Upgrade 2 volumes + blend pot

[Rwanou]

Hi All,

I want to upgrade my old bass electronics, because it's cheap and fun, but mainly because there may be an issue, although I can't tell for sure.

My setup is rather unusual as my bass is passive and has 2 volume pots for the J and P pickups + a blend pot (with middle dent). A 4th pot is for the tone.

The main issue here is that when the blend pot is in the middle, the main volume is sensibly lower than when it's full way J or P. I need to turn up the Amp volume quite a lot to get equivalent sound. I would expect the out level to be the same whatever the rotation of the blend pot. I cannot tell for sure wether it was always like that or it changed with time.

Also my blend pot has 2 resistors and 2 capacitors wired. I read somewhere here that the capacitors prevent treble to bleed too much, what about the resistors?
Values are: resistor=180k, capacitor=1nF

here's a rough sketch of the wiring. I left the 2 volume pots out.

I was thinking of replacing the 2 volumes with 500 lin pots and the tone with 250 log. For the blend I don't know which is best, 500 or 250. I could also just have a 3 way ON-ON-ON switch, although most have a smaller mounting diameter (could easily be fixed with nuts).

Any thought or idea welcome!

 

[ctmullins]

What brand/model are your two pickups? Are they from different manufacturers? If so, and based on your “volume drop” description, they may be out of phase. If the middle position is drastically lower in volume, with a very thin sound, then this could be the cause. The fix is to swap the hot and ground wires on just one of the pickups.

The capacitor/resistor combination is a standard “treble bleed” circuit. It prevents loss of treble when turning the volume knob down. The resistor alters the taper and level of the effect, and should be there.

Regarding the two-volume-and-blend setup, that’s not typical, and is a bit redundant, since it’s electrically equivalent to each pickup having two volume controls. I would simplify that circuit to either one volume per pickup, or one blend and a master volume.

Actually I would simplify it to a three-way switch, as you also mentioned. You could use a mini-switch inside a rubber O-ring to fill the control hole, or you could enlarge the hole and use a Gibson-style toggle switch.

 

[Aloe]

you don't need two volumes + a blend. better change it to two volumes + master volume.

aside from this, likely you have an AC blend pot, not the MN, that's why everything is quieter in the middle.

 

[sunbeast]

Hi All,

I want to upgrade my old bass electronics, because it's cheap and fun, but mainly because there may be an issue, although I can't tell for sure.

My setup is rather unusual as my bass is passive and has 2 volume pots for the J and P pickups + a blend pot (with middle dent). A 4th pot is for the tone.

The main issue here is that when the blend pot is in the middle, the main volume is sensibly lower than when it's full way J or P. I need to turn up the Amp volume quite a lot to get equivalent sound. I would expect the out level to be the same whatever the rotation of the blend pot. I cannot tell for sure wether it was always like that or it changed with time.

Also my blend pot has 2 resistors and 2 capacitors wired. I read somewhere here that the capacitors prevent treble to bleed too much, what about the resistors?
Values are: resistor=180k, capacitor=1nF

here's a rough sketch of the wiring. I left the 2 volume pots out.
View attachment 4471906

I was thinking of replacing the 2 volumes with 500 lin pots and the tone with 250 log. For the blend I don't know which is best, 500 or 250. I could also just have a 3 way ON-ON-ON switch, although most have a smaller mounting diameter (could easily be fixed with nuts).

Any thought or idea welcome!

What is your purpose for the dual volume + blend scheme? The dual volumes on their own can do all the blending you need, which is why you typically see either one or the other blending scheme.

The dip in output with both pickups on is totally normal in passive basses, and is the same thing that happens in a humbucker with a coil switch going from single-coil (equivalent to either bridge or neck) and parallel dual-coils (equivalent to both pickups on). A passive parallel path of two similar pickups practically halves the output voltage of the pickups when combined compared to either soloed (of course human hearing isn’t linear, so we hear only something like a 3db drop with a halving of output voltage). The only way to avoid this with parallel pickup mixing is to use an active blend circuit that compensates for it in middle position.

 

[ctmullins]

What is your purpose for the dual volume + blend scheme? The dual volumes on their own can do all the blending you need, which is why you typically see either one or the other blending scheme.

The dip in output with both pickups on is totally normal in passive basses, and is the same thing that happens in a humbucker with a coil switch going from single-coil (equivalent to either bridge or neck) and parallel dual-coils (equivalent to both pickups on). A passive parallel path of two similar pickups practically halves the output voltage of the pickups when combined compared to either soloed (of course human hearing isn’t linear, so we hear only something like a 3db drop with a halving of output voltage). The only way to avoid this with parallel pickup mixing is to use an active blend circuit that compensates for it in middle position.

