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New headphones amplifier
uart
post Aug 8 2009, 12:50
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Last week I decided to built me a simple opamp based headphones amplilfer (for mounting within my computer). The reason is that my computer's only got onboard realtek audio. It's not a bad implementation though (gigabyte motherboard) and it was good enough for me except for one thing. When I connected my headphones the bottom end frequency response dropped away. My headphones have an inline volume control so their impedence changes depending on the volume setting, it was ok at low to mid volume settings (higher impedance) but started dropping the bass a bit at higher volume settings (lower impedance).

Anyway today I got the amplifier finished and I'm pretty happy with the results, it sounds great to me smile.gif. I uploaded the rmaa results HERE if anyone wants to take a look. BTW This rmaa loopback test was run on the realtek onboard audio and with the headphones attached and at a reasonably high volume setting.

I designed the amplifier myself from low cost components, pretty much built it from left over stuff from other projects that I had laying around. If anyone is interest I can also upload the circuit diagram and some pictures of the unit.

This post has been edited by uart: Aug 8 2009, 12:59
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odigg
post Aug 8 2009, 14:51
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QUOTE (uart @ Aug 8 2009, 07:50) *
I designed the amplifier myself from low cost components, pretty much built it from left over stuff from other projects that I had laying around. If anyone is interest I can also upload the circuit diagram and some pictures of the unit.


I'd like to see the circuit. I'd also like to see a headphone loaded RMAA of the onboard sound without the amp to see just how much bass rolloff you are talking about.


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uart
post Aug 8 2009, 15:34
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I've added the circuit diagram and some pictures in the uploads section HERE.

The first picture (after the circuit diagram) is the crudely made circuit mounted on the back of one of those little metal back plates that cover the slots in the back of a computer tower.

The soldering was so ugly I decided to put it inside a "Faraday cage" really just so I wouldn't have to look at it. I made it's little silver cage out of cardboard covered with aluminium foil and then sticky tape. The second picture shows it with it's little silver condom in place. This picture is the most embarrassing because I've got the holes where the RCA connectors protrude though cover with silver (coloured) electrical tape. I did this to control the ground points and avoid any ground loops. I just did such an ugly job there but I tidied it up a bit later.

The last picture show the rear view, the two wires coming out the back are for +/- 12 volts from the computer power supply. I didn't directly connect the ground (zero volt) PSU wire, again to control ground loops. Ground enters the circuit at a single point only, through the RCA input cable.

Sorry that I didn't save a copy of the RMAA results of the loaded soundcard without the phones amp, but it looked very similar to this one that I just found on the net HERE. It was similar to the white trace at low volume levels, very similar to the green trace at about normal volume levels (where the phones impedance is a couple of hundred ohms) but was more like the blue trace in that image when the volume was really pushed.

This post has been edited by uart: Aug 8 2009, 16:26
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odigg
post Aug 8 2009, 15:55
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QUOTE (uart @ Aug 8 2009, 10:34) *
Here is the circuit diagram [attachment=5290:headphones_amp1.jpg]


Thanks!

I feel like crying when I see stuff like this. This costs maximum $15 in parts? If you go into the headphone amp world, people will say $100 is a very low cost for a "entry level" headphone amplifier, but you have just demonstrated it really does not cost much to make a good (measure well) headphone amp and that headphone amps can be made simply.

I feel like crying because of how disgusted I feel with the headphone eqiupment industry.

This post has been edited by odigg: Aug 8 2009, 15:55
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uart
post Aug 8 2009, 16:08
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QUOTE (odigg @ Aug 8 2009, 07:55) *
QUOTE (uart @ Aug 8 2009, 10:34) *
Here is the circuit diagram [attachment=5290:headphones_amp1.jpg]


Thanks!

I feel like crying when I see stuff like this. This costs maximum $15 in parts? If you go into the headphone amp world, people will say $100 is a very low cost for a "entry level" headphone amplifier, but you have just demonstrated it really does not cost much to make a good (measure well) headphone amp and that headphone amps can be made simply.

I feel like crying because of how disgusted I feel with the headphone eqiupment industry.


Yeah I was thinking the same thing when I tested this, it sounds really good for such little cost smile.gif. One thing to remember though is that I have saved a lot by having it mounted inside my computer. I've cut out the cost of the enclosure and most of the power supply circuitry (by tapping into the +/- 12 volt wires from the computers ATX bundle).
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