If I suspect my sub could benefit from a phase reverse but have no phase switch?

[Phill]

Could I simply reverse the phase of all my other speakers?

Surely it would work. I would just wire (+)ve to (-)ve on all my other speakers.

Phill

[Cable Monkey]

The only real reson for a phase switch is to give more versatility when positioning. If you can't hear as much base as you should, then try reversing the phase. Simply reversing the phase of your other speakers will still leave you bass shy. Without a phase switch your only option is to try putting the sub elsewhere. I think!

[Phill]

Thanks for the info but lack of bass isn't my problem.

My problem is boomyness arround the crossover point. Can't bad relative phase between the main speakers and Sub cause this?
In which case, changeing the phase of the other speakers acheives the same end.

Phill

[GAmbrose]

Have you tried the THX audio tests on various DVD's such as Terminator 2, Moulin Rouge, Die Hard etc?

Thats deals with Subwoofer crossover and out of phase speakers.

Gary A

[Neon]

You can indeed reverse the polarity on all your other speakers. This will have exactly the same effect as a phase reversal switch on the sub.

If you've got boominess at the crossover point, it sounds more likely that they are in phase, but the crossover point is set too high on the sub.

[Phill]

I have T2 ultimate Edition Region 1, I'll check if it's got test tracks on it, that might be helpful. Actually, I think one reason for having a phase reverse switch is to deliberatly put the sub out of phase with the main speaker to prevent messy constructive interference.

I shall get my experimenting head on tonight and have a play.

Cheers
Phill

[GAmbrose]

Whilst we're at it, I have a KEF 2005 system now and a Pioneer VXD859RDS Reciever.

Now, on the surround settings on the reciever, the Crossover point is 80Hz, but on the actual Sub there is a dial which lets you change between 80-120 hz.

I take it these should both be set at 80? I tinkered with the THX crossoverpoint tests you get on some DVD's and didn't notice any difference between 80 and 120 on the actual sub to be honest, so is the reciever overriding it?

Gary A

[Neon]

It's more complicated than this, because you've also got to consider the speaker roll-off characteristics. In other words, if your amp rolls of at 80Hz, but the main speakers are small and roll off at 100Hz, it's that which will take precedence.

As has been suggested, the best best is to use audio test tracks to set the system up by ear. The sound should blend smoothly all the way down - no peaks or dips. You may find that placing the sub to fire sideways (across the width of the room) or diagonally into the room helps to smooth things out. If you have a downward firing sub, then try different places in the room.

[Phill]

GAmbrose,

If I understand it correctly, and I may not ;) If you connect your Sub to an dedicated Sub output on your AV amp, the roll-off dial on your sub won't be affective, you may as well set it to the highest frequency because your AMP is handling which frequencies go to the sub and which go to the other speakers. Only if you use high level inputs or connect it into a tape loop is it important to set the upper frequency.

At least I think that's how it works.

[Neon]

Phill, you're right, but if all your speakers are set to small, for example, then the amp doesn't just send LFE to the sub, it also sends the "missing" bass as well. So you do need to adjust the sub's cutoff to match the other speakers.

This is why it's important to ensure the amp's settings match what the speakers are capable of. If the amp thinks they're large, it will send bass down to 80Hz to them, but if in reality they can't reproduce anything below 100Hz, there'll be a hole in the bass between 80 and 100Hz. I'm over-simplifying, of course, but you get the idea.

I hope that makes sense - I'm currently under the influence of alcohol!

[Phill]

Originally posted by Neon

I hope that makes sense - I'm currently under the influence of alcohol!

Well done, Sir!


While we're here. What happens when in pro-logic mode rather than DD 5.1.

Does the amp send the frequncies below 80Hz to the Sub?
If you set your speakers to small, the amp stops sending low frequency output to the speakers. I know because my system sounds worse when I set my speakers to small. Surely then, the sound going to the sub is not also being sent to the other speakers so again it makes no diffeence. Or am I over-simplifying?

cheers
Phill

[Neon]

If you set your speakers to small, the amp directs the missing bass to the sub. That's how these systems with tiny speakers work as well as they do. If your system sounds worse, then either your sub isn't set up properly - or your definition of worse is different :)

[Phill]

My speakers aren't satelite speakers though. I have Tannoy m3 up fornt and m1 in the rear with an Eltax centre speaker. What I suppose is happening is that the sub isn't as good at handling frequencies between 80-150Hz as my other speakers are so the High bass-low mid-range part of the sound isn't as good. The guy at Audio-T in Oxford told me to expect this because the drivers in full-range speakers are generally better than those for subs.

The sound definately sounds fuller and the bass is tighter when I set my speakers to large.

But I digress, my argument is that the amp is always controlling which frequencies go where and so the cross-over is redundent in the sub, it deosn't have to filter out frequencies that aren't being sent.

At least that's my understanding, seein gas I've owned a sub for about four days now, I accept that I may not fully understand it all yet. :)

[Neon]

Ah-ha! The mists are clearing! You don't say what sub you have, but unless you spent a lot on it, your M3s will certainly be handling the upper end 80Hz+ more cleanly than it can.

The cut-off frequencies are not immediate - typically they will be 2nd or 3rd order filters - rolling off at 12 or 18dB/octave. So, setting the amp's cutoff to 80Hz means that at 40Hz the level's only 12 or 18dB down. Combine that with the roll-off of the sub's filter (at 160Hz it's 12/18 dB down) and you can see that you can get a peak (in-phase) or trough (out-of-phase) around the cut-off.

It sounds like you want to adjust the sub's cutoff further up (100Hz, 120Hz, whatever) to move the overlap out of the way.

[Phill]

Indeed, suddenly everything becomes clear. Thanks for the explanation Neon. I'll have another go at setting up seeing if I can improove it further.

Just in case there are others reading this thread and don't know, I think it's worth explaining that 1 octave is equal to a doubling or halving of the frequency, it's not linear. I don't mean to critcise your explaining skills neon, please don't take offence.

BTW, In case you're interested, my subs a Jamo a3sub.1. It's budget, I only paid £150 ex-demo from Sevenoaks.

Thanks again,
Phill