out sound card...
References: <[email protected]> 
"mc"  writes:

> You cannot damage a soundcard by connecting its output to the input of a 
> stereo.  

Theoretically you should not be able to damage a sound card 
by connecting its output to the input of the stereo. 
It is for what the line output on the computer is designed for. 
It is designed to be connected to the input of stereo or 
multimedia speakers. 

But in real life there are possibilities to damage soundcard 
when making the connection! I have seen that happen and 
I know wy it happens. 

The problem is the following part in it:
1. The typical audio connectors (3.5 mm jack, RCA) easily
   make the connection for the signal conductor before the 
   signal ground makes contact. This is bad.
2. Equipment are not always completely isolated from electrical
   power or securely groundes to same ground potential

The combination of those can cause damaging current going 
through the sound cards output and/or hifi equipment line 
input on some unfortunate cases. 

If one equipment is at considerable different potential than another, 
then when you make the connection to other equipment, the potential 
between them tries to be the same through the audio cable. 
Normally the interconnection cable ground shield does this job 
nicely when cables are always connected. But when you insert 
the cable to the equipment when it was not initially connected, 
unfortuantely the signal wire gets the first connection 
before the ground. The potential difference between the equipment 
tries to go away through that signal wire connection, causing 
a quite high surge to the circuitry on the equipment connected 
to the line (surge goes through their input/output circuitry). 

The potential difference can be considerable. For example 
if a PC is powered from ungrounded 230V AC outlet, you can easily 
measure around 110V AC between the PC case and real ground 
(there would be nearly 1 mA curent if you short circuit case to ground). 
This is caused by the AC mains filter capacitors (few nF capacitors 
wires live-ground and neutral-ground, ground being connected 
to PC signal ground/case + mains connector ground pin).

If the hifi equipment is grounded (for example tuner connected to 
antenna outlet or wired to TV/VCR connected to antenna outlet), then 
you can get quite considerable (around 110V) surge coming to the audio 
circuitry of the PC and/or hifi equipment. The surge nergy is 
determined by the side of filtering capacitors (at worst case you 
can have up to nearly 150V positive or negative peak voltage in 
relation to ground on the ungrounded PC case and charged to 
those filtering capacitors if you make connection exactly at 
the mains highes volage peak). 
This can damage equipment if not very well protected! 

To make connections safely you have the following choises 
for methods:

1. Use audio connectors that are guaranteed to make the ground 
   connction first when plug is inserted and cut is last 
   when removed. Professional audio applications use XLR 
   connector that does that.

2. Have all equipment securely grounded together. This makes 
   sure that there will not be damaging potentials between 
   equipment. This is used in many professional systems. 

3. Make the connections and disconnections only when 
   all the equipment are not plugged to the mains outlet 
   and not plugged to antenna outlet. Now every equipment 
   is "freely floating" and no mains leakage that causes 
   potential differences (there is still small possibility 
   of problems caused by ESD)

5. Have all equipment built in such way that they do not 
   have any potentially dangerous amount of mains leakage. 
   This means consumer hifi equipment that are 
   dual-insulated designed originally with ungrounded 
   power connector. Have all "leaky" equipment powered 
   through safety isolation transformer. Now there 
   can be potential differences still, but leakage 
   capacitances are so low that the nergu stored to 
   them (the one the causes surge in interconnection)  
   is so low that do not damage well designed equipment.

6. Use a signal isoltation transformer between your equipment. 
   An audio line isolation transformer will pass the audio 
   signal but keeps the ends of the isolator otherwise 
   isolated (no interconnection of equipment cases and 
   surge related to it)


Those are the basic techniques for making sure that the 
connecting/disconnecting audio cables will not fry equipment. 
There are those possibilities. Using any of them should 
make the connection safe then there is no fault anywhere. 


Note: The following setups not covered by above tips
are also "safe" not to damage on interconnection:
- one dual insulated low leakage equipment and one "leaky" equipment
  (typical case: ungrounded PC multimedia speakers and ungrounded):
- one dual insulated low leakage equipment and one grounded equipment
  (hifi system not connected to antenna outlet and grounded PC)


> Headphone level is, for all practical purposes, the same as line 
> level, and even if it were not, the worst that would happen is that the 
> stereo would be too soft or too loud or possibly distorted.

You are absolutely right on this.
 
> If he plugged it into something that was not an input, the situation is 
> entirely different, of course.

You. You are githt on this. Usually a short time accidental inteconnection 
of two line outs together (soundcard output and hifi output) does 
not damage anything. I can say usually no damage expected here. 

> If the repair place says something about "the wrong power," get them to be 
> precise and then tell us.



-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/