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This MUST be easy to do - amp tech question

Discussion in 'DIY Amp Forum' started by JamesE, Dec 10, 2019.

  1. JamesE

    JamesE Strat-O-Master

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    Very cheap amp I want to use for foldback, BUT it doesn't have an O/P for the mixer. OK, it's got a headphone socket, but of course, when I plug anything in the headphone jack, the speaker cuts out.

    Pleas advise how I can wire up a socket to provide an output to the mixing desk?
     
  2. JamesE

    JamesE Strat-O-Master

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    DSC_4721.JPG
    DSC_4722.JPG

    Despite it's bargain basement heritage ( I got it free when I bought a guitar), the clean channel is fine when boosted with a pedal. Just don't use the "Overdrive" switch!
     
  3. fezz parka

    fezz parka Strat-Talk Supporter

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    You need to get at the headphone out jack and rewire it so it doesn't kill the speaker.
     
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  4. Tone Guru

    Tone Guru Senior Stratmaster

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  5. JamesE

    JamesE Strat-O-Master

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    Just solder two wires to the jack?
     
  6. JamesE

    JamesE Strat-O-Master

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    Is that an SM57? Must be worth 4x what the amp's worth!
     
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  7. fezz parka

    fezz parka Strat-Talk Supporter

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    It'll be a switching jack. Find out where the lead is that mutes the speaker and bypass it.
     
  8. BuddhaFingas

    BuddhaFingas Strat-Talker

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    Getting the impedance right is usually the twitchy bit, followed by the grounding method used (or not used) in the amp.

    Depends on the impedance you are feeding into, & where the headphone jack is being driven from in the amp's circuitry.

    If the phone jack is driven by the same amp stage out as the speaker, rigging the jack to stay live puts your external load across the speaker, which will drop total impedance (parallel impedances divide, just like caps in series). Depending on the load, this could do funny things or even risk the amp driver chip.

    Some of the inexpensive modern low-power amps use differential drive stages to the speaker to up the delivered power, where neither side of the speaker (or the phones) is actually "grounded" at all - - two output stages, operating 180 degrees out of phase.

    Grounding one side of such an output would be rough on the amp chips. Such drives need an isolation transformer or other form of ground resolution to safely tap into a grounded input like a mixing desk.

    I chased after this issue with my VT20X. Wanted to bring it into a PC sound-card to record my horrible playing.

    Great little amp, but the headphone jack was soldered directly to the multi-layer PCB, which made trying to tap there impractical.

    And the output is deferentially driven by FOUR output amp drivers in quadrature; a stereo amp push-pull chip bridged to drive both sides of the (odd-impedance) speaker, no ground.

    I ended up doing a current-limited resistive voltage-division tap with a DC isolation cap. It works okay, but has to be adjusted to accommodate different volume outputs from the amp.

    It's far from ideal, and kind of risky if adjusted wrong.
     
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  9. Tone Guru

    Tone Guru Senior Stratmaster

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    I made the terrible assumption that a facility that has a mixing desk also has microphones.
     
    Last edited: Dec 10, 2019
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  10. JamesE

    JamesE Strat-O-Master

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    Your suggestion is of course very good, but I just want to make it as simple as possible. And, although the clean channel sounds ok, the loudspeaker probably isn't of a particularly high standard - hence I'd rather just carry an extra cable than a microphone, stand and cable.
     
  11. JamesE

    JamesE Strat-O-Master

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    Oh dear. I thought it was just going to be a case of soldering a couple of wires on and drilling a hole for a jack socket.
     
  12. fezz parka

    fezz parka Strat-Talk Supporter

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    Just use the headphone out. The switching jack stops the signal from going to the speaker. Find where it is on the jack and bypass it. Then fashion a jumper to with a stereo TRS plug on one end and mono on the other. Plug the mono end into a hiZ input on the board. ;)
     
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  13. BuddhaFingas

    BuddhaFingas Strat-Talker

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    It --might-- be. It all depends on the construction of your amp & what you want to attach it to.

    Is the jack wiring accessible inside the amp? Mine wasn't; would have required modifying the main PCB and eliminating the headphone function. Didn't want to do that.

    Is the headphone jack's "common" an actual ground? Having to use the speaker drive, mine wasn't.

    Is the input impedance of the board high enough to not load the amp drive down? The PC audio input I was dealing with was borderline - only a couple kilo ohms, not really "high-Z". Headphones usually run a few hundred ohms. Higher impedance has less impact. But I had to go across the low-impedance speaker drive without a real ground. It got messy.

    If your answers are "yes," it's as easy as Fezz is saying - his answers are absolutely right for "traditional" amp construction. Which is what I was hoping to do, but the VT20X's design made the easy way impossible. Yours may or may not be built the same way.

    Forewarned is forearmed. Some of the modern, inexpensive amps are tricksy.
     
    Last edited: Dec 11, 2019
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  14. JamesE

    JamesE Strat-O-Master

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    That's the headphone socket in the centre, encased in a little plastic box. I'm not going to try to hack into it in case I damage something, so I'm thinking about the Y-cable.
     
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