Extension Speaker Hum Problem

Discussion in 'Amp Tech Center' started by average_picker, Nov 1, 2019.

  1. average_picker

    average_picker TDPRI Member

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    Hey folks. I've been away a while, though I generally read way more than I post anyhow. I'm back to ask your assistance with a bad hum issue.

    I just finished building a speaker extension cabinet from scratch, first time trying it, happy enough with the end result.

    However, to the issue:

    I mounted a brand new Cannabis Rex. I'm trying to run it from the "Speaker Output" jack on the back of a Fender Bassbreaker 007 combo listed at 7 watts, 8 ohm min.

    With extension cab plugged in, everything seems fine for the first 10 to 20 seconds after turning on the amp. Then a loud, obnoxious hum swells up. My phone app says the hum is at 65.8 Hz. The hum is not present through the stock speaker when the extension cab is unplugged.

    So far I have tried a ground lift plug on the wall, two different cables between amp and cabinet, swapping the + and - leads from the jack to the speaker itself. Volume and gain controls have no effect on volume or pitch of the hum.

    I'm planning to take it to church tomorrow where I have my Princeton Reverb RI stashed and try it with that.

    Music makes sense to me, electronics do not, but I'm trying to learn.

    Help me out folks, what do I need to learn here?
     
  2. Dacious

    Dacious Poster Extraordinaire

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    Tried teaching it the words? Sorry, somebody had to....

    :lol:

    Internal speaker does not hum, external does - faulty speaker possibly. But faults usually mean no sound or cone cry or voicecoil rub.

    However, I have to say - what you're hearing is probably there with the little speaker but that is less apparent with the dinky inefficient stocker. Single ended amps - more hum.
     
  3. Paul G.

    Paul G. Friend of Leo's

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    Are you using speaker cable and not an instrument cable?
     
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  4. average_picker

    average_picker TDPRI Member

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    Using instrument cable, same as from guitar to original amp.
     
  5. wabashslim

    wabashslim Tele-Holic

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    Are you running the extension AND the internal together? Impedance mismatch.
    A speaker cannot make hum on its own. It has to come from the amp. Grounding has nothing to do with it. But the extra speaker load is probably putting a strain on the marginal power supply, drawing more current than it's designed for. Can you disconnect the internal speaker when you plug the extension in? Is the Rex 8 ohms?
    That's my guess...
     
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  6. average_picker

    average_picker TDPRI Member

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    For more info, upon warm up I get the usual tube amp "hiss" which is fine and expected. But the problem hum comes a little after that yet.

    C-Rex is 8 ohms, 50 watts.

    The Speaker Output jack is supposed to mute the internal speaker on the 007.
     
  7. Milspec

    Milspec Friend of Leo's

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    Just unplug the combo speaker and run the speaker cable to the extension cabinet (as long as it is the correct impedence). I have seen many amps with bad extension sockets or ones that just don't jive with the onboard speaker in the combo.

    I am reminded of the exact opposite effect on a Epiphone Electar combo that I have. Hums like a banshe with the combo speaker, but when the extension cabinet is plugged in (combo speaker does not cancel) both speakers are very clear...no hum.
     
  8. average_picker

    average_picker TDPRI Member

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    Well, the stock Celestion Ten 30 appears to be hard wired into the chassis. And I'm not willing to pull that apart yet.

    I did disconnect the stock speaker wires at the speaker end. However, the problem persists.

    I'm unable to upload my recording of the issue, unfortunately.
     
  9. Milspec

    Milspec Friend of Leo's

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    My guess is the extension jack has an issue. Most people never use them and they sit idle for years. My personal mantra is that everything falls apart when not used. Spray some deoxit in that extension jack and insert the cable several times to see if that helps. Might just be idle time and dust as the flaw.
     
  10. average_picker

    average_picker TDPRI Member

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    I unplugged the leads from the stock speaker and patched them to the leads to the C-Rex. And the problem still occurs.

