Limp Mode Debunked?

[EWL5]

I believe most of us are already familiar with the "limp mode" that Amir experienced from his 4-ohm bench tests of certain PAC AVRs that came out in 2021. In the Pioneer 505 re-test thread, I had recently posed the theory that this "limp mode" may be caused by a permanent 4-ohm setting under a low impedance load regardless of whether the AVR was in the "6 ohms or greater" default mode or "4 ohm mode" (I wondered aloud if the result would have been the same if Amir had selected the 4-ohm mode in his testing).

The following is from the original 505 review:

Clipping appears to start at 35W and approximately 75dB THD+N.

We know that Amir never tested the RZ70 (or the Pioneer 805 or Integra DRX 8.4 clones) but there is a comprehensive review by Gene over at Audioholics. In that review, the RZ70 undergoes a series of load tests and Gene goes on to prove that the RZ70 is not limited (i.e. no "limp mode") even under 4 ohms resistance. This was of course a breath of fresh air for all in the community and the RZ70 went on to earn well-deserved praise and admiration. In the same review, Gene actually goes out of his way to flip the switch to put the machine into "4 ohm mode" to see the impact on the RZ70 and also prove his well-regarded recommendation to never flip this switch on any AVR regardless of your speaker's resistance!

Here is that curve under "4 ohm mode" on the RZ70:

Clipping appears to start just past 40W and when you convert the 0.02% THD+N to dB, it comes out to 74dB!

Could this be the smoking gun that proves my theory that the "limp mode" for the 2021 PAC AVRs is no more than a permanent "4 ohm" mode for low resistance speakers?

Gene's review is here if you've never read it:

Onkyo TX-RZ70 11.2CH AV Receiver Review

The Onkyo TX-RZ70 11.2CH AV receiver boasts 140 watts/ch, 8K HDMI inputs, Auro 3D, FREE Dirac Live and the BEST bench test results we’ve seen in its price class. Is this the best AVR at under $3k?

www.audioholics.com

 

[peng]

I believe most of us are already familiar with the "limp mode" that Amir experienced from his 4-ohm bench tests of certain PAC AVRs that came out in 2021. In the Pioneer 505 re-test thread, I had recently posed the theory that this "limp mode" may be caused by a permanent 4-ohm setting under a low impedance load regardless of whether the AVR was in the "6 ohms or greater" default mode or "4 ohm mode" (I wondered aloud if the result would have been the same if Amir had selected the 4-ohm mode in his testing).

The following is from the original 505 review:

View attachment 521822

Clipping appears to start at 35W and approximately 75dB THD+N.

We know that Amir never tested the RZ70 (or the Pioneer 805 or Integra DRX 8.4 clones) but there is a comprehensive review by Gene over at Audioholics. In that review, the RZ70 undergoes a series of load tests and Gene goes on to prove that the RZ70 is not limited (i.e. no "limp mode") even under 4 ohms resistance. This was of course a breath of fresh air for all in the community and the RZ70 went on to earn well-deserved praise and admiration. In the same review, Gene actually goes out of his way to flip the switch to put the machine into "4 ohm mode" to see the impact on the RZ70 and also prove his well-regarded recommendation to never flip this switch on any AVR regardless of your speaker's resistance!

Here is that curve under "4 ohm mode" on the RZ70:

View attachment 521823
Clipping appears to start just past 40W and when you convert the 0.02% THD+N to dB, it comes out to 74dB!

Could this be the smoking gun that proves my theory that the "limp mode" for the 2021 PAC AVRs is no more than a permanent "4 ohm" mode for low resistance speakers?

Gene's review is here if you've never read it:

Onkyo TX-RZ70 11.2CH AV Receiver Review

The Onkyo TX-RZ70 11.2CH AV receiver boasts 140 watts/ch, 8K HDMI inputs, Auro 3D, FREE Dirac Live and the BEST bench test results we’ve seen in its price class. Is this the best AVR at under $3k?

www.audioholics.com

Who's arguing, or doubting?

 

[kyuu]

Could this be the smoking gun that proves my theory that the "limp mode" for the 2021 PAC AVRs is no more than a permanent "4 ohm" mode for low resistance speakers?

I'm not clear what the difference is supposed to be here. Almost all modern speakers dip down to 4 ohms or lower over some part of their impedence curve, so if the AVR does this if it is somehow detecting a low impedance load regardless of the setting of the "4-ohm mode" switch then still seems like a problematic feature. The limp mode could be triggered at any time, and as Amir noted there is no clear indication when it goes into this mode and it does not automatically clear.

Still a big no thanks for me when there are other options without this issue.

 

[EWL5]

I'm not clear what the difference is supposed to be here. Almost all modern speakers dip down to 4 ohms or lower over some part of their impedence curve, so if the AVR does this if it is somehow detecting a low impedance load regardless of the setting of the "4-ohm mode" switch then still seems like a problematic feature. The limp mode could be triggered at any time, and as Amir noted there is no clear indication when it goes into this mode and it does not automatically clear.

Still a big no thanks for me when there are other options without this issue.

Yes, the issue is it's an involuntary "4 ohm mode" if you happen to be lucky enough to have challenging speakers.

Plenty of folks out there who say this "limp mode" doesn't exist and is in Santa and Easter Bunny territory!

 

[peng]

I'm not clear what the difference is supposed to be here. Almost all modern speakers dip down to 4 ohms or lower over some part of their impedence curve, so if the AVR does this if it is somehow detecting a low impedance load regardless of the setting of the "4-ohm mode" switch then still seems like a problematic feature. The limp mode could be triggered at any time, and as Amir noted there is no clear indication when it goes into this mode and it does not automatically clear.

Still a big no thanks for me when there are other options without this issue.

If it was designed half competently, it would not be triggered by just overcurrent alone, but also time. Such a protection scheme typically would be based on an time/overcurrent curve. So, momentary dips to 4 ohms or even 2 ohms should not trigger the so called limp mode. For example, it may get get triggered if it sense overcurrent that correspond to around 4 ohms impedance for say 10 seconds, or around 2 ohms if it stays around there for even just 1 second. Again, that's just numerical example I make up for demo purposes only, what kind of time/current curve they used is an unknown to us.

Regardless, the flaw here is not the time/current curve, or algorithm, but the fact that they somehow decided to have the limp mode latched, once triggered, and could only be reset by cycling power (according to Amir's finding). That's the real, or even fatal, flaw, otherwise it could be a feature that I might even welcome.

 

[EWL5]

One would hope that Onkyo has learned their lesson and make sure their AVRs bench better for the RZx1 generation.
Otherwise, I know what additional test I'd ask Amir to do when he gets his hand on one!