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Fast DC protection for HPamps.
Fast DC protection for HPamps.
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Old 19th September 2019, 10:27 AM   #1
RickTH is offline RickTH
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Default Fast DC protection for HPamps.

So here's an idea I've been thinking about , instead of the slow servo-like DC
protection circuits I seen here on the forum and elsewhere , a faster one that not
just detects DC on the output but other calamities too , like distortions when you
drive the output to the rails. This works not by comparing the average voltage of a signal (music) , but the difference between in and output. (adjusted for the amplification of course)

An instrumentation amp amplifies the differential voltage between input and output
by 20-50x and outputs this to a window comparator . Low offsets , like mV won't
make the threshold . Large ones will after a small RC filter ( < 100 us).
This could reduce the reaction time to less than a millisecond (with fast opamps/Inamps) + 5 ms for the output relay to switch off. This is an order or 2 better than the servo + filtering types and just might save those ridiculously expensive HP's.
My background in electronics is not that much and I haven't used an
instrumentation amp before , integrated or with 3 discrete opamps.
I'm sure I'm not re-inventing the wheel here , but I haven't seen this before.
What do you think , before I start soldering a test circuit. Is there something I didn't
think about , ... pitfalls ?

The 2 top opamps are a basic HP amp with a gain of 3. Ignore the part after the diodes , it's just a level shifter to go to a S/R flipflop and the relays switch.
I want the relay switched off after detection , not coming back after de DC has gone.
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Old 19th September 2019, 01:59 PM   #2
Ken Newton is offline Ken Newton  United States
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Your circuit is not applicable to D.C. coupled amplifier protection, where input D.C. may be present. There must be an D.C. blocking input cap., yes?
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Old 19th September 2019, 03:14 PM   #3
RickTH is offline RickTH
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Quote:
Originally Posted by Ken Newton View Post
Your circuit is not applicable to D.C. coupled amplifier protection, where input D.C. may be present. There must be an D.C. blocking input cap., yes?
I guess you didn't notice the capacitor at the input that will keep all DC out from the source. All that is left is offset from the opamps.

I'm more concerned about phase shift between in and output that might be an issue when amplifying the difference between them.
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Old 19th September 2019, 06:06 PM   #4
Ken Newton is offline Ken Newton  United States
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Quote:
Originally Posted by RickTH View Post
I guess you didn't notice the capacitor at the input that will keep all DC out from the source. All that is left is offset from the opamps.
Sorry, that sentence of mine was not worded as clearly as it could have been. Actually, I had noticed the input cap. My observation was just that your circuit protects against internal D.C. faults. It is not applicable to protecting D.C. coupled amplifiers from D.C. input offsets.
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Old 19th September 2019, 06:28 PM   #5
synonymous is offline synonymous  United States
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Input and output comparison would be the end-all for protection.
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Old 19th September 2019, 06:49 PM   #6
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by RickTH View Post
So here's an idea I've been thinking about , instead of the slow servo-like DC
protection circuits I seen here on the forum and elsewhere , a faster one that not
just detects DC on the output but other calamities too , like distortions when you
drive the output to the rails. This works not by comparing the average voltage of a signal (music) , but the difference between in and output. (adjusted for the amplification of course)

An instrumentation amp amplifies the differential voltage between input and output
by 20-50x and outputs this to a window comparator . Low offsets , like mV won't
make the threshold . Large ones will after a small RC filter ( < 100 us).
This could reduce the reaction time to less than a millisecond (with fast opamps/Inamps) + 5 ms for the output relay to switch off. This is an order or 2 better than the servo + filtering types and just might save those ridiculously expensive HP's.
My background in electronics is not that much and I haven't used an
instrumentation amp before , integrated or with 3 discrete opamps.
I'm sure I'm not re-inventing the wheel here , but I haven't seen this before.
What do you think , before I start soldering a test circuit. Is there something I didn't
think about , ... pitfalls ?

The 2 top opamps are a basic HP amp with a gain of 3. Ignore the part after the diodes , it's just a level shifter to go to a S/R flipflop and the relays switch.
I want the relay switched off after detection , not coming back after de DC has gone.
Clever. I wonder if you could implement this idea with a 555 timer chip.

It's not a new idea, but I haven't seen it implemented. It could maybe be reconfigured to control a compressor on a power amplifier to prevent clipping.

I've designed and built DC protection circuits that used a comparator based circuit and referenced ground. They were "fast enough" and worked without glitches. Speed can be optimized by choosing the right resistors; the more current driving the base, the slower the transistor will come out of saturation when that current is removed.

It will also shut down on clipping or gross distortion, both of which shouldn't be happening on a headphone amp.

Last edited by Fast Eddie D; 19th September 2019 at 06:57 PM.
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Old 20th September 2019, 12:34 AM   #7
RickTH is offline RickTH
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^
It triggers a flipflop , so triggering a 555 timer isn't a problem.

The meaning of the circuit is to protect the HP's not just against (too big a) DC offsets on the output but also in case of opamps that break down and give a short to either supply rail . It shouldn't come back on after a certain time. Servo based or the RC protection is far too slow for that . Since the goal is to switch off the output relay , the saturation of the transistor is irrelevant.
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Old 20th September 2019, 02:21 AM   #8
xrk971 is offline xrk971  United States
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Fast DC protection for HPamps.
I am using this system here designed Jhofland:
Click the image to open in full size.

It catches DC offsets and triggers an opto-coupler, which then drives an SSR. Works very nicely. Also gives a startup delay to prevent turn on thump. The SSR has no shattering added.

More info here:
Cheap as Chips OPA1688 Low-THD Muscle Amp

Click the image to open in full size.
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Old 20th September 2019, 03:09 AM   #9
RickTH is offline RickTH
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^ Yes xrk971 , I already saw that circuit researching solutions here on the forum , but this is the RC one that is too slow. It's ok for small DC's , switching off the relay or supply for the time a DC voltage is detected , but it offers little protection for real faults like opamps shorting to the supply rails.
I know there is little chance of that happening , but still , connecting a 500 $/€ HP (or even a 100-200 $/€) , you want to be absolutely sure . Maybe the 5 ms it takes for the relay to switch off, may be already to long . Why would an opamp break down ? Bad handling , ESD , bad/too long soldering, ...or even overdissipating like putting it in a 3 x 3 mm package (OPA 1622). Resistors aren't foolproof . If one breaks in the feedback or in the DC correction you use , it could force the output to extremes .

I shouldn't have added the transistor , because it's irrelevant what comes behind the window comparator. Some want a timer , others just want an off switch during DC detection , transistor or opto-coupler , it doesn't matter. For me it's a S/R flipflop. It's the comparing in and output by an instrumentation amp that is the essence here.

Last edited by RickTH; 20th September 2019 at 03:31 AM.
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Old 20th September 2019, 04:55 AM   #10
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by RickTH View Post
^
It triggers a flipflop , so triggering a 555 timer isn't a problem.

The meaning of the circuit is to protect the HP's not just against (too big a) DC offsets on the output but also in case of opamps that break down and give a short to either supply rail . It shouldn't come back on after a certain time.
Now I see a slight advantage to the circuit. But a circuit referenced to ground will still trigger, will it not?

Quote:
Since the goal is to switch off the output relay , the saturation of the transistor is irrelevant.
Noted.

The reason I mentioned this is that I'm working on protection circuits that use the 555 and a handful of transistors. Some circuits cam be optimized with this knowledge.

Last edited by Fast Eddie D; 20th September 2019 at 04:57 AM.
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