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Old 5th February 2010, 02:59 PM   #1
maurycy is offline maurycy  United States
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Default LM1875 for headphone amp

I am looking to purchase this board (link) so I can put together headphone amplifier. I want also to have speaker out so I can connect small speakers to it. This is for a bedroom system (low volumes). Is the board, I linked to suitable for this headphone modification? This LM1875 board has common ground for the speaker out.

If that's not good, what other kit would you recommend? I need to get the amp running fairly quickly without hunting for parts so that's why I am looking at a kit.

Thanks.
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Old 5th February 2010, 03:10 PM   #2
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Quote:
Originally Posted by maurycy View Post
I am looking to purchase this board (link) so I can put together headphone amplifier. I want also to have speaker out so I can connect small speakers to it. This is for a bedroom system (low volumes). Is the board, I linked to suitable for this headphone modification? This LM1875 board has common ground for the speaker out.

If that's not good, what other kit would you recommend? I need to get the amp running fairly quickly without hunting for parts so that's why I am looking at a kit.

Thanks.
One thought,

What would happened if you plug in the headphone jack while playing music? This means the amplifier get shorted while doing this.

With kind regards,
Bas
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Old 5th February 2010, 03:12 PM   #3
maurycy is offline maurycy  United States
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Well, there is going to be a switch as explained in the headphone mod. So it is either going to be headphone or speaker output but not both. I am just not sure if this kit is suitable for this mod.

Thanks.
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Old 5th February 2010, 03:27 PM   #4
AndrewT is offline AndrewT  Scotland
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Default noise specification.

What are the sensitivity of your headphones?
What is the noise output of the amplifier?
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regards Andrew T.
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Old 5th February 2010, 03:34 PM   #5
maurycy is offline maurycy  United States
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Andrew,

Here are the specs for the headphones (JVC HA-RX700):

Frequency Response 8-25,000Hz
Nominal Impedance 48ohms
Sensitivity 105dB/1mW
Max. Input Capability 1500mW(IEC)

As to the amplifier, I have no idea. I am just looking for something that I could build that will give me ability to connect speakers and headphones (not at the same time). Power requirements are low as the speakers are rated at 5W RMS only.

Thanks.
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Old 6th February 2010, 09:40 AM   #6
AndrewT is offline AndrewT  Scotland
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for 1mW into 48ohm headphone you need 220mVac to get 105dB.

If you want noise below 25dB, which may be just audible in a quiet listening room, then the noise output from the amp must be 80dB below that 220mV signal, i.e. <=0.022mV.
Check the datasheet for the proposed chipamp. What is the audio band noise output?
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Old 8th February 2010, 02:53 AM   #7
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And don't forget to include the voltage divider into the calculations Andrew is suggesting.
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Old 8th February 2010, 01:51 PM   #8
maurycy is offline maurycy  United States
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Guys, you are talking now about things that are way beyond my knowledge. I can solder, read diagram, identify parts but I have no idea what "audio band noise output" is. I have read thru LM1875 datasheet but wasn't able to find anything that was rated in mV and could indicate it is "audio band noise output".
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Old 8th February 2010, 02:00 PM   #9
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The LM1875 is probably a huge overkill for a headphone amp. You may want to look at something like a TDA2822M or similar, instead.
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Old 8th February 2010, 04:34 PM   #10
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The LM1875 is good for driving headphones with the adapter you linked, as well as low volume listening with speakers. I would personally buy a kit from BrianGT before I would buy from an unknown eBay source, and I don't think you need a built in preamp for what you want to do, although it depends on your source.

I used the headphone adapter you linked to with my chipamp (LM3886), and it works fantastic. One thing he doesn't mention in the article that I did was to use a 4-pole switch to disconnect the speaker grounds as well (so that they didn't act as antennae) and grounded the headphone socket to the case so there was only one return to the star ground.
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Last edited by Redshift187; 8th February 2010 at 04:38 PM.
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