Omicron, our compact ultra-low-distortion headphone amplifier, includes a simple, compact, accurate, flexible, effective, reliable and affordable headphone protection against DC voltages and turn on/off transients. Since there has been interest in using Omicron's protection with other headamps, we implemented the Omicron protection as a separate board:
The board is 48×48mm (1⅞ inch), mounting holes are in the corners of a 40×40mm square, all parts are easily available and through hole, all connections are routed in a single copper layer.
Details will follow.
The board is 48×48mm (1⅞ inch), mounting holes are in the corners of a 40×40mm square, all parts are easily available and through hole, all connections are routed in a single copper layer.
Details will follow.
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The schematic is virtually identical to the one published last year, with a couple of resistors and a ground connect jumper added:
The signal from each channel is low-pass filtered with R9 C5 R11 C3 and R8 C6 R10 C4 and fed to a window comparator. If the DC component of the input signal falls outside of the window set by R2 R3 R4 R5 (+/-80-mV with the values shown and +/-17V power supply), the comparator switches off the Schmitt trigger T2T3, which turns off the relay and disconnects the load. The protection is fast - a 1V step at the input disconnects the load in 30 milliseconds.
When DC disappears, C7 slowly charges via R12, so the relay turns on with a delay of about 1 second with the values shown. The same delay protects the load from turn-on transients.
T1 optionally provides fast turn-off, in cooperation with the power supply. The gate of T1 is connected to a rectifier with a small filter capacitor along these lines (Omicron power supply includes a suitable rectifier):
Normally, the gate is at or below the negative rail (-17V) but is pulled to ground by Rbleed when AC power disappears, which discharges C7 and turnes off the relay.
The part list is simple, with just 12 lines and 30 components in total (excluding the board and connectors). Most likely, you have most if not all parts already. The total cost at today's prices on Mouser is less than $8, with the relay being the most expensive component at $3.
The signal from each channel is low-pass filtered with R9 C5 R11 C3 and R8 C6 R10 C4 and fed to a window comparator. If the DC component of the input signal falls outside of the window set by R2 R3 R4 R5 (+/-80-mV with the values shown and +/-17V power supply), the comparator switches off the Schmitt trigger T2T3, which turns off the relay and disconnects the load. The protection is fast - a 1V step at the input disconnects the load in 30 milliseconds.
When DC disappears, C7 slowly charges via R12, so the relay turns on with a delay of about 1 second with the values shown. The same delay protects the load from turn-on transients.
T1 optionally provides fast turn-off, in cooperation with the power supply. The gate of T1 is connected to a rectifier with a small filter capacitor along these lines (Omicron power supply includes a suitable rectifier):
Normally, the gate is at or below the negative rail (-17V) but is pulled to ground by Rbleed when AC power disappears, which discharges C7 and turnes off the relay.
The part list is simple, with just 12 lines and 30 components in total (excluding the board and connectors). Most likely, you have most if not all parts already. The total cost at today's prices on Mouser is less than $8, with the relay being the most expensive component at $3.
For BTL aka "balanced" amplifiers, you'd need a different DC detector, sensitive to differential voltage. There is an excellent example elsewhere on this forum. Out of curiosity, what balanced headamp did you build?
If you use a DC blocking capacitor, you need no DC protection, although may still need protection against turn-on thump. However, a protection circuit is totally transparent until a fault happens, while a big coupling capacitor is not. Such capacitor would be out of place in any amplifier with split supply rails.
If you use a DC blocking capacitor, you need no DC protection, although may still need protection against turn-on thump. However, a protection circuit is totally transparent until a fault happens, while a big coupling capacitor is not. Such capacitor would be out of place in any amplifier with split supply rails.
I thought well designed headphone amplifier does not need protection. Headphones are not demanding any big power, few milliwatts is all it takes, and even if the design is for a watt, thats negligable power, not much current, most reasonable headphones are 50 ohms and higher, nothing like classA amps. All it takes is one cap on the output. I have many great sounding cap coupled headphone amps, why such aversion against caps? There are literally thousands in recording studio.
With a split rails headamp, one can take any of the three paths: (1) keep fingers crossed and hope no fault can ever happen (off topic: my oven stopped working after a thunderstorm today); (2) add a big cap at the output just in case something does happen; (3) add a relay and a few other parts to protect your cans in case of a fault. I am not averse to any of these choices; it's your cans on the line, not mine. Should you choose (3), the schematic and board above may be helpful.
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