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PostPosted: Sat Aug 26, 2017 2:47 am 
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Location: Berlin, DE
Hi, in my first post- need help or advice for designing headphone amplifier circuit.

I have to design a headphone amplifier circuit. We are told the impedance of the headphones and the minimum voltage they require and also the voltage source. After researching I have found that the LM386N-4 is a popular choice. (According to a datasheet of LM386N-4 - https://www.icrfq.com/part/2377557-LM386N_4.html ). I've seen many circuit diagrams but what I don't understand is how to pick the voltage gain needed for the amplifier.

I originally thought that you would need to know the input audio signal voltage and when you multiplied that by the gain that would need to give the minimum voltage required by the headphones, but it appears that what matters is the current which I don't get.

And are you able to design such a circuit without knowing about the audio input signal? If you do need to know about it what is it you need to know about it and why?

I would greatly appreciate any help as I am very lost.
Many Thanks.


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PostPosted: Sat Aug 26, 2017 5:17 am 
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Something like this?
Attachment:
headphone amp.jpg
headphone amp.jpg [ 116.75 KiB | Viewed 1194 times ]

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Ohms

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PostPosted: Sat Aug 26, 2017 5:28 am 
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The supply voltage Its most likely going to be +15V 0V -15V Unless its a portable design
Based on split 15V with a 120V main supply a 25VA transformer should suffice
A schematic diagram of the circuit would help if you have if not why not just build one of the many designs that can be found on the internet
There is a ton on ebay and can be had as blank boards or full kits


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PostPosted: Sat Aug 26, 2017 6:27 am 
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Or, for the chip you specified.
Attachment:
LM386N-4.gif
LM386N-4.gif [ 8.43 KiB | Viewed 1167 times ]

http://www.minidisc.org/headbanger.html

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Ohms

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PostPosted: Sat Aug 26, 2017 4:02 pm 
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I'll admit, I don't think I've ever seen a minimum voltage spec for headphones; usually impedance, sensitivity and maximum power handling. The current demands depend on the impedance of the headphones. The voltage needed (and therefore gain) can be calculated using Ohm's Law for power.

I designed and built a headphone amp using discrete transistors operating in classA. The most voltage I could get without any feedback was nearly 6 volts with a 30V supply, which was enough to make turning the volume up past 10 o'clock a little too loud. Unfortunately, the output resistance was quite high so there was a noticeable hiss with low impedance headphones; with high enough impedances there was no noise at all.

With my circuit, I was able to control gain through the collector resistors. Using the LM386 you'd be controlling gain with feedback. Current should not be an issue provided you have sufficient heat sinking and a large enough output capacitor.
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PostPosted: Mon Aug 28, 2017 3:45 am 
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Thanks a lot, all of you.

Really It's a helpful information for me. Thanks again for giving me a helpful diagram, which is really helping me a lot!

Kind Regards
Menzel


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