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Fast DC protection for HPamps.
Fast DC protection for HPamps.
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Old 20th September 2019, 12:23 PM   #11
RickTH is offline RickTH
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Quote:
Originally Posted by Fast Eddie D View Post
Now I see a slight advantage to the circuit. But a circuit referenced to ground will still trigger, will it not?
Only a slight advantage ?

It doesn't matter what the circuit is referenced to . Use a levelshifter or an opto-coupler and anything goes. I'm not a fan of the old 555 or it's grandkids 7555/TLC555. For timers I prefer 4538 or HC4538. My whole protection circuit is in HCMOS , over and under voltage detection and now , trying to get the DC protection right.
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Old 20th September 2019, 01:39 PM   #12
xrk971 is offline xrk971  United States
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Fast DC protection for HPamps.
Would 5ms full rail blip kill a headphone? That’s like a 200Hz whack of a rimshot sound.
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Old 20th September 2019, 02:11 PM   #13
RickTH is offline RickTH
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Originally Posted by xrk971 View Post
Would 5ms full rail blip kill a headphone? That’s like a 200Hz whack of a rimshot sound.
With +/-15 V supply and opamp output going to 13V max , a 24ohm HP would get around 540mA = 7watts , definitely dead. A 300 ohm gets 560mW , may survive it for those 5ms . I'm not going to try it.
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Old 20th September 2019, 03:11 PM   #14
xrk971 is offline xrk971  United States
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Fast DC protection for HPamps.
I don’t think it’s that simple. You have to look at maximum suspension movement and how many joules of energy the voice coil (VC) can take, plus how many mA can the power supply really deliver. Very few headphone IC amps can are capable of delivering more than 150mA and what you are saying is an internal failure in shorted to rail mode. 5ms of 13v at some typically limited current is probably not going to destroy a large headphone with typical 96dB sensitivity. 114dB per mW IEMs on the other hand will die if you even look at them wrong.

7w over 5ms is 35mJ of energy. Voicoil of a big over the ear can with 45mm dia VC can probably take 35mJ dissipation. Can the suspension handle it? Maybe.

Nevertheless, I agree that if your circuit is comparing out vs in modified by gain, and if that’s out of bounds then it triggers. This doesn’t prevent damage from perfect fidelity of out vs in but exceeding suspension Xmax or thermal energy max of the headphone driver VC.

Also, planar magnetics like Hifiman are very resistant to over voltage and can dissipate quite a bit of peak burst power.
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Last edited by xrk971; 20th September 2019 at 03:14 PM.
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Old 20th September 2019, 07:46 PM   #15
Elvee is offline Elvee  Belgium
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I made a quick estimation on a real-world example, a 6mm dia. VC:

Fast DC protection for HPamps.-vc-jpg

The gross volume is about 2mm³; let's say for convenience that the actual copper volume is 1.1mm³.
Since its density is ~9g/cm³, the total mass is ~10mg.
With a heat capacity of 380J/°K*kg, this means that in order not to exceed a temp rise of 150°C for a single pulse in adiabatic conditions, the energy must be < 0.01*150*0.38 = 0.57J, or 570mJ.

The estimation is conservative, and a number of heat sinks, like glue, varnish or mylar have not been taken into account.

With a 7W input, about 80ms is required to reach 570mJ.
This puts things in perpective (and with 80ms, the conditions will not be adiabatic anymore)

Mechanical stress could be more problematic, depending on the construction
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Old 21st September 2019, 02:31 AM   #16
RickTH is offline RickTH
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^ So we're pretty safe with the 5 ms it takes the relay .

You also need to consider big caps. While the power supply may not deliver a big current , the caps can . Have you seen the spark when you short (or discharge with a few ohms) a 1000 uF cap ?
The tiny voice coils of those in-ear or even the slightly bigger ones like on LV's pic will not survive . To my shame I have to admit I made costly mistakes with HP's .

Of course if you're confident your HP can take a hit , the slow circuit like Jhofland will do just fine.
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Old 21st September 2019, 05:19 AM   #17
EUVL is offline EUVL  Europe
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I presume you want to have protection for your headphone against a high power amp ?
Or you have IC based headphone amps driving IEM’s ?

