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  • The ol' Hum Problem

    Hello,

    I've having issues with hum in a JTM45 build from Triode Electronics (http://site.triodestore.com/JTM45LAYOUT.pdf) following their layout as closely as possible.
    • Without a cable plugged in, I can dime the volumes and the hum is negligible. With a cable plugged in, even without a guitar on the other end, the hum occurs. When I touch the sleeve of the cable, it gets quieter, but still apparent (same with touching the guitar strings). When I touch the tip, I get the nice loud pop. Moving around the cable does no effect the hum as it would if it were acting like an antenna.
    • Hum is present on both channels, but only when a cable is plugged in to the respective input.
    • The hum volume is controlled by the volume of the channel. Turning it all the way down gets rid of the hum.
    • The hum can be tailored by all the EQ knobs to get your favorite hum tone.
    • The amp is plugged straight in to an outlet with three prongs (ordered an outlet tester to see if it's really grounded). The outlet is shared with a surge protector that powers a CPU, monitor/TV and a few other random audio equipment.
    • With the surge protector off, the hum occurs. With surge protector on, but attached equipment still off, the hum gets a nice fat bass layer. When turning on CPU, TV/Monitor, etc, the hum gets more and more complex.
    • I have a 'free app' on my phone that measures frequency. With only the amp on, I get straight jumps between 175 to 301.5Hz. Same frequency with surge protector on. With the other electronic equipment on, it seems to settle around 236.9/239.6Hz with the occasional flicker to 175 and 301.5Hz. I'm guessing it's not a coincidence that ~240Hz is roughly halfway between the two extreme frequency measurements and a multiple of 60Hz.
    That's as far as I got. The house is an older rental in a crowded residential area outside of Pasadena CA, if that matters.

    I didn't have much of a chance to play the amp after building it a few weeks ago, so I can't say if this has been a problem since the beginning, or something that just happened. I had rewired my guitar and thought it was that causing it, but it was not. I also recently installed a large metal rack shelf next to the amp. I read that these things could act like antenna, but I don't get any hum changes when I move the cable towards or away the shelf.

    I'm thinking my next step is to pull V1 to see if it's coming from there as it's the only active component that comes before the volume controls. If it's not coming from there, I'm guessing it's a grounding problem, but I'm not sure what steps to take if that is the case.

    Any help is much appreciated!
    Last edited by H.R. Shove and Stuff; 05-08-2016, 06:16 PM.
    Love,
    Matthew

    ✌ ♥☺

  • #2
    Originally posted by H.R. Shove and Stuff View Post
    Hello,[*]Without a cable plugged in, I can dime the volumes and the hum is negligible. With a cable plugged in, even without a guitar on the other end, the hum occurs. When I touch the sleeve of the cable, it gets quieter, but still apparent (same with touching the guitar strings). When I touch the tip, I get the nice loud pop. Moving around the cable does no effect the hum as it would if it were acting like an antenna.[*]Hum is present on both channels, but only when a cable is plugged in to the respective input.[*]The hum volume is controlled by the volume of the channel. Turning it all the way down gets rid of the hum.[*]The hum can be tailored by all the EQ knobs to get your favorite hum tone.

    Any help is much appreciated!
    This happens with all high impedance amps. There is no malfunction here besides the open cord acting like an AC attracting antenna. You have to have a completed circuit with a guitar plugged in to prevent hum. Both the cord and the guitar have to be well shielded too. Cheap guitar cords often have less then 75% shielding and a a guitar like a fender with single coils often has no shielding at all besides the strings being grounded.

    Touching the sleeve of the guitar cord is just like touching your strings. Your body becomes a big Shielding plate which helps ground the AC radio waves. I'm surprised you don't know these basics given the fact you're into building amps. This is really basic 101 stuff.

    You may want to google up "Faraday cage" and what it does to eliminate hum.
    Since the advent of AC electricity grid the entire globe has 50/60HZ Radio waves emanating from all the power wires, especially those big Texas Towers that transport AC voltage long distance. You cant get away from it no matter what you do.

    An amps chassis acts not only as a common ground, but it also acts as a Faraday Cage which collects the EMF and safely ground it before it gets to your DC circuitry inside the amp. Having all your wiring in an amp tight against the chassis, especially the signal wires that feed your gain stages is essential in minimizing hum. The guitar cord is shielded. The shielding is an extension of the chassis that surrounds the input signal wire.

    With no load on the cable, the maximum gain is feeding the first stage. When a guitar is connected the impedance goes down as does the gain and much of the signal wire is shielded by the outer wire wraps on the pickup.

    A typical radio uses an amp for the speakers. The only difference is the antennal that feeds the amplifier is exposed to the air so it picks up allot of AC waves. The coils and caps in the radio act much like an audio equalizer does, except they are tuned much higher to filter Radio Frequency waves so only narrow bands get to the amplifier. Your AC hum is filtered out by these coils along with the broader band of waves. This way you don't get a bunch or stations being received at the same time.

    Of course it gets a bit more complex then that. AM has a carrier wave at a specific frequency the coils are tuned to and then you extract the stations from that wave. FM uses Frequency modulation instead of amplitude modulation, digital is a whole different animal of course but the basics are still there. AC waves are within the audible hearing range of 20~20Khz so its difficult to filter them without affecting the bass frequencies once they get inside the amp. This is why your have to go through the extra trouble of preventing them from getting to the signal wire in the first place.

    If your volume Knob affects the hum level, you have front end AC contamination you need to eliminate. If the volume has no effect on the hum, its usually caused by the DC power supply caps failing to block/filter the AC coming from the wall. You can also have an issue with a Power transformer emitting EMF into the circuit with its placement near the signal wires.

    If you're into building amps, a useful pro shops use is an RF generator. These generators can be used to tune radio coils, but they are also great for cleaning up an amps noise and making it quiet. You can use a probe the is essentially a steel rood and a coil around it. You hook a scope to your speaker with the amp running then you set the generator for 10HZ. You can then run the probe around to find out where AC is leaking into the amp.

    Many times when you buy electronic gear and you see transistors and caps bent over at strange angles, its to change the phase angle of the AC waves getting into the amp. "Do not go through and straighten them up to look nice" If your amp is quiet, leave it alone. It may have been tweaked by a tech with a signal generator probe to make it quiet. If it wasn't, you don't want to touch the components anyway. Bending the leads can crack a solder joint or damage the component inside, especially on older amps where the components have been heated and cooled many times.

    Like I said, this is all Electronics 101 stuff you learn in school or you gather as you gain experience working with other techs who know their stuff. Your amp is operating normally from what you've described. I'd only add, you can change how sensitive your cable is with the input impedance resistors that are usually the first thing the inputs connect to. These may be one meg resistors that can be lowered to 500K 100K or even 10K. This should reduce cable hum, but it can also affect your gain and tone. You can cross reference different amp designs then experiment to see what works best for your instrument. Get a good cable first though. Garbage in, Garbage out as they say. The amp is simply doing its job and reproducing the hum you're feeding it.
    Last edited by WRGKMC; 05-09-2016, 08:50 AM.

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    • #3
      What happens to the hum when you adjust the volume control on your guitar?
      As a human being, you come with the whole range of inner possibilities
      from the deepest hell to the highest states.

      It is up to you which one you choose to explore
      .

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      • #4
        Problem solved, ground connection on the preamp not soldered together! Thanks everybody
        Love,
        Matthew

        ✌ ♥☺

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        • #5
          "The hum can be tailored by all the EQ knobs to get your favorite hum tone." This made me laugh out loud!
          I was kicked out of music class for passing notes...
          Tuned out, turned in and dropped off

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