Connect with us

Headphones from amp speaker wires

Discussion in 'General Electronics Discussion' started by L B Masters, Nov 30, 2016.

Scroll to continue with content
  1. L B Masters

    L B Masters

    19
    0
    Nov 30, 2016
    View attachment 30585 Hi all, hope I'm in the right place.

    I'm new to electronics, but I can follow directions,lol.
    I have a situation where I wired a headphone jack to the speaker wires
    of my ACX450 Behringer acoustic guitar amp.
    45 watt output, 4 Ohm speaker.

    I used 1/4" mono jack with the contact that disconnects the speaker when the phones are
    plugged in. I've read all the warnings about running an amp with no load to the speaker wires,
    but it's not actually cutting the load, but switching the load to the phones.
    Besides most techies say a solid state amp isn't affected by no load operation like tube amps are.
    In any case, I WILL NOT run the amp through the phones until I get the correct value and placement of resisters
    to get the volume close to the level it would be through the speaker for practice.
    The problem I have is that It's drastically overdriving the phones.

    Does anyone know of a diagram to wire this jack for the correct for
    a resistance range that will allow me to use phones at a higher volume than 1 or 2 at the most.
     

    Attached Files:

  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    I would start with a 100Ω resistor in series with the phones. If it is still overdriving, try higher.

    Bob
     
  3. Audioguru

    Audioguru

    3,480
    740
    Sep 24, 2016
    The resistor calculation depends on the impedance and sensitivity of each earphone that you did not say.
    Does the amplifier produce 45 Whats per channel or 45 real Watts?
     
  4. L B Masters

    L B Masters

    19
    0
    Nov 30, 2016
    Very good question Guru,
    As to the earphone resistance, I didn't think it had to be that exact because people buy different headphones all the time without even thinking about it.
    I have two old pairs, one is 32 Ohm the other 40 Ohm.
    I'd like to buy a good pair, but I'm waiting to get this figured out first.
    Guess it's a catch 22 situation.
    Isn't there a range I can shoot for that is within a standard for headphones?
    Now that you got me double checking, on the watts output,
    the specs booklet covers 3 models. There is no flat output wattage shown.
    This is what they show for power rating in the system specs.
    The ACX1800 2x90 / 2x4 Ohm
    I'm assuming is where the name ACX1800 model came from
    The ACX900 2x45 / 2x4 Ohm is then named the ACX900
    The first two are called STEREO amps, which explains the 2x90 and the
    2x45.
    But mine says 1x45 / 2x4 Ohm which is sounds like one channel
    but they advertise it as the
    ACX450 2-channel Acoustic Instrument Amplifier with dual FX and FBQ Feedback Detection.
    I only bought it because it had a separate mic section with it's own EQ's
    to play an acoustic guitar and sing through at the same time.
    I'm totally confused now,:( I'm guessing 45 total watt output.
    Sorry for the book.
     
    Last edited: Dec 1, 2016
  5. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    A solid state amplifier should be stable with no load.
    Power (W) = V*V/R so the voltage is 13V maximum. The resistor is series with the phone needs to be high enough to prevent damage. Do what BobK says in #2. I presume that the amp is fitted with a volume control and do not harm your hearing.
     
  6. Audioguru

    Audioguru

    3,480
    740
    Sep 24, 2016
    The true power output of an amplifier is continuously with low distortion (no clipping) from 20Hz to 20kHz. Many amplifier ads exaggerate the number by saying the power at one frequency with 10% of awful clipping distortion for only a moment before the power supply voltage sags. Then they quote "peak" power which is the previous fake number doubled. Your "45 Whats" amplifier could produce only 12 real Watts.

    Assuming that your headphones are 32 ohms per ear and have a sensitivity of 1V for 100dB (very loud) then both earphones are parallel connected to the 45W amplifier through an attenuator that reduces the 13V to 1V.
    A resistor that is (32 ohms/2) x 12= 192 ohms (use 200 ohms) can be used in series with the two paralleled earphones. The power in the resistor will be 0.72W so use a 2W resistor.

    If the headphones are cheap then you will probably hear them resonating like a bongo drum then a headphones amplifier with a low output impedance should be used to damp the resonating.
     
  7. L B Masters

    L B Masters

    19
    0
    Nov 30, 2016
    Yes Duke, it has a volume control, lol. It has a nice EQ also and effects section.
    its a nice Behringer but I screwed up when I bought it, I thought it had a headphone jack.
    It's also been mentioned to add the resistor inside a 1/4" male end for the phones.
    allowing this jack to be a fully powered auxiliary out.
     

    Attached Files:

  8. L B Masters

    L B Masters

    19
    0
    Nov 30, 2016
    Ok Fellows,
    Since I'm not worried about adjusting the phones independently with a separate volume control,
    and all I want to do is make them listenable, (is that a word lol) by lowering the volume of what I hear in the phones now.

    You're saying I should be able to experiment with resisters in the line starting with around 100 Ohms leading to the phones jack until I get it where I like it?
    (I take it wired in series), and does it matter which lead, positive (Tip) or negative (sleeve)?
    Without danger of damaging my amp.
    The phones I have are junk so I'm not afraid of trying them, but if it works out I'll just get better ones of the same resistance.
    I'm only worried about THE AMP!!!!!!
     