Well (as @sunbeast is already well aware of) you can also mitigate the perception of a drop in volume by high-passing one of the pickups, such as Rickenbacker does on their bridge pickup. It’s a different tone, but an entirely usable tone.

 

[Rwanou]

Thanks for this interesting feedback. I understand that a blend + 2 volumes is redundant, but this is how the bass came in. The knobs are heavy and feel nice, so I’d rather keep them for the aesthetics. As for the resistor and capacitor it now makes a lot of sense. I was confused as they were wired near the blend pot, but they actually operate on the pickup.

I think the drop of output is closer to 3db as explained by sunbeast. I don’t think the pickups are out of phase, is still get sound when blend in the middle.

Given the cheap price of pots, I may order a 3 way switch and a MN 500k and test both. I remember reading some time ago (maybe on TB?) a setup with a 7 rotary switch witch each position wired to a different blend. This way one can easily get his favorite mix with both volumes 100%. I also saw on reverb my model fitted with a lever switch, although I’m not sure that they are stock.

As the replacement of the volume pots, is 250 lin + 250 log tone fine? I know that there is a LOT to read here, so a short answer is fine (Bourns 500 lin are out of stock)

 

[Rwanou]

Well (as @sunbeast is already well aware of) you can also mitigate the perception of a drop in volume by high-passing one of the pickups, such as Rickenbacker does on their bridge pickup. It’s a different tone, but an entirely usable tone.

how one would achieve this? While I’m at it, I could give it a try

 

[ctmullins]

how one would achieve this? While I’m at it, I could give it a try

Quite simply, wire a small capacitor (Rickenbacker uses 0.0047uF) inline with the bridge pickup hot wire. This will cut the lows out of the bridge pickup, which will reduce the cancellations when both pickups are on. But when the bridge pickup is soloed, the lack of lows isn’t such a great idea, so Rickenbacker makes theirs switchable with a push/pull pot.

 

[JeezyMcNuggles]

Thanks for this interesting feedback. I understand that a blend + 2 volumes is redundant, but this is how the bass came in. The knobs are heavy and feel nice, so I’d rather keep them for the aesthetics. As for the resistor and capacitor it now makes a lot of sense. I was confused as they were wired near the blend pot, but they actually operate on the pickup.

I think the drop of output is closer to 3db as explained by sunbeast. I don’t think the pickups are out of phase, is still get sound when blend in the middle.

Given the cheap price of pots, I may order a 3 way switch and a MN 500k and test both. I remember reading some time ago (maybe on TB?) a setup with a 7 rotary switch witch each position wired to a different blend. This way one can easily get his favorite mix with both volumes 100%. I also saw on reverb my model fitted with a lever switch, although I’m not sure that they are stock.

As the replacement of the volume pots, is 250 lin + 250 log tone fine? I know that there is a LOT to read here, so a short answer is fine (Bourns 500 lin are out of stock)

They aren't out of stock on Amazon, or eBay, or stewmac, or anything else online that popped up on my search yesterday when I ordered two bourns 500 pots and a 500 blend off amazon.

 

[Rwanou]

They aren't out of stock on Amazon, or eBay, or stewmac, or anything else online that popped up on my search yesterday when I ordered two bourns 500 pots and a 500 blend off amazon.

On my side of the Atlantic I don't see in stock much. The stores where I can order have a high delivery price, which defeats the purpose of a cheap upgrade.

Is 250k lin pots a bad idea for my volumes? Or should I really favour 500k by choosing another brand?

 

[JeezyMcNuggles]

On my side of the Atlantic I don't see in stock much. The stores where I can order have a high delivery price, which defeats the purpose of a cheap upgrade.

Is 250k lin pots a bad idea for my volumes? Or should I really favour 500k by choosing another brand?

Fender, CTS, Dunlop, Duncan, Made in China, whatever really. It just a potentiometer. There's really nothing to it. I like bourns pots because there's like zero resistance. But, Dunlop and Duncan have low resistance models too.

 

[Rwanou]

Hi Again,

I ended up upgrading my electronics with a 2 volume solution + 4 way rotary switch + tone:
Each volume controls a pickup. The rotary is a DP4T that switches to the following:

• neck only
• both pickups in series
• both pickups in parallel
• bridge only

In series mode, both volume obviously act as a master volume.
The overall benefits of this setup is of course discussable, but as I said, the main purpose of this project was mostly educational
For those of you who are interested, here is the schema I came up with. The switch symbol is not standard, please refer to the numbers for positions.
Hope it helps!