stereo in/thru single sub - possible?

MickDuprez

Member
2020-01-15 11:36 pm
Hi All,

this is more of a theoretical question at this point but is it feasible to build a passive sub that takes the stereo line outs from the amp (600*600W), process the signal through a crossover and send to the FOH speakers?
Is there a particular term to search the forums for this setup?

I have a pretty good quality 15" speaker from a car stereo system laying around and I'd like to build a cab for it as a learning/hobby project.
The passive 2 way's I have are 15" with horn but low end is lacking in larger areas for live gigs, hopefully a sub will help with a bit more low end for kick drum and bass etc.

any thoughts and suggestions welcome, thanks.
 
piece of cake to derive a mixed signal (three resistors or less, eh). Then you need a XO or low pass filter. Betcha if you just had a sub do L, nobody would know R is missing.

In practice, a million owners of one-subwoofer systems say it works just fine. Not much L and R has been encoded into recordings or not enough to bother keeping distinct. Better to afford on great sub than poor stereo subs, even if strategically placed in your room.

Historical footnote: I think I still have a trick transformer made by University in 1955 that does the magic mixing trick on the outputs of the L and R amps, not needing another amp, if I recall correctly.

B.
 
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MickDuprez

Member
2020-01-15 11:36 pm
Thanks bentoronto.

I was thinking that as my PA is 2 x 8ohm sends that it would be better to divide them through the sub somehow to keep power levels even through the speakers.

Would it be noticeable/incorrect to have one side, say L with sub and 2 way (4 + 4 ohm) and the other side, R just 2 way (8ohm)?

The system is for live gigs (5 piece) and most channels are panned evenly or a little one way or the other but it wouldn't matter that much with bass freq's and I could pan them to the sub side if you think that setup would work.

Cheers.
 
You don't need to use any "tricks" to drive a DVC driver with a stereo signal. Just connect one channel to each coil. That's it.

Research the history of DVC drivers. The very reason they first came into being was for exactly that purpose - combining the output of two channels in one speaker, while keeping the channels electrically separated.

I've never used a DVC speaker, but theoretically there are some faults: being the device electro-mechanical and being it quite microphonic (comma) if you make L play ( loud, eh..!) and R mute, what happens?
Note: the membrane has a volume of air in an enclosed box, on the back of it
 
I've never used a DVC speaker...

As far as I know, the two windings just sum together without a second-order interaction of any significance.

Originally for car audio, I believe, in order to make systems requiring just one sub - and directionality not an issue for a sub in a car (or at home with a low passband).

The downside is the load on the amp. When the two windings are wired independently for separate amps, their separate impedances can fall below 2 Ohms for 2+2 Ohm drivers. Not an issue for most car audio amps.

So depends on how you are accessing the sub or subs signal(s). An electric mix and a single filter and an extra amp from the Salvation Army store makes more sense to me.
 
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As far as I know, the two windings just sum together without a second-order interaction of any significance.

I was thinking of B.e.m.F. in case you didn't notice...
In JCBPIII thread there were about 100 pages some 3 months ago about catching that infamous BEMF.
Double windings are always the key for making that type of comparison in a loudspeaker: it is a passive component but also active :eek:
 
I was thinking of B.e.m.F. in case you didn't notice...

Overcoming my usual reluctance to argue about speculative trivial issues,

1. the condition causing your anxiety about a null signal in one channel does not occur in playing music, weird electronic concoctions aside,

2. from my and others' experiments with motional feedback using DVC drivers, the back EMF signal is very tiny and insignificant, and

3. sound quality might actually benefit from the degenerative feedback coming from the null-signal amp being fed "erroneous" back EMF.

Paraphrasing the old Tom Lehrer song, "... there may be other interactions, but the news hasn't come to Harvard..."
 
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MickDuprez

Member
2020-01-15 11:36 pm
Thanks bentoronto.

I was thinking that as my PA is 2 x 8ohm sends that it would be better to divide them through the sub somehow to keep power levels even through the speakers.

Would it be noticeable/incorrect to have one side, say L with sub and 2 way (4 + 4 ohm) and the other side, R just the 2 way (8ohm)?

The system is for live gigs (5 piece) and most channels are panned evenly or a little one way or the other but it wouldn't matter that much with bass freq's and I could pan them to the sub side if you think that setup would work.

Cheers.

Any thoughts on the above?

I'm trying to learn electronics again and while I have a broad overall understanding, impedance is definitely something I need to work on :)

Cheers.
 
I was thinking that as my PA is 2 x 8ohm sends that it would be better to divide them through the sub somehow to keep power levels even through the speakers.

Would it be noticeable/incorrect to have one side, say L with sub and 2 way (4 + 4 ohm) and the other side, R just 2 way (8ohm)?
Mick,

The sub will need the highs removed to sound decent, and even then, will likely need separate volume adjustment from the main speakers. If you do want stereo mains, a separate amp for the sub would be best.

Your question could be answered better if you gave the model of the "PA".
Sounds like it is some form of powered mixer, you may have the option of inserting a sub processor/crossover on one of the amp channels, which would allow you to mix the balance of the sub, and only pan channels that could use it (like kick drum and bass) into the sub channel.

Art
 

Xoc1

Member
2008-11-08 8:25 pm
Devon UK
One way to achieve this is to phase invert one channel and then connect the sub in bridge mode + terminal to + terminal with a passive low pass filter. Inverting one channel might be as easy as swapping the wires on an XLR input. The mid top speaker wired to the inverted channel would also need the speaker terminals swapped over to restore the proper phase.
Although this will basically work, this might cause a few issues, not the least being the load on the amplifier as it would probably be seeing anything down to 2.7 ohms per channel. The overall response would probably be very bass heavy which might need some EQ to reduce the bass or passive high pass for the mid top speakers. The bass driver might get the full 1200w rating of the amp which might be excessive!
The only time I have seen this sort of set up practically used is with a car amplifier, which often have one channel phase inverted permanently to make bridging easy.
 

MickDuprez

Member
2020-01-15 11:36 pm
Thanks Art for your reply, yes, this is a powered mixer. I've attached a quick diagram of my proposed layout that should make things clearer.

Xoc1
sounds pretty complicated and above my pay grade :D
thanks for the info though, I think I grasp what you're saying :)


That setup is not ideal. L plus R on one channel and sub on the other works though if there's little panning anyway.

This sounds like the simplest idea for a test at the very least, thanks!

My plan is to build or buy an active setup eventually, just thought it would be nice to get a sub in the mix in the meantime, cheers.
 

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