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Old 13th June 2007, 10:34 PM   #11
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Sunsun22:
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A question about the connection. 8 ohms in series and 1 ohm in parallel will means the headphone is now having (62+8) ohms in parallel with 1 ohm. It doesn't sound right? Isn't it?
What I meant was 1 ohm in parallel with the 62 ohm speakers, and then 8 ohms in series with that. Andrew T has done the math.

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The voltage felt by the headphones is approximately 0r98/(8r98)=0.109 (-19.2db). This is potentially very loud. A 10W valve amp will still send 100mW to the headphones.
Absoluely ! As well as sensitivity of headphones and of ears being factors in selecting the 1 ohm resistor, I should have added method of use. What I do is have my volume control set to what for me is a pleasant value with speakers ( for me about 80 dB), and my wish is to be able to plug in my headphones without changing the volume control. Since my amp is putting out probably less than 1 watt, the 8 and 1 ohm values work for me.
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Old 15th June 2007, 12:58 AM   #12
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Thanks Robert, I just look at the connection from the opposite side.

By the way, why 8 ohms? 1/62+1/1=1/0.98R. The output is 8 ohms and it will be looking at the headphone side as (8 + 0.98) = 8.98R. Is it better to use 7 ohms?

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Old 15th June 2007, 06:29 AM   #13
AndrewT is online now AndrewT  Scotland
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Hi,
You might be able to buy a 7r5 but 6r8 and/or 8r2 is more common in the high power types.
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Old 15th June 2007, 02:09 PM   #14
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question.

whats the downside of just connecting a higher impedance headphone to a 8ohm tap, no resistors. The load the output tubes see goes up, so the power transfer is much worse, but there is not a need for much power anyway. Certainly you could do this with 32ohm phones on a 16ohm tap.
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Old 15th June 2007, 02:33 PM   #15
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A pentode amp might object to the light load, possibly resulting in higher distortion or even instability in marginal cases. The other immediate downside of eliminating the resistive divider is noise. Headphones are much less tolerant of residual noise due to their very high voltage sensitivity compared to speakers from a typical listening distance.
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Old 16th June 2007, 01:29 PM   #16
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Default 8 vs 7 ohm series resistor

Sunsun22:
Quote:
By the way, why 8 ohms? 1/62+1/1=1/0.98R. The output is 8 ohms and it will be looking at the headphone side as (8 + 0.98) = 8.98R. Is it better to use 7 ohms?
Yes, 7 ohms would be more accurate, but I just happen to have a bunch of 10watt 8 ohm resistors in my stash of parts. The loudspeakers connected to the amp wont have an impedance of exactly 8.00 ohms across all frequencies, so there is no reason for the headphone ciruit too either.

Perhaps you may want to vary the resistance to see if there is an optimum load for the amp. Normally the operating point is a bit of a compromise between maximum power and minimum distortion ( pentodes ), triodes may want a higher resistance as others have mentioned. But for a headphone you dont need much power, so you would not need to settle for a compromise value. Just a thought, not something I have actually tried myself.
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