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[Kirby Yarbrough]

I have a pair of QSC HPR181i powered subs, and they do very well for the small events I do.  I sometimes do smaller rooms where I'd like to use HPR151i powered subs, because they're lighter, smaller and have more thump at lower levels, but I don't have them yet.  It'd be great to use all four for larger events, but I'd like to know if using two different sized drivers is a good or bad idea.  Both boxes are identically powered and have the same 100Hz crossover.  Thanks in advance for your help.
Kirby

[Edited to clarify I don't have the 15s at this point]

[Dave Barker]

I did a small outdoor event and used a pair of the 181's with 151's stacked on top and 152's on top that and it sounded great.  No problems at all.

[Matt Vivlamore]

You might run into cancelation problems.  Depending on the room and PA layout.

I would run the 18” the standard way and used the 15” as supplemental subs.  I would also place them in an AUX sub fashion.  So that you can adjust them as needed.

[Sam York]

Bad idea really, will probably muddy the sound if anything.

If that's what's already in your inventory then the only way round it I could see would be by using a DSP or active crossover to run them in different frequency bands, i.e. sub and mid bass/kick bins.

Not sure whether you can bypass the internal crossovers as I've never used these, also no idea how they will sound for the same reason, but when you see 15s and 18s in the same rig this is how it will usually be done.

EDIT: a quick glance at the QSC website shows that all the cabs accept a full range input (i.e. so it can be band limited externally) so in theory this would work. They don't include anything as useful as a frequency reponse plot so not sure how well it would work in practice, although from the frequency specs given the 18 appears to go lower than the 15, as you'd expect.

Failing that the only other thing I can think of is getting rid of the 18s, buying four 15s and using those where you'd use 2 18s before, should go as low and be louder. Maybe buy another 2 15s for the big gigs to give you six. Or just buying more 18s and using those everywhere.

This could be rather good actually, anyone out there running a four way HPR rig?

(side note, I'd expect the 12" mid tops to work better in this config than the 15" ones, as they wouldn't have to deal with as much mid range signal)

[Sam York]

Matt Vivlamore wrote on Mon, 13 October 2008 21:08

You might run into cancelation problems.  Depending on the room and PA layout. I would run the 18” the standard way and used the 15” as supplemental subs.  I would also place them in an AUX sub fashion.  So that you can adjust them as needed.

How would that get round the cancellation problems though Matt?

[Bryan Bartone]

The QSC HPR subs have an internal crossover.  So the sub's amp only sees sub frequencies.

Sam York wrote on Mon, 13 October 2008 15:09

Bad idea really, will probably muddy the sound if anything. If that's what's already in your inventory then the only way round it I could see would be by using a DSP or active crossover to run them in different frequency bands, i.e. sub and mid bass/kick bins. Not sure whether you can bypass the internal crossovers as I've never used these, also no idea how they will sound for the same reason, but when you see 15s and 18s in the same rig this is how it will usually be done. EDIT: a quick glance at the QSC website shows that all the cabs accept a full range input (i.e. so it can be band limited externally) so in theory this would work. They don't include anything as useful as a frequency reponse plot so not sure how well it would work in practice, although from the frequency specs given the 18 appears to go lower than the 15, as you'd expect. Failing that the only other thing I can think of is getting rid of the 18s, buying four 15s and using those where you'd use 2 18s before, should go as low and be louder. Maybe buy another 2 15s for the big gigs to give you six. Or just buying more 18s and using those everywhere. This could be rather good actually, anyone out there running a four way HPR rig? (side note, I'd expect the 12" mid tops to work better in this config than the 15" ones, as they wouldn't have to deal with as much mid range signal)

[Sam York]

Bryan Bartone wrote on Mon, 13 October 2008 21:40

The QSC HPR subs have an internal crossover.  So the sub's amp only sees sub frequencies.

Doh. Me being an idiot. I was only considering the filtered output to the mid top cab.

I'd get out my screwdriver and wire the input to be after the crossover, but it'd be a kludge (I customise a lot of things   )

If QSC published the specs of those filters you could probably even work around it, it'd just mean the 15s only going up to 100 or wherever their LPF is, when in a four way setup with 12" mid tops you'd want the mid bass to go higher than that.

I know someone who has a HPR rig and I have a Driverack, I feel an experiment coming on!

[Douglas R. Allen]

I asked QSC this very question and they told me the Phase and Freq response of the two subs was close enough to use them  together. QSC has a chat forum. You should post the same question there as well.

Douglas R. Allen

[Bryan Bartone]

Which makes me happy that I bought the 15's over the 18's.

Douglas R. Allen wrote on Mon, 13 October 2008 15:57

I asked QSC this very question and they told me the Phase and Freq response of the two subs was close enough to use them  together. QSC has a chat forum. You should post the same question there as well. Douglas R. Allen

[Matt Vivlamore]

Sam York wrote on Mon, 13 October 2008 16:09

Matt Vivlamore wrote on Mon, 13 October 2008 21:08 You might run into cancelation problems.  Depending on the room and PA layout. I would run the 18” the standard way and used the 15” as supplemental subs.  I would also place them in an AUX sub fashion.  So that you can adjust them as needed. How would that get round the cancellation problems though Matt?

If the cancel each other out, go ahead and pull down the slider and not use them.

[Bruce Gering]

Sam York wrote on Mon, 13 October 2008 15:09

This could be rather good actually, anyone out there running a four way HPR rig?

Yes. But maybe not exactly what you meant. Check out my recent post-

http://srforums.prosoundweb.com/index.php/m/368340/175/#msg_ 368340

[Scott Smith]

Douglas R. Allen wrote on Mon, 13 October 2008 16:57

I asked QSC this very question and they told me the Phase and Freq response of the two subs was close enough to use them  together...

Seems right to me.  Both boxes are front loaded.  As long as they are correctly lined up together, the pro's should outweigh any con's.