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Old 19th August 2011, 07:10 AM   #1
jaycee is offline jaycee  United Kingdom
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Default Protection on headphone amps

One thing I'm not seeing a lot of on headphone amp designs is any form of protection circuit. I have been using a design which is basically an OPA2134 driving a diamond buffer for a good few months now, but due to lack of time it has never left my breadboard.

Sadly last night I moved the breadboard and accidentally disconnected the +V supply. The output immediately swung to -V, killing my Beyer DT231's in the process

Obviously this would not have happened on a PCB, but parts failure does happen... so I am wondering what would be appropriate to protect the 'phones in this case. Coupling capacitors is the easy choice, but not a preferred one. I'm thinking a VI limiter would suffice here.

Any comments ?
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Old 19th August 2011, 07:22 AM   #2
Mooly is offline Mooly  United Kingdom
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That's tough luck...

Maybe use a low value circuit protector (fuse) and a couple of series back to back zeners across the output. The amp would have to settle quickly though.

Fully built and properly designed on a proper PCB and I think the risk without protection is acceptable.
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Old 19th August 2011, 07:27 AM   #3
sandyK is offline sandyK  Australia
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There are little protection PCBs available on fleabay at quite reasonable prices.
Headphone Amp relay & DC offset protection | eBay
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Old 19th August 2011, 07:30 AM   #4
jaycee is offline jaycee  United Kingdom
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Yeah I know.. should've made the time to build the thing... even if it was on Veroboard!

The real one would be powered through 317/337 regs (the prototype just used some 7812/7912's i had laying around) and I thought about perhaps adding transistors across the Adjust resistor to ground which, if the opposite polarity supply goes missing, switch on thus driving the regulator output low.
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Old 19th August 2011, 07:31 AM   #5
jaycee is offline jaycee  United Kingdom
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Quote:
Originally Posted by sandyK View Post
There are little protection PCBs available on fleabay at quite reasonable prices.
Headphone Amp relay & DC offset protection | eBay
That was my other thought - simply add a DC offset protection circuit in the same manner as a power amp. I didn't know how effective this would be for a low output circuit like a headphone amp though.
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Old 19th August 2011, 07:35 AM   #6
jaycee is offline jaycee  United Kingdom
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Just for reference, this is the circuit - pretty obvious implementation really.

Click the image to open in full size.

There is no gain simply because with my PC's sound output, it's not needed. It doesn't really need voltage gain - what it needed was current gain... the onboard sound will not drive a 32 ohm load without some current limiting kicking in and distorting badly.
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Old 19th August 2011, 08:00 AM   #7
gfle is offline gfle  Greece
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Quote:
Originally Posted by jaycee View Post
That was my other thought - simply add a DC offset protection circuit in the same manner as a power amp. I didn't know how effective this would be for a low output circuit like a headphone amp though.
During testing a good practice is to add a capacitor with the load.
I know this is not acceptable, soundwise, so I have simulated a simple modification of a power amp Dc protection, for HeadPhones.

It'll trigger the relay for DC offset or +-65mV or more.
It's not tested so there may be errors. Could be done better I quess but I can not thing anything simpler than that.
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File Type: jpg dc-PROTECT-HP.jpg (23.7 KB, 461 views)
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Old 19th August 2011, 08:28 AM   #8
hesener is offline hesener  Germany
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How about driving the headphone symmetrically? That may require re-wiring though. But if one supply goes the outputs will swing to the same level, no DC across the headphones...

just my two cents

PS: Nice circuit by the way.... Thinking about doing something similar for my K701's
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Old 19th August 2011, 08:46 AM   #9
jaycee is offline jaycee  United Kingdom
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Quote:
Originally Posted by hesener View Post
How about driving the headphone symmetrically? That may require re-wiring though. But if one supply goes the outputs will swing to the same level, no DC across the headphones...

just my two cents

PS: Nice circuit by the way.... Thinking about doing something similar for my K701's
Common ground so not really possible. I think a regular relay based DC detect will do the job.

The circuit works pretty well. I only used those MJE15032/3 transistors because I had them - BD139/140 will probably work just as well, although they are biased fairly high so the TO-220 package probably works well here.

Here's a picture of the murderer!
Click the image to open in full size.
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Old 19th August 2011, 09:19 AM   #10
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I am building such a device. At least it is on a to-do list.
I will use an Atmel microcontroller to control and TL08x opamp for sensing.
Relay (OMRON G6K or similar) for switching.

Some say the MCU can fault too but then the system wont turn relay on.
Main advantages (or possibilities):
-can sense PSU voltage and react depending those
-can sense DC on output and react depending those
-could sense temperature and react depending those
-can be used to StandBy mode and switch the main power for amplifier (if has different transformer and 230V relay)
-can be controlled by push buttons
-can drive LED for feedback or even display
-can built SMT, no big caps needed but relay drive transistor
-few parts

Controlling ideas:
-can sense DC levels, triggering level can be set precisely
-can wait and see if DC shows up again and decide
-can set alert levels depending on faliure (short turn off, long tudn off, turn off and wait for interact)
-can send signal (LED) to interact if needed
-of course it can be programmed for turn on delay and check DC before turn on.
-possibilities are endless but restricted on MCU ports (legs)

Disadvantages:
Need different power supply. It needs 5V, but draws only 50mA (with relay) I think, maybe can be connected to the positive transformer rail with a 5V regulator.

What do You think?
Of course I would publish all files if there is an interest.

Last edited by Earfanatic; 19th August 2011 at 09:44 AM.
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