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Topic: Loudspeaker cables - horses For courses? (Read 22780 times) previous topic - next topic
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Loudspeaker cables - horses For courses?

Reply #50
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It seems to me like the best thing to have, if one tended to listen to both speakers and headphones equally and only wanted a single device, would be a receiver that had two amp circuits in it. The standard high power speaker driving amp, and a little headphone level amp for when the phones are plugged in.
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THat's one of the things you get as you move up the price scale.

Loudspeaker cables - horses For courses?

Reply #51
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Something like a 20 Ohm and a 2 Ohm resistors in series with amp speaker terminals, with the headphones in parallel with the 2 Ohm resistor, will drive any headphone so that the frequency response deviation is negligible (voltage source-like), providing a 20 dB attenuation. The 20 Ohm resistor must be rated several watts.
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Only thing is it raises the damping factor considerably. Series resistors would do that too, to an even greater extent though. The 20 Ohm resisors would need to be quite a high-wattage rating, roughly 1/5 of the amp's output rating (into 8 Ohms) if you're going to wind the amp up.

I'm curious how you calculated the 20dB attenuation - that'd be a voltage attenuation, ignoring the headphone impedance?
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Damping factor won't be an issue. A headphone has a minimum impedance that is several times greater than the 2 Ohm load it is in parallel with, so the amp sees a near 22 Ohm load. The headphones see 2 Ohm in parallel with a 20 Ohm, so it's nearly 2 Ohm output impedance that they see. Their impedance is quite greater, so electrical damping won't be very bad. But also, headphones are already very damped mechanically, and need very little electrical damping.

As to the 20 dB calculation, yes, that's a voltage (level), attenuation. In practice it will be a little bit more than 20 dB, depending on how low is headphone impedance.

Loudspeaker cables - horses For courses?

Reply #52
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Quote
Quote
Something like a 20 Ohm and a 2 Ohm resistors in series with amp speaker terminals, with the headphones in parallel with the 2 Ohm resistor, will drive any headphone so that the frequency response deviation is negligible (voltage source-like), providing a 20 dB attenuation. The 20 Ohm resistor must be rated several watts.
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Only thing is it raises the damping factor considerably. Series resistors would do that too, to an even greater extent though. The 20 Ohm resisors would need to be quite a high-wattage rating, roughly 1/5 of the amp's output rating (into 8 Ohms) if you're going to wind the amp up.

I'm curious how you calculated the 20dB attenuation - that'd be a voltage attenuation, ignoring the headphone impedance?
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Damping factor won't be an issue. A headphone has a minimum impedance that is several times greater than the 2 Ohm load it is in parallel with, so the amp sees a near 22 Ohm load. The headphones see 2 Ohm in parallel with a 20 Ohm, so it's nearly 2 Ohm output impedance that they see. Their impedance is quite greater, so electrical damping won't be very bad. But also, headphones are already very damped mechanically, and need very little electrical damping.

As to the 20 dB calculation, yes, that's a voltage (level), attenuation. In practice it will be a little bit more than 20 dB, depending on how low is headphone impedance.
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So, for a headphone of 16 Ohms, the damping factor is 8, which isn't brilliant, though I'm not sure how much it really matters. For the same impedance I calculate the power attenuation to be about 17 dB. I'm not sure if 16 Ohms is a typical figure, I think they are often higher, but mine are certainly 16 Ohm (2 pairs). Be interesting to know how this set-up compares in reality against series resistors - someone care to try it?

Loudspeaker cables - horses For courses?

Reply #53
In your case, damping factor wouldn't still probably be an issue, due to headphones being enough mechanically damped. But if your headphones have varying impedance across frequency, a small deviation from flat frequency response at headphone terminals (due to the 2 Ohm output impedance) could be audible. See basic Ohm's law calculation:

http://www.kikeg.arrakis.es/various/zout_zl.xls

If it is, better use a 1 Ohm / 10 Ohm combination instead.

Edit: with this same spreadsheet, compare the effect on using a simple 120 Ohm resistor in series with the headphones.