Printer Friendly Forum Posts - THD vs S/N Ratio

So, my question pertains to the THD %, S-N Ratio, and power output.

I currently believe that the THD is how clean the signal is that is being sent to the amplifier.
The power output is then based off of 1 watt worth of this unclean signal with the actual output not being affected in any way by the percentage.
The signal to noise ratio is what is affected by the THD % value.
As the THD % value increases, the signal to noise ratio decreases.
That factor is not of any major importance, it is merely logical.
What is important is the paticular value of the signal to noise ratio when compared to the THD % value.
When all amplifiers are compared at the same THD %, the cleanliness of the amplifiers can then be effectively determined.

I was wondering if this was an accurate statement.
I feel like the signal to noise ratio or the THD % value should play some part in the end result of the amplifier power output.

Does anyone know how these three things play on one another or if there are any other parts of the puzzle that I might be missing?

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...typically, I just run whatever I randomly pick up off the floor.
1995 Ford Ranger Supercab
MECA member
Team CSS

Well, yes and no. THD and S/N are not really related.

S/N is the amount of "residual" noise in a system in relation to a standard signal, or the noise that is generated by the electronics itself. The higher the S/N the lower the amount of random noise introduced by the system and the "cleaner" the signal. S/N is probably the most important measurement to consider when comparing amplifiers.

THD is total harmonic distortion and is the square root of the sum of the squares of the individual harmonic amplitudes as a percentage of a standard signal level. This can be thought of as the amount of "ringing" or resonance that occurs in the circuitry. THD (or more commonly THD+N) indicates a percentage of introduced harmonics in relation to the base signal level. Lower THD percentage is better, although the human ear cannot normally detect THD less than 1% and as the signal frequency decreases, higher THD is more easily masked by the signal. In subwoofer frequencies for example, 10% THD is often the threshold before humans can detect it.

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OK, we seem to be VERY confused, here...

THD

S/N Ratio

And power is self explanatory.

As you can see these numbers are completely separate and distinct definitions and specifications, having little or nothing to do with EACH OTHER. Neither of them are affected by the other. The effect of a low S/N can be seen (really, HEARD) by simply placing your ear next to a tweeter on an amp with the input shorted.

Another way to look at it: THD is the "damage done" to a signal BY the amplifier, where S/N is the noise ADDED to the signal by the internal circuitry.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

haemphyst wrote:

OK, we seem to be VERY confused, here...

THD

S/N Ratio

And power is self explanatory.

As you can see these numbers are completely separate and distinct definitions and specifications, having little or nothing to do with EACH OTHER. Neither of them are affected by the other. The effect of a low S/N can be seen (really, HEARD) by simply placing your ear next to a tweeter on an amp with the input shorted.

Another way to look at it: THD is the "damage done" to a signal BY the amplifier, where S/N is the noise ADDED to the signal by the internal circuitry.

What a shame, with all this knowledge, you'd expect one to use better amps than Eclipse posted_image

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I am glad to see there really are people out there that know about this stuff. All the local installers are clueless. I wind up doing most of their research for them.

So, I do have a problem with what haemphyst said there at the end.

"Another way to look at it: THD is the "damage done" to a signal BY the amplifier, where S/N is the noise ADDED to the signal by the internal circuitry."

If the internal circuitry is inside the amplifier, then wouldn't the noise introduced by the internal circuitry be "damage done" to the signal by the amplifier. I am sorry, I am just not making much sense of the statement. The statement makes it seem as though they stem from a similar source. Where exactly do each THD and S/N come from?

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...typically, I just run whatever I randomly pick up off the floor.
1995 Ford Ranger Supercab
MECA member
Team CSS

I guess what this is coming from was I got into car audio just before the CEA 2006 compliance. I didn't know what any of the numbers meant, so I took note of all of them. After most everyone adopted the CEA 2006 compliance, I noticed that they decided to test for 1% THD+N. When they did this, I noticed that the S/N ratio went lower. For most every amplifier, it looked to me like they choose a THD+N that would achieve a 100dB S/N ratio. Now, the THD+N % remains at 1% while the S/N ratio drops down to a range of 60dB to 80dB. However, you are telling me that the relationship that I noticed does not exist. So, why did a change occur in the S/N ratio. Did the CEA 2006 compliance change the that the S/N ratio was calculated? There are still amplifiers out there that do not adopt the CEA 2006 compliance and I need a way to bring them into comparison.

