Amp Short Protection?

[RenEH]

I was A/B testing between two different headphones last night on a Magni 3. At one point, the 1/4" jack was about 3/4 of a cm from being fully inserted, but I didn't notice. About 30 seconds later, I could smell burning electronics. I immediately turned off the unit, but it continued to smell for the next half hour. Upon inspecting the unit, it still worked fine.

An email with schiit later confirmed that there is no short protection in the magni 3 and that it could damage the amp, but they didn't comment if it could damage the headphones.

Retarded move on my part for not filly inserting the plug, but are there any amps that do provide some kind of short protection? I have little ones around and this could definitely happen again with me not around.

 

[sergeauckland]

Utter madness. Any product that can't take a permanent short circuit without damage isn't fit to be used.

Considering that headphone jacks often short out one or other channel even if for a short time, they must be able to stand a permanent short.

There are lots of power amps for loudspeakers equally unprotected.

S.

 

[DonH56]

Every time you insert or remove a standard 1/4" or 1/8" TRS headphone jack the output is potentially momentarily shorted.

Easy answer: don't buy Schiit.

 

[solderdude]

An email with schiit later confirmed that there is no short protection in the magni 3 and that it could damage the amp, but they didn't comment if it could damage the headphones.

By the looks of the PCB there is a relay in the output of the amp.
But the part number count suggests this is only a slow on/quick-off 'pop-elimination' circuit.

When schit like shorting the output for a longer period happens chances are the output device or the big emitter resistors may fail after a certain time and when driven loudly.
If this happens on one of the voltage rail sides only it is theoretically possible there may be a DC voltage on the shorted output.
Once that short has been removed (the plug inserted correctly) there could be a damaging DC present and still have no sound.
As the relay, most likely, doesn't have a DC protection circuit in the DC coupled output stage (as the Magni 3 is) then chances are there could be a DC voltage on the output which silently can blow up a driver.
So, yes, it could be possible a driver could be destroyed when all of this happens in the described way.
As long as the amp still makes good sound after the short you don't have to fear damage to the headphone.
Adding the DC protection would increasing the costs of course but not by much when using a cheap IC that does this. The relay is the most costly part in the protection circuit and is already is in the amp.

sergeauckland is right though. Even for longer periods of time a headphone amp should be able to handle a short without blowing up.
I also think any good DC coupled amplifier should have at least DC protection on its output.

 

[RenEH]

Can you

By the looks of the PCB there is a relay in the output of the amp.
But the part number count suggests this is only a slow on/quick-off 'pop-elimination' circuit.

When schit like shorting the output for a longer period happens chances are the output device or the big emitter resistors may fail after a certain time and when driven loudly.
If this happens on one of the voltage rail sides only it is theoretically possible there may be a DC voltage on the shorted output.
Once that short has been removed (the plug inserted correctly) there could be a damaging DC present and still have no sound.
As the relay, most likely, doesn't have a DC protection circuit in the DC coupled output stage (as the Magni 3 is) then chances are there could be a DC voltage on the output which silently can blow up a driver.
So, yes, it could be possible a driver could be destroyed when all of this happens in the described way.
As long as the amp still makes good sound after the short you don't have to fear damage to the headphone.
Adding the DC protection would increasing the costs of course but not by much when using a cheap IC that does this. The relay is the most costly part in the protection circuit and is already is in the amp.

sergeauckland is right though. Even for longer periods of time a headphone amp should be able to handle a short without blowing up.
I also think any good DC coupled amplifier should have at least DC protection on its output.

I appreciate the thorough response, thanks. Can you recommend any amps that specifically have this short protection? I was eyeing a THX 789.

 

[Blumlein 88]

Any way you spell it this company's products just turn out to be Schiit.

 

[solderdude]

Can you recommend any amps that specifically have this short protection? I was eyeing a THX 789.

The THX789 seems well designed and has DC protection as well as thermal protection and current limiters.

