How do I integrate a headphone jack into a LM 3875 Kit - diyAudio
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Old 9th November 2011, 10:22 PM   #1
j.burtt is offline j.burtt  United States
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Default How do I integrate a headphone jack into a LM 3875 Kit

About to order a LM3875 Premium Kit and wanted to make sure I could integrate a headphone jack before I pulled the trigger.

James
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Old 12th November 2011, 05:07 PM   #2
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You can find examples of headphone jack connection here: headphone jack connections - Google Search

The amp itself works very well as headphone amp:
LM3875 as a headphone amp (another try)
Commercial Gainclone kit- building instructions
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Old 12th November 2011, 06:39 PM   #3
j.burtt is offline j.burtt  United States
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Thanks for the reply Peter. All I need to do is install a switch to toggle back and forth between the stock 680R and a 1K2 to lower the gain when using headphones. Connections of the headphone jack would just tie into the outputs? It appears from looking at the board that the outputs of the amp are commonly grounded, which makes connecting the ground from the jack simple.
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Old 13th November 2011, 03:14 PM   #4
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Yes, the headphone jack would be connected directly to the output, no additional resistor needed.

I tie the grounds of both channels indeed: Commercial Gainclone kit- building instructions

As to 680R/1k2 resistor, you don't really need to lower the gain, but if you decide to do it, it's better to add resistance in parallel to avoid any popping noises.
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Old 13th November 2011, 05:11 PM   #5
j.burtt is offline j.burtt  United States
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When you parallel 2 resistors of equal values, the net resistance is half isn't it? At least that's what I remember from electronics class 28 years ago.

So, I could install 1k2 resistor in the board and parallel another 1k2 that is switch activated when I needed the higher gain.

Would 1k3 or 1k4 resistors be recommended as they would provide 650R or 700R when the high gain switch is activated?

Any benefit to sound quality by reducing the gain for headphones, or just more range on the volume pot?

Thanks,

James
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Old 13th November 2011, 05:18 PM   #6
Mooly is offline Mooly  United Kingdom
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With headphones a lot depends on the sensitivity of them...

The values of series resistor to the phones are more a trial and error than exact science. You want something that feels and sounds right with the rotation of the volume control so experiment a bit.

If you look at any service manuals of old amps you will get an idea of how its done and values used. You never alter the gain of the main amp, just alter the series resistors feeding the phones.

Resistors in parallel and series,

Series and parallel circuits - Wikipedia, the free encyclopedia
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