Modding a bass for stereo output

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commodorejohn
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Modding a bass for stereo output

Post by commodorejohn » Sat Mar 04, 2017 8:03 pm

I'm asking this here because you guys are smart and I'm too lazy to go through the rigamarole of setting up an account on a bass forum and meeting whatever new-topic requirements just to post this question. I've got this bass, and I'd like to set it up with stereo output jacks (one from each pickup) so that I can add distortion to the output without killing the low end. The wiring diagram is pretty standard for a Jazz Bass clone:
Image
In theory, it should be as simple as connecting a second TRS jack to the output of the volume pot for one of the pickups and disconnecting it from the tone-control pot and main output jack, right? (I suppose it means no tone control on one of the pickups, but since the whole point is to leave the low end unmolested, I can probably just disconnect the middle/neck pickup from the tone control and not worry about it.)

Should I also connect the volume pot for the middle pickup to the ground wire from the bridge (since it won't be connected through the tone-control pot anymore,) or will this screw anything up?

Finally, I've never been clear on how "pre-patched" jacks work (i.e. on the MS-20 where it's connected one way when you don't have anything plugged in and connected to something else when you do.) Is there a way to use this so that it still functions as a standard Jazz Bass clone when only one cable is plugged in and only breaks the connection to the tone-control pot when the second jack is connected to something? That's not vital, but I'm kinda curious.
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Post by guest » Sun Mar 05, 2017 1:06 am

you should be able to get a "normalled (with switch)" 1/4" jack. take the output of one of the volume pots, run it to tip on the jack, and then run a wire back from the switch to the other volume pot (where it was originally going). connect the sleeve of the jack to ground.
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Post by fuzzbass » Sun Mar 05, 2017 8:18 am

This looks like a Jazz Bass schematic.

The two pickups have individual level controls, but sum to a single tone potentiometer, with a single capacitor. The simple approach is to double the tone control, aka Rick-O-Sound.

What is below is a no going back design - you will lose the neck pickup if you plug in a mono cable. However, you will still be able to run your bass through a fuzzbox, which is ESSENTIAL. :hyper:

First check your control cavity. Is there enough clearance for a dual gang pot in the tone control position?

Materials:
1 addl capacitor, same type
1 replacement tone potentiometer, dual gang pot with single shaft (not "stacked", which has dual shafts).
1 replacement jack TRS type

tubesandmore is a good place for these parts, in USA. Your dual gang pot should have 6mm round shaft.

You could use a stacked tone control like V1 jazz bass, but then you are looking for new knobs too.

Construct two tone controls, identical to the one you see, on the double gang pot. Break the connection that sums the output of the first pickup to the second, and bring to the newly added tone control.

Center pin (wiper) on each tone pot is hot.

Hot from the neck pickup goes to Tip on the jack, and hot from the bridge pickup goes to Ring. You might notice there is error in the schematic, the ground connection runs across the all of the potentiometer enclosures.

If you want to be able to go back to a mono cable, you will need a second jack, and the TRS jack will need to be the switching type mentioned above. This is how Rick 4000 series basses are wired.
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Post by commodorejohn » Sun Mar 05, 2017 10:43 am

Thanks for weighing in :) The dual-ganged tone-control pot is an interesting idea, but I'm not sure I need it on the middle/neck pickup in the first place, as it doesn't seem like that has too much treble to begin with. Is the sound you'd get with no tone control interposed pretty much the same as it gives now with the tone control wide open? (Also, I know filters mess with phase relationships - would having one pickup filtered and the other not screw with the one-pickup-inverted humbucking effect of the Jazz Bass design?)
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JimY
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Post by JimY » Sun Mar 05, 2017 2:03 pm

Humbucking can only work if both pickups are wired to one point. The hum is of opposite phase from one pickup and cancels out with the in-phase one when they are mixed together. You will lose it if you go to separate outputs. Tone control don't affect it.

Not sure if you want..
A second jack & use 2 cables.
Or..
A TRS jack and use a single twin core cable

May I be awkward and suggest another way?
Split the existing single output to 2 feeds in a splitter pedal which may have eq in one feed to suit your distortion. One feed is otherwise clean for your normal bass sound. You will have to be certain that the 2 paths to the amp (assume you have 2 channel amp?) are in phase - some distortions do invert.

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Post by Jarno » Sun Mar 05, 2017 3:22 pm

I'd go without tone controls and just wire each of the pickups through a volume control to ring and tip.
If it's reducing loss of bass when using distortion, there are simpler ways. A clean blend control works, or a fixed filter like the buchla 194 followed by distortion per frequency band AndrewF has a pcb for this.

