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:
The boards are available in the Omicron GB that is open until June 2.
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:
The boards are available in the Omicron GB that is open until June 2.
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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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hello.
what is the function
1. "GND Connect" pin? why not directly connect to ground? why there is "option"
2. R6 and R7?
3. if am using 12v coil relay can i adjust R16? or something else to be adjusted? and at what value for 12v coil relay?
thank you so much
1. The option to connect the power ground to the signal ground adds some flexibility when integrating the protection into an amplifier. Normally, there should be no connection, as the power ground should be connected to the power supply's ground, while the signal ground - to the amplifier's output ground.
2. R6 and R7 sink the comparators' input bias currents. These resistors are not required if the protection circuit is permanently connected to an amplifier's output (as in the Omicron). However, should the amplifier's output be unconnected and R6/R7 not installed, the input bias currents would slowly charge C3 C4 C5 C6, triggering protection.
3. R16 drops the difference between the rail-to-rail power supply voltage and the relay's coil voltage rating. The value, obviously, depends on the impedance of your relay's coil.
For example, for Fujitsu RY-12W-K the coil resistance is 960 ohm, which corresponds to 12V/960ohm=12.5mA. If your rails are +/-15V, you'd need to drop (15+15)-12=18V, and R16 would be 18V/12.5mA=1.44kOhm. From the E24 list of standard values, 1.5kOhm would work well. Note that R16 would dissipate 18V×12.5mA=225mW, so you'd need at least a 0.5W resistor, and 1W would be even better. For a lower impedance coil, the current and the power dissipation would be proportionally higher, and the required resistance would be lower.
2. R6 and R7 sink the comparators' input bias currents. These resistors are not required if the protection circuit is permanently connected to an amplifier's output (as in the Omicron). However, should the amplifier's output be unconnected and R6/R7 not installed, the input bias currents would slowly charge C3 C4 C5 C6, triggering protection.
3. R16 drops the difference between the rail-to-rail power supply voltage and the relay's coil voltage rating. The value, obviously, depends on the impedance of your relay's coil.
For example, for Fujitsu RY-12W-K the coil resistance is 960 ohm, which corresponds to 12V/960ohm=12.5mA. If your rails are +/-15V, you'd need to drop (15+15)-12=18V, and R16 would be 18V/12.5mA=1.44kOhm. From the E24 list of standard values, 1.5kOhm would work well. Note that R16 would dissipate 18V×12.5mA=225mW, so you'd need at least a 0.5W resistor, and 1W would be even better. For a lower impedance coil, the current and the power dissipation would be proportionally higher, and the required resistance would be lower.
The values shown work for 18VAC secondaries. You can make it work with pretty much any voltage, provided that the gate of the MOSFET never goes above or below its source (=negative supply rail) by more than the MOSFET gate-to-source voltage rating (+/-20V typical). To achieve that, choose R1 R13 to make the MOSFET safe, then choose Rbleed so that MOSFET stays off when AC is present but opens quickly when AC disappears.
Not for sale yet, but will be soon.
Hi Alex, what a wonderful and creative circuit. Being already a fan of your omicron i think i found my next project.
My omicron iteration is dual mono, so this circuit will have to be split into single channels.
For that, which comparator should i use?
And is there any way to simplify or eliminate T1's separate psu requirement? My main concern for it is the layout challenge
Lastly how about flipping the relay connection so that when its off it connects the output to the ground and at ON its a lift, effectively removing it from the signal path? Its how myref amp's relay circuit is done and i like it.
Thanks
My omicron iteration is dual mono, so this circuit will have to be split into single channels.
For that, which comparator should i use?
And is there any way to simplify or eliminate T1's separate psu requirement? My main concern for it is the layout challenge
Lastly how about flipping the relay connection so that when its off it connects the output to the ground and at ON its a lift, effectively removing it from the signal path? Its how myref amp's relay circuit is done and i like it.
Thanks
For a single channel, you'd need a dual comparator with open collector/drain outputs and the maximum supply voltage above what you're going to supply it with. Something like the LM393 should work, and there are many other suitable parts.
T1's purpose is to quickly disconnect the load when AC power is lost. With the Omicron, you can simply omit T1 - Omicron shuts down quietly.
Unlike the LM3886, the Omicron's output stage in unprotected from overcurrent other than by the emitter resistors. It survives brief shorts such as when you plug in your headphones, but I would advise against shorting Omicron's output to the ground with a relay. BTW the specified relays do not add any distortion - all distortion measurement and listening tests for the Omicron were performed with the relay contacts in the circuit.
