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Help ! : ) Which resistor to pad down out of control uke bass output?

Discussion in 'Pickups & Electronics [BG]' started by spigmu, Dec 7, 2017.

  1. spigmu


    Mar 25, 2009
    Brooklyn, NY
    I've got a Rondo Hadean UKB-20, solid body active preamped piezo bass electronics, that clips like Jack Bruce at the lowest possible level and Lemmy at 5% higher. The level is far too hot at any level but the clipping is happening before it leaves the bass, not overloading the output. Rondo has suggested a 50k resistor between two the pots to raise resistance (pots are 50k). My question is: will this give me resistance where I need it? Enough? What ohm and watt resistor is proper? I've been looking it up but am only finding info re magnetic pickups not piezo.

    Thanks for any advice!! : )

  2. lz4005


    Oct 22, 2013
    Rondo should be suggesting giving you a refund for selling you a defective instrument.
    Get rid of it and find something similar that works correctly.
    line6man likes this.
  3. spigmu


    Mar 25, 2009
    Brooklyn, NY
    I waited far too long to deal with it, past their policy. I spent too much time troubleshooting hot output clipping input stages of what it was plugged into, and then put it in the closet and practically forgot about it. Rondo has been responsive responding to emails. Their suggestion was to add resistance between the pots with a 50k. Just looking for a way to get it to $175 beater bass status as my xmas gift to myself this week.
  4. Instead of a fixed resistor, use a trim pot. That way, you can adjust the resistance until you get the result that you are looking for.
    two fingers and Matt Liebenau like this.
  5. +1 to line6man. If they're saying 50k maybe get a 100k trimpot and have some room to adjust.

    No worry on the wattage. The pickup and preamp aren't putting out enough power to damage any trimpot you might put in.
  6. spigmu


    Mar 25, 2009
    Brooklyn, NY
    Thanks all! Off on a trimpot safari tomorrow! I am in the mood to learn more (as in doing it, not reading about it) about sticking resistors in things. And my soldering skill is total poopie, and since one of last years resolutions was to get better enough to trust not shorting out what I'm trying to fix, this is motivation to hone that.
  7. spigmu


    Mar 25, 2009
    Brooklyn, NY
    I'm sitting here with my noob multimeter, noob soldering iron and 100k trimmer, and I thought it would be easy to put it between the pots, but there's nothing going pot to pot so I'm stalled. The pot wires all go under the preamp fabric cover, whether they connect to it or not, so I have to undo it to find out what goes where anyway, but before I ruined whatever order there was under there I wanted to post a pic and see if I could get any sympathy guidance. Thanks : )


  8. So it has vol, and active treble and bass controls, right? And they are all 50k? treble and bass are B50K? Vol A50K?

    With a piezo pickup, it will (or should!) go straight into the preamp. So this means that the volume pot is probably on the ouput, after the buffer/gain/active tone. So I'm not really sure how this would help as you have said that the pre itself is clipping. OTOH, I guess it could be between two stages of the circuit... Without removing the heatshrink and looking, it's hard to say.

    Anyway, without actually knowing the circuit, I'd say try removing the wire going to the clockwise lug of the volume pot, (white in the pic I think?) then put your resistor inline with this. You can't really get 50K resistors, but either 47K or 51K should reduce the signal by around 6dB. Having said that, your 2nd pic looks like there's a big blob of solder on that lug, so I'm not sure. That usually means it's earthing the pot casing...?


    Hang on, is this a left-handed guitar?
    Do the pots work anti-clockwise? If that's the case, for a start it should have a C50K volume pot. That might be why it's too loud through the pot's sweep. (Although this won't effect the pre volume when fully turned up.) Do the active treble and bass controls also work ant-clockwise?

    OK, if that's the case, I'd replace the volume pot with a C50K pot (or another B50K, if that's all you can get) instead of the A50K. Then I'd put the resistor inline with the anti-clockwise lug of the volume pot. (Red wire in your pic)...
    ex-tension likes this.
  9. spigmu


    Mar 25, 2009
    Brooklyn, NY
    RobbieK, thanks so much for taking the time here : )

    In my fear of ripping connections out when moving the preamp I left it to cover the third pot : / So it's tone/tone on the left, both B50K and vol on the right A50K. And yes! it's lefty (face palm) : /

    The tones are a bass boost and high mid boost, both of which send the level further into unusable stratosphere. You can only use either at 10% up but for all practical purposes they stay off, especially given that I've been trying to troubleshoot the clipping.

    Since my guitars and basses run the gamut of pots turning every which way I'll check : )

    Thanks again!
  10. spigmu


    Mar 25, 2009
    Brooklyn, NY
    All three turning counter-clockwise increases gain/tone boost. Clockwise decreases vol pot until off and tone pots until flat.
  11. byacey


    May 16, 2008
    Alberta, Canada
    A piezo can develop several volts of output, depending how hard you're playing. The preamp really doesn't need to produce much gain with such a hot signal, so a FET input unity gain buffer is more useful in this instance. If the preamp is introducing much gain, with a 9V battery it's probably exceeding the maximum 3 volt RMS signal at clip. If the battery is weak, the situation further deteriorates and the onset of clipping will be even lower.
  12. Then yeah, I'd replace the vol pot with C or B taper, and try puting a resistor inline with that red wire.