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Post by commodorejohn » Sun Mar 05, 2017 5:15 pm

JimY wrote:Humbucking can only work if both pickups are wired to one point. The hum is of opposite phase from one pickup and cancels out with the in-phase one when they are mixed together. You will lose it if you go to separate outputs. Tone control don't affect it.
Gotcha.
Not sure if you want..
A second jack & use 2 cables.
Or..
A TRS jack and use a single twin core cable

May I be awkward and suggest another way?
Split the existing single output to 2 feeds in a splitter pedal which may have eq in one feed to suit your distortion. One feed is otherwise clean for your normal bass sound. You will have to be certain that the 2 paths to the amp (assume you have 2 channel amp?) are in phase - some distortions do invert.
I'm leaning towards the two-jack solution, partly because I'm specifically trying to mimic the Rickenbacker approach (without, y'know, going to the extent of paying for a Rickenbacker) and partly because I like the idea of having both pickups available separately; every time I've tried to use EQ to separate out part of a signal post-fact it's been at best a suboptimal solution.
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Post by guest » Sun Mar 05, 2017 6:49 pm

with humbuckers, there are 2 sets of coils in a single pickup, so it will be fine to seperate the kneck and the bridge pickup.
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br1qbat
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Post by br1qbat » Sun Mar 05, 2017 7:10 pm

Many a legendary bassist has gone this route, Chris Squire (Mr. Ric 4001) and Billy Sheehan are 2 that I know of off the top of my head. the 2 plug approach is probably the simplest, but yeah, New holes in your bass, but if you have no issues doing that, you'll get exactly what your after. Might be worth googling the ric4003 wiring diagram too

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Post by Jarno » Mon Mar 06, 2017 6:25 am

The only reason I'd personally go for two jacks is when using separate amps, like a guitar amp and a bass amp, or a leslie and a bass amp.

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Post by fuzzbass » Mon Mar 06, 2017 6:34 am

guest wrote:with humbuckers, there are 2 sets of coils in a single pickup, so it will be fine to seperate the kneck and the bridge pickup.
True with P-basses, which have a two coil, humbucking pickup. Sometime true with J-basses. The classic, original J-bass pickup is single coil, but this has been subject to endless changes and tweaks, and many examples are now two coil.

Easy test: listen to see if hum comes up when you turn down one of the J-bass pickups. If it does, you have single coil pickups and the whole of it becomes humbucking when both pickups are at approximately same volume.

If you split two single coils and drive them through the same amp, humbucking can occur electronically. If you drive them through two different amps, humbucking can occur acoustically, but is a much dicier affair.

This didn't bother Squire or Sheehan much. Squire's bass had two single coil pickups that were poorly matched. The neck pickup was a six pole guitar unit. The bridge pickup was exceptionally hot, like the V1 P-bass pickup, and all of this was factory. On this bass it is difficult to use the internal mixing apparatus, because one pickup so dominates the other.

Both Fender and Rickenbacker thought this hot pickup was a problem and did away with it quickly. I use to have a very early 70's Rick, a very pretty affair with checkered binding, genuine mother pearl crushed and formed in laquer for position marking, and a clear, backpainted logo tag. This instrument had the original six pole "toaster" guitar pickup in the neck position, but the bridge pickup had been reworked into a fake plastic horse shit magnet. Instruments like this regularly go for $12-15K on ebay now. But really, the Rick to get is the one with the real, original HOT bridge pickup. You will know it because the pickup cover cannot be removed.
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Post by fuzzbass » Mon Mar 06, 2017 8:43 am

br1qbat wrote:Many a legendary bassist has gone this route, Chris Squire (Mr. Ric 4001) and Billy Sheehan are 2 that I know of off the top of my head. the 2 plug approach is probably the simplest, but yeah, New holes in your bass, but if you have no issues doing that, you'll get exactly what your after. Might be worth googling the ric4003 wiring diagram too
Also, Chris' bass was a stripped down export 4001 model (just like Paul's), and started life without the Rick-O-Sound jack.
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Post by commodorejohn » Mon Mar 13, 2017 2:37 pm

So, coming back to this, I'm looking at the diagram for switching jacks, and if I'm understanding this right, the tip is connected to the "shunt" pin (which they intend to be connected to ground to eliminate hum on an unconnected input) when there's no plug inserted, and connected to the "signal" pin in any case. So if I connect the output of the neck volume pot to the "signal" pin, and connect the "shunt" pin to the input of the tone-control pot, that should give the desired behavior of eliminating the neck pickup from the primary output when a second plug is inserted, and behaving like a standard Jazz Bass clone when the jack is left empty, right?
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Post by guest » Mon Mar 13, 2017 3:57 pm

yes, thats exactly what you want to do.
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Post by commodorejohn » Mon Mar 13, 2017 4:22 pm

Sweet :) Now I just need to get a chance to take it over to a friend's machine shop and put an extra hole in the control plate...
Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
Synthesizers: Roland JX-10/SH-09/MT-32/D-50, Yamaha DX7/V50/TX7/TG33/FB-01, Korg MS-20 Mini/ARP Odyssey/DW-8000, Ensoniq SQ-80

"'Legacy code' often differs from its suggested alternative by actually working and scaling." - Bjarne Stroustrup

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