How to use/set oscilloscope to track down weird overtones in plexi build

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, May 20, 2020.

  1. itsGiusto

    itsGiusto Tele-Meister

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    Hi all, I'm trying to use my new oscilloscope to track down why my plexi build has weird, spitty, modulating overtones (see https://www.tdpri.com/threads/not-sure-if-my-plexi-build-has-problems.984480/ or ).

    How would you go about trying to track down the problem? I figured I could try to probe the circuit and listen for where I hear the overtones, then maybe try to probe that location with the oscilloscope to see if I see anything weird. How do you set an oscilloscope to be able to visualize electric guitar audio, without it looking chaotic?
    I also figure I might FFT it to see if anything weird shows up on the frequency spectrum.

    I also figured I might probe all locations where power is filtered, to see if I see anything abnormal about the DC, since this sounds maybe like crossover distortion.

    Any other ideas?
     
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  2. Peegoo

    Peegoo Friend of Leo's

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    The best way is to use a signal generator to send a signal through the amp, and when you see the original 'clean' sine wave change profile, you know the section you're testing is where the noise is entering the signal. You also need the schematic so you know where to test for signal in each stage of the amp.

    Does your new Rigol scope have a square-wave source built in? Some do. If so, you can use that as your signal source.

    Here's a quick 'n' dirty (but good) look at signal tracing through an amp.

     
  3. kbold

    kbold Tele-Afflicted Silver Supporter

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    ^^^ Peegoo's got it.
    You need a reference: an audio signal like a song or your guitar is not a good reference.
     
  4. Commodore 64

    Commodore 64 Friend of Leo's

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  5. clintj

    clintj Friend of Leo's

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    With a signal generator with a frequency selector, you can try various frequencies while listening and see if you can find one where you hear the noise you're looking for. Because of the amp's EQ curve, not all frequencies will provoke issues.

    Once you can get the amp to start making the noise, start tracing with the scope to see where the sine wave starts looking weird. Make sure you zoom in a little and look at the peaks, as your noise may be hiding in there.

    Sent from my Pixel 2 using Tapatalk
     
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  6. sds1

    sds1 Tele-Afflicted

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    Anyone hear what he is talking about? Kinda that swirly phaser sound in heavy (power tube??) distortion? I hear it in all different amps, normal or no?

    I don't particularly care for it either.
     
  7. itsGiusto

    itsGiusto Tele-Meister

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    I'd love to hear what people think about that sound, and if they know where it comes from, and especially if it's supposed to be there or not (I don't think it is?). I'm not sure its from the power tubes, because I tried tracing the signal and listening to it, post phase inverter and pre power tube, and I THINK I still heard that sound there, prior to the power tubes.
     
    Last edited: May 20, 2020
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  8. itsGiusto

    itsGiusto Tele-Meister

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    Okay, i'll definitely watch the video when I have a moment, and try generating a signal to send through. The reasons why I wasn't sure that was the best path forward is that I AM expecting some distortions - I want the amp to distort, but in the right way, not the wrong way. I know what that sounds like on a guitar, but not what it looks or sounds like on a sine wave, so I'd be unsure how to detect it.

    Yeah, the Rigol scope can generate a 3v square wave (1hz?). I'm not sure it's changeable though. Also, if I'm looking for distortions, not sure that a square wave would be good, since it's already significantly clipped? Maybe I could invest in a signal generator of some sort. Is there any reason not to just use a phone-app for signal generation?
     
    Last edited: May 20, 2020
  9. Nickfl

    Nickfl Tele-Afflicted

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    Have you eliminated the speakers as potential source of your problem? Have you tried the amp with different speakers and speaker witha different amp?

    The sound of your video reminds me of an issue I had with a build that turned out to be a Weber 10" alnico speaker that was bad from the factory with voice coil rub that only showed up when the amp was overdriven. It drove me nuts for months trying to track it down until I figured out the symptoms followed the speaker and not the amp.

    Between the time it took to finish the build and figure out that the speaker was bad it had been about a year since I bought it and by the time I got around to bringing it up with Weber technical support they didn't even bother replying to my email...
     