    If the pickup is overloading the input of the pre though, this won't help. But it's simple to do and worth a try.

    As Byacey mentioned, a pre designed for a piezo pickup will usually have a very high input z, most often done with a fet. So there's no practical way of padding the pickup at the input of the preamp without altering its tone and response quite significantly. And unlike a magnetic pickup, you can't just lower it of course!

    With a bit of luck, that volume pot is wired after a hi z input buffer (with no gain), and the gain in the circuit is later. But knowing these type of circuits well, I wouldn't hold your breath.

    If this was on my bench, I'd run a test tone into it and use a scope to see the gain. There's no real reason for a pre like this to have a lot of boost so I'm not sure why it has such a hot output. The pre might simply be faulty.

    FWIW, also be aware that I've come across a few piezo pickups that buzz and fart and give a distorted sound not from overdriving the preamp, but actually from a poor installation. You may have done this, but I'd certainly remove the bridge and check that there is no splinters of timber or other fine debris in the saddle slot.
    ex-tension likes this.
  13. spigmu


    Mar 25, 2009
    Brooklyn, NY
    OK, everyone is off playing with their presents or otherwise leaving me alone to finally tear into the thing!

    If anyone has a moment and would be kind enough to answer these two questions I would be eternally grateful : )

    I found a few resources for how to do these two simple things with a mono jack but not with the stereo jack currently there. I want to just cut the current preamp out of the equation for now. So...

    1) How would I wire the piezo passively to the volume pot and the stereo output jack?

    or, barring that,

    2) How would I wire the piezo directly passively to the stereo output jack?

    In both cases I'll use an external preamp for now and not worry about putting one in the cavity until I can decide if it's worth it and settle on a good candidate.

    Thanks. Very appreciative of any direction on this : )

  14. That new pic is a bit clearer...

    So the wire from the middle lug of the volume control (black?) goes directly to the tip lug of the jack, right?

    If that's the case, then as I suspected, the volume pot is simply an output attenuator for the pre and padding the signal at this point would be closing the gate after the horse has bolted.

    Unfortunately, padding the signal at the input of the pre will almost certainly alter the tone quite radically. Piezos are very hi z devices (they have an infinite DC resistance) and need to see a very high z.

    It looks like you could easily fit another battery in there, and it's a simple job to wire a second in series with the other. This would give you more headroom. However I'd check the parts (electro caps especially, and maybe the opamp) are rated to 20V. With surface mounted electronics, you'll need a magnifying glass!

    Another option would be to mod the circuit. If you know what you are doing, this is likely a very simple job. Most likely you only need to short the feedback resistor at the first opamp stage. (That's only an educated guess, though.)

    Another option is to make a separate input buffer and put an attenuator between this and the pre. This would most likely be the approach I'd take. A very simple circuit - 5 parts - would do it. Also, it would probably improve the tone and dynamics of the bass a great deal. It's very common that budget instruments like this have a generic preamp designed for magnetic pickups, that doesn't have a very high input z. I've modded many acoustic preamps this way.

    If you aren't that handy with the soldering iron, I'm sure you could buy a little fet buffer from somewhere, then mount a trimpot on it. More than likely, this will give you a really nice sounding instrument.

    I'd recommend against running a piezo passively with a volume pot. Even a 1meg pot will greatly reduce the pickup's output and make it sound very average, tonally. You could try a 2.2Meg pot, I guess, but get a linear (B taper) one, otherwise it will act like an on-off switch unless you feed a preamp with a very high input z. Be aware, that passive piezos are prone to hum.

    Well, yeah, I guess that's an option, but try to use as short a cable as possible between the jack and the preamp. And use a good quality cable, and a pre with a nice high input z.

    I'd buy a little 2.5mm socket like this and wire it inside the guitar. Tip to tip and sleeve to sleeve. Leave the ring connector from the original jack disconnected (it was for the battery). Keep the wiring short or use cable ties etc, so that there's no chance the metal parts of the 2.5mm jack can short on the tip of the original jack.

    I'd also think about wiring a mute/kill switch.
  15. You won't be able to make an accurate assessment of piezo preamps this way. In short, the piezo element is going to sound like crap if it's driving instrument cable, due to the capacitance of the cable. And it's also going to sound like crap with a volume pot, unless you get the value right, as RobbieK mentioned.
  16. Ha. Yeah true. Well it'll still sound pretty average with a 2.2M pot, but that's probably the best compromise if you need an onboard vol with a passive piezo.

    I guess it really depends on what @spigmu wants/expects from this instrument.

    I suppose you could also route for a pickup, and try to find some strings that work...
  17. spigmu


    Mar 25, 2009
    Brooklyn, NY
    Thanks all for the good information. Really appreciate it.
  18. spigmu


    Mar 25, 2009
    Brooklyn, NY
    Thanks, RobbieK. Great video : )

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