Likes Likes:  0
Results 1 to 6 of 6

Thread: SD STC-3P blend center volume drop with SPB-4 and Apollo Jazz

  1. #1
    Junior Member
    Join Date
    Dec 2017
    Posts
    4

    Default SD STC-3P blend center volume drop with SPB-4 and Apollo Jazz

    Hi everyone! Read so much information on this board, but first post here... I've tried Googling and searching the forum, but didn't find any post fit in my situation... I also tried SD "Contact Us" but so far haven't got any feedback.

    I have SPB-4 Steve Harris precision pickup and Apollo Jazz bridge pickup installed on a Fender American Deluxe P Bass together with Seymour Duncan STC-3P passive-voiced preamp/EQ, and did active/passive switch plus "tone after active" mod based on this: https://www.seymourduncan.com/blog/t...he-stc-preamps

    As for pickup lead wiring, I used standard wiring diagram found on instruction paper (white => hot, black/bare => ground). Now I find in passive mode, when blend pot is in center detent, volume drops significantly. I estimated a 9 dB drop as my DAW channel meter goes from about -18 dB on either single pickup to about -27 dB when blend in center. However, in active mode this doesn't seem to be too much of a problem (only about 3 dB different in active mode).

    Based on information I gathered over the Internet, it could possibly be an out-of-phase problem. But before I got the STC-3P preamp, I had the exact same SPB-4 and Apollo Jazz Bridge installed on a MIM Fender Deluxe Active Precision Bass with stock 3-band preamp and stock blend pot. Wiring was pretty much the same, but in either passive or active mode, I don't recall such significant volume drop.

    Can someone help me diagnose if I am indeed having out-of-phase problem before I open up pickguard again to swap one of pickups' wires, or even buy another pickup for wiring pure passive (pickguard for this particular 11-hole p body is kind of hard to find...)? I thought neither SPB-4 or Apollo was split-coil as they only have two or three wires. And since they are both from Seymour Duncan, they should be working together fine... right?

    Thanks in advance for any input!

  2. #2
    Super Toneologist DavidRavenMoon's Avatar
    Join Date
    Apr 2006
    Posts
    1,250

    Default Re: SD STC-3P blend center volume drop with SPB-4 and Apollo Jazz

    Iíll assume that itís a volume drop but you arenít losing low end? Out of phase pickups are very thin sounding.

    Itís normal for some volume loss with some pickup combinations. A P pickup has a much higher impedance than a J. So when you combine them the J loads down the P and you lose volume. This is why I had two P pickups on my P bass back in the 70s. They were evenly matched.

    Luthier Rick Turner said he had his basses set up to give a slight volume boost when using both pickups in parallel because of this.

    You donít hear it as much with the preamp since itís buffering the pickups. That helps keep your cable and controls from adding to the situation.

    The only way to totally prevent this is to use active blending of the pickups; you would insulate the two pickups from each other using series resistors into an active summing stage. Kind of the way a mixer works.

    EMG sells an active blend control that Iím pretty sure works with passive pickups. You can also try putting a 10k resistor in-line with each pickup when itís going to the preamp. It would work on passive mode, but youíd lose level. The old stack knob Jazz basses were wired like that


    Sent from my iPhone using Tapatalk

  3. #3
    Junior Member
    Join Date
    Dec 2017
    Posts
    4

    Default Re: SD STC-3P blend center volume drop with SPB-4 and Apollo Jazz

    Hmmm, I didn't pay attention to bass being thin or just volume drop. It was a 9 dB total volume drop that makes blend center position almost inaudible.

    The interesting thing is, there wasn't such issue in passive mode when I wire these two pickups the same way in a 2017 MIM Fender Deluxe Active Precision. I don't think that bass had active blend. And I don't think the no-load passive tone pot contributes to the problem either as it is wired after blend.

