Author Topic: Audiophile help please - Ohms and power  (Read 12957 times)

0 Members and 1 Guest are viewing this topic.

Offline DW1961Topic starter

  • Frequent Contributor
  • **
  • Posts: 753
  • Country: us
Audiophile help please - Ohms and power
« on: July 30, 2020, 07:08:26 am »
If you have an amp rated at 100 watts RMS total @ 4 Ohms (but can power 4-8 Ohm speakers), if you plug 8ohms speaker into it, do you get 50 watts total in reality?

I thought a 4 Ohms speaker would be more sensitive than an 8 Ohm speaker, thus requiring less power for the same volume. I've been reading what sounds like it is the opposite. If a 4 Ohms speaker is less resistance, wouldn't that make it easier to drive with less power?

Thanks again.
 

Offline DaJMasta

  • Super Contributor
  • ***
  • Posts: 2393
  • Country: us
    • medpants.com
Re: Audiophile help please - Ohms and power
« Reply #1 on: July 30, 2020, 07:39:57 am »
The answer is not really, if you take the basic ohms law V = IR and the basic calculation for power P = VI, you'll find that to get a given power you need a combination of current and voltage applied.  So with a lower rated resistance of your speaker, there needs to be more current applied by the driver, and with a higher rated resistance it needs more voltage to get the equivalent power dissipated by the speaker in either case.

Since your driver says it can drive 4 or 8 ohm speakers, it can likely put 100W into either (since it would be sort of fudging its specifications if it can't!), but for any given amplifier, you're going to be limited by your driver supply current when driving lower impedance (resistance) speakers and you're going to be more limited by your drive voltage capability when dealing with higher impedance speakers.

It ends up being finding an optimum point on a few curves given your driver circuit and your load (the speaker), and usually the maximum output of the driver comes along with some distortion, so there's also probably a THD concern when driving at the edges of your driver's output ratings.


So more generally, a speaker rated for however many watts should sound equivalently loud as another speaker with a different rated impedance properly driven with the same power - though depending on speaker construction and choice, there can be some difference in efficiency of turning the electrical power into vibrations in the air given the physical construction of the given speaker.
 
The following users thanked this post: DW1961

Offline Psi

  • Super Contributor
  • ***
  • Posts: 10386
  • Country: nz
Re: Audiophile help please - Ohms and power
« Reply #2 on: July 30, 2020, 07:41:24 am »
The speaker is just a load.

If you put a 4 ohm load across a PSU that's outputting 12V you get 36W
If you put a 8 ohm load across a PSU that's outputting 12V you get 18W

The 4 ohms speaker draws more current for the same voltage.

So a 4 ohm speaker will pull more power from the amp and be louder than an 8 ohm speaker.

Of course there is a limit, the amp must be able to supply the current.
You can't put a 1 ohm speaker on the amp and expect even more power. Unless the amp is rated to drive 1 ohm speakers


« Last Edit: July 30, 2020, 07:44:36 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 
The following users thanked this post: DW1961

Offline DW1961Topic starter

  • Frequent Contributor
  • **
  • Posts: 753
  • Country: us
Re: Audiophile help please - Ohms and power
« Reply #3 on: July 30, 2020, 07:48:23 am »
The speaker is just a load.

If you put a 4 ohm load across a PSU that's outputting 12V you get 36W
If you put a 8 ohm load across a PSU that's outputting 12V you get 18W

The 4 ohms speaker draws more current for the same voltage.

So a 4 ohm speaker will pull more power from the amp and be louder than an 8 ohm speaker.

Of course there is a limit, the amp must be able to supply the current.
You can't put a 1 ohm speaker on the amp and expect even more power. Unless the amp is rated to drive 1 ohm speakers

Okay, so if you run identical speakers except for ohms, then the lower Ohm speakers would put out the same volume with less power needed?
 

