Doubled Voltage - What Happens to the Noise?

[karis12]

I've been experimenting with LT1054s a lot lately, and was wondering about noisy, unfiltered power supplies and what would happen to their characteristic whining when the voltages get multiplied. Do they double in frequency as well?

The reason why I'm asking this is because I'm using an RC low pass filter as per Jack Orman's suggestion to deal with noise from the power supply, but this method lowers the voltage reading by 1 or 2 volts. From 9 volts, I'm left with around 7 to double, which works out to at most 14 to 15 volts of output. Moving the filter to AFTER the doubler circuit raises the resulting voltage to almost 18 volts, which is preferable to me.

Any help, suggestions or pieces of advice is appreciated. Thank you in advance.

[amz-fx]

When a current flows through a resistance, there is a voltage drop across that resistance. Make the resistor smaller and you have less drop.

The switching noise from the LT1054 charge pump will be 25kHz or more and it is fairly easy to filter out with the R-C after it - 4.7 ohms and 22uF should be okay and there will not be much voltage drop. Since the charge pump has a modest amount of output resistance, you can probably get by without the resistor and just use a larger capacitor than the 10uF typically recommended for charge pumps - go with 22uf or 47uf and see how that works in your application.

If you have a cheap unregulated wall adapter with 60/120Hz noise, this isn't going to help since noise from the voltage source is also passed to the output and 4.7/22 isn't going to get it. However, if there is higher frequency switching noise from a regulated adapter, then the added R-C after the charge pump will filter it out. It depends on your adapter and how noisy it is.

Best regards, Jack

[karis12]

Thank you for the reply, Jack! There's always so much to learn from your circuit designs. Sorry for this being so late. The best way to find out is to try it out yourself indeed, and that's what ended up happening to me and the question I originally posted.

I built myself an EP-Pre which needed a charge pump, and did not include a filtering circuit at first. Using my unfiltered wallwart did introduce lots of whining noise as expected, which prompted me to include a post-charge pump RC filter (100 ohms, 100uf had always worked well for me). The result? Very little voltage drop, no power supply noise, and little to no change to the sound. If anything, it probably sounds cleaner due to less noise, but I might just be overselling it haha