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Carvin Bass Preamp - change frequencies?

Discussion in 'Pickups & Electronics [BG]' started by markblasco, Jun 29, 2007.

  1. I've got a Carvin 5 string bass I built from a kit. It has a 3 band EQ, which is supposed to be bass, mid, and treble. The problem is that the bass and mid knobs sound pretty much exactly the same, as if the wrong parts were put in to determine the frequencies.

    I have enough electronics experience to change out the components in order to get a different set of frequencies, but I don't know which ones to change, and what to change them to.

    Does anyone here have any experience with one of these preamps, or have any ideas about what i will need to change? Thanks!
  2. doctorjazz


    Oct 22, 2006
    Wilmington, NC
    I can't give you any kind of specifics as I have zero experience with this, but it sounds like you've probably got the same capacitor on your bass and mid knobs. You could call and ask them about it, or do some experimenting yourself, Carvin customer service has been very helpful in my experience. Which are they both doing, bass or mids?
  3. They both seem to be giving me more bass. I don't know what frequency it is centered at - I usually just leave it at the mid point and don't worry about it, but I would like to start doing some more tonal experimenting, so changing it would help me out a lot.
  4. A9X


    Dec 27, 2003
    You'll need the schematic and then someone to calculate out the values of a cap or two, then to solder them onto the PCB.
  5. greenboy


    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    They don't sound the same. But if your rig isn't especially capable of lower audio detail because of midbass hump or fairly high cutoff I can see why you'd think that.

    Actually the midrange seems to be a fairly wide bandpass centered at around 250 Hz, and the bass control seems to be shelving below 100 Hz or so.
  6. Greenboy,

    Yes, they do sound the same. I am not running through a bass amp at all - I am running through the DI input of an API preamp into my recording rig, and have monitored through 3 different set of monitors. If I take the mid knob and turn it all of the way up, and then turn that to normal and turn the bass knob all of the way up, it sounds pretty much identical. I have had several recording engineers listen and all of them agree that the two knobs are pretty much identical.
  7. I found this....


    what is the number stamped on your circuit board? is it 501B?

    by the looks of the schematic, the mid control is a plug in to the preamp (looks like they build the same board for a 2-band and a 3-band and just wack in the mid)...is your board like that?

    It'll take me a little time to sort out the frequencies here...anyone a little more experienced that can do it quicker?
  8. greenboy


    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    Sorry, Mark - I skimmed and didn't realize we might be talking about an improperly manufactured one. On my '98 Carvin preamp it's easy to hear the difference.
  9. It's all good. I thought it might have just been me for a couple of years, but several other people have said that they sound nearly identical, so at this point I am sure it is not just in my head.

    PilbaraBass, I will check the serial number on it tonight to find out what the model is.
  10. fivestringer555


    Oct 26, 2003
    Mark- I have the same problem with my LB75F, so you're not crazy! The bass and mid sound slightly different, but not much. Not to mention very little boost/cut on all the eq. Overall, it pretty much sucks! The vol/pan PCB has 1997 stamped on it. I think they've upgraded since then, but I'm not sure. I'm curious to see what you come up with!
  11. Ok...i've crunched the numbers...I hope I'm right...

    I'm basing all of this off the circuit diagram, not even sure yet that this even is your circuit, yet, mark.

    But anyway:

    the frequency for the lows is 98Hz
    the frequency for the hights is 8202Hz

    the frequency for the mids is 161Hz!!! ok...that's just a little TOO close to bass, IMO...300 - 400 Hz would be more like it...

    If it's possible, try replacing R10 on the circuit w/ a 1K resistor...this will give you a mid frequency of 301Hz...

    The filter designs are T-filters through a single op-amp for all 3 bands, so: (1) there's a bit of interaction between tone pots, (2) you won't get much filtering effect (gain)

    Gain for the Bass Boost is 1.1
    Gain for the mid boost is 1.3
    Gain for the highs is 35

    Note: by changing R10 to 1K, mid boost gain is increased to 5
  12. fivestringer555


    Oct 26, 2003
    Didn't mean to hijack the thread, but I'll give it a try! I'll let you know how it turns out. Thanks, mate!:D
  13. A9X


    Dec 27, 2003
  14. you're not hijacking the thread...you're sharing in the same problem...

    hijacking is when you take a thread completely off topic...(something I have a bad habit of doing, myself...:bag:)
  15. 555,

    Please feel free to take as much space here as you would like with your experience. With the holiday tomorrow and a busy week ahead of me, I don't know if I will get to doing anything for at least a few days.

    I just hooked up my bass rig for the first time in years (I have been exclusively using my bass direct into my recording system), and I must say, I am surprised at how good my bass is sounding. It is running through an SWR 300watt head and a 4X10, and I think I am going to start micing the cab for my rock recordings in addition to the DI. I am having so much fun playing it that taking it apart to futz with the preamp has moved down my priority list.
  16. fivestringer555


    Oct 26, 2003
    Ahh... okay!

    Well, I popped in that 1K resistor at R10 and it pretty much did what you calculated. Now the mid sounds more like midrange! You can definitely tell it apart from the bass when tweaked. Not bad for $.99(for a set of 5- still have four left). Mark should have no problem doing this when he gets a chance. Now if we could move the bass down to 60 and the highs to 5K!:D

    Thanks, Pil!
  17. Ben B

    Ben B

    Jul 13, 2006
    San Diego, CA
    I have the 501b in both of my Carvin basses. I think the frequencies quoted by PilbaraBass are fairly close. I can hear a definite difference between the bass and mid, it isn't a huge difference.

    When I first got my SVT3Pro, I compared the sound changes of the graphic EQ on the amp to sound changes made by the bass and mid on my basses. What I found was moving the 80Hz slider on the amp had about the same effect as the bass control on the basses. Moving the 150 and 300 Hz sliders on the amp was about the same the mid control on the basses.

    Moving either but not both the 150 or 300 Hz sliders didn't sound like moving the mid knob on the bass. You had to move both sliders together. So, if I had to take a guess (based on my ears only, not scientific measurements), I would say the mid control on the guitar is a little above 200 Hz and the bass control is a little under 100 Hz.

    If the OP can't hear the difference, the unit might be defective.

  18. Ben,

    Although your tests aren't very scientific, they certainly are methodical, and extremely practical....after all, it is what we HEAR that is important anyway...Well done!

    it's nice to know that my calculations weren't total hogwash and actually stood up to an empirical test...

    the calculations are basically quick and dirty and are taken from the stand point of the tone controls being wide-open, on this type of tone-control (similar to what you find in a big muff) the frequencies slide as you change the boost and cut.

    I agree that the difference between the bass and the mid controls should be audible (we're talking almost a full octave here)....However, it is interesting that many other 3-band preamp builders use significantly different voicings to this.... for instance, the Aguilar OPB-3 uses 40Hz for bass, 400Hz or 800Hz (selectable) for mid, and 6.5KHz for treble...the OPB-2 is voiced at 40Hz and 4KHz, which is the Same as the Sadowsky (the sadowsky is boost only, however, and the Q's are different)...

    I'm just making a suggestion for an easy way (at least I think easy, I still need a photo of the mid-module), to move the mid frequency up to the 300Hz range.

  19. I was looking for the specs on the 501B...couldn't find anything...but I did get a photo...


    locating the resistor and replacing is a straight-forward task...
  20. You people here are great!! I just replaced the R10 resistor with a 1K part, and now there is a HUGE difference. I actually get midrange adjustments! I am a happy camper.

    I am wondering what values you would need to get a range closer to 800Hz?
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