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Negative version of LP2951 LDO

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MagicSmoker:
You don't worry about matching impedance between outputs and inputs in audio, nor do you even want to. In fact, you want to present any signal source with a relatively constant (and relatively high) impedance so peaks aren't distorted from increased loading.

You also want the DC blocking cap to work into a relatively high and constant impedance to minimize its size and maintain a consistent frequency response, so the better arrangement would be to put the cap first - that is, in between the source and the top of the pot - with the wiper from the pot going to the mixer input.

In your example the volume control pot is 10k and the DC blocking capacitor is 1uF so the -3dB point (that is, when the reactance of the capacitor equals the resistance of the pot) will be 16Hz*. For high-fidelity applications you'll want the cutoff under 10Hz and this is best done by increasing the capacitance, rather than the resistance of the volume control pot, because the latter makes the circuit more noisy. I'd use at least 3.3uF for the DC blocking capacitor here, though there is no harm in going up to 10uF so long as you don't use ultra high-k ceramic types (like Y5V and Z5U) as they are piezoelectric and suffer a number of other pathologies that make them terrible for use in audio or other sensitive signal amplification applications.


* - the equation to determine the reactance of a capacitor at a given frequency is Xc = 1 / (2 * Pi * f * C)

exe:

--- Quote from: MagicSmoker on September 10, 2019, 07:43:17 pm ---
--- Quote from: edavid on September 10, 2019, 03:28:24 pm ---1. For a simple design like this, a rail splitter would be as good as split supplies
--- End quote ---

Functionally, sure - it just turns the mid-point of a positive supply into the 0V reference - but this also elevates the "supposed to be grounded" shell of shielded interconnects to the mid-point voltage which can cause problems when connecting to other devices, especially if they share a chassis ground and/or are earthed.

--- End quote ---

Can shield be connected to "real" ground? In my understanding, any fixed potential is good for shielding.

MagicSmoker:

--- Quote from: exe on September 11, 2019, 11:29:41 am ---Can shield be connected to "real" ground? In my understanding, any fixed potential is good for shielding.

--- End quote ---

Shield is usually connected to earth ground in XLR (balanced) interconnects, but shouldn't be - yet still often is - in RCA and similar type interconnects. In the latter cases this results in the dreaded ground loop.

Zero999:

--- Quote from: ZeroResistance on September 10, 2019, 08:50:55 am ---
--- Quote from: Siwastaja on September 10, 2019, 08:30:05 am ---For price reduction, try to think how you'd get rid of the negative rail. If you really need it, does it really need to be regulated?

--- End quote ---

I am trying to build a audio mixer in the virtual earth configuration.
So my thinking was virtual earth needs the non -inverting input to be at ground potential, so that pulls the inverting input to a virtual ground.
Is my thinking flawed and if so is there some means to eliminate the negative power supply in a virtual earth configuration.

--- End quote ---
Just AC couple all of the signal paths and bias the amplifiers to work at half the supply voltage.

The volume control should be between the two amplifier stages. I've converted the second one to a unity gain buffer and put the volume control before it.

ZeroResistance:

--- Quote from: Zero999 on September 11, 2019, 12:56:51 pm ---
Just AC couple all of the signal paths and bias the amplifiers to work at half the supply voltage.

The volume control should be between the two amplifier stages. I've converted the second one to a unity gain buffer and put the volume control before it.

--- End quote ---

This is still great! but would it still count as an virtual earth mixer?
The earth point has now shifted to 1/2 VCC.
From what I understood about the concept of a virtual earth mixer, is that if all the input signals see a true virtual earth. There won't be any crosstalk between them.

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