Is THD additive?

PolksTillIDie

I keep trying to get this basic question answered with online searches without success: is THD additive across audio components? Is the THD I hear as I listen to my system the sum of all the THDs of the individual components?

Is there possible any "filtering" of upstream distortion by my ultra-low distortion M-80?

mhardy6647

PolksTillIDie wrote: »

I keep trying to get this basic question answered with online searches without success: is THD additive across audio components? Is the THD I hear as I listen to my system the sum of all the THDs of the individual components?

I... I don't know! (sounds of Marky being hurled into the Gorge of Eternal Peril)


Basically, the component with highest THD will dominate what you hear (sort of the "weakest link" phenomenon).

PolksTillIDie wrote: »

...
Is there possible any "filtering" of upstream distortion by my ultra-low distortion M-80?

The answer to your second question is "no". You can't make distortion go away downstream. You may reproduce it more accurately, though.

And the THD of loudspeakers is at least an order of magnitude higher (typically two orders of magnitude -- or three) than any self-respectin' upstream (electronic) component.

EDIT: It is probably worth mentioning that THD per se, while is is a straightforward and easily calibrated objective measurement of amplifier performance, is kind of meaningless. THD (as a single figure) doesn't give any insight into the harmonic distribution of the distortion components. I say this because some forms of harmonic distortion (though distortions they be!) are not unpleasant sounding. The second harmonic is exactly one octave above the fundamental. High levels of 2nd HD are both tolerable and (even can be) pleasant to hear! Odd-order harmonics are dissonant and are objectionable at much lower levels. A push-pull amplifier topology cancels out even-order harmonics; all of the HD from a push-pull amp is in the form of "dissonant" odd-order components. Conversely, a single-ended amp's distortion spectrum may contain even and odd order harmonics. A much higher THD level may be tolerable in an SE amp compared to a PP amp.

Traditionally any level of THD less than 1% was considered "hifi".


Hope this helps!

mhardy6647

Oh, one other comment about THD.

Measuring THD as a function of output power is a pretty good, straighforward, and objective way to determine the output power of a given amplifier into a given load. THD will increase rapidly as the amplifier transitions form linear operation into "clipping".

Here's some real world data from Stereophile for a fairly fancy ($8495) solid state power amplifier.

Fig.6 Mark Levinson No.333, distortion (%) vs output power into (from bottom to top at 10W): 8 ohms, 4 ohms, and 2 ohms.

source: https://www.stereophile.com/content/mark-levinson-no333-power-amplifier-measurements

The "clipping" points into each load impedance is (or at least should be) pretty visually apparent.
JA put it thus:

As can be seen from fig.6, it clipped (defined as 1% THD+N) at 400W into 8 ohms (26dBW) with both channels driven and a wall voltage of 117V. This almost doubled to 700Wpc into 4 ohms (25.4dBW) with both channels driven, and almost doubled again at 1250W into 2 ohms (25dBW), one channel driven. Note that the AC line voltage drooped to 114V for these last two measurements, implying that with a "stiff" AC supply (as Madrigal recommends), the No.333 will pump out even more watts.—John Atkinson

[emphases added]

pitdogg2

[emphases added]

Is that first or second order ?

mhardy6647

regular ol' plural, so second.
one emphasis, two emphases -- like basis and bases.
English, it's what's for dinner.

PolksTillIDie

mhardy6647, that last graph is interesting and somewhat anticipates why I'm asking this question. I'm trying to decide if upgrading the coils on my SDA-1C's to get even better bass is worth dropping down to 4 ohms and losing some ground on THD. If its additive of all the inline components its worth it probably. If its multiplicative, then I might have to listen to decide if its worth it. Luckily, its just about $100 to find out and if I don't wanna risk it I could always sell the coils to one of you guys.

PolksTillIDie

Since my last post on this topic I've gone ahead and massively modded the SDA-1Cs with Erse inductors and all is well at 4 ohms. All thanks to westmassguy for the upgrades. Highly recommended.

So, what? Me worry?