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  • Kustom kba strange noise

    Hi all, long term lurker,
    I have a kustom Kia bass amp that came to me making a nasty buzzing noise, I managed to track it down to a switching jack on the input, the amp is now quiet but if the guitar is unplugged the noise is there again.
    All the other amps I own, Marshall valve amp, lanes bass head solid state practice amps, make a little more noise when a guitar is unplugged but not on this level, this is a really harsh buzz which alters in level with the Channel gains ,the same thing happens on both channels.
    I will try to post a copy of the schematic

    Any help would be great.

  • #2
    I have found the schematic on the net, it's on the elektrotanya.com site, I'm not sure how to put it in the post.

    Comment


    • #3
      Is this the one?

      Kustom_KBA80B.pdf

      The first thing I'd look for would be a issue with the preamp power supply i.e +15V and -15V.

      If you have a meter measure the ACV on those the +15V -15V rails - put a 47nF capacitor in series with your meter lead when doing this to block any DC.
      Experience is something you get, just after you really needed it.

      Comment


      • #4
        Thanks for the speedy reply, just to confirm when there is a bass plugged in the amp works fine on both channels, the noise when the open ended cable is plugged in is like a really bad earth hum, the peak detect led lights up as well, I could try to measure the frequency if that helps at all.
        I have a DMM and an old 20 MHz scope will check the power supplies for ac tomorrow.

        Yes that's the schematic, I couldn't find the layout that you included.

        Comment


        • #5
          If ti's OK with a bass the forget the power supply comment then. I would expect to pick up noise using a open ended cable - don't do that

          Seriously though, did something change? Does it only happen with your guitar? What happens if you turn down the volumes of your guitar?

          EDIT: I just re-read you original post and I see had misread it. I had thought you said that after fixing the input jack you got noise when you plugged in. Like Jazz says in that case the input jack shorting switch contact is still bad.
          Last edited by nickb; 10-16-2016, 07:49 AM.
          Experience is something you get, just after you really needed it.

          Comment


          • #6
            If the amp 'makes noise' with nothing plugged in, then the jack is not grounding properly.

            There are normally closed contacts as part of the jack that must close when the plug is removed.

            If the contacts do not close properly, all bets are off, as you now have a 'floating' input.

            kustom_kba80b input jack.pdf

            Comment


            • #7
              Lucky guy, SGM would have written *volumes* about the "overheating 100V Japan version" and the "STK power amplifier chip"
              Juan Manuel Fahey

              Comment


              • #8
                Hi, the amp has had a new switching jack on the input,
                This stopped it making noise with nothing plugged in to the amp,
                Which was the reason it was given to me.
                I understand that a lead plugged into the amp but not into a guitar
                Leaves the input floating in the same way as the broken jack socket did.

                What I'm asking is, since all the other amps that I own make hardly any extra
                noise when the input is floating, but the kustom brings on the peak led, is there
                a fault somewhere in the circuit or is it maybe just the design / gain of the amp?

                This is really annoying when changing guitars, as the noise is so loud.

                Comment


                • #9
                  Originally posted by fireman View Post
                  Hi, the amp has had a new switching jack on the input,
                  This stopped it making noise with nothing plugged in to the amp,
                  Which was the reason it was given to me.
                  I understand that a lead plugged into the amp but not into a guitar
                  Leaves the input floating in the same way as the broken jack socket did.

                  What I'm asking is, since all the other amps that I own make hardly any extra
                  noise when the input is floating, but the kustom brings on the peak led, is there
                  a fault somewhere in the circuit or is it maybe just the design / gain of the amp?

                  This is really annoying when changing guitars, as the noise is so loud.
                  Wen changing guitars always turn down the volume on the amp first otherwise you risk damaging the speaker. If you insist that is the only way for you then get leads fitted with a Neutrik "Quiet Connect" jack at the guitar end. These have a build-in shorting switch.

                  Never have an open cable. Alse see comments in post 5 - has it always been like this?

                  The design of the input stage is not great in my opinion. It has a nasty peak in the input impedance around 500Hz making it susceptible to picking up buzz. Also the low end is quite extended and in particular has a slight peak around mains hum frequency 50-60Hz. I think things would be much improved if you were to simply remove the 470K resistor R2 on the input.
                  Experience is something you get, just after you really needed it.

                  Comment


                  • #10
                    thanks nickb,
                    I try to avoid having open ended cables, the reason for posting is that the level of noise is really high even when the channel gain is really low, this made me think perhaps there is another problem with this amp, I did wonder if a pedal with a buffer in it would help but im at work at the moment so unable to try it.

                    the amp has always been like this since ive had it.

                    just noticed that the input is marked high gain on the schematic, may be its just the design and they all do it, it just seems exesive to me.

                    I will try lifting the R2 and see if its any better.
                    cheers.

                    Comment


                    • #11
                      As said above, you *always* turn volume down before unplugging the guitar end of the cable, period.

                      No excuses or justifications for not doing so.
                      Juan Manuel Fahey

                      Comment


                      • #12
                        I think this thread has started to go off on a tangent, this isn't really about swapping guitars, I just used that as an example.

                        what Im trying to say is, the level of noise that this amp produces, compared to my other amps seem excesive, which makes me think that maybe it has something wrong with it.

                        the noise is similar to what you would get from touching the tip of a lead that is plugged in to an amp, but its really loud even when the gain is on 1-2.

                        Comment


                        • #13
                          Let's not talk at all anymore about how the amp acts without a guitar connected. That will get us back on tangent.
                          What is the symptom you are trying to correct when the guitar is connected?
                          Too much noise along with the guitar signal?
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


                          Comment


                          • #14
                            I'm starting to feel as though I'm wasting everyone's time here, I apologise if you all feel the same way.

                            This amp was given to me, the input jack was faulty an the amp was very noisy with nothing plugged in,
                            I replaced the jack and now the amp works fine,

                            it is silent when nothing is plugged in,

                            with a guitar plugged in, it amplifies it and doesn't make any extra noise.

                            When just the guitar lead is plugged in there is a very loud low frequency hum/buzz and the peak led is lit, this is with the gain on 1-2.

                            This doesn't seem right to me, as none of my other amplifiers do this.

                            And my question is,
                            Does this seem right to you ?

                            Comment


                            • #15
                              Originally posted by fireman View Post
                              I'm starting to feel as though I'm wasting everyone's time here, I apologise if you all feel the same way.

                              This amp was given to me, the input jack was faulty an the amp was very noisy with nothing plugged in,
                              I replaced the jack and now the amp works fine,

                              it is silent when nothing is plugged in,

                              with a guitar plugged in, it amplifies it and doesn't make any extra noise.

                              When just the guitar lead is plugged in there is a very loud low frequency hum/buzz and the peak led is lit, this is with the gain on 1-2.

                              This doesn't seem right to me, as none of my other amplifiers do this.

                              And my question is,
                              Does this seem right to you ?
                              If it sounds OK with your instrument plugged in, it's not a problem.
                              Experience is something you get, just after you really needed it.

                              Comment

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