    So even straight from amp to new speaker something is still wonky.
     
  11. average_picker

    average_picker TDPRI Member

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    More specs if it helps:

    - The jack is a Switchcraft TS2 Mono
    - Wiring is Pro Co 12-2 Stage Speaker Wire
    - Connectors are from the hardware store, and I soldered them onto the wiring so as not to work loose.
     
  12. Milspec

    Milspec Friend of Leo's

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    Can you try another speaker cable? It would be nice to rule out that variable.
     
  13. average_picker

    average_picker TDPRI Member

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    I can try tomorrow with another amp. But since the issue occurs even when essentially wired straight from amp to new speaker I'm curious what a different cable might do?

    Or are you thinking I should try re-wiring my cab?
     
  14. Paul G.

    Paul G. Friend of Leo's

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    You can't use instrument cable to connect a speaker. At best, you will get hum, at worst the capacitance will cause damage to your amp at high volumes. Get a speaker cable (unshielded), and use that.

    The two cables have completely different construction and are not interchangeable.
     
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  15. Paul G.

    Paul G. Friend of Leo's

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    YOU CANNOT USE AN INSTRUMENT CABLE TO CONNECT A SPEAKER TO AN AMPLIFIER!

    What else can I say?
     
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  16. average_picker

    average_picker TDPRI Member

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    Ok, yes, I understand to try using a speaker cable.

    However, I literally bypassed the speaker output jack completely and wired the new speaker directly to the amp as if it were the only speaker. Wired direct from the amp to the new speaker. No jack or external cable involved.

    See post #10. And #13.

    Even wired direct from amp to speaker I had the same problem.

    So, why the all caps?
     
  17. Paul G.

    Paul G. Friend of Leo's

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    The wiring to an external speaker is simple, 2 wires, one to each terminal. Tip should be +, sleeve -, but if reversed you only change the direction of motion of the speaker, nothing else changes. A faulty speaker will usually not hum. What did you use to wire the cab? Should be unshielded, 2-conductor 16 ga. or better. All cabling to speakers must be 2-conductor, unshielded. Since another speaker hums as well, it's not the speaker. What kind of wire did you use to directly jump to the C-Rex?

    By leaving everything else alone and using the correct cable, you could have rooted out anything having to do with your cabinet or speaker. If you used the cab at volume with an instrument cable, you may have damaged your amp. If you wired your cab with shielded cable, you may have damaged your amp. If you jumped to another speaker with shielded cable, well, you know.

    If you're not familiar with electronics, you shouldn't have gone directly to soldering.

    Now, am I understanding the internal speaker does not hum? Even now, after running the amp with the guitar cable? If so, you may have gotten lucky.


    Hum is usually caused by a bad (or no) ground, or a ground loop. Make sure the sleeve of your external speaker jack (on the amp) shows continuity to the chassis. If it doesn't, loosen it, wiggle it and retighten to try to get contact. If it does show continuity, the problem is internal and should be looked at by a tech.
     
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  18. average_picker

    average_picker TDPRI Member

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    I used 12 gauge stage speaker wire in the cab.

    The hum present in the new speaker is NOT present in the stock speaker.

    I jumped the amp to speaker by directly connecting the amp wires out to the cab wiring in.

    I soldered the connectors to the wires, NOT the wires to the speaker. Nothing was permanently connected.

    Internal speaker on amp does not hum even after all my experimenting and trouble shooting.
     
  19. average_picker

    average_picker TDPRI Member

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    First off, thanks all for the thoughts and help so far.

    Tested the C-Rex cab connected via legit speaker cable to my Princeton Reverb. Ran great! No issues during that test.

    Connected C-Rex to the 007 with the same speaker cable and the hum issue arose again. So, it's something in that amp.

    Stock internal speaker on the 007 still does not have the issue alone as when the extension is plugged in.

    Aside from a loose speaker output jack / ground issue there, anything else I or a tech might look at to narrow things down?
     
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