First tweak ever...Active to passive conversion and...

[Saxn]

...a series/parallel switch. I think I'm hooked!

Apologies in advance for pic quality but all I had was my phone and I wasn't about to break the flow of the project to hunt down anything better... was already on a tight timeline!

Anyway, in my ongoing quest to know thing off of my 'to do' list, I finally got round to jumping on a project that has been waiting in the wings nearly as long as I have been playing my first bass. Here's the skinny, and it started here:

1-1 by Saxn75, on Flickr

That's a Yamaha RXB374, an active bass with master volume, pickup blend, and bass/treble boost and cut controls. It has been a pretty good player, but I have always had the sneaking suspicion that she could do a bit better. After doing a fair amount of research, I decided to mod it up a little with an active to passive conversion... because to be honest I invested in a pretty nuce bass amp and I just KNEW that the preamp on that would absolutely destroy the one built into my Yammy in terms of sound quality, noise floor, etc. So finally two days ago the day came, and I went to the local music joint and picked up four 250k pots for the conversion. I rescued a couple of capacitors (.024 uF) from a defunct PA system to use for the tone controls and went to work.

Taking a look at the back:

2-1 by Saxn75, on Flickr

You can see all the crap that has to come out. As I began gutting the poor girl I realized that I now had an empty battery compartment to work with... and I ALSO had a mini DPDT toggle switch in the junk drawer... hmmmm.... a plan began forming. I now envisioned an active/passive conversion, with independent pickup volume and tone controls, running to a series/parallel switch. Tone options for DAYS!

So I ripped out her innards...

4-1 by Saxn75, on Flickr

And took off the pickups...

5-1 by Saxn75, on Flickr

And saw that not only was the battery compartment not shielded at all (to be expected) but that Yamaha had shielded the pickup and electronics cavities with a conductive paint of some sort, but due to a poor choice of material, inadequate coating, or both I did not believe (and STILL don't believe) that the EMI shielding was adequate, so I decided to add in the step of beefing that up a little bit. I used aluminum tape for this and wound up with the following:

7-1 by Saxn75, on Flickr

6-1 by Saxn75, on Flickr
All grounds checked good, so on we go.

I also ran into a bridge grounding issue that I WISH had taken photos of and sent to Yamaha QC, but I didn't. Let's just say it was no wonder that the amp popped every time I put my fingers on the strings! All fixed now though...

I had to do some drilling to get the pots in, but that was a minor diversion, and after fitting the pots and the "afterthought" series/parallel switch into place I began the long process of soldering and insulating... BLEH! The single most tiring part of the ordeal summed up in the following two pics:

9-1 by Saxn75, on Flickr

10-1 by Saxn75, on Flickr

She was ready to test, and after a short stint playing on a single string I found two minor gremlins in the works, one in the neck pickup and one in the hot line to the output jack... both of which were the result of getting in a hurry and not following mil-spec soldering protocol! I will confess: I didn't tin the wire and that allowed stray copper strands to bridge the circuit and short it out. There, I said it. But once that was all squared away, I had a brand new PASSIVE Yamaha...with independent volume and tone controls to boot! She don't look dramatically different:

11-1 by Saxn75, on Flickr
But she plays a treat AND I am proud to say that she's a lot quieter between tunes! Looking forward to gigging it soon.

And now on to the next thing on the list...

EDIT: Yeah, I know there's a hole in my sock. My grandma would be devastated. Sorry.

 

[Bob C]

No picture.

 

[Saxn]

Ah CRAP... sorry. Let me figure out why.

 

[SGD Lutherie]

Yamaha electronics are not very good in my option. I've worked on guitars with the humbucking pickups wired out-of-phase with them selves, so they were noisy and thin sounding, and did some setups on the high end six string basses and found the pickups to be noisy and uninspiring.

The wood work is nice though.

By the way, I see no need to shield the battery compartment. Why would you? No signal goes through the battery.

 

[Saxn]

Good observation for no pics available SGD! The answer is that I am now using the battery compartment as a cavity to house any additional switches I will be installing. I already have a series/parallel put in there. As they say a picture is worth a thousand words, and I unfortunately had to leave this morning before troubleshooting the pic problem. Hopefully in a few minutes...

