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Detailed info on simple volume mods on SBMM/SUB basses?

Discussion in 'Pickups & Electronics [BG]' started by nickap, Jul 21, 2019.

  1. nickap


    Oct 16, 2016
    Hi all, I have a SUB Ray4 bass which I really like, except for the often-reported overly high output level, which can cause distortion through some amps. I've been searching for options to reduce the output level without changing the tone (which I'm very happy with). On the forums there are various mentions of things like "the volume mod" but after much searching on this site, and on the Music Man forums, I've yet to find detailed information on exactly what the volume mod is and how to do it. Can someone please let me know what my simplest options are? Links to wiring and/or instructions for the mod(s) would be perfect, if they exist. I know that I could replace the entire preamp, but as I really like the sound as it is, I'd prefer to explore other options first. Thanks!
  2. nickap


    Oct 16, 2016
    Okay, immediately after posting this I did find instructions on the volume-after-preamp mod here. But I would still love to get any other feedback or suggestions about this and other basic mods that anyone is willing to share!
  3. Firstly I'm surprised your bass is so hot. Both the 2 and 3 band mm pre circuits have no extra gain when the tone pots are centred. Perhaps the pickup is wired in series for your model ..?

    Anyway as the vol pot is on the output, it's a simple mod to attenuate the output. You need to find the wire (or circuit board track) that runs from the opamp to the clockwise lug of the vol pot. You need to cut this wire (or track) and put in a resistor in line. For 6dB of attenuation, use a resistor that is the same or close to the value of the pot. For 3dB, go with a resistor that is around half the value of the pot.

    If you post some nice clear photos of the electronics on your bass, I might be able to be more specific.
  4. OK, I just saw that pdf in the link from that old thread. Simply swapping the middle and clockwise lugs on the volume pot won't work, and actually may damage the preamp. With it wired this way you are shorting the opamp output to earth when the vol is turned all the way down. This is a problem especially for the 2 band as iirc, the lm4250 has no output short circuit protection...
    Last edited: Jul 22, 2019
  5. nickap


    Oct 16, 2016
    Good to know - thanks! I'll post a photo of the actual electronics as soon as I can - I've been assuming that it's a stock setup, but I got it used so it's even possible someone has been messing with the electronics already.
    RobbieK likes this.
  6. Yahboy


    May 21, 2008
    There are different preamp version on sub ray 4. Mine is new headstock decal model, I found the preamp are different than earlier one. The later one are same preamp use on Ray25 and cone with B25K volume pot.
  7. nickap


    Oct 16, 2016
    Looks to me like I have the original preamp and wiring - the volume pot is 500K, and it is wired before the preamp.
    20190721_204103. 20190721_204119.
    Yahboy likes this.
  8. Yahboy


    May 21, 2008
    I believe your sub ray 4 are not Indonesian version, because indo made Sterling sub has no side input jack and no rear cavity cover.

    Mind share some photo from your sub ray 4 ?
  9. nickap


    Oct 16, 2016
    So after a little research I'm thinking I actually have a SUB SB4, not a Ray4. As far as I can see though, the electronics are identical ... And the volume issues appear to be the same; what I've experienced is exactly what others have reported about the Ray4.
    Yahboy likes this.
  10. nickap


    Oct 16, 2016
    Apparently the SB4 does have the pickups wired in series, while on the Ray4 they are parallel
  11. Interesting. That doesn't look like the famous 2 band stingray preamp circuit. Plus the vol pot is wired backwards.

    Who knows?.. I'd measure the resistance of the pickup. If it's more than say, 6k, then it's almost certainly series wired.

    Anyway, to attenuate the output, you could shift the vol to the output of the pre. Use a 10k or 25k pot, but as I mentioned, use a resistor the same value of the pot in line with the white from the pre to the pot. This will give 6dB attenuation when the vol is full.

    Or if you change to a post-pre vol you could also attenuate the pickup signal before the pre with a couple of Resistors wired as a voltage divider. A pair or 220k's would work.

    You could also leave the 500k pot where it is, but use a couple of resistors on the output as a voltage divider. A pair of 10k's would work.

    Another option is to use a second small pot or trim pot on the output and mount it in the cavity. (10k to 25k linear taper) then set this where you like. Half way will give 6dB.

    I guess a final option would be to rewire the pickup to parallel. But you'd probably have to disassemble the pickup. Plus it will significantly brighten the tone.
  12. nickap


    Oct 16, 2016
    Thanks, this is really helpful. Re. the pot mounted in the cavity, I have a spare 25K pot but it is audio taper, not linear. I'm guessing I could use that, but it would just make it a little harder to really fine-tune the output, is that correct?
  13. GilmourD


    Jul 11, 2010
    Rutherford, NJ
    If you wanna hear the difference between the preamp before the volume fix and after the volume fix:


    Attached Files:

    Zoobiedood likes this.
  14. If you plan to use your 25k pot as a second, post-pre attenuator stowed in the cavity, then yeah audio taper is not ideal. Linear will give you fine adjustment. In this context, as I mentioned a linear pot only gives 6dB at half way (depending on the input z of the amp or whatever you are plugged into).

    Otoh, if you are planning to replace the existing pre-preamp vol with your 25k pot wired on the output, then an audio taper is ideal. In fact it is how the regular 2 and 3-band stingray preamps are wired.
  15. nickap


    Oct 16, 2016
    So I finally found time to work on my bass ... tried a couple of different options, and recorded the bass at each point so that I could compare.
    -- First I tried simply replacing the volume pot with another 500K one, audio taper, which as expected made no change to the sound - it also didn't provide any better control of level, suggesting that the one that was faulty was also audio taper, not linear.
    -- I then did the volume mod, replacing the 500K pot before the preamp with a 25K pot after the preamp. This provides much smoother control over the level. It did affect the sound, very slightly - the bass has a touch less low end after the mod. I actually like this better than the original sound! - it retains most of the low end but also accentuates the "growl" of the bass.
    -- Finally I soldered a 5K trim pot right before the output jack, which allow me to control the overall output level. I don't hear any difference to the sound after this mod (other than level of course) so this appears to be doing exactly what I want - bringing the level of the bass down so that it doesn't overdrive the amp input, but without affecting the overall sound/tone of the bass. I may still want to adjust this pot a bit -- the output may be TOO low at this point -- but it works well and doesn't affect the sound.
    So overall I'm feeling very good about the mods ... :)

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