Bass preamp in a stompbox

[Shlafenflärst]

Hi everyone !

Rather unusual question, I think, but I'm willing to give it a try.

I don't like active basses. They sure have good sides, but I prefer my preamp on the floor, not inside the bass. But I do like the preamp on my Sire M2 (Marcus Miller Heritage-3), and I wish I could use it with all my basses.

So, since I'm planning to change the electronics on that M2 to make it completely passive, I'm wondering if I could put the preamp in a stompbox, with extra components if necessary. Has anyone ever done that ? Do you think it's possible ?

Thanks in advance !

[Fancy Lime]

Sure, no problem. Putting what used to be on-board electronics in a stomp box is usually much easier than the other way round. However, if you want to use a power supply, you may need to add some additional power supply filtering to reduce noise. You can also change the opamps from the low current types used in most on-board electronics to prolong battery life (most popularly, the TL062), to their low nois equivalents (TL072).

Cheers,
Andy

[Shlafenflärst]

Thanks !

I won't use batteries, one of the reasons why I don't like active basses is precisely that, I will only use power supply. So I'll need to filter that. I found this, but they talk about 9V supply, I need 18V, what do I need to change ?

And for the sound part, I just need to connect the input jack where the pickups were connected, and leave the output one where it is ? I also want to add a volume pot (the one from the bass is stacked with a tone pot that does absolutely nothing to the sound, there's no point in keeping that), is it better to put it before or after the preamp ? I'll also replace the active/passive toggle switch with a footswitch, that shouldn't be too complicated.

[DnD]

Ciao
Only use isloated (plastic) jacks if you are gonna case it in a metal box and if you want to add a bypass switch, check if out is already connected to ground via 1M resitor to avoid pops when engage

Regards
Alessandro

[Shlafenflärst]

Thanks !

Why do I need isolated jacks ? I get that it helps avoiding ground loops, but not every stompbox has isolated jacks... is that because the preamp comes from a bass ? Also, I still need to connect the frame to the ground somehow, right ?

[vigilante397]

I'm also curious, why do you think it needs isolated jacks? I have literally never isolated a stompbox, including preamp-in-a-pedal builds. It's typical to use non-isolated jacks to get the benefit of enclosing the circuit in a grounded chassis.

[Nitefly182]

There is no reason it would need isolated jacks. This is no different than any other pedal you might build. Its going to be an active tone stack and a gain stage most likely.

The extra power supply filtering and changing the opamps are good ideas though. Batteries are a very clean source of DC for low voltage circuits so preamps usually don't need much filtering and they could do with saving space. Add some basic filtering to the DC jack, connect your in/out to a pair of jacks or a switch if you want switching, and you should be good to go.

[Shlafenflärst]

Thanks !

For the filtering, as I said, I found how to do it, but I don't know which components to use, since I need a 18V power supply.

Should I need a different capacitor ? Also, what type of capacitor is the best for this ?

And how do I chose the inductor ? I don't know anything about inductors, I don't even know where to find one...

Also, I thought I could add a diode to that, to protect the preamp in case i mistakenly use a power supply with inverted poles (I always make sure I don't, but you never know...), is that useful ? And if so, how do I chose the diode ?

[vigilante397]

Should I need a different capacitor ? Also, what type of capacitor is the best for this ?

I usually use a 100uF and a 100nF in parallel. If you're running an 18V input you'll want at least 25V caps, personally I would go 35V or 50V just to be safe.

And how do I chose the inductor ? I don't know anything about inductors, I don't even know where to find one...

I wouldn't bother with an inductor. They're great for filtering, but generally overkill for most pedals.

Also, I thought I could add a diode to that, to protect the preamp in case i mistakenly use a power supply with inverted poles (I always make sure I don't, but you never know...), is that useful ? And if so, how do I chose the diode ?

Definitely a good idea. One fairly standard practice is to use a low forward voltage diode like a 1N5817 or similar in series with the input supply. It will drop the voltage a little bit, but it will protect the circuit if you accidentally plug in the wrong polarity.

BONUS POINTS: If you have a lot of extra space you could use four diodes and set up a bridge rectifier, which means you still have the voltage drop, but you can use either polarity power supply and it will work. I've been meaning to start doing that for convenience, but I keep forgetting every time I do a design

[Shlafenflärst]

Thanks !

I wouldn't bother with an inductor. They're great for filtering, but generally overkill for most pedals.

So just a resistor, then. Again, anything to change because of the 18V ?

Definitely a good idea. One fairly standard practice is to use a low forward voltage diode like a 1N5817 or similar in series with the input supply. It will drop the voltage a little bit, but it will protect the circuit if you accidentally plug in the wrong polarity.

I have a 1N4007 and two 1N4148, that wouldn't fit ?

BONUS POINTS: If you have a lot of extra space you could use four diodes and set up a bridge rectifier, which means you still have the voltage drop, but you can use either polarity power supply and it will work. I've been meaning to start doing that for convenience, but I keep forgetting every time I do a design

Yeah, I thought about that. The preamp is rather small, so I should have enough space to fit that in.

