Author Topic: (solved) Attenuator values for headphone circuit  (Read 274 times)

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Offline serprinssTopic starter

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(solved) Attenuator values for headphone circuit
« on: January 12, 2025, 09:58:31 am »
Hello

I am trying to add a headphone output for a cheap boombox device.

I found this https://sound-au.com/project100.htm circuit but the resistor values only go down to 10w at 4Ω. The amp in my device says it outputs 2.5w at 4Ω (datasheet here:https://www.eeweb.com/wp-content/uploads/projects-member-projects-utc8227-1367701357-171108-032246.pdf)

I worked out that this is an L pad attenuator however as I am not familear with this topic I am not sure how to calculate what value of resistor I need.
« Last Edit: January 15, 2025, 05:10:14 am by serprinss »
 

Online magic

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Re: Attenuator values for headphone circuit
« Reply #1 on: January 12, 2025, 10:13:13 am »
It tries to target the retarded 120Ω standard which nobody ever cared about. In reality, the lower the output impedance, the better.

2.5W into 4Ω is about 3Vrms. Some larger headphones could basically run from this directly, no resistors whatsoever.

If you want to push it down, say 1.5Vrms, then you need two equal resistors such that their sum is about 4Ω (if you disconnect the speaker) or several times higher than 4Ω (if you don't). That's all, no R3 necessary at all.

For 1Vrms, same thing but 2:1 ratio resistors. And so on.
« Last Edit: January 12, 2025, 10:17:38 am by magic »
 
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Offline lakis70

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Re: Attenuator values for headphone circuit
« Reply #2 on: January 12, 2025, 11:56:02 pm »
Whats the impedance of your headphones?
 

Offline serprinssTopic starter

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Re: (solved) Attenuator values for headphone circuit
« Reply #3 on: January 15, 2025, 05:21:11 am »
That worked great. thanks.

to clarify for anyone finding this in the future I arranged two 2Ω resistors in an l pad circuit similar to the 2nd circuit found at https://sound-au.com/project100.htm.

I used a double pole double throw (DPDT) socket which means you have two inputs each going to one output when something is plugged in, and another when it is not. this meant I could connect the positive left and right audio leads and connect the speakers to the side that is connected when it is unplugged and the socket itself to the other (through the resistors).
 


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