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Old 4th August 2012, 12:32 PM   #1
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Default Short line level signal

Im wondering.

Can I short a line level signal without destroing the source?

For example an iphone or a regular consumer cd player or a computer...


This question may sound stupid but I need to know and dont want to try it before.

Thanks!
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Old 4th August 2012, 12:38 PM   #2
Mooly is offline Mooly  United Kingdom
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Line sources usually yes, you can. 99% of opamp outputs are short circuit proof by design.

I'm sure there must be exceptions somewhere to this and I don't know what an iphone output is like. If it's a Class D speaker output fed directly to a socket then perhaps not.

Does that help...
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Old 4th August 2012, 06:38 PM   #3
Enzo is offline Enzo  United States
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My working assumption (always risky) is that in this day and age they anticipate the headphones jack on a portable device getting shorted as plugs are inserted and removed, and design accordingly.
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Old 4th August 2012, 06:50 PM   #4
jcx is online now jcx  United States
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TRS jack/plug interface sucks - it it common for the Tip, Ring to be shorted together by one of the jack contacts during plugging in/out

so any competent consumer product design using a TRS jack/plug has to survive at least momentary shorting of the channels

some op amps may have problems - especially with extended short circuited outputs

the AD8397 is somewhat popular in diy headphone amps for its 300 mA drive and rail-to-rail output swing - but it can be lunched by output shorting - if you have some current limiting the chip thermal shutdown may kick in but the output devices are too fast/high current to be saved from a low Ohm short
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Old 7th August 2012, 03:50 PM   #5
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Thanks for you replies guys. But I still feel I havent got the anwser I want to try this.

jcx: its common the tip gets shorted while inserting and taking out but Im wondering if I short one or both channel for a longer time. What do you think?
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Old 7th August 2012, 05:34 PM   #6
DF96 is offline DF96  England
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You have been given wise advice, which get as close to answering your question as is reasonably possible. To give the definitive answer you appear to be seeking would require an exhaustive search through every line level source ever implemented. A competent consultant would want a huge fee for this work, and in the end would still add get-out clauses to his answer.
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Old 12th August 2012, 10:44 PM   #7
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by McDaisy View Post
Thanks for you replies guys. But I still feel I havent got the anwser I want to try this.

jcx: its common the tip gets shorted while inserting and taking out but Im wondering if I short one or both channel for a longer time. What do you think?
The answer is "It depends." Likely there is over current protection built in but maybe not. The only why to know is to find a schematic for the device in question. You 'd expect some kind of short protection but maybe not.

BTW my bother just brought something over this morning for me to look at. Turns out an electrolytic cap had exploded. The designer, I'm sure in order to save 2 cents designed a power supply that would fail instantly if you put a battery in backwards. So you never know. All depends on how cheap the company building the device is.
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Old 23rd January 2013, 12:08 PM   #8
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Default How to mute balanced line

I've got a similar question if anyone can help.

I would like to have 4 line level outputs from a cheap Behringer mixer permanently muted, until separate momentary switches unmute them individually. Would a simple solution of a normally closed switch across pins 2 and 3 work - as with switches on mics? Or is that bad practice, as this forum suggests, for the output stage at line level? 99% of the time it will be shorted (muted).

The purpose is so a single talkback mic can communicate with 4 different earpieces individually via 4 switches. A bonus would be a fifth switch that talks to ALL (opening all 4 shorts at once)

Any suggestions before I possibly cause any damage would be greatly appreciated!

Adam
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Old 23rd January 2013, 12:32 PM   #9
Mooly is offline Mooly  United Kingdom
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Hi Adam,
you might be better starting a new thread for this as its specific to a particular unit.

As a general guide, although shorting an output to mute it might be considered poor practice, the reality is that most opamps are short circuit proof. As always, knowing the exact circuit configuration helps.

You could always add a small resistance (say 100 ohms) to each feed to make it more "acceptable". That would help preserve DC conditions even though the output were shorted. If the outputs are AC coupled then its even less of a problem tbh.
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