"Limp mode" for Beginners

[EWL5]

There's been enough confusion on this topic that I think it rightfully deserves it's own thread:

In 2021, a number of "better than entry" and mid-range AVRs were introduced by Premium Audio Company (PAC), some of the first in the industry to include Dirac room correction in the price! Over the years, these receivers have been lauded by professional and amateur reviewers alike as they provided great value for the price. However, it appears there was a "catch" to the total package. Back in February 2022, Amir of audiosciencereview.com discovered an anomaly in his bench test of an Onkyo RZ50. In his words:

This testing cost me anther hour of sleep (went to bed near 4:00 am!). I started with measuring 4 ohm power as I usually do and got ridiculously low level of output before clipping. I checked all the settings, set the amp to 6 vs 4 ohm and back, power save mode off, etc. but nothing would fix it. On a hunch, I disconnected the power cable, waited a bit and powered the unit back on. I got full power twice but then it limited:

Clearly there is some monitoring going on internally causing the amplifier to go into ECO/power limiting mode. With 8 ohm load, I didn't see this problem:

Given that vast majority of speakers today are 4 ohm or even lower, this is a serious problem. I could understand it if the amp was cooking but it was quite cool running with the upper fan off. Traditionally this mode would be triggered if you set the amplifier to 4 ohm mode but per above, this did not have an effect here.

Given this problem I could not run my max and peak power. But did run my frequency sweeps post a power cable removal:

Onkyo TX-RZ50 Review (Home Theater AVR)

This bench test could easily have been a "one off" but subsequent bench tests of the Pioneer VSX-LX505, re-test of the 505, Onkyo 7100, and RZ30 by Amir produced the same results!

https://www.audiosciencereview.com/...oneer-vsx-lx505-avr-review.40731/post-1438037

Pioneer VSX-LX505 Power Remeasurement

Back in 2023, I reviewed the Pioneer VSX-LX505 AVR. It received the fully broken verdict from me: Reason was that it would severely power limit in my sweeps: Folks theorized that this was maybe fixed in firmware. Well, a member was kind enough to loan me a sample and he assured me is...

www.audiosciencereview.com

https://www.audiosciencereview.com/...onkyo-tx-nr7100-avr-review.56300/post-2054862

Onkyo TX-RZ30 AV Receiver Review

This is a review and detailed measurements of the Onkyo TX-RZ30 9.2 home theater AVR. It was kindly drop shipped to me by a member and costs US $1,100. As you can see, we have the same cookie-cutter design with no attempt at differentiation. As if to copy that into controls, the volume knob is...

www.audiosciencereview.com

What makes this discovery significant is that it didn't just impact 4 receivers but the potential to impact their respective clones as well:

"Better than Entry" clones
Onkyo 7100 (bench tested Aug 2024 and "limp mode" coined in review)
Pioneer VSX-LX305
Integra DRX 3.4
RZ30, spiritual successor to 7100

Mid-range clones
Onkyo RZ50
Pioneer VSX-LX505
Integra DRX 5.4

Many naysayers have said this is simply a form of protection mode and that there's nothing to worry about. However, a traditional protection mode would shut off the AVR and leave it in a state where it may require sending to service or at least a hard reset! Amir rightly coined it a "limp mode" because what it does is it allows the AVR to continue to operate to low impedance loads but at much reduced capacity! As observed by Amir, this limp mode is defeated once you pull the plug and restart the AVR! Anyone whose car has ever entered "limp mode" knows that it only does enough to get you safely to the side of the road for help!

In addition to "limp mode", I may have also called it "stealth protection" in the past (hopefully for obvious reasons). I've recently settled on "unintended 4 ohm mode" because it's the most neutral and is able to convey what's going on w/o judgment.

TLDR Summary for the "unintended 4 ohm mode":

• will activate for the first time on challenging impedance loads (nominally 4 ohms)
• audio will play normally at reduced quality (detection of reduced quality entirely on the owner)
• there is no indication of this mode on the AVR
• there is no indication in the web user interface (not even "protect")
• AVR can only reset by pulling the plug and re-plugging

I've already documented subjective comments from owners in my AVS thread (you can search for it if you'd like as I will not link to it).

 

[Ruediger]

There is also the misleading 4/8 ohm switch on some Yamaha amps. You'd think putting it to 4 is to match the impedance better somehow. But all it does is put the amp into "super weak mode".

I would consider amps that do that on their own as broken. Unless it is clearly stated that they can't do low impedance and the user must expect protection kicking in.

 

[Farva0066]

I wonder if there is more to this than simply encountering an X-impedence load? And is it something that many users would encounter in real-world usage? I'm no expert, but sweeps are rather demanding compared to home theater usage. Must the AVR be at 50% output on a speaker that is sub-6 ohm from 100hz - 500hz (like an RP Klipsch), and the soundtrack is very-heavy in that range for... 5 seconds? Or whatever hypothetical you could come up with.

I'm not saying it's "not a big deal," is ethical, or should be downplayed. But I am saying that such an AVR may still be the correct purchase for a given consumer -- based on *exactly* what causes this to trigger, and if they ever see those conditions in their use case.

There haven't been (widespread) user reports of seeing this behavior in the wild (that I'm aware of). I find it unlikely a company would sell a product that would fail in this way during "normal" usage.

Having just simp'd for the manufacturer for paragraphs... I would flip out it a new AVR I spent a lot of money on, and a lot of time researching/setting up, failed on me every other time I used it. Esp if I didn't encounter the failure until the return window had passed.

 

[EWL5]

I wonder if there is more to this than simply encountering an X-impedence load? And is it something that many users would encounter in real-world usage? I'm no expert, but sweeps are rather demanding compared to home theater usage. Must the AVR be at 50% output on a speaker that is sub-6 ohm from 100hz - 500hz (like an RP Klipsch), and the soundtrack is very-heavy in that range for... 5 seconds? Or whatever hypothetical you could come up with.

I'm not saying it's "not a big deal," is ethical, or should be downplayed. But I am saying that such an AVR may still be the correct purchase for a given consumer -- based on *exactly* what causes this to trigger, and if they ever see those conditions in their use case.

There haven't been (widespread) user reports of seeing this behavior in the wild (that I'm aware of). I find it unlikely a company would sell a product that would fail in this way during "normal" usage.

Having just simp'd for the manufacturer for paragraphs... I would flip out it a new AVR I spent a lot of money on, and a lot of time researching/setting up, failed on me every other time I used it. Esp if I didn't encounter the failure until the return window had passed.

You only have to ask former owners like @dlaloum why they've moved on and it will be clear. Besides, if it was such a strong, robust AVR, it should've had no problem running ART and yet we need to wait until 2027 for the first ones to appear. By that time, D&M would've had at least a 1.5-2 year advantage running ART!

There's nothing more damning than when Andrew Robinson was told by Onkyo that the RZ30 simply didn't have the "horsepower" to run ART!

 

[B-Flow]

This is specifically, and the only, reason I went with a Denon 3800 (such as it is) over the Z50.

I do wonder what exactly the PAC designers are referencing for functions like limp mode, fans, etc. It clearly has nothing to do with heat (at least not directly so). I've never seen an AVR ever use its fans in real-world use (Anthem notwithstanding), even when they were subjectively very hot. While bench testing a couple Onkyos back in the early 2000s I discovered the fans would suddenly come on while test tones were played, driving test loads, and the volume passed a certain threshold. I could litterally get the fans to turn on and off just by tweaking the volume a hair durring the steady-state tests.... irrespective of temperature. So, with those at least, they seemed to be monitoring current-delivered as the trigger for the fans. Their approach was not "fans turn on to keep things cool" bur rather "fans turn on when the box draws more than X amp". I wonder if a similar triger is used for limp mode. Doesnt make it right of course. If the unit is drawing more current from the wall than it can safely handle, the unit should shut down and flash a red LED (or whatever) like a normal AVR, not spontaneously downgrade itself.

 

[audio_tony]

There is also the misleading 4/8 ohm switch on some Yamaha amps. You'd think putting it to 4 is to match the impedance better somehow. But all it does is put the amp into "super weak mode".

The switch typically switches in a different transformer secondary winding.

The transformer will have two separate secondary voltages - one for 4 ohm loads and one for 8 ohm loads.

This is done to limit dissipation in the output stage when driving low impedance (4 ohm) loads, by reducing the overall power supply voltage.

It's been quite common in Yamaha amps in particular for years. Marantz do it in the PM7200 - but that's to switch between class A and A/B modes.

EDIT: corrected typos!

 

[B-Flow]

It's been quite common in Yamaha amps in particular for years.

Not just Yamaha. Virtually every AVR from any manufacturer has a 4-8ohm (or sometimes 4-6ohm) setting though there are a few notable exceptions (Anthem comes to mind). Even flagship models like Denon's A1H have it!

On older models it was a physical switch on the back but most models now implement it as a setting buried in the setup menu (or in the case of Denons, behind an obscure button press combination). Either way, it does the exact same thing.

 

[peng]

Not just Yamaha. Virtually every AVR from any manufacturer has a 4-8ohm (or sometimes 4-6ohm) setting though there are a few notable exceptions (Anthem comes to mind). Even flagship models like Denon's A1H have it!

Anthem does not include that feature because they choose to not rate their avrs for 4 ohm loads.

On older models it was a physical switch on the back but most models now implement it as a setting buried in the setup menu (or in the case of Denons, behind an obscure button press combination). Either way, it does the exact same thing.

 

[B-Flow]

Anthem does not include that feature because they choose to not rate their avrs for 4 ohm loads.

Even though they dont quote power into 4ohm as part of their spec, my understanding was that they are stable/safe with 4ohm speakers (the back panel labeling says "speaker impedance: 4-8ohm"). It would be interesting to know if the MRXs were actually subjected to 4ohm tests as part of their CSA and cUL safety certs.

 

[peng]

Even though they dont quote power into 4ohm as part of their spec, my understanding was that they are stable/safe with 4ohm speakers (the back panel labeling says "speaker impedance: 4-8ohm"). It would be interesting to know if the MRXs were actually subjected to 4ohm tests as part of their CSA and cUL safety certs.

Agreed, but officially Anthem does not say anything about power into 4 ohms, below is from their website:

It clearly shows speaker impedance should be higher than 4 ohms though 6 ohm is okay. Obviously it is safe to connect a 4 ohm load, just don't go crazy on the volume dial Lol.. That spec, like the power consumption specs are among the ones that are vague and even confusing, and manufacturers don't seem to care about that, perhaps they think it is to their advantage to leave such specs ambiguous.

• Number of Power Amps​

  11

• Maximum Continuous Output (1% THD) (20Hz - 20kHz, Two Channels Driven @ 8Ω) Channels: 1~5​

  140 W

• Maximum Continuous Output (1% THD) (20Hz - 20kHz, Two Channels Driven @ 6Ω) Channels: 1~5​

  170 W

• Maximum Continuous Output (1% THD) (20Hz - 20kHz, Two Channels Driven @ 8Ω) Channels: Remaining​

  60 W

• Maximum Continuous Output (1% THD) (20Hz - 20kHz, Two Channels Driven @ 6Ω) Channels: Remaining​

  75 W

• Speaker Impedance​

  >4Ω

 

[EWL5]

Anthem does not include that feature because they choose to not rate their avrs for 4 ohm loads.

I could be mistaken but I think nearly all manufacturers leave out a 4 ohm rating (including Onkyo).

 

[peng]

I could be mistaken but I think nearly all manufacturers leave out a 4 ohm rating (including Onkyo).

I think that is true, the difference is, Anthem is one that (may be others too) explicitly stated “>4 ohm speaker impedance”, whereas Denon, Marantz (may be some others, Onkyo, Yamaha? I did not check on those), are clear about using 4 ohm speaker will be fine, with the 4 ohm setting. We all know they just do that to cover themselves, knowing that if the 4 ohm setting is used, voltage/current will be limited, hence no rating given for that scenario, so yes, some users might be misled, and not aware their AVR would be in V/I limited mode when using the 4 ohm setting.

Anthem's no better, except they chose not to play that game.

 

[dlaloum]

You only have to ask former owners like @dlaloum why they've moved on and it will be clear. Besides, if it was such a strong, robust AVR, it should've had no problem running ART and yet we need to wait until 2027 for the first ones to appear. By that time, D&M would've had at least a 1.5-2 year advantage running ART!

There's nothing more damning than when Andrew Robinson was told by Onkyo that the RZ30 simply didn't have the "horsepower" to run ART!

The claim that the RZ30 and the rest of that AVR Generation from Onkyo/Integra/Pioneer didn't have the "horsepower" to run ART was clearly BS....

Early interviews with Dirac engineers/leaders discussing what was required to run ART clearly stated that if a processor had enough power to run DLBC, then it had enough power to run ART.

DLBC runs on the Onkyo RZ70, and the processor is identical from RZ70 down to NR7100 - so in terms of AVR processing power, they should ALL be capable of running ART.

The decision not to release ART on these platforms comes down to a decision with regards to costs for integration on the current generation and (far more likely) a marketing decision to push the next generation and use ART as a key part of the marketing for it.

I moved from the Integra DRX3.4 to a Denon AVR simply so I could get ART.

With regards to other aspects - I was using the Onkyo/Integra to drive my 8ohm, easy to drive surrounds and heights, and I was using external power amps to drive my hard to drive front L/C/R.

The more "beefy" Denon X4800 has proved to be adequate to drive ALL my speakers, and I have retired my power amps for the time being.
I am certain that the RZ70/DR8.4 would have handled my speakers without any trouble too - and I would have stuck with the house of Onkyo/Integra, if they had supported ART on their platform.... They lost out on an upgrade sale by not releasing ART. I was not willing to wait an additional 2 years.

At present, looking at developments and what is in the pipeline, I cannot see a need for upgrade or side-grade, for the forseeable future (next 5 years+).

 

[dlaloum]

This is specifically, and the only, reason I went with a Denon 3800 (such as it is) over the Z50.

I do wonder what exactly the PAC designers are referencing for functions like limp mode, fans, etc. It clearly has nothing to do with heat (at least not directly so). I've never seen an AVR ever use its fans in real-world use (Anthem notwithstanding), even when they were subjectively very hot. While bench testing a couple Onkyos back in the early 2000s I discovered the fans would suddenly come on while test tones were played, driving test loads, and the volume passed a certain threshold. I could litterally get the fans to turn on and off just by tweaking the volume a hair durring the steady-state tests.... irrespective of temperature. So, with those at least, they seemed to be monitoring current-delivered as the trigger for the fans. Their approach was not "fans turn on to keep things cool" bur rather "fans turn on when the box draws more than X amp". I wonder if a similar triger is used for limp mode. Doesnt make it right of course. If the unit is drawing more current from the wall than it can safely handle, the unit should shut down and flash a red LED (or whatever) like a normal AVR, not spontaneously downgrade itself.

Sadly, since the take over by PAC of Onkyo/Integra, they ceased the practice of publishing the service manuals - so we cannot refer to circuit diagrams.

However, from looking at where the heat has been over multiple generations since circa 2005, I can state with substantial certainty that the heat was primarily from processor chipsets (HDMI, DSP) and not from power amp stages.

The cooling design, the fans and temperature sensors, seemed to have been designed by old school audio engineers, who focused on the power amp circuits.... but considered the logic circuits to be a non issue - hence there were no heatsinks / fans on the processor chipsets - and the issues with heat and reliability ended up being consistently on the processor boards.

The current generation seems to have brought the temperature of the processors under control - most AVR's now fit heatsinks to processor chips.

The "Nanny Protection" seems a holdover of the focus on the power amp circuits - and it appears to completely ignore the processor boards.