Whether your amp performs well may depend on a test nobody is required to run.
A 1970s receiver spec sheet gave buyers more than watts, THD, and frequency response. It also showed intermodulation distortion (IMD), a number meant to show what happened when an amplifier handled more than one tone at once.
That made it one of the few published specs aimed at the messier side of music playback, where many sounds pass through the circuit together.
However, IMD has quietly disappeared from modern receiver pages.
Here’s how that old spec vanished, what it measured, and why its absence leaves buyers with a less complete picture.
Why THD Can’t Tell the Whole Story
The audio industry’s case for dropping IMD from spec sheets is not baseless. THD and IMD come from the same basic problem inside an amplifier.
If that non-linearity is low enough for THD+N to measure clean, the reasoning goes, IMD should also be low enough to ignore. That logic explains why manufacturers could treat IMD as redundant once THD became the cleaner, easier number to publish.
The problem starts when music gets complex.
THD is usually measured with a single tone. It adds frequencies at mathematical multiples of that tone, so the added products remain harmonically related to the original signal. That does not make harmonic distortion harmless, but it helps explain why the ear often treats it as less jarring.
On the other hand, IMD appears when two or more tones pass through a non-linear circuit at the same time. So instead of only adding neat multiples of the original tones, the amplifier also creates sum-and-difference products between them.
Those new frequencies can land in places that have no musical relationship to the source.
That’s why Elliott calls these products “far more objectionable” than harmonics.
Those old IMD numbers gave buyers at least one clue about how an amplifier behaved with more than one tone. The tests were crude, and they did not capture everything about real music. Still, they asked a different question from THD.
The Spec Wars
Those old IMD numbers gave buyers at least one clue about how an amplifier behaved with more than one tone. In the 1970s, that clue was not hard to find because amplifier specs had become part of the sales pitch.
In November 1978, HiFi Stereo Review tested Pioneer’s flagship SX-1980 and found it clipping at roughly 300 watts with intermodulation distortion measured at 0.045 percent. The Yamaha CR-2020 published 0.05 percent THD at its rated 105 watts per channel, with IMD listed in the same spec set.
Those numbers appeared because the FTC had just given the industry a common ruler. Before the 1974 Amplifier Rule, receiver advertising was a free-for-all. There was no standard for measuring power output, and the same amplifier could claim 20 watts under one testing method or 30 watts under another.
The FTC fixed power claims by requiring continuous RMS ratings with both channels driven into 8 ohms, and it mandated THD disclosure at rated power. IMD was not part of that regulatory floor.
That left IMD in a different category: not required, but still useful in a market where Pioneer, Yamaha, Marantz, Sansui, Kenwood, and Technics were fighting over every visible sign of amplifier performance.
So once power and THD were standardized, every remaining number became a way to look stronger than the next receiver on the shelf.
IMD survived because competitors were still printing it. But the moment receiver marketing moved away from amplifier measurements, there was no rule forcing the number to stay.
One Blogger Tested Both
In 2020, independent audio measurement blogger Archimago filled part of that gap by comparing a 1978 Pioneer SX-880 with a 2011 Onkyo TX-NR1009 multichannel audio-video receiver.
Pioneer’s SX-880 was a mid-range stereo receiver that originally cost around $425. At 2V output, it measured around -80dB on IMD. Onkyo’s TX-NR1009 measured -65dB on the SMPTE IMD test, making the vintage receiver look 15dB cleaner by that one method.
Meanwhile, other IMD tests made the gap look much smaller. For instance, Onkyo’s Linkwitz IMD result was -80dB, matching the Pioneer, while triple-tone distortion landed within 1dB between the two. One comparison even told different stories depending on which IMD test was applied.
None of this proves modern multichannel receivers measure worse across the board, though. Instead, it shows why dropping IMD from spec sheets removes useful detail. A single THD+N figure would not show that one IMD method found a large gap while two others showed near-parity.
Where the Weight Went
A 1970s stereo receiver had a simpler job. It took music in, amplified two channels, and used most of its space, heat capacity, and parts budget for that purpose.
Released in 1976, the Pioneer SX-1250 put much of its 64-pound chassis toward two-channel amplification, with independent power supplies, separate rectifiers, and a large transformer feeding four 22,000-microfarad filter capacitors.
Those details mattered because amplifier hardware was still the product buyers were being asked to compare.
In comparison, modern AVRs carry a much wider workload. A single receiver may handle seven to eleven amplifier channels, HDMI switching, surround decoding, room correction DSP, video features, wireless networking, app control, and licensing costs. The amplifier section now shares the chassis with a long list of systems that did not exist in the receiver wars.
This means that, in the 1970s, a receiver could win attention with lower distortion, bigger power supplies, and cleaner amplifier measurements. But today, the sales page has more room for channel count, Atmos support, HDMI features, wireless platforms, and room correction than for a voluntary multi-tone distortion test.
IMD faded in that environment.
Fifty Years Without a Floor
The FTC built its amplifier rule around the problem it could prove. Power claims had become messy enough to confuse buyers, so the agency standardized how manufacturers had to rate and disclose them.
IMD fell outside the fix. Manufacturers were publishing it during the receiver wars, so the agency had less reason to treat it as a consumer-protection problem. Competitive pressure kept the number visible for a while, but competitive pressure is fragile. Once brands stopped fighting over amplifier measurements, the voluntary spec had nothing holding it in place.
This rule continued to evolve without bringing IMD back. The FTC’s 2024 amendments updated power-output testing and capped rated THD at 1 percent, yet intermodulation distortion still appears nowhere in the regulation.
To be clear, buyers do not need to treat IMD as a magic number. A low THD figure is useful, but it is only part of the picture. Third-party IMD or multi-tone measurements are worth checking when available, especially for receivers that pack many channels and features into one chassis.
But a spec sheet that only publishes the cleanest, easiest numbers should leave readers asking what else was left out.