Offline Psi

  • Super Contributor
  • ***
  • Posts: 10386
  • Country: nz
Re: Audiophile help please - Ohms and power
« Reply #4 on: July 30, 2020, 07:54:58 am »
The speaker is just a load.

If you put a 4 ohm load across a PSU that's outputting 12V you get 36W
If you put a 8 ohm load across a PSU that's outputting 12V you get 18W

The 4 ohms speaker draws more current for the same voltage.

So a 4 ohm speaker will pull more power from the amp and be louder than an 8 ohm speaker.

Of course there is a limit, the amp must be able to supply the current.
You can't put a 1 ohm speaker on the amp and expect even more power. Unless the amp is rated to drive 1 ohm speakers

Okay, so if you run identical speakers except for ohms, then the lower Ohm speakers would put out the same volume with less power needed?

Nope.
Volume is power and power is volume.
You cant get free power or free volume.
If you run two identical speakers except for ohms one will use more power and be louder and one will use less power and not be as loud.

You can get more efficient speakers which are louder than other speakers at the same power.
But this is not ohms related, this is because they are made out of lighter materials so need less power to move air.
Both speakers are getting the same power, but in one of them more of that power is converted into sound.
« Last Edit: July 30, 2020, 08:01:31 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline Psi

  • Super Contributor
  • ***
  • Posts: 10386
  • Country: nz
Re: Audiophile help please - Ohms and power
« Reply #5 on: July 30, 2020, 08:05:28 am »
I think i can see where you might be getting confused.

The volume knob on your amp isn't setting the power directly, is not an "amount of power" knob.
Its scale changes depending on what speakers you have on it. It's more of a voltage knob.

If you have 8 ohm speakers the max position on the vol knob might be 100W
but if you change to 4 ohm speakers the max position is now 200W and half way will be 100W and be same volume as max was with your 8 ohm speakers.
Something like that..
« Last Edit: July 30, 2020, 08:08:12 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline magic

  • Super Contributor
  • ***
  • Posts: 7454
  • Country: pl
Re: Audiophile help please - Ohms and power
« Reply #6 on: July 30, 2020, 08:14:49 am »
I haven't read the whole thread and don't know if this answers the question, but

The volume knob on your amp isn't setting the power directly, is not an "amount of power" knob.
Its scale changes depending on what speakers you have on it. It's more of a voltage knob.
::)
It is exactly a "voltage" knob. It simply controls effective end-to-end voltage gain of the whole preamp - power amp chain. At one setting the output volts might be 100x the input volts, at another it might be 1x. The power stage will supply whatever current to the load to maintain the expected voltage.

Exception may be some "toob" amps or other audiophile contraptions with crazy high output impedance.
 

Offline Michaelaudio

  • Regular Contributor
  • *
  • Posts: 212
  • Country: de
Re: Audiophile help please - Ohms and power
« Reply #7 on: July 30, 2020, 10:22:30 am »
Its a combination of efficiency and Impedance  (and frequency response) Lets say u have one 8ohm woofer and one 4ohm, 4ohm with 83db @2,83V/m and the other 8ohm with 93db @2,83V/m efficiency. U apply 2,83v to both speakers(and the same frequency), the 4 Ohm speaker will draw more current from the amp than the 8ohm one, but would still be not as loud as the 8Ohm couse the speaker is way less efficient. There are 10db diference of efficiency between the speakers (10db more means double the volume for ur ears)
 

Offline DW1961Topic starter

  • Frequent Contributor
  • **
  • Posts: 753
  • Country: us
Re: Audiophile help please - Ohms and power
« Reply #8 on: July 30, 2020, 07:43:28 pm »
I want to get back to the speaker aspect of this, but this might help me understand. Here is the TEXAS INSTRUMENT 3116D amp power specs:
https://www.ti.com/document-viewer/TPA3116D2/datasheet

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.


I'm wondering why only 60 watts instead of 72 total, since 8 ohms / 24V / 3 A = 72 watts?

