Guitar Amp Headphone socket

Discussion in 'General Electronics Chat' started by jaybefr, Dec 31, 2017.

  1. jaybefr

    Thread Starter New Member

    Aug 21, 2017
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    I am trying to learn about audio electronics and am doing an online course. The outcome is to build a simple 10W practice amp for a guitar. (circuit diagram attached) I have managed that successfully. I would like to add a 1/4" headphone socket that will silence the main speaker and be controllable from the amp's volume pot. I would be grateful if someone could advise me how to go about it. My headphones are 200mW 60ohm.

    Thanks in advance
     
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    What is the part number for IC3?

    Add a coupling capacitor in series with R5. This will prevent any DC offset out of the first opamp and input offset voltage error in the second opamp from being amplified and possibly causing clipping at the output.

    ak
     
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  3. MrChips

    Moderator

    Oct 2, 2009
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    What type of connector is on your headphones, 1/4" or 3.5mm stereo?

    Here is a circuit diagram off the internet:

    [​IMG]

    I would do it slightly differently for stereo headphones.
     
  4. Audioguru

    Expert

    Dec 20, 2007
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    With a 12V supply and a single-ended (not bridged) amp the output swing is probably 9V p-p. Then the RMS output is 3.18V and the power to an 8 ohm speaker is 1.26W or 2.5W into 4 ohms.

    If the two earphones are paralleled to make a 30 ohm load then the power to them is a whopping 337mW and will damage them and make you deaf. Add a 22 ohms resistor in series with each 30 ohm earphone then their max power will be 112mW each.
     
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  5. MrChips

    Moderator

    Oct 2, 2009
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    Correct, but a little tricky trying to do that with standard stereo headphone jacks as can be seen in the schematic in post #3.

    I haven't come up with a solution as yet.
     
  6. jaybefr

    Thread Starter New Member

    Aug 21, 2017
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    Hi, It's 1/4" stereo jack for the 'phones but the amp is mono obviously.

    Thanks for the circuit diagram. It looks useful.

    And Happy New Year!
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    Try this circuit:

    Headphone jack.jpg

    This configuration puts the left and right headsets in series (and out of phase). It does not use the sleeve (GND) connection of the jack.
    You can omit the resistor shown. If the volume is too loud you may add the resistor (perhaps starting around 100Ω depending on the impedance and loudness of the headphones).

    Note: Don't do this with a transformer output tube amp because the output impedance will not match up with what the tube circuit expects to see.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Here is another way:

    Headphone jack2.jpg

    This puts the headsets in parallel and in phase and uses the GND connection.
    Select the resistor value to set the volume level (start with 33Ω and test).

    Note that the sleeve of the headphone jack is not at the same GND as the speaker GND. If you are using a metal chassis the two grounds must be isolated from each other.
     
  9. jaybefr

    Thread Starter New Member

    Aug 21, 2017
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    Thanks Mr Chips for the time and effort that you have obviously put into this. I guess you are a bit like me and just like working through problems. Unfortunately, I don't yet have the Knowledge or understanding of electronics to do this myself. I'm sure from your suggestions I will work something out. I also just realised that I may have overcomplicated the issue, there is absolutely no reason why I couldn't insert a physical switch to switch between speakers and phones, much like I have on my hifi. Thanks again.
     
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