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Old 18th December 2011, 06:28 PM   #1
mmerig is offline mmerig  United States
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Default Step-down from mains to line level

I would like to drop power from a stereo set of wires from a power amp (120 watts per channel into 8 ohms) to line level so a line-level headphone can be connected. The headphone impedance would be about 8 to 32 ohms.

A voltage divider would require a high power resister, wouldn't it? If so I would rather avoid this because of potential noise, heat, and cost.

Would a decent audio-quality step-down transformer do a better job (one for each channel)? If so, what would be the specifications?
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Old 18th December 2011, 06:40 PM   #2
Mooly is offline Mooly  United Kingdom
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It not such a problem. An amp like that would need around 330 ohm 2 watt series resistors to the headphone output socket. There's no noise, heat is virtually non existant on normal listening, and cost is ultra cheap.

It's the standard industry accepted way of doing it. The impedance isn't such an issue either.

Try it and see
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Old 18th December 2011, 08:01 PM   #3
Lavcat is offline Lavcat  United States
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What I do is connect my 'phones to the amplifier outputs directly without using any pads. Of course one has to be careful with the level.
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Old 18th December 2011, 09:28 PM   #4
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ESP easy headphone adapter (project 100):

Headphone Adaptor for Power Amplifiers
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Old 19th December 2011, 02:11 PM   #5
mmerig is offline mmerig  United States
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Thanks for all of your helpful suggestions.
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Old 19th December 2011, 02:26 PM   #6
AndrewT is offline AndrewT  Scotland
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I prefer a ladder attenuator.

Let's suppose the headphones will work well with Rs= 10r.
Now create the ladder attenuator.
The 10r is the lower resistor. It is in parallel to the headphone input impedance.
Let's assume the Headphone must never see a voltage peak exceeding 5Vpk.
Let's further assume that the amplifier is capable of putting out 30Vpk.
The upper resistor of the ladder needs to be 30/5 -1 * 10 = 50r.
Let's check the resistor current to ensure we are not overloading the resistors.
assume the headphones are 10ohm (just to make the sums easier), the parallel lower load is 10//10 = 5r.
The upper resistor is 51r (the nearest to 50r). the total resistance is 56r.
The maximum output voltage is 30Vpk.
Ipk = 30/56 = 833mApk.
The power dissipated in the 51r is 0.833^2 * 51r / 2 (for sinewave signals) ~ 18W.
That's a mighty big resistor to obtain that maximum of 5Vpk at the headphones.
BTW,
5Vpk at the headphones for more than a few seconds will damage your hearing.

If the average listening level is -20dB ref. maximum, then that resistor has to dissipate an average of ~180mW of power.

See the problem.
Normal listeners listening to normal music at normal levels will never overheat the 600mW resistor.
Design can't easily account for abnormal listeners.
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Old 19th December 2011, 02:33 PM   #7
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If you read the threads carefully regarding placing resistors into the headphone circuit, no-one is wrong. The headphones will work but you will lose fidelity.

If you aren't after Hi-Fi then ignore my suggestion.

If you are after Hi-Quality-Hi-Fi then a dedicated Hi-Fi headphone amp is a must and it needn't cost the Earth.

I'll leave it at that unless you answer looking for a Hi-Fi solution.
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Old 19th December 2011, 06:23 PM   #8
mmerig is offline mmerig  United States
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I am after hi-fi. Here is the motivation for the original question:

My wife likes to listen to music on headphones while riding a bike on a trainer-stand, which is quite loud. I am getting her noise-cancelling heaphones.

The bike is in a room about 15 feet from the pre-amp and power amp; there are wires from the power amp running to this room, but not-line level wires. The power amp wires are the most convenient source for the headphones.

To me, the problem seems to be impedance and power-matching (either voltage and/or current).

Is a headphone amp appropriate when starting with such a high-power source?

The ESP project 100 looks like a good choice. Why would it be low-fidelity if the output impedance and power are matched to the headphones' input?
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Old 19th December 2011, 07:15 PM   #9
Mooly is offline Mooly  United Kingdom
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Just try the series resistors... for 10p/0.1€$ what have you too lose.

Amps costing 1000 €$ use that method.
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