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Protecting amps from TRS Jack?

post #1 of 4
12/31/10 at 2:03pm
Thread Starter 

 

Is there a decent non manual way to protect amps from the output shorting of TRS jacks?

 

I've been reading up on it and see that adding even an ohm or two resistor to the left and right channel of the TRS output jack should prevent an amp blowup on inserting/removing headphones.  I read the wattage should be high (like 5w?).  I am having trouble finding metal film resistors with those characteristics.   On my CK2III most of the other resistors are vishay metal film, so thought I would stick with those if possible.   Will sticking a resistor right at the output have a noticeable negative impact on sound?

 

My primary concern is my wife and company aren't likely to follow directions and turn down the volume before switching headphones.  The CK2III seems to have handled the occasional abuse well, but if i build a more powerful amp I'd like it to be accident proof ;)

 

Other than going pure XLR's any thoughts on this issue?

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post #2 of 4
12/31/10 at 3:34pm

Output resistors are the easiest answer, after XLR's. 

 

It does seem counterintuitive to build an amp with a very low output impedance / high damping factor only to put an output resistor on it. The question of whether this resistor will have an effect on SQ is an important one. IME the resistor does change the sound of both low and high impedance headphones depending how large it is.... and frequently for the better. Build a simple resistor cable (or get creative with alligator leads) and experiment with your CKKIII and a few pair of headphones.

 

If you do want to stay with a very high damping factor / low output impedance, XLR plugs are said to have lower contact resistance than TRS plugs. Its after the feedback point, every fraction of an ohm adds up.

 

If its any help every beyer I have ever heard has sounded better from a 120 ohm output impedance amp. They all have a characteristic spike in the upper mids that smooooths out with a 120ohm source. Very nice.

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post #3 of 4
12/31/10 at 4:33pm
Thread Starter 

 

Thanks for the lead on output impedance.  I did not even think about it from that perspective and see a lot of discussion on that, so that will be some fun reading tonight.  I had heard about impedance matching, but didn't give it a second thought.

 

I'll have to get my work area set back up and do some trial and error as well.

 

 

Protection wise is there some math that I should do?  Like the output transistors will be protected if there is a short from the TRS jack if there is a x ohm resistor that can handle x watts.  I don't see a lot of discussion on this... what do commercial do make their jacks user proof?

 

Is the relationship here the lower the ohms the higher the wattage needed, so if i go with 120ohm on the output I can use a 1/8 or 1/4 watt resistor?

 

 

 

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post #4 of 4
1/1/11 at 8:54am

1W or 2W is fine.

 

2.2ohm for R23/R24/R47/R48 instead of the default 0.47 ohm resistor.

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