I've scanned through all of the Onkyo & Integra "official" threads and off-shoots and I really think this warrants a separate topic of discussion.
I have an Onkyo TX-SR705 and never noticed any hum issues until I knew what to look for but I don't know what should be expected. After reading so many complaints I started looking at my own receiver more closely and have found that I have a very noticeable hum coming from all of my speakers when BOTH of the following two conditions are met...
1.) The volume is set to anything above -15db. The hum at reference level (0db) is VERY noticeable and annoying.
2.) ANY DSP mode is engaged. The receiver output is (nearly) completely silent while in "Direct" and "Pure Audio" modes.
Although we really never listen to a movie or music at reference level it is my understanding that every "THX Certified" receiver should meet certain quality standards while playing at this volume. Since (I believe) mine falls short I was wondering if maybe my expectations are too high, if I have a problem with my unit, or if this is a common trait among some or all of the new Onkyo lineup? I have written Onkyo AND THX on this and have yet to hear back.
I do not believe my issue is related in any way to video noise or other outside electronic inference because the hum exists even when ALL of my components are disconnected from the receiver. If you have a new Onkyo or Integra receiver (and a few minutes) please take the time to try this and report your results...
1) Turn off all of your components EXCEPT your receiver.
2) Switch to any input (other than the radio or phono sources).
3) Select any DSP mode (DD, DTS, etc...)
4) Turn the volume up to 0db.
5) Listen for speaker hum
6) Switch to "Pure Audio" or "Direct"
7) Listen for hum again
The steps make it sound like a chore but it should only take less than a minute. Please report your results, good or bad here. In my case there is a SUBSTANTIAL loss of hum once the receiver is no longer processing the signal (Direct and Pure Audio modes).
I would also be very interested to hear from the Integra owners out there, see if that extra money made a difference.
Since my initial post I have done some more testing, here is what I found...
First, all tests below were performed starting at a 0db volume and without ANY components connected, I even unplugged my AM/FM antennas. The ONLY things connected to my receiver were 5 speakers (4 towers, 1 center).
Here is what I found...
At first the problem *seemed* like it may be related to EQ settings but I doubt the EQ itself is the issue. It may just be that the amplification of certain frequencies exaggerates things? Read on, I'll explain.
Here are the tests I performed and my results...
1st: EQ tests
With Audyssey engaged the issue is MUCH more pronounced. When I turned off the EQ entirely the hum was not completely gone but it was not noticeable unless you were close to a speaker and really listening for it (good enough for me to say "no hum" but an audiophile would disagree). When I started playing with the EQ manually I found that if I bumped up the level past 0db on the lower bands the hum began to come back, if I bumped up the higher bands the hum was still there but the tone changes to almost a soft buzz. This was consistent no matter what input source or DSP mode was selected (this does not apply to "Pure Audio" and "Direct", I'll get to those later). When I disabled the EQ again I tried changing only the bass/treble tone controls, similar results (increasing the bass raised the hum, increasing treble raised the buzz) the only thing different while testing the tone controls was that the hum disappeared under this scenario with THX engaged. THX seems to make no difference if the manual EQ or the Audyssey EQ is engaged.
2nd: Volume tests
I then reset the tone controls to 0 again and began playing with volume. With the EQ off and the master volume set to 0db I was back to (almost) no hum. I then began turning up the volume until the hum began to get very noticeable again, around the +10 level (with the EQ still off). I turned it back down to 0db and tried raising the InteliVolume, same results as the master volume (as would be expected). Then I reset the Intelivolume and tried the volume on each speaker independently, same results per speaker.
3rd: Crossover tests
It's important to note that during all of the above mentioned tests I had no sub-woofer connected and my speakers were all set to "Full Band". I reset the volume again (my EQ was still off at this point) and the hum disappeared again. I then turned the volume up to +15 so that hum was nice and annoying and I switched my speakers from "Full Band" to 80Hz, the hum was slightly reduced and it changed to a softer buzz type of sound (just like increasing the higher bands on the EQ).
4th: Pure Audio (and Direct) mode
I then switched to Pure Audio mode and repeated all of the above tests. At no time was there EVER any significant hum to speak of. Turning the volume up past +10 produced a VERY slight sound but that was only audible within inches of the speaker, at a normal listening position the system always remained completely silent. The same can be said for Direct mode.
It would seem the 705 may have a flaw related in some way to the DSP processing but it DOES seem to operate (barely) as per the THX reference level spec when all EQ and Volume configurations are at or below 0db, there is no significant hum/buzz until SOMETHING is raised past the 0db mark (any volume or EQ setting). The problem is that in order to maintain this quality the 0db mark must be used as an across the board MAXIMUM, in other words if you would like to raise a given frequency band to the highest setting it should remain at 0db and all other bands should either be lowered OR the "InteliVolume" should be lowered to compensate for any setting raised above 0db. My only question is does Onkyo agree and consider a certain degree of speaker hum normal in this case? I have asked and am still waiting on an answer.
I have read that some users have returned their 705 and upgraded to the 805 to resolve their hum "issue". I'm wondering if this really solves anything or if the increase in output power simply makes most people a little less-inclined to push the power limits because raising the volume to a point that would introduce hum raises the listening level to an uncomfortable volume? Just a guess?
I've got until Monday to return my 705. I was thinking of getting the 805 but noticed it's about 4" deeper, it won't fit in my rack.
I've also been thinking about trying to track down an Integra DTR-6.8 but haven't been able to so far. Had a slight glimmer of hope when I came across the Onkyo 705P
model but it looks like it's just a standard 705 with a rack mountable option. :/