For instance, 8 Ohms @ 19V = 45 watts

How do they get 60 watts at 24V?

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?

Damn I'm having a hard time with this. It seems like you would want to increase power for the 8 Ohms spec, but that's my confusion dammit. It's like the 4 Ohm will use more power, gets more power, and the 8 Ohm NEEDS more power, but gets less. Shit!
« Last Edit: July 30, 2020, 07:48:18 pm by DW1961 »
 

Online Siwastaja

  • Super Contributor
  • ***
  • Posts: 9339
  • Country: fi
Re: Audiophile help please - Ohms and power
« Reply #9 on: July 30, 2020, 08:03:47 pm »
Efficiency is a completely different parameter.

Similarly, you can have a LED light bulb designed for 12V, 110V, or 230V. All can have good, or poor efficiency.

The speaker impedance is an arbitrary parameter that can be designed quite freely. A lower impedance allows lower voltages to be used yet still deliver quite some power to the load. If you do have enough voltage available, the speaker designer can design it for higher impedance. The benefit is less losses in wiring. Unsurprisingly, lower impedances are seen in automotive, where the limitation of 12V battery was relevant (modern high-power amps boost the voltage internally to overcome this limitation, though); in PA, higher impedances are seen, so that higher voltages are used, so that the relative voltage drop over the long wiring isn't a problem - the exact same thing why power delivery networks use higher voltage than what you have at your home.

All of this does not relate to the sound level in any way.

Sound level relates to the power and efficiency. Whatever power can be delivered at any impedance, it's just about designing the speaker and the amplifier. And, good efficiency can be had at any impedance, as well (ignoring extremes which will make the target difficult; say a 0.1-ohm or a 100000 ohm speaker would likely be inefficient with all the real-world constraints).There is quite some efficiency variation in speakers, and it's not always specified at all. An efficient speaker puts out quite some volume at, say, 10W, while another one might require 50W to sound the same and mostly produce heat.
« Last Edit: July 30, 2020, 08:15:07 pm by Siwastaja »
 
The following users thanked this post: DW1961

Offline magic

  • Super Contributor
  • ***
  • Posts: 7454
  • Country: pl
Re: Audiophile help please - Ohms and power
« Reply #10 on: July 30, 2020, 08:10:40 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)
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.
 
The following users thanked this post: DW1961

Offline Benta

  • Super Contributor
  • ***
  • Posts: 6420
  • Country: de
Re: Audiophile help please - Ohms and power
« Reply #11 on: July 30, 2020, 08:13:11 pm »
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.
 
The following users thanked this post: DW1961

Offline DW1961Topic starter

  • Frequent Contributor
  • **
  • Posts: 753
  • Country: us
Re: Audiophile help please - Ohms and power
« Reply #12 on: July 30, 2020, 10:00:25 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)
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.

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?"

« Last Edit: July 31, 2020, 12:25:14 am by DW1961 »
 

Offline helius

  • Super Contributor
  • ***
  • Posts: 3688
  • Country: us
Re: Audiophile help please - Ohms and power
« Reply #13 on: July 30, 2020, 10:30:17 pm »
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).
« Last Edit: July 30, 2020, 10:31:48 pm by helius »
 

Offline ejeffrey

  • Super Contributor
  • ***
  • Posts: 4034
  • Country: us
Re: Audiophile help please - Ohms and power
« Reply #14 on: July 30, 2020, 10:43:30 pm »

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?

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.

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.


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.
 
The following users thanked this post: DW1961

Offline Dundarave

  • Regular Contributor
  • *
  • Posts: 160
  • Country: ca
Re: Audiophile help please - Ohms and power
« Reply #15 on: July 30, 2020, 11:02:51 pm »
Another, simpler (I think) way to look at this is from an impedance-matching perspective:

For maximum power transfer between any amplifier circuit and its load, the output impedance of the amp needs to match the speaker.  Any other load impedance will, theoretically, reduce the amount of power delivered to the load.  It’s as simple as that.

