Axiom Home Page Forums General Discussion Home Theater Most Musical HT Receiver

Amer,do you mean that even if you set your 787 to 4ohms that it would still shut off on occasion?

If you're set on the separates, the most obvious suggestion to keep costs reasonably in line would be the Outlaw 950 plus as many M200 amps as you'd need. Let me suggest something different. Consider the Sony DA4ES receiver(available for $650 including shipping), which does all the formats and has an exceptionally wide selection of surround modes and other bells and whistles. If there wouldn't be complete satisfaction running the M80s at either 8ohm or even 4ohm receiver settings, get a pair of M200s to power the M80s, using the DA4ES pre-outs.

John,

I don't think the 787 has a switch for 4 ohm loads (if it does it will be very pleasant news to my ears and wallet). Its only rated down to 6 ohms by Onkyo, and the manual clearly states that running under 6 ohm loads could damage the reciever and invalidate the warranty.

Thanks for the info on the Sony and outlaw stuff, I will definately look into it.

Amer,there isn't an 8/4ohm switch on the back, but I know that the 797(which Chess has)and I'd presume the 787, both have a sub-menu during setup which allows selecting a 4ohm setting. Check your manual and try it. Since the 4ohm setting limits the voltage and therefore the current flow and available power, it should only be used when necessary, although owners manuals, in an excess of caution, usually say to use it whenever 4ohm speakers are present. Let us know what you find.

John,

I can't seem to locate any 4 ohm selection in the manual or as I flip though the setup options on screen. I checked the speaker and audio setup options, and couldn't find a thing. Not sure what section its under on the 797 menu (perhaps chess can let me know where it most likely would be).

On another interesting note, on the last page of the manual it Onkyo rates 'dynamic' power at 250W down to 3 ohms on this sucker. Interestingly enough they rate the continuous average power with all channels driven at 100W RMS for 6 channels driven....surprising since power consumption is rated at 440W. The mystery deepens..........

Went to the Onkyo site and looked at both the 787 and 797 manuals. My presumption wasn't correct; the 797 has a sub-menu during setup which allows setting 4ohms, but the 787 doesn't have such a setting and apparently has no type of adjustment at all for 4ohm speakers. You'd think that this would imply that there would be no trouble with 4ohm speakers and no adjustment was needed, but that's apparently not the case. The only thing I saw in the 787 manual that might even remotely be helpful was a bass-limiter in the sub-menus. It's unlikely that the bass is being overdriven and drawing too much current, but that was the only thing that was even close.

In reply to:

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Certainly if you already have a separate amp for your Hales, then just use the receiver as the pre-amp for processing and let the Adcom handle the power as you seemingly plan to do.

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Right, chesseroo. My Adcom has only 2 channels, so I will still have to use at least 3 channels of whatever receiver's power amp. According to my current plan, those 3 channels will drive a VP150 and a pair of QS8, all rated at 6 ohms. I hope Pioneer and other brands will "approve" this plan. Unfortunately, $1K is about as much as I can possibly spend on the amps at this time (including a pre/pro and power amps). So I do not consider separates right now, although things like Outlaw 755 and M200 are certainly very attractive.

Cheers!

I beleive that's to be expected from the Onkyo ds787, as they specify min. impedance of 6 ohms. On the other hand the DS797 is last years model of the current sr800. It has a setup screen for impedance selection; ie, for less than or greater than 6 ohms. I was not concerned with the unit shuting down. The concern is tonal quality, reduced volume (?), etc., if switched to the low impedance. Anyone have experience with this ?? Sorry! Didn't mean to take this thread away from the original topic!!

Now I am certainly not the most knowledgable when it comes to electronics.... but I know that I always look for the Dynamic Headroom spec. Most integrated receivers don't list this spec...probably because there isn't much and providing it would do more harm than good.

Correct me if I am mistaken, but I am under the impression that the dymanic headrom spec/rating is a good indicator of the amps inherent power characteristics....especially in terms of reserve power (for spikes such as chees eluded to). Is my impression accurate?

It would be nice if manufacturers listed the dynamic headroom of their receivers.

Yes certainly, Mike... But the problem is that there are numerous possible definitions for "dynamic headroom" ratings. When they say "dynamic," how long does the "peak" power or current has to be sustained? - should it last for 1 millisecond or 1 second? - with what level of allowable harmonic distortions? - with how many channels simultaneously driven? - and with what frequency ranges? I have never seen a dynamic headroom spec with all these qualifications.

As far as I know, there is no established industry-standard methods for the measurement of dynamic headroom. This means that there is no way of comparing these numbers in a meaningful manner among different brands. For the same reason, comparing loudspeakers' "frequency response" ratings among different brands is almost meaningless, even if one assumes that all manufacturers publish honest numbers.

There are certain numbers from which you can guestimate the amp's relative power reserve (although it would by no means be quantitative). For example, I understand that H/K publishes what they call "high instantaneous current capability (HCC)." It is certainly nicer to have these numbers than not. But again, without defining what "instantaneous" means, the number conveys little information. Plus, in this case, without having a rating for power-stage's maximum peak-swing voltage, you cannot even start translating the Ampere rating into the actual power delivered to the speaker. For example, H/K's new AVR7200 is said to have an HCC rating of +/- 75 Amperes, which is admirable. But if you do simple calculations, this can imply that the amp is capable of supplying the whopping 45000W of instantaneous peak-to-peak power into an 8-ohm load (75^2 x 8), or 22500W into a 4-ohm load. Of course, this is not at all the case, because the amp's output voltage will clip way before the current limit is reached.

Another rating that I see often cited in higher-end amp specs in relation to dynamic power handling is the total capacitance per channel of power-supply condensers. A $1000-class receiver tend to have 3000-5000 microfarad per channel, while a good separate power amp may boast >20000 microfarad per channel. Again, it is nice to know these numbers, but the capability of an amp's power supply is determined by numerous factors, with the condenser capacitance being only one tiny bit of information.

Obviously, manufacturers regard these specs as powerful marketing tools. Accordingly, they choose to publish only a carefully selected set of ratings, without providing us the whole picture... A huge dilemma for "well-informed" consumers!

Cheers!