question about crossover frequencies.

[dr.nutter]

Hi.

I have just added a sub to my set up, and am a little confused by what frequencies are been sent across which cables, and which crossover settings affect them. If anyone could help me out, I think that I'll have a much better idea about what is going on as I try to set it up.

The sub is an MJ Acoustics Pro50 mkII, and it is hooked up to a Denon AVR2802. The main speakers are a pair of Kef IQ7s with an IQ6 centre and a pair of IQ1's as surrounds.

The sub is connected to the amp with a hi-level cable connected to the binding posts, as well as the line level LFE connection.

The settings on the amp are: <all speakers>=large, crossover=80Hz, Sub mode="LFE + MAIN".
I then have rolloff and level controls for each of the 2 inputs on the sub, plus a 180 deg variable phase.

I guess that my question concerns the settings on the amp (just considering a stereo source). Where is the amp sending the signal below the 80Hz crossover? Is it all been sent through the LFE cable? That was my initial assumption, but I can still get seemingly low frequency output using only the hi-level gain on the sub (indeed, I was advised by the store to use
only the hi-level input to set up the sub initially), so I guess not.

2nd question is, should the roll-off for the LFE be set to the crossover frequency of the amp? I don't realy understand the point of this control, since I presume that no frequencies higher than the crossover are been sent on this input...?

I'm so confused

If anyone could clarify this all for me, I would be very grateful.

Thanks in advance.
Dave.

 

[HiFiRuss71]

The permutations of settings are indeed confusing.

Some of the confusion comes from understanding the difference between the subwoofer channel and the LFE soundtrack. The subwoofer is just another channel, just like any of the other speaker channels on the amplifier. The LFE soundtrack contains bass effects that are only designed to reproduced via the subwoofer channel. Equally, all of the other speaker channels are full range soundtracks with their own bass.

The confusion comes when setting the amplifiers bass management and understanding in which direction the bass is sent. Where you have a subwoofer and it is turned on (set to 'Yes') in the amplifiers menus, then the LFE soundtrack will be sent to the sub - end of story.*

Speakers are more confusing as a result of the terminology involved. 'Large' actually just means a speaker capable of reproducing bass below 80Hz which isn't too tricky even for a compact stand-mount. However, setting a speaker to 'Large' means it will be sent a full range signal, regardless of where the crossover is set. All that will happen is that the speaker will get a full range signal, regardless of whether it can actually reproduce it and the subwoofer will reproduce the LFE soundtrack alone.

If you select 'LFE+Main' (as opposed to 'LFE' only), then the speakers will still be full range but the speaker bass below the selected crossover frequency will be sent to the subwoofer channel aswell. To clarify, if you set the crossover to 40Hz, the speakers will still be full range, but their bass below 40Hz, will also be duplicated by the subwoofer. Get it?

In a modern receiver, setting a speaker to 'Small' (originally meaning a speaker only capable of 80Hz and above) will result in all bass below the crossover point being redirected to the subwoofer channel and this is what I would advise you do. Whilst it's hard to believe that a pair of floorstanders can be 'Small', it's more a case of ensuring that the bass is directed to the box most capable of reproducing it and that is normally the sub.

Some receivers (My Denon 3806 for instance) as well as allowing the normal speaker crossovers to be set, also have a crossover setting for the sub. This should always be set to it's maximum which is normally 120Hz. Setting the subs crossover in the receiver has no effect on the bass redirected from the speakers (their crossover does that) but the LFE soundtrack can contain it's own bass up to 120Hz, so let it! A lot of receivers don't have this option, but it's worth knowing about.

As the receiver has gone to all the trouble of this complex bass management, there's no need for the subwoofer to do anything in this regard, so set it's crossover to maximum, or defeat (If I recall correctly, MJs actually say LFE when set to maximum, which is defeat) as the subs crossover is now irrelevant.

So, in your case, set the speakers to 'Small', the amps crossover to 80Hz, the subs crossover to maximum (in the amp and on the sub) and because of the speakers being set to 'Small', 'LFE+Main' shouldn't matter any more, but set it to 'LFE+Main' anyway as insurance.

Hope this helps,

Russell

*If the subwoofer is set to 'Off' or 'No', some amps will redirect the LFE soundtrack to the speakers, but normally with some sort of dynamic range compression to protect the speakers/amp from huge bass transients. Whether this is true, or the details of how, I couldn't care less because this is the subwoofer forum and if you don't have a sub, then you're missing out on big fun either way.

 

[recruit]

moved to Room Acoustics, Audio & Video Calibration forum.

 

[dr.nutter]

Hi Russell.

Thanks very much for your post, it has cleared many things up.

If I do as you suggest, i.e. set the speakers to small and the crossover to 80Hz (or 120? I only have 1 setting on the amp, and I'm not sure if its the one that should be 80, or the one that should be 120) with the subs LFE roll-off on defeat, then (I think...) that I will be sending all of the bass out to the sub, with none being sent out on the front channels.

If I do this, then how does the high level input come into play? With the speakers set to small then surely there's no bass coming in on this input? This is confusing, as the shop tried to impress on me the fact that the Hi-level input was the key to matching the subs bass with the floorstanders.

Cheers
Dave

 

[zaentzpantz]

You are right, when you use the high level sub input and the Denon settings are set for small front speakers then the sub will not get any LF.
I would do away with the high level inputs and just use the low level phono input fed from the LFE output on your Denon.
I would advocate a different setup, which you may like to try and which works well for me.

1. Set the speakers to "large" so that your front speakers get full range signals.
2. Set the bass management to LFE & main. The Denon will then send a filtered LF signal to the sub whatever you are listening to.
3. Use the phono input on the sub, and adjust level and crossover frequency at the sub to take over from your front speakers at their natural rolloff frequency. Beware of both sub and speakers adding around 50-80Hz and creating a boom, the sub should just take over as the front speakers start losing bass.
4. Set the Denon LFE frequency to 120Hz.

This way you can listen to music etc with or without the sub, and you can control the sound of the sub to just fill in the low stuff where your Kefs can't get down to. And you won't get that annoying boom and thump of too much sub.
My sub is set to about 45Hz and just takes over from the large front speakers which get thin below 50Hz. There's no apparent change in sound quality whether the sub is on or off until something big happens. Then you know.