That’s the theory.  The reality is that the impedance of anything is dependent on the frequencies involved, but practically speaking, matching an 8 ohm speaker with an 8 ohm amp is always going to deliver the most power to the speaker.
 
The following users thanked this post: DW1961

Offline DW1961Topic starter

  • Frequent Contributor
  • **
  • Posts: 753
  • Country: us
Re: Audiophile help please - Ohms and power
« Reply #16 on: July 31, 2020, 12:30:02 am »
Another, simpler (I think) way to look at this is from an impedance-matching perspective:

For maximum power transfer between any amplifier circuit and its load, the output impedance of the amp needs to match the speaker.  Any other load impedance will, theoretically, reduce the amount of power delivered to the load.  It’s as simple as that.

That’s the theory.  The reality is that the impedance of anything is dependent on the frequencies involved, but practically speaking, matching an 8 ohm speaker with an 8 ohm amp is always going to deliver the most power to the speaker.

Yes, I do understand that the  impedance changes depending on what sound is begin generated, but keeping it static is all my head can handle. Thanks for that simplistic explanation. However, I do need to do some calculations, so I'm missing something in the formula (see post below).
« Last Edit: July 31, 2020, 12:45:41 am by DW1961 »
 

Offline retrolefty

  • Super Contributor
  • ***
  • Posts: 1648
  • Country: us
  • measurement changes behavior
Re: Audiophile help please - Ohms and power
« Reply #17 on: July 31, 2020, 01:35:29 am »
Another, simpler (I think) way to look at this is from an impedance-matching perspective:

For maximum power transfer between any amplifier circuit and its load, the output impedance of the amp needs to match the speaker.  Any other load impedance will, theoretically, reduce the amount of power delivered to the load.  It’s as simple as that.

That’s the theory.  The reality is that the impedance of anything is dependent on the frequencies involved, but practically speaking, matching an 8 ohm speaker with an 8 ohm amp is always going to deliver the most power to the speaker.

 I have the understanding that audio amps normally have a much lower output impedance rating then the speakers they drive by a factor of 50 to 150. In fact there is a quality specification parameter called damping factor covers this topic
 ( https://en.wikipedia.org/wiki/Damping_factor ).

As most solid state audio amps normally use significant negative feedback (Vs tube amps} to maintain a linear output voltage it should be considered as having a very low output impedance Vs normal speaker impedance. Your point is about when most efficient power transfer occurs, but most all audio amps are not designed and used for best efficiency, that doesn't sell amps.  :-DMM
« Last Edit: July 31, 2020, 01:37:12 am by retrolefty »
 

Offline Dundarave

  • Regular Contributor
  • *
  • Posts: 160
  • Country: ca
Re: Audiophile help please - Ohms and power
« Reply #18 on: July 31, 2020, 02:41:56 am »
Another, simpler (I think) way to look at this is from an impedance-matching perspective:

For maximum power transfer between any amplifier circuit and its load, the output impedance of the amp needs to match the speaker.  Any other load impedance will, theoretically, reduce the amount of power delivered to the load.  It’s as simple as that.

That’s the theory.  The reality is that the impedance of anything is dependent on the frequencies involved, but practically speaking, matching an 8 ohm speaker with an 8 ohm amp is always going to deliver the most power to the speaker.

 I have the understanding that audio amps normally have a much lower output impedance rating then the speakers they drive by a factor of 50 to 150. In fact there is a quality specification parameter called damping factor covers this topic
 ( https://en.wikipedia.org/wiki/Damping_factor ).

As most solid state audio amps normally use significant negative feedback (Vs tube amps} to maintain a linear output voltage it should be considered as having a very low output impedance Vs normal speaker impedance. Your point is about when most efficient power transfer occurs, but most all audio amps are not designed and used for best efficiency, that doesn't sell amps.  :-DMM

Lol, I’m not going to argue the point, as I was only talking about “maximum power transfer”, but an amp driving a 4 ohm speaker to have a reduced impedance factor of 50 to 150 below 4 ohms has got to have a pretty insignificant output impedance!
 

