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Wiring onboard preamp for outboard?

Discussion in 'Amps and Cabs [BG]' started by luknfur, Feb 29, 2004.


  1. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Trying to wire an onboard for outboard use and not getting results. You amp guys are the solder junkies so this should be a cakewalk.

    The hangup is the jacks of course. An in and out for the outboard. What I've done is wire the jack out same as it would be for onboard - using the idea that's the way it comes out of the bass to the amp.

    The ingoing I've wired as you would a passive single pup with volume only: The volume recieving the preamp wire which would normally go to the volume control - left lug (looking top down with lugs toward you) and the center lug to the hot lug of the jack (stereo jack but shorter of the two hots), ground lug of the jack grounded to the back of the volume pot. Grounds running between pots (2 band so volume and stacked tone).

    Alternatively, I'm thinking the jacks should maybe tied together but the current was what I thought would work.

    I daigram I suppose would be ideal but I think this is comprehensible enough.

    Did a search but saw nothing along this line but I know this has had to have come up in the past.

    Also if you know a link that would likely resolve the issue, that would work.

    Thanks
     
  2. The stereo jack is for the output. The tip is the positive output of the preamp. The shield is the negative output (ground) of the preamp. The ring goes to the negative of the battery.
     
  3. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Appreciate it. Will give it a go promptly and see if some transformation takes place.

    Apparently, I was relatively close. I really didn't see where tieing the jacks together would produce results.
     
  4. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Looking at my wiring, it's wired as you said for the output jack. The only difference being that I wired the negative off the preamp to the back of a pot which is in turn run to what you're calling the ring terminal (which is what it's actually called no doubt).

    So I'm guessing the problem is the way I've got the input jack/volume pot wired, which is the volume recieving the preamp wire which would normally go to the volume control if onboard - left lug (looking down from the pot back with lugs toward you) and the center lug to the hot lug of the jack (ring as you say), ground lug of the jack grounded to the back of the volume pot. Grounds running between pots.

    For convenience I've got this set in a piece of carboard as you'd wire a harness but the jacks and the pots are all tied through grounds.
     
  5. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Okay, have sound now. Apparently I had the wire to the ring and tips crossed on the input jack. Both are stereo jacks but was skipping one terminal on the input. Can't really tell if the tones working till it's in a box (coming up) cause the things are a little floppy on cardboard.
     
  6. luknfur

    luknfur

    Jan 14, 2004
    DIXIE

    Houston we have lift off. Appreciate the feedback. Good timing to boot.
     
  7. luknfur

    luknfur

    Jan 14, 2004
    DIXIE

    Was wondering if you have any idea how to wire a buffer into an onboard or even if it can be done? Picked up a Bart NTMB with a buffer and I've tried ways I figured it would work but since I can't if it's working or not I assume it's not doing anything - the preamp in outboard form itself is working.
     
  8. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Buffer? What's that, a separate little black box? God, I hate those Bart preamps, they're not user friendly at all.

    Does the buffer go "before" the preamp, or after it?
     
  9. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Thought it was Nashvillebill for a second. Had me concerned. Buffer apparently actually boosts the signal, even though from the name you'd think the opposite. Anyway, it's part of the preamp. ties directly to the pups and links to a blend and the preamp itself. Seems I recall someone saying you can't have a buffered outboard.
     
  10. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Okay, got it. Yeah, that makes sense. Probably the buffer converts the high impedance signal from the pups into a lower impedance signal suitable for processing. And, it may boost the signal level too.

    Yeah, that would be something you'd want to leave in your bass, if at all possible. The reason being, that the low impedance signal is much better for driving long cables. Otherwise, your instrument will be essentially "passive", which means it'll be susceptible to all the usual hum problems of passive instruments.

    I assume each buffer has a single output connection? If so, just run that through the blend and volume controls in the usual way, and take the output from the volume control to feed the instrument cable. Are you removing the tone controls from your bass, or leaving them in?
     
  11. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Buffer has dual inputs (from each pup) and output for each connection of blend pot. I can't leave the buffer in the bass and take the preamp out (at least not to my knowledge) cause it runs off the preamp. I guess I'll pull the buffer from the outboard preamp completely if I don't hear from Nashville soon. If it sounds different than before, at least I'll know it's doing something the way I have it wired. I've got another NTMB in outboard form but it has a different mid unit so I can't compare the two as is cause even aside from the buffer, they're different. And they definetly sound different. The buffered one has a smooth tone to it.

    I started out with the intention of pulling the preamps and just leaving volume pots in the basses (since two pups), otherwise I'd just have run a direct out. But before I even got started I found myself jacking with switches (phase, series/parallel, whatever) and started plugging holes with those. But then I had a bass with Bart M34C soap bars I decided to sell so I ran it with two volume/two tone (ala Les Paul) and ran a DPTT switch running the outer single coils, standard duals, and inner single coils respectively. What's wierd is, I guess because of the switch, the tone controls ended up being interactive affecting both volumes - which would be redundant except the capacitors on each tone control are lightyears apart so you get two completely different treble bleeds. The bottom line is I ended with an amazing variation in tones from that control setup. And it's dead quiet cause it's all humbucking.
     
