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Audiophile help please - Ohms and power

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magic:

--- Quote from: DW1961 on July 30, 2020, 07:43:28 pm ---    2 × 50 W Into a 4-Ω BTL Load at 21 V (TPA3116D2)
    2 × 30 W Into a 8-Ω BTL Load at 24 V (TPA3118D2)
    2 × 15 W Into a 8-Ω BTL Load at 15 V (TPA3130D2)

--- End quote ---
For supply voltage of ±21/24/15V maximum sinewave output is approximately 14/16/10V RMS, respectively. Do the math (P=V²/R) and you get the numbers they give.

Benta:
I understand your confusion, and some power specs for amplifiers are extremely misleading. But you're on the right track.

First, solid-state audio power amplifiers (quaint 70s expression, no?) are almost perfect voltage sources (as opposed to tube amps with output transformer).

This simplifies things, as the electric power absorbed by your loudspeaker (not to be confused with acoustic power!) relates directly to its impedance.
If your amp delivers 10 VRMS into a 4 ohm speaker, this equals 25 WRMS (U2/R). For an 8 ohm speaker, it will be 12.5 WRMS.

10 VRMS equals ~14.1 VPEAK, meaning the amp's power supply should be around +/- 20 V (to have headroom).

"Marketing" power numbers such as "music", "peak" etc. can safely be ignored.

DW1961:

--- Quote from: magic on July 30, 2020, 08:10:40 pm ---
--- Quote from: DW1961 on July 30, 2020, 07:43:28 pm ---    2 × 50 W Into a 4-Ω BTL Load at 21 V (TPA3116D2)
    2 × 30 W Into a 8-Ω BTL Load at 24 V (TPA3118D2)
    2 × 15 W Into a 8-Ω BTL Load at 15 V (TPA3130D2)

--- End quote ---
For supply voltage of ±21/24/15V maximum sinewave output is approximately 14/16/10V RMS, respectively. Do the math (P=V²/R) and you get the numbers they give.

--- End quote ---

If I understand, that seems a weird way to explain the output power drop. 24V is RMS. It seems like in order to get the amplifier output, there is another calculation after you get input RMS voltage? It looks like 1/2 peak power is "continuous?"

helius:
RMS power refers to a periodic AC waveform, but audio amplifiers are powered from DC rails. So the output voltage waveform will have a peak (not RMS) level limited by the supply voltage minus some diode drops. Converting peak to RMS is the familiar divide by sqrt(2).

ejeffrey:


--- Quote ---Also, back to speaker Ohms, if you had that amp and had two identical speakers, but one with 4 Ohms and one with 8 Ohms with the above amp specs, how is that going to effect speaker output?

--- End quote ---

It won't.  The difference between an 8 ohm and 4 ohm speaker is the voice coil.  An 8 ohm speaker will have more turns of thinner wire than a 4 ohm speaker.  When driven with the same power (higher voltage but lower current) the coil will produce the same magnetic field, and thus the same sound.

Speaker impedance is within reason a matter of choice and what is convenient for an amplifier.  Common transistors are happy with 10s of volts and a few amps, so a few ohms is a good target.  If you go to the low end (say 1 ohm or less), then the ohmic losses of your speaker wires and PCB traces start to be a problem, and even the inductance of your speaker wires can become a factor.  At the high end (say 32 ohms or higher) you start needing uncomfortably high voltage in your amplifier to get reasonable power.  This can be an issue for safety as well as limiting your transistor choices.

8 ohms is a pretty happy place for general purpose audio use and therefore became the almost universal standard for home stereo.  Low resistances became common in car audio to get maximum output out of the low supply voltages available (~12 V total) without a boost converter: a 4 or 2 ohm speaker can get a moderate amount of output power from a single 12 supply, especially in a bridge tied configuration.  Even when you have a higher supply voltage, for any given amplifier a lower impedance speaker will draw more power from the amplifier until it gets too low and something breaks.  So people looking for MOAR POWR! will generally pick the lowest resistance speakers their amplifier can drive.


--- Quote from: DW1961 on July 30, 2020, 07:43:28 pm ---Supports Multiple Output Configurations

    2 × 50 W Into a 4-Ω BTL Load at 21 V (TPA3116D2)
    2 × 30 W Into a 8-Ω BTL Load at 24 V (TPA3118D2)
    2 × 15 W Into a 8-Ω BTL Load at 15 V (TPA3130D2)

Since they increased the voltage a little for the 8 Ohm spec, it's still about half of what the 4 Ohm spec is given the same voltage.

--- End quote ---


The maximum RMS voltage is ~1/sqrt(2) the DC voltage and from there you get the power in these configurations.  These specified in BTL configuration where both wires of the speaker are driven with opposite polarity rather than leaving one terminal at ground.  Therefore, a 24 V supply can produce up to + or - 24 V across the speaker.  An old school design would use +/- 24 V rails to accomplish the same thing.

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