Question about Dual Polarity Power Supp. (Ashamed to ask)

[cpemma]

Originally posted by TaaJ
I did know that speakers alternate negative and positive. I wans't aware so much about cpemma talked about negative and positive signal halves (I'm assuming the it would look like a jagged sine wave?)

So say if the amp has a 44volt potential difference, it could potentially use 22 volts to amplify the negative signal or the positive signal.

May be easier to follow looking at the LM1875 datasheet, which has two very similar examples, a split-rail version on p1, and the single supply equivalent on p2.

Split-rail, the input capacitor is often optional, so the input signal can come straight in to pin1, get amplified and go straight to the speaker from pin4. Amplifier voltage gain is 21, so +1V input (which could be the peak of an input sine wave) is amplified to +21V , -1V input (trough of the same wave) is amplified to -21V. Providing the power rails are a bit greater than +/-21V, say +/-25V, you've a working amplifier.

Single-rail, you have to add a few more parts. The two equal resistors R1 & R2 split the supply voltage down the middle, and add half-supply voltage to the signal at pin1. Taking 50V as the supply, a +1/-1V sine wave input becomes a +26V/+24V signal and is amplified to +47V/+3V. (The reference point for this amplification is 25V, often called a "virtual earth"). Still within the supply rails, so no problem. But before sending the amplified signal on to the loudspeaker from pin4, you need to remove the 25V DC offset, which will push the speaker cone hard in one direction even with zero volts signal input, so the large capacitor C6 is added as a DC-blocking filter.

Ignoring any sound quality issues, the single-rail system needs more parts, more expense. And for the same power amplifier, touching a supply wire gives twice the shock.

[TaaJ]

Originally posted by cpemma

May be easier to follow looking at the LM1875 datasheet, which has two very similar examples, a split-rail version on p1, and the single supply equivalent on p2.

Split-rail, the input capacitor is often optional, so the input signal can come straight in to pin1, get amplified and go straight to the speaker from pin4. Amplifier voltage gain is 21, so +1V input (which could be the peak of an input sine wave) is amplified to +21V , -1V input (trough of the same wave) is amplified to -21V. Providing the power rails are a bit greater than +/-21V, say +/-25V, you've a working amplifier.

Single-rail, you have to add a few more parts. The two equal resistors R1 & R2 split the supply voltage down the middle, and add half-supply voltage to the signal at pin1. Taking 50V as the supply, a +1/-1V sine wave input becomes a +26V/+24V signal and is amplified to +47V/+3V. (The reference point for this amplification is 25V, often called a "virtual earth"). Still within the supply rails, so no problem. But before sending the amplified signal on to the loudspeaker from pin4, you need to remove the 25V DC offset, which will push the speaker cone hard in one direction even with zero volts signal input, so the large capacitor C6 is added as a DC-blocking filter.

Ignoring any sound quality issues, the single-rail system needs more parts, more expense. And for the same power amplifier, touching a supply wire gives twice the shock.

Great post for me, makes alot of sense.

Thanks everyone!