 

[RenEH]

The THX789 seems well designed and has DC protection as well as thermal protection and current limiters.

Thanks!

 

[sergeauckland]

The Behringer A500 is properly protected against short circuits as I tested several times when putting a screwdriver across the outputs at full power. The amp was fine, the screwdriver less so.

Quad amplifiers are also fully protected. If they are, it will tell you in the spec. If it doesn't say, then most likely they're not.

S

 

[RenEH]

FYI I got a reply from Massdrop, the thx 789 has no such protection. Any other suggestions?

 

[amirm]

FYI I got a reply from Massdrop, the thx 789 has no such protection. Any other suggestions?

I suspect they are mistaken as I pushed the unit into overload and it shut down. There also appears to be a protection circuit near the headphone jack: https://www.audiosciencereview.com/...n-of-massdrop-thx-aaa-789-headphone-amp.5327/

We need to ask @AndrewMason to know for sure.

 

[RenEH]

Thanks, for the record this was their response.

“Protection from that kind of short would blow a fuse, and would still render the unit unusable until it was repaired. PEBCAK errors are notoriously hard to engineer around or plan for.”

 

[amirm]

Thanks, for the record this was their response.

“Protection from that kind of short would blow a fuse, and would still render the unit unusable until it was repaired. PEBCAK errors are notoriously hard to engineer around or plan for.”

Well, that is proof that their response is wrong. Fuses are rarely used in that application. Instead, an overcurrent protection circuit detects the situation and will shut down the power to the amplifier, and/or disconnects the load.

 

[solderdude]

Indeed, the response is wrong.
The used output op-amps are short circuit protected by themselves (and thermally) and there is a circuit, for each channel, that measures DC on the output it seems.

 

[Panelhead]

That is good to know. The chip amplifiers are usually protected by the chips internal circuitry. Discrete have to rely on an external over current/dc output sensing to protect the amp and load.
I much prefer to have both. Had a bad solder joint open up the feedback loop in an anplifier once while playing. Dumped about 20 volts dc on the output before an output relay could open. The speaker was a 100 dB horn loaded single driver. Sounded like a bomb exploded in the house.
Pisitive thing was nothing was damaged. Found the connection and touched up. Everything worked fine afterwards. Except my ears, rang for a few hours.

 

[bboris77]

Does anyone know if Schiit's higher end amps like the Lyr 3 have this form of short-circuit protection? I emailed their support and will post back. I recently had a situation where my HD600's 1/8 plug was not plugged all the way into the 1/4 adapter. I only had sound in one channel and when I tried increasing the volume, the amp relay clicked and went into the reboot mode which lasts about 40 seconds. I plugged the 1/8 plug all the way into the adapter, and after a reboot everything was fine.

 

[bboris77]

A quick update - I just heard from their tech support and they confirmed that the Lyr 3 has "internal protection" and should not be damaged in case you accidentally short the input plug. In fact, what happened in my case where the relay shut down the output, is considered normal operation of this protection mechanism.

 

[restorer-john]

...I could smell burning electronics...

The minimum you should be doing is checking the emitter resistors (4) to confirm they are still within tolerance. They may have drifted somewhat high after their smoking episode.

 

[AresHarvest]

Indeed, the response is wrong.
The used output op-amps are short circuit protected by themselves (and thermally) and there is a circuit, for each channel, that measures DC on the output it seems.

You can see this in action on the 789 when plugging or unplugging SE headphones. Protection temporarily engages - white LED momentarily turns red to indicate this.

I haven't seen it happen with the XLR output, but then again it wouldn't happen just by plugging or unplugging, unless there was a short somewhere in the cable.

 

[tired_guru]

An email with schiit later confirmed that there is no short protection in the magni 3 and that it could damage the amp, but they didn't comment if it could damage the headphones

Died after reading this. This is ridiculous.

Easy answer: don't buy Schiit.