NAD T752 Bass management issues with Analog Stereo

Jonathan M · 03-25-2003, 07:18 PM

Hi All,

Just purchased my NAD T752 and have discovered a "feature" that is not in agreement with what is written in the manual.

With an analog stereo input signal, there is no bass management in effect. The left and right signals are summed and sent to the sub-out which is low passed at 500Hz or so. The mains are producing the full signal.

I don't know if this also happens with a digital input, as I have yet to test this.

You can check by removing the pre-out/main-in loop plugs on the main and hooking the sub out up to the left main in, and play some music input a stereo analog input. This results in you listening to what is being sent to the sub, which on my unit is the summed signal, low-passed at around 500Hz or so with a fairly shallow slope (12dB/octave or less).

ie. When listening to an analogue input, the bass management is not being used. I assume this is because all bass management is done in the digital domain, and I have yet to check this out with a digital input.

Anyone else with the T752 (or any other receiver - I'm sure there are others that do the same thing!) want to check this out / make suggestions?

Chriss M · 03-25-2003, 09:18 PM

most (all?) receivers do not have analog crossovers. Usually you get one of three things. The receiver digitizes everything and applies bass management while the signal is digital, the recevier provides a pure analog mode and sends a crossed-over copy to the sub, or the receiver provides a pure analog mode and does not send any output to the sub.

Seems like you nad is doing # 2. does it give you the option of turning the sub off ? If you do want sub output and don't want any bass doubling, your only option might be to use a digital input. maybe some NAD owners will have a more specific answer for you.

Jonathan M · 03-25-2003, 09:42 PM

Thanks for the reply, Chriss.

I am aware that this is likely designed like this, as the decoder chips themselves usually implement the crossovers for bass management - entirely in the digital domain. At least it is not digitizing the signal first.

I will try a digital signal at home, and I am currently developing a method to create test tones on a CD using either DTS or Dolby Digital. I don't know whether this will work or not yet, but it looks like I'll be able to get test tones on whichever channel I like and burn to CDRW for playback in my DVD player. This will allow me to test the response of the unit to all possible inputs, to generate the best possible solution for this (I currently have 4 planned that should work - it's just a matter of nailing down what happens in every different input mode to come up with the best solution.)

I have built my own sub amp, so adding sufficient xovers etc. is easy.

I am still interested in hearing from anyone else with this unit or others who are willing to test what their particular receiver is doing. This would be a valuable piece of knowledge IMO for future purchases.

Chris PC · 03-25-2003, 10:20 PM

On my Marantz, I can use the receivers SMALL setting and internal crossover when playing a CD player connected via "analog" rca's. Of course, there is a lot of D to A, A to D and D to A going on.

Mike Up · 03-26-2003, 12:14 AM

This is getting scary. The Rotel preamp didn't offer any low frequency filtering on it's stereo analog inputs according to posts, but did offer a subwoofer output that could cause either bass doubling or bass cancellation according to your room setup.

Since my priority is music, I want correct bass management on the analog inputs as I listen to albums, cassettes, and VHS concerts I like to hear in stereo. I'm just astonished that there's still makers, and reputable makers who can't even adhere to the standards of basic bass management. I guess once you find a brand you like, maybe sticking with it is the best since you never know what you'll get with another company.

BTW, you could always set your speakers as large and run the speaker outputs to the subwoofer and then connected the speakers to the subwoofer's speaker level outputs. I know my Velodyne has a 85Hz highpass crossover for the main speakers, although it's at a rather shallow 6db/octave. Still better than heavily distorting speakers that weren't made for heavy bass reproduction.

Good luck.

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Jonathan M · 03-26-2003, 12:28 AM

I guess as analog connections get fewer and fewer, these issues will become less of a problem, but I agree that it is a pain.

My problem with doing large mains (small everything else) and no sub and actively crossing myself (Which I can do easily) is that I don't want to miss out on the LFE channel, and I am as yet unconvinced that it will also be redirected to the mains when the sub is off and the mains are large. If it is, then it's all good. If it's not (Which I suspect is the case) then it is a little trickier...

As I said, I've got 4 solutions that will fix the problem, however I want to test everything out first before I post the solution.

BTW: I have a way to play DD tracks from a CD - I have yet to test it 100% but it appears as though it works from my initial tests. Thus, we can create test tones on any of the channels (including LFE) that we like in order to figure out what is going on.

03-26-2003, 09:52 AM

Jonathan,

You do not lose the LFE channel if you run your sub off the Main left and right signal. Just select Mains Large and Sub Off/No and the processor sends all LFE to the Mains.

I have been running my systems like that for years... first with Mirage OM-6's (built-in subs) and now with my Revel B15 and M20's. The only disadvantage is that if you want accurate music, you cannot run your bass 'hot' (unless you play with your sub's volume control every time you watch a movie). For me this isn't a problem at all because I balance my system to be as flat as possible and then listen to music and movies at that setting. If you like crazy over-the-top bass (which apparently a lot of people do) your music will suffer.

Jonathan M · 03-26-2003, 03:04 PM

Hi Craig,

Yes, I have just checked this (via a Dolby Digital CD that I burned) and can confirm that the LFE (.1 channel) is live and well when the mains are set LARGE, everything else SMALL and sub OFF.

The Tone controls are still in existence (and the NAD has good tone controls - just bass increase without effecting the midrange too much) so they can be used to give the bass a little kick if one so desires.

Luckily, the NAD comes with Pre-out/Main-in loops for the front 3 channels, so I can actively cross to perfectly match my speakers (2nd order butterworth highpass at 80Hz, 4th order L-R lowpass at 80Hz to match sealed mains with an f3 of 80Hz and Qts of 0.707)

I am going to make some comprehensive frequency response measurements of each of the bass management options seeing as now I can burn CDR's with Dolby Digital (or DTS) tracks on them. I'll post the results when complete.

Jonathan M · 03-26-2003, 03:08 PM

Mike -

Quote:

I'm just astonished that there's still makers, and reputable makers who can't even adhere to the standards of basic bass management. I guess once you find a brand you like, maybe sticking with it is the best since you never know what you'll get with another company.

I think you'll find that most manufacturers that do offer bass management on the analog inputs do so via an additional ADC/DAC conversion - very few do it in the analog domain. I guess this is a case of which is the bigger evil - the extra ADC/DAC stage or no bass management...

Chris PC · 03-26-2003, 11:30 PM

So I wonder whether the T762 is much better than the T752 in terms of power, sound quality and stereo playback. Good little receiver that T752. Does it suffer from the same QC issues as the T 762?