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Audiophile Related - Understanding power, peak, contious and dBs

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DW1961:

--- Quote from: TimFox on August 02, 2020, 09:23:15 pm ---The physical statements are correct. (You should be more careful about the use of common terms such as "adding" and "multiplying";  "adding twice the power" means "tripling the power" in plain English, since 1 + 2 = 3.  Doubling the power does add 3dB to the result).
The final statement about the volume is not a physical statement, but the result of psychoacoustic experiments with the perception by human listeners, and is related to the Weber-Fechner law.

--- End quote ---

Yes, understood about terms, for sure.

OK, so now about my thoughts on the power ratings for an amp, still using our TA 3116D class amp.

The amp ratings are rated at 4 ohms:

–  2 × 50 W Into a 4-ΩBTL Load at 21 V(TPA3116D2

The two other chips are rated with different voltages and ohms.

For example:



Spec Sheet Source: https://www.ti.com/lit/ds/symlink/tpa3116d2.pdf?ts=1596389304658&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPA3116D2

Note that the chips are different. This is from the same spec sheet.

Then we have the spec sheet graphs which do not specify which chip, but seem to be for all (3) chips listed on the spec sheet.



It seems like they are rating it at an average using the graph for 4 ohms. It looks like the power output is an average between THD of 1-10, as is indeed 100 watts total at 4 ohms @ 21V. (Graph to the right of the 8 ohms graph.

If we use the 8 0hm graph, it would seem the average is 29 watts per channel or 58 watts total @ 21V. With a little fudging for graph reading inconsistencies, we could say roughly that it is putting out about half the rated power at 4 ohms as it is at 8 ohms @ 21V.

I'm going to stop here to make I'm not in error.


TimFox:
I think you mean that the power into 8 ohms is approximately half the power into 4 ohms, which is reasonable, since the devices are close to constant-voltage sources.

DW1961:

--- Quote from: TimFox on August 02, 2020, 11:44:50 pm ---I think you mean that the power into 8 ohms is approximately half the power into 4 ohms, which is reasonable, since the devices are close to constant-voltage sources.

--- End quote ---

Yeah that's what I meant.

Okay, so how can I say this?

So what I am understanding is that it is good to have the amp rated at 4 ohms, since driving an 8 ohms load is easier on the amp?

Also, if you connect 4 8 ohms speakers in parallel to that amp chip, then you would share 100 watts with each speaker, and the amp could power it without a problem, since it would be seeing the load as 4 ohms?

If so, yes, the 4 - 8 ohm speakers would be operating at 25 watts each, whereas at 8 ohms and two speakers ( using the calculation that running at 8 ohms and 21 v = 58 watts total) would get 29 watts each.

That sounds like a pretty good trade off for having twice the speakers and only losing 4 watts each speaker?

Just as an aside, that little amp chip is pretty impressive given heat, power efficiency, and THD at 50% volume?

TimFox:
Since transistors normally operate at low voltages (compared with tubes), and 4 ohms is the lowest popular speaker nominal impedance, it is common to design the circuit and power supply with 4 ohms in mind.  The amplifier may run cooler into an 8 ohm load, while delivering less power than into a 4 ohm load.
Two identical 8 ohm speakers in parallel will present a nominal impedance of 4 ohms to the amplifier, and each one will receive half the total power.  Four 8 ohm speakers would give 2 ohms, and you need to see if the amplifier is happy with that low load resistance.  Re-do your arithmetic about four speakers.

DW1961:

--- Quote from: TimFox on August 03, 2020, 01:18:41 pm ---Since transistors normally operate at low voltages (compared with tubes), and 4 ohms is the lowest popular speaker nominal impedance, it is common to design the circuit and power supply with 4 ohms in mind.  The amplifier may run cooler into an 8 ohm load, while delivering less power than into a 4 ohm load.
Two identical 8 ohm speakers in parallel will present a nominal impedance of 4 ohms to the amplifier, and each one will receive half the total power.  Four 8 ohm speakers would give 2 ohms, and you need to see if the amplifier is happy with that low load resistance.  Re-do your arithmetic about four speakers.

--- End quote ---

Tim,

Researched that and got it. (8 ohms/4 speakers = 2 ohm load)

Thanks much for hanging in there with me. I appreciate it.

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