  10. sds1

    sds1 Tele-Afflicted

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    Cool, am I hearing it right you think though? In the video it's subtle but I think that's what you are hearing. If so then it's definitely an overdrive thing... when it gets thick and syrupy you get that swirly effect. I don't like it but much like fizz it's one of those things that will disappear in a mix. So not fatal in severity per-se, but I'd love to know how to avoid it.
     
  11. itsGiusto

    itsGiusto Tele-Meister

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    Yeah, I've tried with multiple different speakers, same problem.
     
  12. itsGiusto

    itsGiusto Tele-Meister

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    Yeah, I think you're hearing it right. I could see how it sounds like a phaser.
    I was thinking myself that it sounds like crossover distortion from a fuzz, or maybe an octave fuzz. It kinda sounds like a fuzz is on all the time to me, very chainsaw-style distortion. Not more smooth, like I would have expected this amp to be.
     
  13. Commodore 64

    Commodore 64 Friend of Leo's

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    Again: https://www.newark.com/tenma/72-505/audio-signal-generator-frequency/dp/66F3575

    $39, you can change the amplitude of the signal, sine or square, and choose many different frequencies.
     
  14. itsGiusto

    itsGiusto Tele-Meister

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  15. Nickfl

    Nickfl Tele-Afflicted

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    That's what I do. I have a free phone app called "function generator", let's you select all the relevant variables. Some people find a standalone unit more convenient or worry about damaging their phone or computer. I only occasionally use it, maybe a couple times a year, so the phone app works fine for me.
     
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  16. Commodore 64

    Commodore 64 Friend of Leo's

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    Last edited: May 20, 2020
  17. The Ballzz

    The Ballzz Tele-Afflicted

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    What a great thread! :cool: Thanks to all who shared. Lot's of great, well thought out questions and lots of clear information, answers and tips from all! The differential trick, using both channels is pretty cool and its new to me! I can see all sorts of uses for that.:D
    Thanks Again To All!
    Gene
     
  18. itsGiusto

    itsGiusto Tele-Meister

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    Well, here's a reason maybe to use a signal generator - there definitely seems to be a lot of noise mixed in with the signal coming from my phone! It was messing with the triggering function of my oscilloscope, I had to have it trigger on only the low-pass filtered signal. I think there was probably a lot of quantization error, either in the production of the wave, or in the oscilloscope's capturing of the wave, because the phone's output audio signal was so weak - the wave often looked very "boxy", or like lots of little steps instead of more continuous.

    While I'm waiting for a 100:1 attenuation probe to come on Saturday so I can probe the high-voltage areas of my amp, I've been running sine waves through distortion pedals, so I can get an idea of what the distortion is supposed to do, and what it's not supposed to do, to hopefully inform my ability to diagnose the problem in my amp later. It's so fascinating seeing how the waveforms and fft spectra are impacted by these pedals!!

    The tubescreamer turns an 800hz sine wave into a triangle wave if the gain is rolled down, and a square wave if it's rolled up. The boss blues driver asymmetrically shapes the sine wave into something that looks like the top part of a "c". A tycobrahe octavia turns the sine wave into a square wave, if the octave is not engaged, and if the octave is switched on it seems to kinda take the center of the wave and jam it up into the top part of the wave.
     
    Last edited: May 22, 2020
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  19. elpico

    elpico Tele-Holic

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    Does your phone sound okay playing music over the earbuds or plugged into the line-in of your car or stereo? If so the problem isn't that you're using a phone. (I use one all the time for this). A phone can drive any input a guitar can, and up to a much higher level than a guitar.

    edit: just to make sure, you've got both volume controls turned up right? The phone's system volume needs to be turned up with the buttons on the the side and then whatever app you're using likely has an on screen control of it's own. Both need to be turned up to get ~1Vac out of the headphone jack.
     
    Last edited: May 22, 2020
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  20. Commodore 64

    Commodore 64 Friend of Leo's

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    I also like the discrete signal generator because I can clip the lead on to any grid in the preamp and PI to isolate that stage and look at it.
     
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