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...typically, I just run whatever I randomly pick up off the floor.
1995 Ford Ranger Supercab
MECA member
Team CSS

Yes, CEA 2006 changes the way all amplifier testing is done. It sets a new standard to replace EIA 517-B that is more consistent from amplifier to amplifier and establishes many standards that did not exist before.

The way S/N is calculated is the biggest change. Under EIA, the S/N could be calculated at max amplifier output, so there was no consistency from amp to amp. Under CEA 2006, S/N is calculated at a standard 1 watt output, which all amps can produce and which allows comparing apples to apples when comparing two different amps. Plus, the hiss that normally makes up the noise in S/N is audible at 1 watt output and may not be at higher wattage levels. It is a much better way to calculate the noise value.

Compliance with CEA 2006 is completely voluntary, so only those manufacturers who choose to do so will list their ratings as CEA 2006 compliant. I will no longer purchase nor recommend purchase of an amp that does not rate per CEA 2006.

THD and S/N are not related to one another. They measure two different things.

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haemphyst wrote:

THD is the "damage done" to a signal BY the amplifier

i.e., how much is the original waveform changed by the amplification stages - THAT is THD.

haemphyst wrote:

where S/N is the noise ADDED to the signal by the internal circuitry.

i.e., the "hiss" you hear in a tweeter with your ear up against it.

S/N is a spec that will exist as long as the amplifier is ON - it is noise that is generated by the gain stages INSIDE of the amplifier, where THD will only be present when the amplifier is "amplifying". If there is no signal to distort, there can be no total harmonic distortion. 10% (possible THD number) of 0 (input signal to be amplified) equals zero.

See, separate "damages" done to the signal, but not the same, and completely NOT interrelated.

dwarren wrote:

What a shame, with all this knowledge, you'd expect one to use better amps than Eclipse

Owowowow... and from HALF A STATE AWAY!.. Damn, Dan, that stings. posted_image

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Thanks guys. I think I was tryin' to make an assumption on a change I observed without full knowledge of the situation. I didn't realize that the CEA 2006 compliance changed the way that the S/N ratio was determined. As far as I could tell, the THD % was the noise introduced and the S/N was the value that came about. I realize now that they are not related in that manner. Thank you for straightening me out.

Do they affect the output at the speaker in any way. I am guessing that if I were to say that the signal to noise ratio or THD+N % were changed that the output of power in watts would change as well, then you might try and come through the computer and strangle me...lol. So, because it is there, it must do something to some output somewhere so, is it the speaker or the amplifier?

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...typically, I just run whatever I randomly pick up off the floor.
1995 Ford Ranger Supercab
MECA member
Team CSS

wormy] wrote:

o they affect the output at the speaker in any way. I am guessing that if I were to say that the signal to noise ratio or THD+N % were changed that the output of power in watts would change as well, then you might try and come through the computer and strangle me...lol. So, because it is there, it must do something to some output somewhere so, is it the speaker or the amplifier?

The distortion and noise characteristics of an amplifier are independent of the power output. Solid-state amps with higher THD and/or lower S/N usually just indicates inferior quality materials, poor design, and/or crappy construction.

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Would it affect the SPL output in anyway in a competition is what I am getting at. I just need to know how to determine exactly what amplifier I want to own for the next hundred years or so. I don't have the money to continue to play around so I have to pick one and stick with it and I figure I should pick a single high powered amp and stay with the lower classes.

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...typically, I just run whatever I randomly pick up off the floor.
1995 Ford Ranger Supercab
MECA member
Team CSS

I need to make sure I pick the best amp I can, so I would like to make sure I understand all of the specifications at the outset. If there are any others that you feel I should look into, please make your suggestions and explain them accordingly. I appreciate the help.

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...typically, I just run whatever I randomly pick up off the floor.
1995 Ford Ranger Supercab
MECA member
Team CSS

Competition and "owning an amp for the next hundred years or so" are two entirely different things!

How about this: You tell us what your need is exactly, what speakers you plan to drive, and how much money you want to spend, and we can make a suggestion or two.

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My honest to god need is to learn. I plan to compete to learn as much as I can. I like helping people as best as I can. I figure the best way to learn is through a combination of hands on experience, constant exploration of thoeries that others turn into books, and by asking those who have done both already. I don't have alot of money that I can afford to waste over the next couple of years so I would like to make sure I exhaust all of the research options before I waste my time and money. Speakers I understand very well. I can easily find information in that field, although, I may start asking a few questions there in the future, but in this paticular thread, I only meant to check into THD and S/N, not get on your bad side. Sorry...

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...typically, I just run whatever I randomly pick up off the floor.
1995 Ford Ranger Supercab
MECA member
Team CSS