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Vox AC15 custom - extension ouput

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  • Vox AC15 custom - extension ouput

    Hi,
    Something broke in my Vox AC15 custom, maybe you are familiar with this kind of problem...
    I tried to plugin in an acoustic guitar with a pickup (a mistake), there was boom noise,
    and after that the amp extension output (the one that works together with amp's speaker) became weak and noisy.
    This is the output I use to connect to sound card and record, it was connected to the input of the sound card while this happened.
    I'll appreciate it if you can help with this.
    Thank you!

  • #2
    The boom noise was caused by the 9volt battery power for the pickup on the guitar. Perfectly normal if the volume on the amp is turned down first.
    Can you elaborate on the socket getting weak and noisy?
    Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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    • #3
      The extension output is in parallel with the internal speaker. If the amp and speaker works fine on its own, then there may be a problem with your sound card input - it could have been overloaded when you plugged in the acoustic guitar. Have you verified where the problem actually lies?

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      • #4
        the amp extension output (the one that works together with amp's speaker) became weak and noisy.
        This is the output I use to connect to sound card and record, it was connected to the input of the sound card
        NO NO NO NO NO, you connected a SPEAKER level signal (say 15-20 Volts RMS) to a PREAMP level input expecting 100-200 MILLI Volt signal.
        Crazy.

        You most probably blew it to pieces.
        Juan Manuel Fahey

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        • #5
          Thank you guys very much for your answers.
          I'm glad to know that if the amp's speaker sounds good then the problem might be somewhere else. I'll elaborate more on the connections.
          I connected the output of the amp (which now I know should be connected only to a matching impedance speaker) to the sound card.
          The input to the amp was a looper/drum-machine pedal.
          Now I try to isolate the problem, that is
          1) Try to record the loop in the pedal via the sound card and via another interface. Maybe the problem is in the sound card, and maybe the pedal loop itself is noisy, in which case, I can't figure out how that happened..
          2) Try the output of the amp in a different way and see if I can get clear sound.
          Thanks!

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          • #6
            For the next time: build a speaker-to-preamp level attenuator and send only the attenuated signal to be recorded/processed.
            Amp will still need a speaker attached not to damage power tubes and/or output transformer.

            Attenuation comes from a 2 resistor network, 22k in series and 1k to ground, which puts out about 1V RMS, typically to drive another power amp.

            For a closer to instrument level signal IŽd use 22k > 220 ohm, for a friendlier 200mV level or so.
            Juan Manuel Fahey

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            • #7
              Originally posted by iu2 View Post
              Thank you guys very much for your answers.
              I'm glad to know that if the amp's speaker sounds good then the problem might be somewhere else. I'll elaborate more on the connections.
              I connected the output of the amp (which now I know should be connected only to a matching impedance speaker) to the sound card.
              The input to the amp was a looper/drum-machine pedal.
              Now I try to isolate the problem, that is
              1) Try to record the loop in the pedal via the sound card and via another interface. Maybe the problem is in the sound card, and maybe the pedal loop itself is noisy, in which case, I can't figure out how that happened..
              2) Try the output of the amp in a different way and see if I can get clear sound.
              Thanks!
              You've got to listen to what Juan is saying and stop plugging an output intended for a speaker directly into any effects, drum machines or other recording gear inputs expecting instrument or line level signals. Do not try to find a way it will work by plugging things in differently. Do not do it as you were again. Ignore this at your own peril. Ignore the good advice on locating your actual problem at your own frustration. A sufficient short due to failure in anything plugged into the extension output also risks damage to the most expensive components of the AC15.

              You CAN build a very simple interface circuit to go between the extension output and your recording gear. Juan has already described that also. It's easy and there are too many potentially expensive risks if you don't do this.
              "Take two placebos, works twice as well." Enzo

              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

              "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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              • #8
                Thanks again.
                Playing the looper straight to my phone revealed that the noise was already in the looper.
                The amp sounds as good as always.

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                • #9
                  FWIW you can also use a potentiometer with Juan's suggested circuit to make it continuously variable from zero output to line level. Just use the 22k resistor from the hot terminal for the jack to a 1k potentiometer wired as voltage divider (like a volume control). Pot wiper goes to the hot terminal for the jack that will feed signal to your recording gear. This will give you some control over input clipping levels and minimizing noise floor.

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                  Last edited by Chuck H; 01-02-2022, 08:06 PM.
                  "Take two placebos, works twice as well." Enzo

                  "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                  "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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