 

[Saxn]

Okay, pics should be up and running now... can someone tell me whether or not I got this flickr thing worked out??

 

[Saxn]

Just a little bump. Can anyone see the pics now?

 

[ctmullins]

Yup - good pics! And congrats on a successful mod!

But - how is it wired, again? Passive, with two volume controls and two tone controls? Generally, two tone controls is a non-optimal setting for a passive bass, due to excessive pickup loading, and difficulty isolating the circuits - do a search, others have described it better than I can.

Series/parallel is a great mod, though!

 

[Saxn]

Hmmm... I WAS a little concerned about signal bleeding 'upstream' through the other tone pot. And yes, I can tell that one tone circuit definitely affects the other one after playing around with it a bit. Would this still be a problem if I managed to isolate the pickups from one another? I am guessing not.

 

[Saxn]

Ok, so have been looking at options and it appears the only way I will be able to get the dual tone control option to work a little better would be to build a passive summing circuit, which I think would result in too much signal attenuation.

So the question remains... what to do with the additional knob?? I hate useless bits hanging around on anything, and right now that's exactly what I have.

Would changing one capacitor to make a different band pass (i.e. a high pass and a low pass filter downstream of both pickups) do the trick?

 

[SGD Lutherie]

So the question remains... what to do with the additional knob??

You can install a rotary switch and use it to change tone caps. It would work like those ToneStyler controls. The common types are 4, 6, or 12 position switches. One position could also be bypass for an extra bright tone.

You could also install a midrange choke, which is commonly called a Varitone when used with a rotary switch, but they also made them with a pot, like on the Gibson Ripper bass. That would give you a passive mid cut control.

You can also wire it up like a passive bass cut control, but I'm not sure how useful that is on a bass.

 

[Saxn]

Oooh I LIKE the idea of a rotary switch with multiple capacitors! Do you know off-hand if anyone has ever tried (I am sure it's been done SOMEWHERE) to put in a variable resistor and a variable capacitor together make a changeable "tank circuit" band-pass?

 

[ehque]

By the way, I see no need to shield the battery compartment. Why would you? No signal goes through the battery.

+1.

Generally, two tone controls is a non-optimal setting for a passive bass, due to excessive pickup loading, and difficulty isolating the circuits

+1.

I would have gone vol-vol-tone-S/P switch for your 4 holes, so that you wouldn't need another hole...

Are the humbuckers 4-, 3-, or 2-conductor? You could use the extra hole(s) to S/P-switch each humbucker by itself, or you could also use switches to switch from single coil to humbucker, depending on how many conductors the humbuckers have.

 

[Saxn]

They're 2 conductor, so not a lot of coil options

I am having a lot of interesting ideas forming from SGD's post though... it would take some time, but I WOULD like to explore that 'tank circuit' idea... unless someone's tried and failed already.

 

[SGD Lutherie]

Oooh I LIKE the idea of a rotary switch with multiple capacitors! Do you know off-hand if anyone has ever tried (I am sure it's been done SOMEWHERE) to put in a variable resistor and a variable capacitor together make a changeable "tank circuit" band-pass?

You can't get variable caps in the range that works for audio.

You would also have a lot of insertion loss in a circuit like that. If you want to do something like that, just make the bass active again.

But what you can do is wire up a rotary switch with some caps for the tone control, or the same switch and caps wired to an inductor. You can use audio transformers as an inductor. That would give you a mid cut. You could also use a push-pull tone control that switches between the regular low pass operation of the control, and the notch filter by switching in the choke coil.

I used to wire up my basses like this back in the 70s, so I have a bunch of ideas for passive bass electronics.

 

[Saxn]

I can see you've thought about this quite a bit, yes... sorry for brainstorming all over the thread, but sometimes my spitballs stick! One last question here and I'll leave it alone... insertion loss caused by an LC circuit would be determined by what? The resistance on the L side? You can see I'm on a big learning curve here...

Thanks for your input, SGD. You've been a big help.