Hello everyone,
I'm exploring some headphone amplifier designs, all OTL. It seems simple enough to attenuate any DC voltage from the output voltage terminal by putting in an appropriately sized coupling capacitor. However, to preserve low frequency gain, the capacitor values I'm looking at imply an eletrolytic.
That electrolytic capacitor could eventually be all that's isolation a listener from dangerous DC voltages. Given that headphones go on one's head, I want to fully understand the safety implications here.
My understanding is that when electrolytic caps fail they tend to fail open circuit-- which OK from a safety point of view.
Can anyone comment on how safe a headphone amp with an electrolytic output capacitor is?
An example is the 30 uF cap in each of the schematics shown in the link below.
Aikido SLPS & Hybrid Headphone Amplifier
I'm exploring some headphone amplifier designs, all OTL. It seems simple enough to attenuate any DC voltage from the output voltage terminal by putting in an appropriately sized coupling capacitor. However, to preserve low frequency gain, the capacitor values I'm looking at imply an eletrolytic.
That electrolytic capacitor could eventually be all that's isolation a listener from dangerous DC voltages. Given that headphones go on one's head, I want to fully understand the safety implications here.
My understanding is that when electrolytic caps fail they tend to fail open circuit-- which OK from a safety point of view.
Can anyone comment on how safe a headphone amp with an electrolytic output capacitor is?
An example is the 30 uF cap in each of the schematics shown in the link below.
Aikido SLPS & Hybrid Headphone Amplifier
A well made electrolytic capacitor won't fail any time soon. If you are really worried about it, just use a film cap. Anything 100uf and below can easily be found as a film capacitor.
You could also try using a bipolar PSU and use a DC servo at the output and just get rid of the output cap all together.
You could also try using a bipolar PSU and use a DC servo at the output and just get rid of the output cap all together.
I would suggest using TVS diodes for this purpose, many are rated at kW of peak pulse power (they are basically bidirectional zeners optimized for protection) and should force some other component to fail - ie use a suitable fuse in the power supply. Chose TVS voltage large enough for your max signal voltage at the 'phones.
That is exactly the problem.
Cap coupling from a plate or cathode that is at 1/2 of B+ voltage will output that voltage until the cap charges up.
And, when the amp is turned off, when the B+ drops, then the cap discharges through the earphones (at 1/2 of B+, only it is a minus voltage (-) this time).
A current limiting resistor in series with the cap, and pair of series Zeners connected in opposite polarity, with one end at the cap output, and the other end connected to ground will limit the transient amp On and transient amp Off voltages. The voltage is limited to the Zener voltage value.
If you are worried about the Zener leakage current when the voltage is at say, 1/2 of the Zener voltage rating, you can do the following:
Use 'steering' diodes:
A diode in series with a Zener (Anode to Anode).
A diode in series with a Zener (Cathode to Cathode).
Connect those pairs across the output connector 'hot' to 'common'.
Again, be sure to use a current limiting diode in series with the output cap.
The thing to remember is that a capacitor is a dead short at the beginning time when it is being charged (and also at the beginning time when it is being discharged).
Until the transient is over (charged fully, or discharged fully, the current is only limited by the series resistance of the rest of the circuit).
Part of the problem is what series resistance do you use.
That depends on the current rating of the Zeners, and also on the DC resistance of the earphones (8, 32, 600 Ohms, etc.).
Protect the Zeners, but use a low enough resistance so that the earphones get enough signal.
There will be no 'Universal' series resistance for all earphones DC resistance.
Cap coupling from a plate or cathode that is at 1/2 of B+ voltage will output that voltage until the cap charges up.
And, when the amp is turned off, when the B+ drops, then the cap discharges through the earphones (at 1/2 of B+, only it is a minus voltage (-) this time).
A current limiting resistor in series with the cap, and pair of series Zeners connected in opposite polarity, with one end at the cap output, and the other end connected to ground will limit the transient amp On and transient amp Off voltages. The voltage is limited to the Zener voltage value.
If you are worried about the Zener leakage current when the voltage is at say, 1/2 of the Zener voltage rating, you can do the following:
Use 'steering' diodes:
A diode in series with a Zener (Anode to Anode).
A diode in series with a Zener (Cathode to Cathode).
Connect those pairs across the output connector 'hot' to 'common'.
Again, be sure to use a current limiting diode in series with the output cap.
The thing to remember is that a capacitor is a dead short at the beginning time when it is being charged (and also at the beginning time when it is being discharged).
Until the transient is over (charged fully, or discharged fully, the current is only limited by the series resistance of the rest of the circuit).
Part of the problem is what series resistance do you use.
That depends on the current rating of the Zeners, and also on the DC resistance of the earphones (8, 32, 600 Ohms, etc.).
Protect the Zeners, but use a low enough resistance so that the earphones get enough signal.
There will be no 'Universal' series resistance for all earphones DC resistance.
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The thing to really do here would be to use a high value electrolytic of say 470uF or so, in conjuction with a muting relay across the output to shunt any nasty HV on start up straight to ground.
Having said that and looking at the headphone output in question, it seems that having a triode between the HV and the output, as opposed to say a resistor that would start passing current as soon as the HV appeared, will be sufficient to prevent HV spikes appearing on the output after start up. Cathode followers always power up very nicely due to the inherent delay in conduction as the filaments warm up.
Having said that and looking at the headphone output in question, it seems that having a triode between the HV and the output, as opposed to say a resistor that would start passing current as soon as the HV appeared, will be sufficient to prevent HV spikes appearing on the output after start up. Cathode followers always power up very nicely due to the inherent delay in conduction as the filaments warm up.
Baudouin's schematic is just fine. Could I also recommend a ½ amp fast-blow fuse in line, too? That way, if the 12 V Zener stack is conducting too aggressively on a blown capacitor, well … the fuse will also pop. Probably saving the Zeners, and debugging them, and all that.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
Yes, it's this. The question here is not how to completely eliminate any possible DC offset from ever coming out of the headphone jack, it's whether this is an actual issue.
In practice that 30uF cap probably isn't big enough, and at 100uF you'll need to build the circuit and fine tune the value of that 10K resistor at the output to pull down the voltage spike sufficiently.
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