• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

WIRING HELP!

pondscum29

pondscum29

Feb 23, 2006
230
38
Sacramento, CA
#1
OK, I'm stuck. I was trying to wire a passive bypass into my bass, which is equipped with a Bartolini NTBT preamp. I drew a wiring diagram of the original setup prior to embarking on this endeavor, and I drew a diagram of the proposed rework which entails:

1. combining the bass & treble into a concentric pot, freeing up a spot for:
2. a 250k pot/50pF as a tone control as in a jazz bass.
3. the bypass is a 250K push pull pot in place of the existing 250K volume pot.

Attached are both my wiring diagrams, color-coded per the preamp (if it's a little off, cut me a break as I'm slightly colorblind).

OK, my dilemma. After wiring it, when I turn the amp on, all I get in active is a noise like a ground hum, which goes away after a second. Then I get only very faint volume, which remains the same no matter how I turn the volume knob. In passive, nothing (well, it may be REALLY faint).

Am I missing something obvious? I've checked all the grounds and everything looks to be solid. :help:

If I did something incorrectly, did I fry the preamp? :confused::bawl:
 
#3
50pF?
I would be surprised if that that any effect at all as a tone control.
Or did you mean 50nF?


Also, the switch for the tone control will not take it out of the circuit in passive mode.
You have it wired straight to the output jack, so it will always be in the circuit unless you disconnect it.
 
#4
Furthermore, the negative terminal of the battery does not get grounded, as your diagram indicates. It goes to the ring terminal of the battery, where it can be switched to ground by the insertion of a guitar cable.

Also, it would probably work better (less switching noise) to switch between active and passive modes from the output of the preamp and volume control, rather than the input of the preamp and volume control.
 
#5
Thanks! Everything I've seen states .05uF, and I you made me realize I moved the decimal 3 places too far. That's easily fixed.

I used to just have a SPDT toggle hooked up (like the yellow side of the P/P pot), and that worked as intended, bypassing the preamp as a passive circuit, and consequently the B/T controls. I was just trying to raise the bar and hook up a tone control. I figured whichever circuit I was using, would be the one output to the jack. Am I incorrect?
 
#6
pondscum29 said:
OK, I'm stuck. I was trying to wire a passive bypass into my bass, which is equipped with a Bartolini NTBT preamp. I drew a wiring diagram of the original setup prior to embarking on this endeavor, and I drew a diagram of the proposed rework which entails:

1. combining the bass & treble into a concentric pot, freeing up a spot for:
2. a 250k pot/50pF as a tone control as in a jazz bass.
3. the bypass is a 250K push pull pot in place of the existing 250K volume pot.

Attached are both my wiring diagrams, color-coded per the preamp (if it's a little off, cut me a break as I'm slightly colorblind).

OK, my dilemma. After wiring it, when I turn the amp on, all I get in active is a noise like a ground hum, which goes away after a second. Then I get only very faint volume, which remains the same no matter how I turn the volume knob. In passive, nothing (well, it may be REALLY faint).

Am I missing something obvious? I've checked all the grounds and everything looks to be solid. :help:

If I did something incorrectly, did I fry the preamp? :confused::bawl:
Click to expand...


You did not wire this correctly. The output of the preamp is connected to the passive circuit and loads it. First lets start with the bypass. The way to do that is that if you look at the original circuit note the green wire coming from the right terminal of the volume. Originally it goes to the output of the preamp. So what you want to do is cut that wire. The piece coming from the right volume terminal goes to the switching terminal of 1/2 of the push pull (I think thats the center terminal on the switch). Hence the volume pot terminal will switch between two places. One is the preamp output (where you cut it loose from...active mode) and the other which is the passive mode would be connected to yellow wire coming out of the blend control. That yellow wire STAYS connected to the preamp input as it should be high impedance and won't matter. That achieves an active/passive switching though like a G&L the battery will continue to drain in passive mode. However it does give you emergency passive backup with a dead battery. Which is good.

The next question would be the tone control. For that you'd need the other side of the push/pull switch which attaches the cap/tone pot series combo to the famous right volume terminal as mentioned above in passive mode. In active mode it connects to nothing. The other end of the cap/pot series tone circuit goes to ground, natch.

Or you could use the second side of the switch to open the battery ground lead and turn it off in passive mode INSTEAD of the tone thing. I personally would do the tone thing since all EQ quits in passive mode.

Give that a try. Your basic problem is that the preamp OUTPUT will load things even if it is off. However it's INPUTS should remain high impedance no matter what so you don't have to switch those wires.

There is a rare chance the preamp inputs will load the circuit when the battery is off. But since this circuit does not kill the battery, that is only a problem when the battery is dead. To test that simply make sure the active/passive switch is working as I described the wiring, and then pull the battery. If it still works, you are in luck. If not then you'll have to deal with switching the input lines as well.

Let's hope you haven't fried it...yet!

Good luck!
 
#7
here's the concept of a true bypass switch, from a pedal diagram. (you'll want the second one)

dpdt.gif


the "input jack" will be the signal from the pickups and the passive volume and tone.

the idea is that the switch not only disconnects the "effect" (preamp) input and output totally, it shorts the effect's input to ground, which eliminates noise.
 
#9
walterw said:
here's the concept of a true bypass switch, from a pedal diagram. (you'll want the second one)


the "input jack" will be the signal from the pickups and the passive volume and tone.

the idea is that the switch not only disconnects the "effect" (preamp) input and output totally, it shorts the effect's input to ground, which eliminates noise.
Click to expand...

Let me add one more comment. If you look at these pedal circuits (which applied to your bass means the input jack actually goes to the blend pot and the output jack goes to the jack on your bass) you will notice in the first version that if you connect a tone pot and cap to the "jumper" (an outside pot terminal to ground and center pot terminal to tone capacitor; Other end of tone cap to the "jumper") This will give you a tone control that will only be "on" when you are in passive mode. But has the disadvantage of not grounding the preamp input in passive mode. If you do the same thing with the second circuit and connect to the "jumper", now the tone circuit will always be working. That may or may not be bad according to how you intend to use it. That would be because the roll-off of a passive tone circuit may not quite give the same effect as the active treble roll-off. Especially with certain values of tone caps. Anyway you've got lots to experiment with now!

Good luck.
 
#11
OK, I'm almost done. Had to take a hiatus to bring in the birth of my son. :hyper: Now I'm just chipping away when he allows me to. Here's how I have it wired now, but I'm not hearing a tone change in passive. I'm missing something obvious; I just know it. And yep, it's a .047uF cap now - a big ol' orange drop. :D I thought I wired it the way all the diagrams show, but... :hmm:
 

Latest posts

Top Bottom
Back