Thanks for taking an interest in this thread. Here are a few answers and comments.
That parallel r/c is a weird one. What value is the r and the c?
Yes, it is strange. I showed the cap markings and resistor values in the second diagram in Post #1. I am pretty sure the cap is 100 uF. Yes, I know that is (absurdly) large. But, since no one so far has been able to explain the idea behind that capacitor, the size is somewhat secondary, no?
Actually many bassists use the active-passive switch a lot.
Really? I mean, I own and play a Fender passive Jazz Bass; so, I appreciate what passive basses can do. The Tobias, however, was designed to be played with an active preamp. You will have to perhaps trust me that the non-active Bartoini pickups by themselves, which is what the A/P switch was intended to provide, are not special, do not produce what Fender passives provide, have no other tone controls associated with them, and are almost hopelessly and pathetically weak. In short, these pickups without their preamp could never be anything but backup. I can't see any bass player choosing to do that as part of their "sound;" but, YMMV.
A bad spike can damage speakers. Also a popping switch will freak out sound engineers and producers if they aren't expecting to hear it. And really, it's just good practice to have all the parts of your rig working correctly.
Yup; no doubt; and, yes, that's the point of this project.
Why those resistors aren't on the circuit board however is beyond me. Perhaps the circuit was never intended to work with an active/passive switch. When an audio circuit is biased for a single supply, there are input and output coupling caps. The preamp side of these is usually at 4.5v (or thereabouts), while the pickups/pots/jack side will be at earth potential. And this DC offset will cause pops with typical active/passive switching.
I'm not sure how to respond to some of those comments, mainly because I really don't know what kind of devices are inside the potted TCT module. However, I find it odd that one of the Bartolini diagrams shows the grounding resistors on the switch, while the other doesn't show them. I'm also not sure why switching the preamplifier in and out of the circuit should inherently cause a pop. My Alembic Series I electronics do not cause a pop when the preamplifier is toggled off and on. But, as I said, we don't know what is inside the black box; so, maybe you are correct. Bartolini's comments, please refer to Post #8, agree with you.
Having a 1M2 on the input and a 150k on the output are entirely appropriate values for the job of bleeding the coupling cap's charge to earth. And shouldn't effect the tone.
Ahhh; now, you have arrived at the MOST interesting part.
"Shouldn't" is the entirely correct word to use. I get that, and that is clearly Bartolini's thought as well. But, please look carefully at what I did for re-wiring in Post #9. I apologize for the lousy drawing; but, the red rectangles are meant to indicate where I removed wires and the yellow and green wires are my adds. The changes effectively wire the switch out of the circuit by using the center posts as junction points, and removing the shorting jumper on the lower switch contacts leaving them open.
As I stated earlier, I initially left that odd RC network on the upper switch position so that I could test it's effects on the sound of the bass vs having no grounding networks just by actuating the switch. Well, of course, the capacitor was messing up the tone. So, I cut the capacitor out of the circuit. Now I can switch those grounding resistors in and out of the circuit. Maybe they are not intended to affect the sound of the bass; but the empirical result is pretty much night and day. Those resistors definitely make the bass MUCH less dynamic.
So if you remove these resistors, and there aren't any others already on the preamp board, the active/passive switch will very likely pop. There's no harm in removing them to try, just be sure to have your amp turned down quite low when you are testing.
If there are already resistors on the board, you will register an ohmic resistance between the input/output and earth. If none, then your meter will give an infinite resistance reading. (Obviously, you'll have to remove the external resistors and set the switch to bypass to test this.)
In fact, I am pretty much inclined just to leave the switch wired the way I currently have it. Putting the A/P capability back in place would be nice, since that was the original thought of the preamp layout, and I am to some extent trying to get the circuit back to what it should be. OTOH, my main concern with just removing the resistors is more that it might somehow internally damage the TCT module; and I am not interested in that outcome. OTOH, I am not going to compromise the sound of the instrument so that I can have an A/P capability that will never get used. So, I am kind of caught in the middle.
As for the resistors on the mid pot, well, without seeing the schematic, especially for that MTC-375 module, I'm not sure. The mtc-375 is clearly a passive network, because there's no +ve or earth wires going to it. I'm pretty sure the tct is based on a fender tonestack, so I'd think you could remove the 47k and the black 375 wire all together and it will function as a regular fender amp mid pot. IOW, won't do much. But I'd try it that way anyway. (You can leave the 27k there. With the black wire removed, it won't have any effect.) Then add the the 47k. This will change the sweep of the pot slightly, but more importantly will deepen the inherent mid scoop of the fender tonestack. (IOW, with the pot all the way clockwise, you should hear a slight mid cut when adding the 47K) Then add the module wire and listen for the difference...
Thanks for the suggestions. Based on EG's comment above, it seems likely that the MCT is actually a 250 Hz module. Otherwise, you are correct. I may think about testing you ideas down the road. However, two factors are probably going to cause me not to try further changes. First, those are the bias resistor values recommended by Bartolini; and while your hypothesis about the components in the module may be correct, Bartolini are not likely to recommend incorrect biasing values. Second, the addition of the biasing resistors has improved the tone to the point that I don't need to change anything. I now get a very credible and powerful p bass tone from the neck pickup, a very cutting staccato sound from the bridge pickup, and all the slap and plectrum sounds I could want from the balanced pair of pickups. In short, the Bartolini recommended bias values provide the tones I am after.
Again, thanks for putting some effort into what has perhaps become a somewhat arcane thread at this point. I really appreciate your thoughts and enjoy discussing the project.
Also, I remember Mike or maybe Bob Lee saying that the early, un-marked MCT modules were 250 Hz. Now that they are available again, I've got spares in each value, but not the switchable 250/375/500 unit.
) Perhaps you could post a pic?