    My understanding is passive blend at center = both pickups 100%. So when I have volume drop at blend center, will I also have the same problem if I wire these two purely passive with volume volume tone and leave both volumes full-on because of the impedance mismatch of the two pickups?

  4. #4
    Super Toneologist DavidRavenMoon's Avatar
    Join Date
    Apr 2006
    Posts
    1,250

    Default Re: SD STC-3P blend center volume drop with SPB-4 and Apollo Jazz

    Quote Originally Posted by purple_string View Post

    My understanding is passive blend at center = both pickups 100%. So when I have volume drop at blend center, will I also have the same problem if I wire these two purely passive with volume volume tone and leave both volumes full-on because of the impedance mismatch of the two pickups?
    Hereís a question; what brand of blend pot are you using? Do you know the taper?

    I find the best blend pot is a Bourns with an N/M taper. But you have to wire it the right way itís not reversible due to the taper.

    For the Bourns pot you have to wire it this way.


    Iíve also experimented with lifting the grounds on the blend. You get less loading, but never totality shut the non used pickup off. But you donít hear it.

    Another thing is the pot value. You should be using a 500k blend due to the parallel resistance of all the pots combined. A 500k blend is a 250k load on the pickups. A 250k blend would be 125k. Now add in a passive volume and the resistance drops more.

    But even with no blend and just a pickup selector switch, you will almost always get a drop in output. Thatís because the impedance drops. If you had two 8k pickups, in parallel thatís 4K.




    Sent from my iPhone using Tapatalk

  5. #5
    Junior Member
    Join Date
    Dec 2017
    Posts
    4

    Default Re: SD STC-3P blend center volume drop with SPB-4 and Apollo Jazz

    The blend pot comes with Seymour Duncan STC-3P kit. Per wiring diagram (https://docs.google.com/gview?embedd...C_Standard.pdf), it should be 250kOhm. And stock blend wiring is the same as the Bourns diagram you posted.

    Interesting point of view of the resulting impedance of two pickups in parallel. It makes a whole lot of sense to me. But the odd thing is, I don't find the same volume drop on my LP guitar with SD Jazz neck (7.5kOhm) and JB bridge (16.6kOhm) when switch at middle (5.2kOhm). Many guitars are equipped with these pickup configs. I don't see any similar complaint... Is it only sensitive to bass guitars?

  6. #6
    Junior Member
    Join Date
    Dec 2017
    Posts
    4

    Default Re: SD STC-3P blend center volume drop with SPB-4 and Apollo Jazz

    [Update:] I brought and dropped in a Bourns MN 250K blend pot like DavidRavenMoon suggested. I referenced SD wiring diagram and other forum posts. I think David's Bourns diagram got the neck and bridge reversed... Got ground connected. Not sure if I really hear any negative effect.

    Anyway, all passive VBT now. There's no volume drop at all when blend is in center detent. It's about the same volume 100% P, 100% J, and center blend PJ reading on channel meter on UAD Console. Also got a thicker foam with built-in springs from eBay to get Apollo Jazz bridge pickup closer to the strings to balance volume between P and J. (The foam that came with SD box was kind of thin for this bass.) Consistent result as in my previous MIM Fender Deluxe active Precision with whatever the stock blend pot was, as well as my Steinberger Spirit bass (TV Jones humbuckers) with Alpha blend pot from StewMac.

    Now I got three round holes on the pickguard to play peek-a-boo... It'll take me sometime to consider worthy buying a new pickguard for $35+ plus shipping from WD. Only WD seems to carry a 11-hole pickguard that fits Fender American Elite P...

    I'm seriously considering the Seymour Duncan STC-3P I bought had either defective or wrong blend pot. Or maybe the factory soldering was messed up. Anyhow, I've already returned the preamp to Stratosphere where I bought half price. So there's no way I can really determine what was causing the significant volume drop. (Not that I'm really capable of determining it... )
    Last edited by purple_string; 05-24-2019 at 09:54 AM.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •