Dead Yamaha RX-A2020 - only 5 months old :-(

Discussion in 'AV Receivers & Amplifiers' started by PH001, Feb 27, 2014.

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  1. PH001

    PH001
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    Was playing some 2ch music at 0db at the weekend (via Focal Chorus 726V).... yes it's quite loud but sounded nice and clean, no distortion. All of a sudden the amp shut down and would not restart (just blinking standby LED).

    Checked speakers which are all still OK and measuring 5 ohms or so. Amp was only playing for about 10 mins so not even that hot, and it's very well ventilated anyway.

    Eventually managed to get into the diagnostics and "I-protect" was flagged. Further investigation revealed the main power darlingtons on the front left amp channel are short circuit.

    Not too impressed. I opted for Yamaha as they have a great rep for reliability but to have the main semis go short without warning doesn't seem like great design to me.

    Anyway, unit is obviously still in warranty but will have to go to a general authorised repair house to be sorted as Yamaha themselves don't have a UK service presence I don't think. It will probably end up at Sontec in Norwich.
     
  2. PH001

    PH001
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    I'v been thinking further about this failure (and perhaps more importantly how to prevent it happening again) and am wondering if the main speakers (Focal Chorus 726V) impedance is part of the problem? These are described as nominal 8 ohm speakers but actually they dip down as low as 2.9 ohms at around 110Hz.

    The Yamaha amp is set to 8ohms but there is an option to reduce this to 6ohms I think. Does anybody know what actual difference this makes?
     
  3. dante01

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    Its rated all the way down to 4ohm so the receiver shouldn't have any issues in relation to powering your speakers. Your particular problem doesn't appear to be widespread with other units so you may just be an unlucky rare case of product failure? I'd check your wiring prior to connecting everything back up to the receiver when it returns just to ensure that nothing is shorting external to the receiver.

    Impedance switching is merely implimented to meet US certification requirements and does nothing more than reduce the rail voltage. Yamaha themselves have stated that impedance switching hasn't reduced or increased the number of reported failures to them since its introduction so it can be assumed that it has had no effect in terms of protecting the hardware? It is more likey to cause your speakers to be underpowered than protect the amp powering them. Leave impedance switching alone irrespective of the impedance of the speakers being powered. More here:

    Impedance ohms and their relevance to speakers and AV amplifiers | AVForums
     
    Last edited: Mar 26, 2014
  4. PH001

    PH001
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    That's pretty much what I had read previously. 4 ohms is one thing but the Focals dipping down to 2.9ohm is a bit naughty really. I seem to remember reading something once that said to be rated as a 'nominal 8 ohm speaker' they had to stay above 6ohms throughout the entire freq range.

    I'm certain the wiring is not shorted anywhere as it is all on Nakamichi banana plugs and being an electronics engineer I'm very aware of that soft of thing.

    The Yammy is supposed to be proected against shorted speakers anyway so even a shorted connection doesn't really excuse the failure.
     
  5. dante01

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    The clue is in the rating, "nominal". All speakers are given a nominal impedance rating and what you've measured is nothing unusual. No speaker has a fixed and definable impedance and all speakers will have a variable load depending upon the frequencies they are handling at any particular moment in time. The only time you'd worry about this is when dealing with speakers with a 4ohm nominal rating that predominantly remained below 4ohm.
     
  6. PH001

    PH001
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    Below is for the 826V but it is virtually idential to the 726V.

    While the impedance does remain above 8 ohms for most of the bass and treble regions, it varies a lot, with a minimum of 2.6 ohms at 119Hz. There is also a current-hungry combination of 3.83 ohms magnitude and –53° electrical phase angle at 94Hz, a frequency where music can have high energy levels. I think most people would argue that this speaker should be used with a good amplifier which is stable at 4 ohms.


    Focal Chorus 826W, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
    [​IMG]
     
  7. dante01

    dante01
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    As I said, the rating is nominal as it is with the impedance rating given to all speakers. The receiver has no issue power speakers far more difficult to power than your speakers. I think others would have come to the same conclusion as you if the speakers had in fact been mistakenly rated 8ohm instead of 4ohm? The rating is correct and they are 8ohm speakers, not 4ohm speakers.


    Impedance ohms and their relevance to speakers and AV amplifiers | AVForums


    Focal do actually give the minimum impedance in their specifications and state this to be 2.9 ohm, but their typical impedance is much higher and correctly stated to be 8ohm. The speakers are very easy to drive and have a healthy 91.5db sensitivity rating. The RXV2020 should have no issues with powering these speakers.

    The RXV2020 is stable at 4ohm and can power speakers with a nominal 4ohm impedance.
     
    Last edited: Mar 26, 2014
  8. ChuckMountain

    ChuckMountain
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    I currently have a 3010 Yamaha but had previously played with a Yamaha DSP-A1 and AX-1 both of which I partnered with a Primare amp which was a similar power output to them. This drove my 6ohm Kef speakers at higher levels much better (having said that was 5 channel surround though). 0db is loud but again it goes all the way to +16.5dB so it shouldn't have done anything more than powered off if it was overheating?
     
  9. PH001

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    This wasn't an average thermal failure it was a dynamic one. It was about half way through the first track so the heatsinks were barely warm and they are well protected thermally anyway. I guess it was due to going outside the safe operating area of the power darlingtons. It will have have been the combination of volts and current I expect. If it can happen once it will happen again. Have now purchased an emotive xpa-2 to drive the front pair!
     
  10. dante01

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    So why isn't there a glut of overheated RXA2020 AV receivers? As I said, your issue seems to be an isolated case as opposed to being the norm. No manufacturer has a 100% fault free track record and you are unlucky enough to have had a faulty unit.

    Yamaha make very robust amplifiers and their products are probably some of the most capable you'll get for the money. Audioholics regularly test Yamaha products and usually rate them higher then Yamaha themselves rate them.

    What you measured is pefectly normal for 8ohm speakers so why aren't RXV2020s failing left, right and centre?
     
  11. PH001

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    It's not a case of simple overheating.

    All the evidence suggests this is not a faulty unit, in that it was not manufactured with faulty components. It has always worked flawlessly. At just 5 months old we can also rule out component ageing.

    I could say with some confidence that most if not all rx-a2020's would fail under the same conditions..... playing into less than 3 ohms at 0db, probably in combination with the 'wrong song' with lots of energy around 110Hz.

    It is the combination of the voltage and current likely to take the power darlingtons out of their safe operating area. It's one reason why perhaps the amp should be set to 6 ohms when playing into such loads. It stands to reason that if the line voltage is reduced to the darlingtons then you stand a better chance of them surviving.
     
  12. ChuckMountain

    ChuckMountain
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    Fair enough it is unlikely to be overheating in the conditions that you state however the components will be generating a fair amount of heat when running at these power levels.

    With a sample set of 1, unless there is a fundamental design flaw that you have observed directly at a board level, then you cannot draw a meaningful conclusion from that.

    Your components could have unfortunately been less tolerant than they should have been to a near full load and hence failed. That wouldn't necessarily show up as a fault under lower volume usage.

    What piece of music did you play out of interest?
     
  13. PH001

    PH001
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    I'm not sure it's a design floor as such, I think Yamaha would argue that 2.6ohms is outside its specification - partcularly when set to 8ohms internally. A good design would self protect from that but it's not easy to implement such protection very well - otherwise there would be no point in the option setting in the first place. I certainly think there is more to it than simply meeting US certification requirements.

    There is a possibility that you could have tolerance values of all the components adding up the wrong way giving you a marginal design on some units but again I don't think that's likely, as it's fairly easy to model at the design stage.

    My advice to anybody running their Yamaha down to loads at less than 3ohms at high volumes is to set their amp to 6ohms. I think the possible increased risk of clipping into your speakers is a small price to pay to save your amplifier stage. It's certainly what I will be doing.
     
  14. dante01

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    Yamahahave said in the past that the implimentation of impedance switching has made no difference to the number of subsequent reports of faults or damage occuring. Audioholics asked Yamaha about this very thing here:

    Setting the A/V Receiver Impedance Selector Switch | Audioholics


    Impedance switching simply reduces the rail voltage and can potentially under power speakers and damage them as opposed to the amp powering them. Manufacturers did not introduce impedance switching of their own free will and it was only ever introduced in order to satify regulations and certification in the USA and Canada.


    UL/CSA lab certification:

     
    Last edited: Mar 27, 2014
  15. ChuckMountain

    ChuckMountain
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    It's also about nominal impedance, these KEF's are rated at 8 Ohm and similar to what I used, they are a similar sensitivity to yours and dip down to a similar level of low impedance. If you look at other graphs they have similar dips around the same level and you would expect to see more failures if the Yamaha couldn't handle this.

    [​IMG]
     
  16. PH001

    PH001
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    Firstly I would imagine that Yamaha would be very unlikely to openly discuss any failures in a public manner whatever the reason.

    The first part of that post concerns itself with overdrive in terms of maximum power draw from the psu which again is somewhat different to secondary breakdown of the power transistors which is a very 'dynamic' failure.

    There isn't a lot of easy to understand info on the web regarding safe operating areas, I did find this....


    What’s all this SOA stuff anyway?

    The safe operating area of a transistor describes all maximum values of combinations of Ic (or Id) and Vce (or Vds) that will not destroy the transistor or FET. But there is also a 3rd dimension, time. Some combinations of Vce and Ic are allowed only for very small periods, like only 10's of milliseconds or only a few milliseconds.

    Especially bipolar power transistors are very sensitive to a destructive overload phenomenon called Secondary Breakdown. Since the die (the active chip) on the transistor is never perfectly homogenous, some small spots conduct more current than other spots. Those spots get hotter than others, which means they conduct even better, etc. The result is a very hot small spot that 'hogs' the current which can destroy the transistor. The effect is stronger with higher Vce so especially at high Vce the allowed Ic gets quite small. This is reflected in the SOA curve.

    You can see that at say, 60V Vce, the allowed Ic is only 150mA or so, which represents only 9 Watts of dissipation. And this a a transistor with a nominal Pc of 50 Watts! On the other hand, you CAN dissipate 50 Watts in this transistor at Vce = 60V, as long as you limit it to 1uS….

    Not all loads are created equal. The situation is getting worse when we use a real loudspeaker load with crossover filter for an amp. Such a load causes a phase shift between the amp output voltage and the amp output current (the load current) The phase shift may cause the output voltage, for example, to become negative, while the current is still positive, coming from the top half of the amp. That means that the top transistor has even more Vce while still having to source current, and Secondary Breakdown becomes a real danger.

    There are several remedies:

    • Use the transistors with lower supply voltages;
    • Use several parallel transistors so they can divide the load current between them;
    • Built-in a protection circuit that limits the output load current when the device comes in the danger zone.


    ..and there is a good but a little more technical article here, particularly section 3.1:

    3.1 Second Breakdown
    Data sheets usually have a full set of graphs and charts, showing the various device parameters as a function of voltage, current and frequency. In the design phase, all are important, but the most important of all are the two that are most often overlooked by hobbyists and experimenters - thermal derating and safe operating area.

    From the data sheet for the MJL4381A, Fig. 3.1 shows the SOA curve for these devices. Non-repetitive peak currents of up to 30A are permissible for 10ms, but only for collector voltages up to 30V, and only with the junction temperature at 25 degrees. This is a peak power of 300W (the device rating is 230W), but it must be stressed that these conditions must not be allowed to continue beyond the time specified - 10ms is not very long!

    [​IMG]
    Figure 3.1 - SOA Curves for MJL4281A/4302A

    If the time is extended, then the peak current is reduced for a given voltage, and for 1 second, the maximum rated current (15A) may only be drawn at collector-emitter voltages below 15V. This region is limited by the maximum rated current of the transistor, and will never allow continuous operation at maximum power. Remember thermal derating? This is where it must be applied.

    So far, all this looks pretty good if you look at it in conjunction with the demands outlined above, and it even looks as if it would be safe with 4 ohm loads at greater than rated ±35V. Appearances can be deceptive though! Remember that all peak currents and power dissipations referred to were for a junction temperature of 25 degrees - no transistor can maintain that temperature in real life, since there is thermal resistance between the die and case, and further thermal resistance between case and heatsink. See Heatsink Design for more information on thermal resistance and heatsinking of transistors.

    The devices must be derated by 1.84° C/W above 25° (see Fig 2.1), which gives zero dissipation at 150° C. The thermal resistance from junction to ambient air (via the case, insulating washer and heatsink) can be expected to be around 1.5-2° C/W (for a big heatsink), so all dissipation limits quoted can be expected to be as little as 1/2 of those shown in the specifications.

    That means that the 230W transistor is really only capable of a dissipation of around 120W at typical (relatively high) operating temperatures. As a result, at ±35V with a 3 ohm resistive + 3 ohm reactive load (representing a typical 4 ohm speaker either side of resonance), the maximum limits will be exceeded with a continuous (steady state) load!

    Although this is completely true, in reality there are two things that will ensure that the amp remains functional (for many years) - the nature of music itself, and the collapse of the power supply under sustained load. However, continuous operation at full power into a reactance that gives a 45° phase angle will cause the amp to fail, even with ±35V supply rails.

    The variable nature of music, where the frequency and instantaneous amplitude are continually changing, means that potentially destructive signals do not last long enough to cause a problem, however increasing the supply voltage or reducing the load impedance further will almost certainly cause device failure. As you can see from the chart, brief excursions into the 'unsafe' area are permissible - look at the 100ms and 10ms limits.

    Likewise, the bigger the heatsink, the better. The thermal resistances that cause the semiconductor die to operate at a much higher temperature than you may expect are the limiting parts of the equation - and they cannot be eliminated - at least not sensibly. It is generally considered uneconomical to provide a refrigeration system to keep the transistor temperature at low enough temperatures to avoid problems.

    6.0 Conclusion
    It is obviously imperative to avoid second breakdown, and there are many ways that various designers have selected to do so. Protection circuits, Class-G (using two or more supply rails of each polarity), variable supply voltages, and even switched supply voltages - these are common in many home theatre amps, and a switch is sometimes used to select the voltage to suit the load impedance (which simply reduces the supply voltages when 'low impedance' is selected).

    There is also the '"brute force' method, where there are so many power transistors that the cables will melt before any one transistor's ratings are exceeded, but this is uncommon except in extreme high end amps where the added cost is not considered a problem. Many amps provide no protection at all, other than ensuring that dissipation limits are observed, but a shorted speaker lead (or a lower than recommended load impedance) can cause the amp to fail.

    Regardless of the method used, it is important to ensure that the designers' recommendations are followed - good output transistors are expensive, and few of us can afford the luxury (??) of blowing up amplifiers for the hell of it. While a design that exceeds the transistor ratings may last for many years, there will eventually be a combination of circumstances that will cause failure. Parties are a prime cause of blown amps and speakers, and if they feature regularly in your activities, a cheap system (that can play loud, but is very basic and has passable fidelity) is highly recommended. Its failure is not something you would cry over, and the main hi-fi system remains intact.

    Finally, it is important to stress the importance of the SOA curve for any transistor used in an output stage (including driver transistors !). Any design that appears to be able to get more power from smaller transistors has almost certainly pushed the devices to (or beyond) their limits, and when driven hard into a difficult load, it will most probably fail - this is an expensive exercise if it takes the loudspeaker with it (not at all uncommon). Ultimately, a 'worst case' design procedure assumes that the amp will be driven hard into a difficult load, and with undersized or barely adequate heatsinks. Such a design will survive - others will not.
     
    Last edited: Mar 27, 2014
  17. ChuckMountain

    ChuckMountain
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    The article comes back to my suggested point where secondary breakdown is causing the issues due to possible minor manufacturing differences have caused failure when operating at\or near limits. Under lower volumes then your amp would would work well and not notice until you go loud with the wrong type of music. Must been a classic if you are not naming it ;)
     
  18. dante01

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    So Audioholics are telling lies??? :confused:


    Audioholics is a very well respected online site and the question was asked of Yamaha as part of an article they did rather than it being simply offered up in a forum debate or thread. If you want to accuse them of being liars then I think it best you seek legal advise or shut up entirely.

    If failures were being caused by not using impedance switching then those failures would be due to customers misusing said equipment. The failures would have nothing to do with Yamaha products having inherent faults. Why the hell would Yamaha need or want to hide this fact?

    And again. if what you are suggesting is the case then why isn't this forum overrun with complaints about Yamaha products shorting out?

    You had a faulty unit and it is being repaired under warranty. No there's not an inherant fault and the receicver can quite happily power your nominal 8ohm speakers without issue so get over it. If this was not the case then again, why has no one else reported the same issue???? Does this issue only effect those with electrical engineering experience or is more a case of that experience causing you to look at issues that don't actually exist? It's like a car mechanic taking his car's engine apart to see why he developed a puncture.
     
    Last edited: Mar 27, 2014
  19. PH001

    PH001
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    What a strange antagonistic attitude you have Dante. Which is a shame as your posts are usually quite intelligent and informative.

    Of course Audioholics aren't liars, they are just reporting what they have been told. It's what they have been told that I might dispute.

    Make of it what you will. My professional opinion is that this amp failed due to secondary breakdown of the power transistors due to a sub 3ohm load with a high volume transient. There is an outside chance that this was due to tolerancing as Chuck Mountain suggests, but there is little doubt in mind mind that it would fail again if subject to the same thing.

    The upshpot is that this failure is unlikely to have occured at the 6ohm setting, but then again there might have been speaker damage due to clipping. Take your pick.
     
  20. PH001

    PH001
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    ..yes I suppose so. It was Jealous Guy by Roxy music (the live version) o_O
     
  21. dante01

    dante01
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    You appear to be an isolated case so the evidence suggests it not t be an inherant problem or there'd be multiple instances of the same problem, or are you suggesting no one else plays audio that incorporate the same frequencies at the same levels as you? This receiver is nearly 2 years old now and there's hardly ever mention of any hardware issues relating to it over that time period. Play the same music at the same volume when your receiver returns, I can't guarantee the issue will not reoccur, but I'm pretty sure it wont. :)

    Honestly, your speakers are no different to any other speakers rated at 8ohm. Besides this, the RXA2020 will actually power 4ohm speakers without any need to use the impedance switching.
     
  22. PH001

    PH001
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    A much more polite post.

    No one would dispute Yamaha amps are some of the most reliable around but we can't ignore the fact that they do fail, albeit very rarely. The evidence is right in front of me. I think it is an isolated case for sure but I maintain that it was not faulty components that were the root cause.

    It probably requires a set of circumstances to all come together at once with maybe odds of a million to 1 for somebody else to replicate. Even so, it's not something I will be trying again.
     
  23. ChuckMountain

    ChuckMountain
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    Although to be fair it should be easily repeatable or at least the circumstances in your case, although I guess in the immortal words are you feeling lucky punk :)

    Problem is on QAing components they can't test to destruction as its a bit late by then. I suppose its a bit like Intel chips, on the same wafer you end with variances leading to differences in top end clock speed. Ok you don't get the same destructive failure but its a similar principle.
     
  24. dante01

    dante01
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    Anything manufactured by any manufacturer can and will fail. No product is imune from this, but many products do exhibit faults that are inherent in each and every unit. Your issue doesn't appear to be an inherent problem given the lack of reported faults that match yours. Your circumstances are not that particularly rare or specific to you. Are you the only person to play audio loud or to own music by Roxy Music? I've seen and heard Yamaha amps push speakers well beyond 104db without issue and Roxy Music don't own the rights to the frequencies apparent within their work.

    I do actually own some audio that will kill most speakers, but it comes with a warning sticker! Maybe write to Brian Ferry to suggest they do likewise? Damn you Eno :devil:
     
    Last edited: Mar 27, 2014
  25. PH001

    PH001
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    That is a good analogy.
     
  26. PH001

    PH001
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    Just to conclude this thread. I did find our what killed my RX-A2020. Basically the crossovers in my Focal 726V got cooked. The cap was vented and the inductor probably shorted out due to the cooked enamel. This probably resulted in a low impedance which the Yammy didn't like. Incidentally the Emotiva I later tried on these speakers went into safe shutdown... which is how I knew it was a speaker problem in the end. Doesn't really excuse the Yamaha I guess in that it should be able to protect itself from this kind of thing.

    What threw me a bit is that the impedance measured fine with them not powered. I guess it needed a bit of heat in the crossover bits to cause them to short.

    Here is a pic of the xover.... Focal.jpg
     
  27. stephenbarnes

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    If you think the Yamaha shut down due to impedance you know what you need to do. The only time I blew speakers is with a Yamaha AVR.

    Your Yamaha is only a 42W per channel amp

    Those speakers deserve better quality amplifiers.

    The Yamaha is 170W into 2 channel, into 8ohm. But no 4 ohm figure
     

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  28. dante01

    dante01
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    Sorry, but that just isn't the case. The Yamaha is a receiver that originally cost £1,500 and was one level down from their flagship model. The Focal speakers are of a level you'd expect to use with such a receiver.

    The speakers were at fault, not the receiver.

    The speakers are not 4ohm speakers anyway and it was a fault with them that caused the issue. They are 8ohm speakers with a sensitivity of 91db so the RXA2020 would have little if any issue powering them.
     
    Last edited: Jun 4, 2015
  29. stephenbarnes

    stephenbarnes
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    Only if you have 2 connected. I wouldn't use those speakers in AV mode on the Yamaha.
     
  30. dante01

    dante01
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    Why?

    I think you are obsessed with power amps? There's no need for any more power than the RXA2020 can supply to power the Focal speakers. The speakers had developed a fault and not the receiver. The speakers were tested with an external power amp and the power amp was forced into standby. If people wanted a power amp they buy a processor to accompany it and not an AV receiver.

    Yamaha could turn around and refuse to repair the receiver under warranty given that the issues was not caused by a fault with the receiver.

    There's nothing particularly highbrow about the 726V speakers and they come from Focal's entry level Chorus range. The RXA2020 and similar level receivers are used to power higher end speakers to reference levels within moderate sized listening rooms. I'd have no problem suggesting these speakers or speakers from the Chorus range with lower level receivers. The receiver didn't cause damage to the speakers, the speaker were faulty and caused damage to the receiver and the speakers would have caused any amplifier issues irrespective of the cost or capabilities of that amplifier. The speakers were performing without distortion prior to the issue arising and the fault occuring with the speaker. Are you saying that the RXV2020 caused the issue with the speaker? If not then why is a different receiver or source of power required? The only issue with the receiver was that the receiver's protection circuiy=try didn'yt react fast enough to prevent internal damage, but there's no gurantee that any receiver would react any faster.

    AV mode? There's no such mode. It is an AV receiver and its primary purpose is AV.
     
    Last edited: Jun 4, 2015

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