Offline ejeffrey

  • Super Contributor
  • ***
  • Posts: 4034
  • Country: us
Re: Audiophile help please - Ohms and power
« Reply #19 on: July 31, 2020, 04:24:32 am »
Another, simpler (I think) way to look at this is from an impedance-matching perspective:

For maximum power transfer between any amplifier circuit and its load, the output impedance of the amp needs to match the speaker.  Any other load impedance will, theoretically, reduce the amount of power delivered to the load.  It’s as simple as that.

The maximum power transfer theorem only applies to linear (and stable) sources and loads, so while it kind of hints at the right answer (making the load *closer* to the source impedance increases power output), it is wrong if taken literally.  If you put a power matched speaker on a typical audio amplifier the power transfer will be zero as you will blow a fuse or power transistor.

Quote
That’s the theory.  The reality is that the impedance of anything is dependent on the frequencies involved, but practically speaking, matching an 8 ohm speaker with an 8 ohm amp is always going to deliver the most power to the speaker.

(Solid state) audio amplifiers are almost always very low impedance -- a small fraction of an ohm, not 8 ohms or similar.  An amplifier described as an "8 ohm amp" just means that is the type of speaker it is designed for.
 

Offline DW1961Topic starter

  • Frequent Contributor
  • **
  • Posts: 753
  • Country: us
Re: Audiophile help please - Ohms and power
« Reply #20 on: July 31, 2020, 04:58:08 am »
    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)
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.

24V RMS 2*/8 ohms = 72 total Then /2 = 36 watts per channel.

Why do you say that 24 volts they use is maximum and not RMS? If it's a 24V power supply, then that's RMS, isn't it?  I mean you don't rate power supplies by peak voltage. That's what Where in the link is the voltage being lost? If I can get this, then were done with the Amp discussion. I'll get it.

I'm reading this and trying to figure it out:
"Now let’s look at the RMS values. In our example, we know the RMS voltage is 10 volts. We saw earlier that 10 squared is 100, and 100 divided by 5 [ohms] gives us a calculated power of 20 watts."
https://geoffthegreygeek.com/understanding-amplifier-power/
« Last Edit: July 31, 2020, 05:00:35 am by DW1961 »
 

Offline ejeffrey

  • Super Contributor
  • ***
  • Posts: 4034
  • Country: us
Re: Audiophile help please - Ohms and power
« Reply #21 on: July 31, 2020, 05:27:14 am »
24 volts is the DC supply voltage to the amplifier.  It's a constant input voltage, so peak, RMS, average are all the same.  When the amplifier has to produce a varying waveform from the 24 V input source, it can produce any voltage up to (slightly less than) 24 V. The voltage is dropped across the amplifier's power transistors.  So if you want to produce a sine wave with maximum power output, it would have a 24 V peak.  In that case the RMS voltage would be 17 V.  (17 V)^2/(8 ohm) is 36 watts into an 8 ohm speaker.  In reality it won't be able to go all the way to 24 V, there will always be some voltage drop on the power transistors, hence the 30 W rating.
 
The following users thanked this post: DW1961

Offline DW1961Topic starter

  • Frequent Contributor
  • **
  • Posts: 753
  • Country: us
Re: Audiophile help please - Ohms and power
« Reply #22 on: July 31, 2020, 05:38:17 am »

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?

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.

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)




"It won't.  The difference between an 8 ohm and 4 ohm speaker is the voice coil. . . .  When driven with the same power (higher voltage but lower current) the coil will produce the same magnetic field, and thus the same sound."

But they aren't being driven with the same power for that 3116D amp. One gets 50 watts per channel and one is getting 30. They do have diff. voltage and current, but the power output is different.

