Neutron (Mutron iii clone) variable power options for dying battery effect

[MatthewD]

Hi, I am still a beginner in the electronics world so I would appreciate some help understanding power options for the Neutron Envelope Filter pedal, which I intend to build as my next project. To be clear, what I am about to propose is the first time I have ever tried to "design" part of a circuit myself.

I am interested in experimenting which the Neutron Filter (Mutron iii clone) using low voltage. As some of you may know there has been a rumor on the net for, well... decades regarding low voltage and the Mutron. So I would like to run a test or experiment to check it out.

I have a MXR ISO Brick power supply which has an 18v outlet that is currently unused that I am itching to make use of.

Here is my first attempt at solving the problem, frankly I don't think this will work. I know with two batteries you can create negative voltage by connecting them in series, but will this work the way I have drawn it below where the +18v input is simply split? I am guessing it probably isn't this simple at all.



So I am hoping someone with more experience with electronics can give me some advice...
Perhaps there is there a better way? I am basically happy with most of the circuit... its just getting the negative voltage at the start that is problematic.

Option 2
I am also considering using a LTC3260 Inverting Charge Pump, this is limited to 50mA per output (so 100mA total)... I am not sure if this is enough for this pedal?
https://www.analog.com/media/en/technical-documentation/data-sheets/3260fa.pdf

Option 2a
Another inverter option called the ICL7662, also not sure it this will output the required milliamps.
https://datasheets.maximintegrated.com/en/ds/ICL7662-Si7661.pdf

Option 3
This would involve using a 555 timer circuit, which I understand can handle 18v input... but it looks like the output is square wave which I don't think would be ideal. Or does it matter?
https://www.instructables.com/id/How-to-Make-a-Voltage-Inverter/

So any advice would be appreciated, including a different approach that I have missed, I am hoping to keep it analog though... I know there are some digital chips out there as well. I am keen to use the LM317 type circuit because as a beginner I actually understand what it's doing.

Thanks,
Matt.

[antonis]

I know with two batteries you can create negative voltage by connecting them in series, but will this work the way I have drawn it below where the +18v input is simply split? I am guessing it probably isn't this simple at all.

Indeed it isn't 'cause you need an intermediate point as ground reference..
(+9V in your case in the configuration of +18V / +9V / 0V ..)

Of course, you can split 18V into 2 X 9V, taking point between 9V parts as ground..

[MatthewD]

Thanks so much for that Antonis, that is exactly what I was looking for.

I have discovered a new problem with my circuit though... the LM317 and LM337 require an input voltage 3v above the output voltage, so I would need to input 12v.

My proposed solution is to run a daisy chain cable from my 18v supply, and build two of the circuits you provided.
The first one would have...
R1 20 ohms
R2 10 ohms

The would output +12v, I would discard the -6v

The second one would have
R1 10 ohms
R2 20 ohms

The would output -12v, I would discard the +6v

Otherwise Plan B would be to use a DC Voltage Converter.

Thanks,
Matt.

[antonis]

Proceed into plan B..

By "discarding" - & + 6V respecivelly, you'll result into 2 completely different circuits (the negative will not work, where the positive doesn't need neither R2 nor R1, cause they don't form a voltage divider anymore (R2 is "open"..)
Even in the case of forming a divider with R1/R2, in the mean of divider junction feeding positive reulator input, input voltage shoud strongly depends on load current..

If you had 18VAC, you could marginally result into +/- 9VDC regulators output (+/- 12VDC In) by full wave rectification and splitting midpoint for GND..
(18 X 1.414 - 2 X diode forward voltage drop..)

Or by utilizing two voltage doublers of opposite polarity for the unregulated voltages..



Of course, the lower the XX Voltage the higher the regulator power dissipation [ P = (VIN - XX) * I ]

[PRR]

You have regulated floating 18VDC(?). You do not need regulators. If you want +/-9V, then you want a rail-splitter. Antonis in #3 will work if the + and - demands are not too different or dynamic.

You did not give a link for this Mutron. If it is what I think it is, the heavy surges into the LED may unbalance the split.

[MatthewD]

I have just had another idea which I hope is a simple solution.

If I started with the circuit that Antonis provided, and placed a potentiometer on both the + and - outputs. I have read online that potentiometers have high internal resistance which is good for simulating internal battery resistance.

I would also wire two switches in parallel with the potentiometers so I can bypass them completely.

This would allow for the voltage adjustment, I am however worried that the unbalanced load problem may be an issue... is it perhaps worth giving it a try?

Here is a link to a pdf showing potentiometers being used this way...
https://moodysounds.com/wp-content/uploads/2014/10/dyingbatterysimulatorpot.pdf

Here is a link to the Neutron build guide...
http://www.geofex.com/PCB_layouts/Layouts/neutronpub.pdf

Thanks again for the help.

[antonis]

I have read online that potentiometers have high internal resistance which is good for simulating internal battery resistance.

Potentiometers haven't any internal (or intrinsic or hidden) resistance and charged batteries exhibit a very low impedance (resistance considered in series with ideal voltage source..)
For simulating low voltage level battery, you need a voltage dividing configuration (rather than a series pot wired as variable resistor) almost independed of current rest circuit current drwan..
The above can be uitilized by a pot of body resistance much lower than rest circuit's one..
(the above, of course, results into elevated current consumption..)

I am however worried that the unbalanced load problem may be an issue...

Depended of unbalancing "degree", a stiffer rail splitter migh settle things

[MatthewD]

I think you are right Antonis, I have researched this a bit more and found this article from R.G. Keen...
http://www.geofex.com/article_folders/pedalbdpwr/pedalbd.htm

He uses a second potentiometer to simulate a batteries internal resistance as it drains.

So the proposed circuit would start with the rail splitter circuit you provided (thanks )
Then the + and - side would each have a potentiometer wired as a voltage divider.
Lastly there would be a series potentiometer on both + and - side to simulate dying battery resistance (this is what Mr Keen calls "RBAT" in his article)


I should also mention I have been thinking about a alternate plan as a backup. I have been thinking that one reason this is challenging is that I have limited myself to 18v. So my alternate plan would involve abandoning the 18v power supply and using a wall mount ac to dc adapter which would supply 24v DC at 500mA. I would then use one of the two circuits that you have shown me to supply +-12v... this would be enough to supply the LM317/LM337 circuit. But I would like to try with 18v first.

[antonis]

Honestly, I can't see any reason for stucking to presicely +/- 12VDC..