[DnD]

Ciao, I suggested isolated jacks because we're talking about a cheap preamp installed in a bass, where cannot be any ground loop, and with only hi impedance Pup's as input. How it will behave in a pedalboard chain with other stuff? This will, with pwr filtering and 1M resistor to out and gnd, keep the noise lower.
Regards
Alessandro

[Shlafenflärst]

I found the box :



Now I'll have to eat what's inside.

One question that hasn't been answered : would it change something to put the volume pot before or after the preamp ?

[samhay]

One question that hasn't been answered : would it change something to put the volume pot before or after the preamp ?

Yes, but without a schematic for the preamp, it will be difficult to tell by how much.
The bigger issue you have is that the preamp has pickup blend. You can't make this work with a standard mono output from the bass.

[Shlafenflärst]

The pickup blend is not on the same PCB as the preamp. If I'm not mistaken there is a mono signal (two wires, black and white) from the PCB with the blend, volume and tone to the one with the preamp.
(there would be an issue if I wanted to take the preamp from a V3 or V7, since the pots are all aligned they have everything on one PCB)

I can't find schematics on the internet, I'll try to draw something and post pictures when I remove it from the bass.

[Shlafenflärst]

Hi everyone ! Just removed the preamp from the bass, here it is :

The black and white wires on the top left come from the other PCB, on which are the balance, volume and tone pots. So they should be connected to the input jack.
On the top right is the active/passive toggle switch, which should be replaced with the footswitch.
On the bottom left are the bass pot, and obviously the output jack.
And the red and black wires on the bottom right come from the power supply.

The PCB has tracks on both sides, and some go under components, too complicated for me to draw schematics. I hope I can work without those...

So, for the volume button, the question remains, but I'm wondering, what if instead of putting it on the input or the output, I replace the blue pot on the PCB ?

Thanks in advance !

[Shlafenflärst]

Wow, that was exactly a year ago ! I had forgotten about this project, but for some reason i felt suddenly motivated !





Made some tests with what i had on my hands, and it works perfectly fine ! A couple of things, tho :

- Adding a volume pot to the circuit doesn't do anything the volume pot on the bass or a volume pedal won't do. It will only turn the volume down. Unless i decide to add a gain section to the project, there's no use for that.

- The blue trim pot on the PCB is used to balance the output level between active and passive modes. Since it doesn't go all the way down to 0 and it can only get slightly louder than the original setting, it wouldn't be good as a proper volume pot, maybe as a level pot, but I wouldn't use it much.

So all I have to do now is to build the power filter. I might already have everything I need except the DC jack.

[anotherjim]

Depending on where you are on the globe, most hobby component supplies stock a limited range of electrolytic caps. You have all the standard capacitance ratings but they may only have each one for one voltage rating. Most are at least 16v which means most stompbox and maker/robotics projects are covered. You will find that say 1uF might be 50v, 10uF might be 25v while 100uF are 16v. As more capacitance is squeezed in a cap of a given size, the voltage rating falls. For your 18v power you need 25v minimum. There is no penalty for using a higher rating such as 50v or 63v except they may be in larger cans.
For most audio circuits, the capacitors might be the only thing where the voltage rating could catch you out.

I don't know the circuit you are thinking of, but if the preamp is for x2 9v batteries, are you sure it isn't a split bi-polar supply of +9v/-9v? If it's using opamps, it might well be split like that.

[Vivek]

Putting what used to be on-board electronics in a stomp box is usually much easier than the other way round.
Cheers,
Andy

At a bar, one man boasts : My wife is so smart, she had a Bikini that she did not want, she made a little bow tie for me out of it.

Other man boasts : That is nothing ! I had a little bow tie that I did not want, my wife made a Bikini out of it !!

[Shlafenflärst]

Depending on where you are on the globe, most hobby component supplies stock a limited range of electrolytic caps. You have all the standard capacitance ratings but they may only have each one for one voltage rating. Most are at least 16v which means most stompbox and maker/robotics projects are covered. You will find that say 1uF might be 50v, 10uF might be 25v while 100uF are 16v. As more capacitance is squeezed in a cap of a given size, the voltage rating falls. For your 18v power you need 25v minimum. There is no penalty for using a higher rating such as 50v or 63v except they may be in larger cans.
For most audio circuits, the capacitors might be the only thing where the voltage rating could catch you out.

I don't know the circuit you are thinking of, but if the preamp is for x2 9v batteries, are you sure it isn't a split bi-polar supply of +9v/-9v? If it's using opamps, it might well be split like that.

I have harvested a lot of component from various PCBs, including many capacitors and resistors. I'm not sure yet but I might have all I need. I even have a diode bridge so I won't have to worry about the polarity of the DC jack. And if I miss anything, I know where to buy components.

As to the original power supply, the two 9V batteries are wired in series, so it is indeed 18V.

[Shlafenflärst]

Back here with a question : I have pretty much everything figured out, I just need to chose 2 resistors, one for the power input and one for the LED.

I plan on using a 18V 100mA power supply, which gives me 1,8W, so do I use 2W resistors ? Or more as a safety precaution ?

And then, I need to calculate the resistor that goes with the LED. I found the formula R=(Vs-Vf)/If where Vs is the input voltage, Vf is the voltage the LED needs, but is If the intensity of the power supply or the one the LED needs to work ?