  12. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Ah, okay. The plot thickens. :) I'm getting a better picture now. Well, several thoughts come to mind. But before I share them, what are you using for a "regular" preamp (or are you running straight into a power amp)?

    And, when you say the bass is "dead quiet", is that with or without the preamp (or I mean, is that with the preamp in the bass, or with the preamp in the outboard rig)?

    Okay, so one thought would be, it's very easy to build a buffer yourself. Are you handy with a soldering iron? If so, you can build a two channel buffer for about ten dollars in parts, and maybe three or four hours' total effort. If you're interested, drop me an e-mail and I'll send you a schematic. Then, you could run a low impedance active signal from your bass to the preamp, guaranteeing a perfectly quiet setup.

    It seems that the problem is retaining the pickup blend and volume controls in the bass, right? 'Cause the pickups go into the buffers first, and then to the controls. My guess is, that the pickups will sound different without the buffers. Part of the reason the buffers are there is undoubtedly to allow the tone control circuitry to operate in "low impedance" mode. That generally results in lower noise and more favorable output characteristics. So, I'm guessing that the Bart tone controls are in the 10k-100k range, rather than the 500k pots you typically find in passive basses. When you load down high impedance pickups with much lower tone control impedances, they won't (generally) sound the same. But, it might be worth checking them out this way, just to satisfy your curiousity. You won't hurt the pickups any by doing that.

    So, that's what I would try to do. I'd keep the buffer (or some variant thereof) onboard the bass, and then just take the wiper of the volume control and run it to the jack. Then don't connect the buffer on the outboard preamp, just use whatever wire normally connects to the wiper of the volume control and feed it with the signal from the instrument cable. If you don't want to build another buffer for your bass, you could get another copy of the exact same Bart electronics, and just use the buffer part inside your instrument.

    Running "purely passive" out of the bass would be my last choice. Recently I've had to run some pretty long instrument cables, and every minute I was thankful I have an active instrument.
     
  13. Unfortunately I'm afraid I'm not much help here, sorry. The Bart preamp I have was out of my Gibson Les Paul bass, which had big soapbar p'ups. The bass was a little noisy, even after carefully shielding the instrument cavity, so I tried taking the preamp out and running purely passive. The DC resistance of the p'ups was about 6 to 10K as I recall. Unfortunately the bass was stolen, but I still have the preamp floating around somewhere in my junk room I think. I had drawn up a schematic, but there wasn't much to be seen, Bart had potted everything. I'm guessing there was an inductor for the midrange as a separate potted "chunk" (for lack of a better word). As I recall the pots were 100K.

    I vaguley recall seeing some schematics on the Elliott Sound Projects web site that may help with a lower impedance setup. IIRC the really old Gibson LP Recording basses had an XLR out. Or another option, see how Alembic did it with their early preamp setups.
     
  14. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Actually, when I said the bass was dead quiet, I was talking about running ithe bass straight to the amp, no preamp onboard or outboard. And by dead quiet, I was talking in terms of no hum, not no sound. But I've got a buffer. It came in the Bart NTMB harness. So I don't need to make one. If fact of got a Bart DTA1.3 buffer sitting from an XTCT-LZ preamp harness. What I could use is another mid. I'm accumulating buffers and blends out the yang.

    I'm just trying to figure out how to incorporate the buffer into the outboard verson of the preamp. To make an onboard into an outboard with no buffer, you basically just chuck the blend control and run the wire that went to the blend from the preamp to the volume control and wire an input jack and you're good to go. But with the buffer being between your pups and blend, you've got 4 wires looking for a place to go. Bartolini.net has the basic situation under the XTCT-LZ or NS2TMB-18 diagrams in their install insructs.

    Since the buffer has leads running to the preamp, I'd think the buffer could not be seperated from the preamp, as in leaving it in the bass. Even if it could, I'd still need to keep a battery in the bass and I'm trying to get most if not all my pups passive and chuck the onboard battery thing. I don't anticipate the need for long cords but I can always rig up an active configuration if it should ever arise. The more I play passive the more I prefer it - maybe cause all my basses had onboard preamps till I started this little sojourn. But the pups just sound better to me the less junk they're run through. But I want the preamps handy just in case they seem to work better with a particular rig or set of pups. If over time I don't use them, I know the way to ebay (and San Jose').

    The pots were all 50K in the original harness, except volume.

    Hmmm? Nashville done let me down.
     
  15. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    Bummer, but appreciate the response and the leads. Sorry about the bass. I had a Guild F-50 Blonde swiped. People can say what they want about the middle east but personally I kind of like the idea of chopping off the hand of someone caught stealing. If for no other reason cause they can only do it twice without getting real creative. But once I remember seeing a guy with no arms drive a farm tractor. So maybe it would be more like 3 times.