  9. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    If it is a solid state amplifier, then it should be OK. If you are worried, then run the output into a resistor, say 3.3 to 10Ω of sufficient size to absorb the power, connect the phones across this with the 100Ω resistor in series. Then the amp will be running into a defined load and if the 100Ω resistor is made variable then that will give some control of phone volume. Whether the resistor is in the tip or sleeve wire is irrelevant. Too low a resistance is unlikely to damage the amp but could melt the resistor or phones.

    If it is a valve amplifier (with output transformer) then you must use a load.
     
  10. L B Masters

    L B Masters

    19
    0
    Nov 30, 2016
    Hi Duke, you said "If it is a valve amplifier (with output transformer) then you must use a load. "
    How can a dummy like me know that? will either of these spec sheets tell anything?
    "You say you must have a load" aren't the headphones drawing a load?
     

    Attached Files:

  11. Arouse1973

    Arouse1973 Adam

    5,164
    1,087
    Dec 18, 2013
    Your amp is solid state not valve.
    Thanks
    Adam
     
  12. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    A valve amplifier runs on a high voltage and a low current. To make the output suitable for a speaker, a transformer is used to drop the voltage and increase the current. If you are going to run the amp, then follow the instructions, you do not need to know why. The transformer stores energy and if this is not transferred to the output, then very high voltages can be generated which can be damaging.

    Headphones will supply a load but if you compare 4Ω to 100Ω phones you will see that it will be so small as to be insignificant and high voltages will fry the resistor and phones.

    A solid state amplifier runs on a suitable voltage and current which can be passed directly to the speaker.

    Edit: Adam says that the amplifier is solid state so no problems without a load.
     
  13. L B Masters

    L B Masters

    19
    0
    Nov 30, 2016
    I want to thank all of you gentleman for the help on my project.
    I hope you all have a merry Christmas.

    And Yes I said CHRISTMAS
     
  14. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Happy Solstice to you too!

    Bob
     
  15. L B Masters

    L B Masters

    19
    0
    Nov 30, 2016
    Sorry Bob,
    I lied, there is one more question.
    Something I never thought of was,
    what about a set of headphones with a built in volume control?
    does the pot just create an adjustable resistance?
    would this be any more dangerous to the amp?
     
  16. Audioguru

    Audioguru

    3,480
    740
    Sep 24, 2016
    How can the fairly high resistance of a volume control on headphones damage a modern solid state amplifier? If you do not attenuate the high output level from the amplifier then the volume control and/or the headphones will be destroyed. The headphones jack on an amplifier is always fed from a series resistor as an attenuator.
     
  17. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    With a 100Ω or more resistor in series, a short circuit would not be a threat to the amp.

    After Audio Guru's calclulation, I would start with a 220Ω 1W resistor instead of the 100 that I suggested. In all likleyhood, the amp is not really 45W RMS and you will have to reduce it to get the volume you want, but better safe than sorry.

    Bob
     
  18. L B Masters

    L B Masters

    19
    0
    Nov 30, 2016
    I did an experiment with an old pair of 32 Ohm headphones.
    I forgot to ask what wattage the resisters were supposed to be, Duh, so I just grabbed some
    1/2 watt 100 Ohm, and some 47 Ohm 1/2 watt resisters.
    It took 3 - 100 Ohm resisters to get the headphones volume down much, but it's still not close to the same as
    the amp unplugged. It actually could have been down a more.

    Is it ok to connect them, in series on the ground?
    and you're saying I should use 1 watt resisters starting at 220 ohm?
    I'm going to buy some new phones of a higher Ohm rating than 32, probably at least 40
    but price may hold me back. Would the higher rated headphones help the situation?
     
  19. Audioguru

    Audioguru

    3,480
    740
    Sep 24, 2016
    I asked abut the impedance of your headphones because some are 8 ohms and others are 600 ohms. Most are 32 ohms and 40 ohms is almost the same.

    45 real Watts into 4 ohms is 13.4V. A 220 ohm resistor in series with both 32 ohms earphones in parallel causes 0.71W in the 220 ohm resistor at full power so a 1W or 2W resistor should be used.
     
  20. L B Masters

    L B Masters

    19
    0
    Nov 30, 2016
    Switchcraft 13A 1-4 Mono Jack Female Panel Mount Transfer Circuit_files.JPG View attachment 30719 !!! UPDATE !!!

    Guru, Bobk,
    You guys were right.

    For the lack of a good selection at Radio Shack, I ended up using
    3- 1watt 100 Ohm resisters in series to obtain the approximate
    matching volume control level number on my amp knob vs the
    my 40 Ohm headphones. Now when I set my amp volume at a
    comfortable sound level through the speaker, I can plug the headphones
    in and they sound about the same to my ears without worry of
    overdriving them or my ears. But these were bench tests of the two separately.
    The next hurdle I ran into was, wiring a standard headphone jack.
    A standard headphone jack breaks the contact to the speaker and of course
    then makes its own contact through its tip and sleeve.
    But it doesn't switch over to the new resisted circuit I made without
    making a new contact along with breaking the one to the speaker.
    I found a jack that switches completely from the speaker
    circuit to the resisted headphones circuit when plugged in.

    Thanks for all your help, I think I have it whipped now.
     
    Last edited: Dec 5, 2016
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-