Not Mauro's MyRef:
Here is the part list, a.k.a. BOM, for the board:
The list was designed to use inexpensive, commonly available parts. Couple of notes:
| Qty | Value | Device | Package | Names | Example P/N |
|---|---|---|---|---|---|
| 1 | LM339 | Quad comparator | DIP-14 | LM339 | LM339, LM339E, LM239, LM2901, LM2901E, LM2901V, NCV2901, MC3302 - if substituting other parts, make sure the pinout is the same |
| 2 | N-CH | N-channel MOSFET | TO-92 (DGS) | T1, T3 | BS170, 2N7000, 2N7002, ZVN3310A, etc. - watch for different pinouts |
| 1 | PNP | PNP BJT | TO-92 (EBC) | T2 | Any small signal PNP with a suitable pinout, e.g. 2N3906 |
| 1 | GREEN | LED | Through-hole | LED | Standard through-hole LED |
| 1 | 1N4148 | Diode | DO-35 | D1 | 1N4148, etc. |
| 1 | G6A | Small signal relay | 20x10mm through-hole (see below) | K1 | RY-24, G5V-24, etc. |
| 2 | 240 ohm | Resistor | Axial D=2.4mm L=7mm | R3, R5 | Standard metal film or carbon film 1/4W resistors, e.g. YAGEO MFR-25 series |
| 2 | 1 kOhm | Resistor | Axial D=2.4mm L=7mm | R1, R16 | R16 may need to be adjusted depending or the relay coil rating and supply rail voltages |
| 10 | 51 kOhm | Resistor | Axial D=2.4mm L=7mm | R2, R4, R6-R11, R13, R14 | |
| 1 | 150 kOhm | Resistor | Axial D=2.4mm L=7mm | R15 | |
| 1 | 1 Mohm | Resistor | Axial D=2.4mm L=7mm | R12 | |
| 7 | 1uF | Ceramic capacitor | Radial LS=5mm | C1-C7 | FG28X7R1E105K, etc. |
| 1 | 2-way header | LS=2.54mm | GND_CONNECT | 22-03-2021, etc. | |
| 2 | 3-way header | LS=2.54mm | AMPLIFIER, HEADPHONES | 22-03-2032, etc. | |
| 1 | 5-way header | LS=2.54mm | POWER | 22-03-2052, etc. |
The list was designed to use inexpensive, commonly available parts. Couple of notes:
- The most suitable parts are those you already have and don't need to buy. The list above allows broad substitution.
- The LM339 is usually the cheapest quad comparator avauilable. If you want to use something else, make sure it can work with your supply rails (some parts are low voltage) and has the proper pinout:
- Most small signal N-channel MOSFETs and a PNP BJT in TO-92 will work. Make sure they can handle the power supply rail voltage and the relays's coil current.
- The LED is generic. It lights up dimly when power is on, and fully when the relay is engaged (that is, the load is connected to you amp). If you don't have or don't want the LED, just short its pads on the PCB.
- There are many suitable relays. The PCB is designed for the following footprint:
- If using different power supply rail voltages: You can make this protection work with rails from about +/-5V to +/-18V.
- The upper limit on power rail voltages is set by the comparator at +/-18V. Also, the negative rail should not be exceed the gate-to-source voltage rating of T3, usually 20V, so that's not an additional restriction.
- The lower limit on power rail voltages is set by the comparator; the gate threshold voltage of T3, up to 3V; and of course by the relay coil.
- Resistors R2-R5 set the DC thresholds where the protection trips. We chose +/-80mV as a good balance between fast turn-offs and no triggering at large low-frequency signals. The thresholds depend on rail voltages, so if you using lower rails, you may want to tweak the values a bit.
- R16 sets the relay current and may need to be changed depending on the relay coil rating and power supply voltage, as discussed in post #8 above. Note that if you choose a low-voltage relay with high supply rails, R16 will need to dissipate some power.
- When using the fast-turn-off feature (see post #2 above), do not exceed the gate-to-source voltage rating of T1, usually 20V. That is, the voltage from the small AC sense rectifier should not be more that 20V below your negative supply rail. Normally, this does not require any special effort.
- Connectors are optional. The two-way header should be shorted if the DC protection and the amplifier use separate power supplies with unconnected grounds.
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