A modern over-ear headphone will take 200mW.
So even at 32R, that is a continuous DC of 2.5V and the coil still survives.
For 300R, this goes up to 7.8V pure DC.
Any of the RC-based protection circuits set at say 100mV will trigger in milliseconds at that DC level.
Not to speak of full rail voltage of 15V or above.

Then you also want to protect against clipping.
Assume you have 9V rails and you are just below clipping.
At 32R, this is ~1.3W rms. Or 0.6W at 65R.
Even if your ears survive, your headphone coil will not, although there is no error between input and the attenuated output.

OK, what if you use IEM’s ?
I guess you are not going to use a 200mA Class A output stage for that.
Isn't it a lot easier to put a current limit on your power supply, to say 40mA, than to rely on 5 opamps to protect against 2 in your main amp circuit ?
I am sure you can find lots of those examples with 2 transistors per rail.
For me a lot more reliable than 5 opamps.

Or am I missing something ?


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Patrick
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Old 21st September 2019, 05:30 AM   #18
EUVL is offline EUVL  Europe
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Actually the likes of LT3045 and LT3094 have built-in programmable current limits (with 1 resistor).
And they do not need large output caps.
10µ will do.


Cheers,
Patrick
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Old 21st September 2019, 05:57 AM   #19
RickTH is offline RickTH
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Quote:
Originally Posted by EUVL View Post
I presume you want to have protection for your headphone against a high power amp ?
Or you have IC based headphone amps driving IEM’s ?
No , for protection against malfunction of the amp.
The HP amp is for 24 ohm HP's as well as 300 ohm ones and works on 2 x 15V.
The 24 ohm's will be much more at risk , because of their very small voice coil. But the 300 ohm's one are the most expensive and I don't want to risk them getting damaged.

Quote:
Originally Posted by EUVL View Post
Any of the RC-based protection circuits set at say 100mV will trigger in milliseconds at that DC level. Not to speak of full rail voltage of 15V or above.
Looking at the RC based (or servo based) circuits , I doubt it wil trigger in ms, more like 100's of ms.
See the pic below.

Quote:
Originally Posted by EUVL View Post
Then you also want to protect against clipping.
No I don't , but it will be inherent to the circuit when it compares in and output.

Quote:
Originally Posted by EUVL View Post
Isn't it a lot easier to put a current limit on your power supply, to say 40mA, than to rely on 5 opamps to protect against 2 in your main amp circuit ?
I am sure you can find lots of those examples with 2 transistors per rail.
For me a lot more reliable than 5 opamps.
A current limiter would surely f-up the low noise regulation , and regulation itself, and it wouldn't help against caps getting discharged through shorted opamps outputs.


A lot of why's I would want such protection and if it is useful ...I think it is.
I hoped this thread would be more about the instrumentation amp , how to do it right ,reliability, ... because I have no experience with it , not about the why , or what comes after the detection of an error .

Tomchr (neurochrome) was making another HP amp , with more output protection, I think it was with the OPA1656 or 1688 . I wonder how he would tackle the problem of watertight protection . But I haven't heard of him lately on the progress on that ... and then again , he does this for a living so he won't be forthcoming to show us how he does it.
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Old 21st September 2019, 06:14 AM   #20
RickTH is offline RickTH
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Quote:
Originally Posted by EUVL View Post
Actually the likes of LT3045 and LT3094 have built-in programmable current limits (with 1 resistor).
And they do not need large output caps.
10µ will do.


Cheers,
Patrick
Yeah , I know them. Both are in a very tiny smd , which is a very big no-no for regulators.
LM317/337 limits current too , but at much higher currents.

But your idea could be a simple solution for limiting the current , a R between base and emitter , when it comes at a certain value , it limits or cut power to the relays.
Combined with the slow RC DC detector , can be a valid way to go , if my idea doesn't pan out.
Still the current limiting is different for 24 or 300 ohm HP's. And you can't have bigger caps behind that limiter.
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