I really apologize if I am just hacking this thing to death. I really am trying. I know you can change voltage and current and get the same power rating. But you did say "same power" so I'm going with that :)
 

Online Siwastaja

  • Super Contributor
  • ***
  • Posts: 9339
  • Country: fi
Re: Audiophile help please - Ohms and power
« Reply #23 on: July 31, 2020, 06:03:09 am »
Another, simpler (I think) way to look at this is from an impedance-matching perspective:

For maximum power transfer between any amplifier circuit and its load, the output impedance of the amp needs to match the speaker.

Noooooo!!!11

This stupid thing, taught in university classrooms, goes around and around misunderstood from decade to decade.

Well obviously, from a very limited perspective, it is true. If you have an arbitrary Thevenin equivalent source, say an amplifier which output transistors define output impedance of 10 ohms, then if you do want to get the maximum possible power from it, you match the output load of 10 ohms.

But then you make the logical mistake and turn it around - if you have a 10 ohm load, to get maximum power possible, do you use a 10 ohm output impedance amplifier? Obviously, no! Use 1 ohm, and you'll get even more power. Run the math, it's trivial!

The key you didn't think about in school is this:
Often:
* the power used by the load does actual work (is converted to sound in a speaker, is converted to mechanical energy in a motor, is converted to light in a lightbulb...)
* the power used by the amplifier is wasted power (converted to heat in the amplifier)

From this perspective, matching the impedances so that power use is shared 50%-50% is a ridiculous idea. Instead, if you choose Zamplifier = 0 and Zload = whatever to bring the voltage & current relationship to the most optimal point for practical design, you have just the amount of power you need, with perfect load voltage regulation, and best possible amplifier efficiency.

The imaginary amplifier with 10 ohm output impedance was probably never meant to drive a 10 ohm load, but maybe a 1000 ohm load. Why?

Because amplifiers are often designed to be voltage sources that can drive the output to whatever value within its rails. If it had the equal output and load impedance, then the output voltage would change by factor of 50%. Try to output 10 volts - only 5 volts available to the load. 5 volts wasted as heat in the amplifier (this obviously won't matter in a linear amplifier which wastes power as heat anyway).

This is why amplifiers have the feedback, which is designed to bring the output impedance close to zero. The transistors and PCB traces themselves still have some on-resistance, so the feedback needs some voltage leeway to work with. Want to output exact 10V, to a load which hogs 1A? Say, transistors drop 1V because of their internal on-resistance. Is the output dropped to just 9V, and is the output impedance 1V/1A = 1 ohm? No, and no. The feedback measures the output and adjusts the transistors to be "more on" until the output is at 10V, and the output impedance is hence 0V/1A = 0 ohms.

So whenever you want to drive a load with a voltage source, the correct impedance is not the load impedance, but zero. In case of linear amplifiers driving intermediate levels, it's about voltage regulation, giving the load the correct voltage. In case of switching amplifiers, it's all about heat as well. If you match the source and load impedance, the efficiency of the amplifier is limited to just 50%, which would be very crappy for such circuit.
« Last Edit: July 31, 2020, 06:09:17 am by Siwastaja »
 
The following users thanked this post: Someone, retrolefty, uer166

Online Siwastaja

  • Super Contributor
  • ***
  • Posts: 9339
  • Country: fi
Re: Audiophile help please - Ohms and power
« Reply #24 on: July 31, 2020, 06:14:06 am »
"It won't.  The difference between an 8 ohm and 4 ohm speaker is the voice coil. . . .  When driven with the same power (higher voltage but lower current) the coil will produce the same magnetic field, and thus the same sound."

But they aren't being driven with the same power for that 3116D amp. One gets 50 watts per channel and one is getting 30. They do have diff. voltage and current, but the power output is different.

To drive them with the same power, you need to up the voltage. This may mean changing to another amplifier chip designed to handle higher voltage, but now this chip doesn't need to deal with as much current.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf