Film-Tech Forum ARCHIVE: Best way to connect 3X JBL 3635 subs to one amp?

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Posted by Mark Strube (Member # 4190) on 03-15-2013, 10:06 PM:

I'm running a QSC MX1500A amplifier to three JBL 3635 subwoofers. At this point, an additional amp isn't really an option.

The way I have it currently connected: the amp is in stereo mode with the input paralleled, 2 subs connected to one channel, and 1 sub on the other channel. The channel with 2 subs is clipping a bit, more than I'd like it to (but things do sound great).

Would there be any benefit to bridging the amp and running all 3 subs daisy-chained? My thinking is that would drop it to 2 ohms (before wire travel), and could that amp handle that? Would I end up with more clipping? Maybe I should just forget the 3rd sub for now?

That 3rd sub did allow me to have much flatter response for the room, so I'm hesitant to disconnect it... but I don't like clipping, and I'm sure neither do the speakers. [Smile]

Thanks for any input!

Posted by Phil Ranucci (Member # 3820) on 03-16-2013, 12:09 AM:

There is no such thing as "more than I'd like" when it comes to clipping. If you can hear it, it needs to be fixed.It will destroy your subs.
Check the back of the amp. QSC usually lists minimum impedances for stereo and mono output. I don't think it will like less than 4 ohms in mono, which 3 subs @ 8 ohms paralleled would be the load. You could always series/parallel them.
Also, make sure that all are matched as far as polarity. Your comment about the 3rd sub flattening out response makes me think one is out of polarity. Check the easy stuff first.

Posted by Sean McKinnon (Member # 612) on 03-16-2013, 07:56 AM:

Why don't you switch the amp to mono (bridge mode) and run two subs in series with the third in parralell? I think that would get you to a more manageable load. You could also connect an 8 ohm (or whatever resistace the subs are) resistor and treat is as a 4th "speaker" wiring 2 sets of 2 speakers in series and parallell them together. That way your amp would see 8 ohms if the four subs are 8 ohms each. (or 4 ohms if the subs are 4 ohms)

FYI- It has been a long time since I have done this so I apologize if my math is incorrect. You can confirm the resistance of your series/paralell configuration by measuring across the combined + and - of the two seriesed sets of speakers for resistance (ohms)

Posted by Marco Giustini (Member # 4544) on 03-16-2013, 10:34 AM:

This may be a very stupid question, but when doing series/parallel with an odd number of speaker, does it end up with one speaker receiving more power?
In this scenario, you would end up with two paralleled subs (4 ohms) wired in series with a third one (8 ohm) giving you a final value of 12Ohms (a bit too much for LF I'd say, but this is another story).

Now, electronically speaking, you'd end up with the same Voltage drop between all speakers, but the one wired in series would get the full current, while the two in parallel would split it 50%.
If P= V x I, does it mean that the sub wired in series is getting twice as much power as the other ones? I may be saying rubbish here!

Back to the subject, did you make sure that the two channels are set to provide the same power to the three speakers? Because one channel has a load of 4Ohm and one of 8Ohm, you may need to adjust the gain of the two channels so the subs get the same amount of power.
How to do that... Not sure, either measuring the SPL out of each speaker, or measuring the voltage - maybe applying a sinewave signal.

But another amp would definitely be better!

Posted by Dave Macaulay (Member # 813) on 03-16-2013, 02:07 PM:

Three 8 ohm speakers in parallel gives you 2.7 ohms. Very few stereo amplifiers will run bridge mono into less than 4 ohms, and the MX1500a is NOT one of them.
There's no super excellent way to do this, and you've picked the a decent one: Paralleling two on one channel and putting the third on the second channel alone. This - in theory - gives you 930W to the speakers.
Since it's clipping the simple answer is that you need more amplifier power.
You could get more power to the speaker by adding a 8 ohm power resistor and connecting in series/parallel for a 4 ohm load. This gives you an amp spec-sheet max power of 1500W, with 375 going to the resistor. That's 1125W to your subs. The resistor should be rated at least 100W dissipation.
Getting a second amp is probably the better choice. Run both in bridge mode, 2 speakers on one and one on the other. The 1500a is rated to accept 4 ohms in bridge mono. That gives you 2600W (liberal reading of spec sheet) to the sub speakers with two 1500a amps.
At least have the amps cleaned out and ensure the fans are clean and working. I think the fans are riveted in: you have to drill the rivets out to remove the fan for proper cleaning (isopropyl and a toothbrush) or replacement. Four #8-32x2" machine screws with nuts and lockwashers are required to reinstall the fan.
With typical clogged heatsinks and "hairy" fan, the 1500a will often overheat and shut down with the kind of load expected in subwoofer use during an action movie. This shutdown is easy to miss, the amp turns off until it cools down then all seems fine again. Only if someone complains that the sound is weird would you notice it, unless a staff person sees the "PROT" light on when it's shut down.

Posted by Mark Strube (Member # 4190) on 03-16-2013, 05:42 PM:

quote: Phil Ranucci
There is no such thing as "more than I'd like" when it comes to clipping. If you can hear it, it needs to be fixed.It will destroy your subs.

Hence this thread asking for a solution.

quote: Phil Ranucci
Also, make sure that all are matched as far as polarity. Your comment about the 3rd sub flattening out response makes me think one is out of polarity. Check the easy stuff first.

I checked that during install.

quote: Marco Giustini
Back to the subject, did you make sure that the two channels are set to provide the same power to the three speakers? Because one channel has a load of 4Ohm and one of 8Ohm, you may need to adjust the gain of the two channels so the subs get the same amount of power.

I simply have them all at full gain to allow for the most headroom. I'd imagine decreasing power on one or the other would just cause more clipping when I'm aiming for the same amount of output?

quote: Dave Macaulay
Getting a second amp is probably the better choice. Run both in bridge mode, 2 speakers on one and one on the other. The 1500a is rated to accept 4 ohms in bridge mono. That gives you 2600W (liberal reading of spec sheet) to the sub speakers with two 1500a amps.

Can the 3635's handle this? They are rated at 300W each.

Posted by Scott Norwood (Member # 30) on 03-16-2013, 07:35 PM:

300W is the continuous (program) power rating. They can take much more for short bursts (peak rating). Amplifiers have peak and program power ratings as well, though the peak rarings are mostly useless. In general, you will do less damage by over-powering speakers than under-powering them, according to the live-sound people I know.

Posted by Mark Strube (Member # 4190) on 03-16-2013, 08:56 PM:

Good to know! I never saw a peak rating for those subwoofers so I was nervous to go much over that. I'll see what we have available for amps, and I might mix n match to get more power to those subs.

However this then presents another possible problem: I don't have any extra sub filters. So, 2 of the subs would be running with that, and one without. Should I be concerned about that, or will it sound consistent enough? (We are running mostly DTS, going to D-Cinema soon.)

Posted by Mark Strube (Member # 4190) on 03-16-2013, 10:36 PM:

More specifically: will it significantly hurt their ability to couple if one sub is running without the filter?

Posted by Harold Hallikainen (Member # 5405) on 03-17-2013, 12:36 AM:

On series/parallel, I always thought you wanted to drive speakers with a zero ohm source. Putting them in series gets away from this.

Harold

Posted by Sean McKinnon (Member # 612) on 03-17-2013, 10:32 AM:

I think if it were me I would try using a resistor and wiring a series/parallel configuration as noted above. If your results are not satisfactory then invest in another amp. It is more cost effective to buy a resistor than an amp and i cant see where you have anything to lose by trying it except time. Just my .02

Posted by Steve Guttag (Member # 268) on 03-17-2013, 12:23 PM:

Alright...you all are starting to tick me off [Roll Eyes]

Let's look at the problem.

Somebody "decided" on 3 subwoofers but only one amplifier...mistake #1.

Someone is using a subwoofer who's design is based on size and NOT having real deep bass but is looking for deep bass...mistake #2. Seriously, this is a subwoofer that has a frequency response down to 38Hz (barely wider than a stage speaker) and only handles 300-watts of continuous pink noise. It does have a sensitivity of 100dB 1w/1m. Compared to other single 18" SW...that is pretty darn sensitive...almost as sensitive as a double 18" sub from any brand. It is only 14.5" deep (368mm to our metric friends)...it is shallow so it will FIT into places.

Anyone that recommends wiring subwoofers in "series/parallel" is incompetent and should be discarded as such...as Harold properly stated...you can wire in parallel but NEVER series...when it comes to subwoofers. The series resistance kills the damping of the system and will cause an exaggerated bass response and will lead to driver failure (due to lack of damping at high levels).

The phrase

quote: Mark Strube
The channel with 2 subs is clipping a bit, more than I'd like it to (but things do sound great).

displays a complete lack of understanding too. There is NO level of clipping that is acceptable. Clipping means the amplifier has run out of available power to supply to the speaker...the output will be "clipped" This means that the driver is being asked to, instantly, STOP, then sent in the opposite direction and then stop again. Why not just destroy the system now and get it over with?

When looking at the power rating, with respect to subwoofers...it is continuous pink noise that is the relevant number as that best simulates a typical subwoofer signal. Subwoofers are not dialog channels nor do they just play music...they often carry effects that may be sustained, at high level. Ratings like "program" power are nothing more than a doubling of pink noise power. It is under the presumption that the signal is only momentarily at the high level. Pink Noise often has a 2 or 6 hour rating for it to be continuous. Note too, the continuous rating does not take into account that as the VC heats up, the drivers output will decrease with the same level of input to it is wise to not drive a driver continuously to its limits.

To improve the system's design...the best suggestion was to add another subwoofer. With three 3635 subs tightly spaced (take advantage of mutual coupling), the system has 104.8 dB of sensitivity. If the theatre were say 60-feet deep, this would mean that the system would need 982-watts...which exceeds the rating on the drivers (and the amp).

If another 3635 is added...the sensitivity increases to 106dB. In the same example 60-foot deep room, the requirement of the system drops to 745-watts. You then have enough drivers to handle the requirements (1200-watts of capacity). The MX-1500a has 500-watts per channel into 4-Ohms or can deliver 1000-watts total to the system (two subwoofers on each channel). In this example the system now will meet spec, if set up properly. This also means that the subwoofer response is taken into consideration...and no exaggerated boost is applied to the bottom end to try and extend its bass beyond its design.

Posted by Mark Strube (Member # 4190) on 03-17-2013, 07:07 PM:

Someone woke up on the wrong side of the booth.

quote: Steve Guttag
Somebody "decided" on 3 subwoofers but only one amplifier...mistake #1.

I'm working with what's available. This wasn't an audio design decision.

quote: Steve Guttag
displays a complete lack of understanding too. There is NO level of clipping that is acceptable.

Already addressed... the clipping is the point of my thread. If there was a complete lack of understanding I wouldn't even be asking this question. The speakers will live for now, at the level we're running, it clips probably once or twice during the entire run of trailers. Not great, but I've seen systems (that I didn't design) clipping for years at levels more severe than this without damaging anything. Not saying it can't happen... but let's calm down.

Your last two paragraphs were informative, however a 4th subwoofer is not an option right now, again, as previously stated.

quote: Steve Guttag
This also means that the subwoofer response is taken into consideration...and no exaggerated boost is applied to the bottom end to try and extend its bass beyond its design.

Do you recommend not performing an EQ to flat response with these subs? Should I ensure a rolloff starting at 38Hz going down? What about the affects of 3 or 4 of them coupling, doesn't that allow for a bit lower response without adversely affecting the drivers?

The channel with 2 subs on it is currently wired in parallel.

Posted by Tony Bandiera Jr (Member # 2365) on 03-17-2013, 07:30 PM:

quote: Mark Strube
Someone woke up on the wrong side of the booth.... (And)...but let's calm down.

Here we go again..... [Roll Eyes]

Steve was actually far more diplomatic that I would have been, and he addressed all of the things wrong with what has been said in this thread so far. I'm gonna leave it at that so Brad doesn't send me to my room.

Mark, your situation as outlined is not possible to correct without doing EXACTLY as Steve said. You are trying to do the physically (acoustically) impossible with what you have.

You solution is easy and cheap: ADD another sub of the same model!! With all the conversions and theatre shutdowns going on you can locate a used sub under $400 with minimal effort. Wire two subs each in parallel and drive each pair with one channel of your existing amp. Done.

quote: Mark Strube
Not great, but I've seen systems (that I didn't design) clipping for years at levels more severe than this without damaging anything.

Well good for you. But I call BS as clipping, especially with subs, ALWAYS results in blown drivers sooner or later.

quote: Mark Strube
....however a 4th subwoofer is not an option right now, again, as previously stated.

Oh really? Not what you said before at all:

quote: Mark Strube
I'm running a QSC MX1500A amplifier to three JBL 3635 subwoofers. At this point, an additional amp isn't really an option.

Again, even a used amp can be had for well under $400 if you can't physically fit another sub into the room...BUT as Steve pointed out the REAL issue is you do NOT have enough subs for the space!!

Now to pick on someone else: [Big Grin]

Sean, your resistor idea will NOT work or help in this situation at all for the reasons Harold and Steve both stated. Never, EVER series connect subwoofers!! I honestly am not fond of series connecting surrounds either in the purest sense, but despite the logical and electrical reasons it is bad, for surrounds the side effects are minimal compared to the substantial added costs of amplification for each speaker. (Now if I had my way and for an ultimate setup, you bet I would line up a set of QSC DCA1644's and drive each speaker individually. [Big Grin]

)

Posted by Mark Strube (Member # 4190) on 03-17-2013, 07:52 PM:

quote: Tony Bandiera Jr
Steve was actually far more diplomatic that I would have been, and he addressed all of the things wrong with what has been said in this thread so far. I'm gonna leave it at that so Brad doesn't send me to my room.

He opened his thread with saying how ticked off he was, I was simply suggesting he calm down. [Wink]

I don't see why diplomacy should be an issue when I'm asking for advice, implying that the people I'm asking (all of you) have more knowledge than myself. Take it as a compliment/ego boost/whatever gets you through your day.

Since it's the safest solution for our system, I will be on the hunt for a 4th sub, but it may be some time before I have the ability to purchase it. I'll just run the LFE at a lower gain until then.

Have a great St. Pat's!

Posted by Steve Guttag (Member # 268) on 03-18-2013, 08:57 AM:

I had actually saw this thread growing all week. I only had time to offer my opinion when I did. And seeing the advice offered...particularly the series/parallel stuff did/does tick me off because people are offering up their ignorance, not help. Intent is not sufficient. When one posts advice on corrective measures, they should have something behind that advice...not just good intentions. Or they should qualify their advice in some manner to allow the receiver of that advice to be able to qualify the validity of said advice. Heck, it should be common courtesy!

quote: Mark Strube
I'm working with what's available. This wasn't an audio design decision.

Somebody designed it because somebody had to decide..."let's use 3 subwoofers on this auditorium." JBL didn't just randomly ship three or, if acquired used, someone decided to put three in this theatre. It is understood that you have three to work with.

With this statement:

quote: Mark Strube
If there was a complete lack of understanding I wouldn't even be asking this question. The speakers will live for now, at the level we're running, it clips probably once or twice during the entire run of trailers. Not great, but I've seen systems (that I didn't design) clipping for years at levels more severe than this without damaging anything.

This statement becomes evident:

quote: Mark Strube
I'm asking for advice, implying that the people I'm asking (all of you) have more knowledge than myself.

quote: Mark Strube
Do you recommend not performing an EQ to flat response with these subs? Should I ensure a rolloff starting at 38Hz going down? What about the affects of 3 or 4 of them coupling, doesn't that allow for a bit lower response without adversely affecting the drivers?

When looking at the response, keep in the back of your mind what the natural response of the system is...that is, it is going to roll off sharply below 38Hz. Will you get some boost below from the coupling, yes. That said, ensure that you apply no boost in your tuning below that point and let it roll off. It may even be advisable to apply a high-pass at 35Hz so you don't waste amplifier power (and speaker excursion) with frequencies that the speakers can't faithfully reproduce. This will allow more power for the passband that they can handle.

quote: Mark Strube
Since it's the safest solution for our system, I will be on the hunt for a 4th sub, but it may be some time before I have the ability to purchase it. I'll just run the LFE at a lower gain until then.

Smart move. It allows you to live for today and gives you a plan for tomorrow.

Posted by Sean McKinnon (Member # 612) on 03-18-2013, 12:03 PM:

I will say that the title of the thread was "best way to connect 3x jbl 3635 subs to one amp?" not "how to make my sound system correct" and when I looked up the specs and saw the frequency response I considered it more like a low frequency stage or surround speaker than a subwoofer. I would have thought it better to get close to a resistance that the amp likes to see than run these three "subs" on one 2 channel amp and cause clipping and damage. I did not consider the other effects for reasons i address below. I guess i was trying to save the guy some money which should not be the goal and is outside my usual priorities. I guess I felt like he was not going to buy another amp or sub either way because that was not the question he asked and that trying to improve his situation was the best compromise but I can see that is not cinema done right and for someone who has advocated such is embarrassing.

I would prefer a lapse in judgement to incompetent but it is what it is. You guys are absolutely right and I was completely wrong. I have been out of this industry for quite a while now and was out of the service end when digital cinema was still mostly an experiment on a few screens in the larger cities. I suppose it's time to be realistic and realize that I have not kept up with the industry or kept my skills sharp so rather than give someone bad advice again in the future or cause someone to do something damaging I will choose to bow out and simply be a "lurker" on the technical parts of the forum.

The last thing on this is there seems to be a lot of mis information out there as if you google the subject there are a lot of sites advocating wiring subs in series not specific to pro cinema but a lot on the home theatre car stereo world I would trust Steve and Tony's word over car stereo or home theatre guys any day.

Posted by Richard Hamilton (Member # 321) on 03-18-2013, 12:51 PM:

Mark,
Here is an article that might give you a little more insight. Speaker Wiring/Loading Examples

It explains why not to use a resistor in place of a speaker and different wiring configurations. Towards the end it has some 3 speaker wiring configurations along with warnings.

At the end:

quote:
NOTE: We do not work on Home or Car Audio. We work only with Pro-Audio applications. We cannot help you with Home or Car Audio questions.

Posted by Marco Giustini (Member # 4544) on 03-18-2013, 04:44 PM:

Thanks Richard,

Very educational.

Posted by Steve Guttag (Member # 268) on 03-18-2013, 09:09 PM:

There are two factors in play when dealing with a series/parallel system.

Putting the load at a compatible impedance to the amplifier. Minimizing series impedance that will reduce damping.

Lets take six (6) surround speakers (on one channel) as an example since it will make the situation more clear. Presuming an 8-Ohm speaker, if all six are paralleled, you present the amplifier with 1-1/3 Ohms. Below what most amplifiers can deal with on one channel.

You can make three groups of two or two groups of three. However, there is really only one good method of satisfying both conditions (usable load, minimal series impedance). The best method is to create two groups of three. But beyond that, each group should be paralleled. So if you parallel three speakers to make a group you have 2-2/3 Ohms in the group. You then series the two groups to present the amp with 5-1/3 Ohms. Something most amps can handle just fine.

But a key to this wiring method is EACH surround speaker only has 2-2/3 Ohms in series with it (the other group)...which is the minimal amount you are going to get with this quantity.

If you, instead series three speakers to make a group. You get 24-Ohm group and when paralleled with the other group present 12-Ohms to the amp (workable but inefficient with a solid-state amp). However, you also have each speaker have 16-Ohms of series impedance with it...bad.

You can work the problem using groups of two but again you are going to have a greater series resistance than the paralleled group of three.

The goal is always to minimize (eliminate, if possible) series connections when it comes to audio...particularly on "bass" components where the effects of damping are more important.

Note, QSC, with the DCA1222 purposely designed it (and has done a VERY poor job advertising, publishing the fact) to allow for driving into 1.6-Ohms. The DCA1222 will take five 8-Ohm speakers in parallel without issue. It makes it particularly suited for surround applications.

Posted by Sean McKinnon (Member # 612) on 03-19-2013, 09:01 AM:

How come some cabinets (not necessarily speaking professional cinema but non pro applications) are wired internally with drivers in series? Specifically my buddies bass guitar amp head and cabinet contains 4 4 ohm drivers with 2 drivers each in series and both sets of two in parallel? (I am honestly interested in learning not trying to defend my error)

I was trained that if you have one speaker in a surround array that is blown you can use a resistor as a replacement to protect both the amp and not cause a noticeable volume difference between sides until a replacement or repair can be obtained, I guess what I am trying to understand is the damping effect mentioned above and why the resistance of the speaker itself has such a different effect than than adding resistance through a resistor?

I have also seen double voice coil speakers (again, not in a pro cinema application) that are specifically intended to be connected in series? I apologize for my confusion it is just very unsettling to find out that the what you were taught was the right way (ie; series/parallel configuration when installing a surround array vs. connecting them all in parallel) was wrong. Even though I have moved on to owning and operating my own business and have no plan to ever re-enter this industry I still have an interest in it and I still want to know and understand what is the right way. Any help you guys can give in helping to understand this issue is greatly appreciated.

Posted by Tony Bandiera Jr (Member # 2365) on 03-19-2013, 12:34 PM:

Sean:

The bass guitar amp has a very ragged and "inaccurate" response as it is intended to deliberately color the sound of the guitar. (That is also why two musicians rarely agree on using the same amp for the same instrument, they all sound different.) There is not a single "instrument" amp/cabinet combo that will have a flat and smooth response curve because they are designed on purpose to "enhance" (a questionable statement at best) the sound of the instrument it is designed for.

The temporary use of a resistor to bypass a blown surround speaker is not really that bad of a thing, but using one permanently (like UC Irvine's HIB100 does) is not acceptable. Like I said in my last post, I am not fond of the idea of series connecting surrounds, but the audible artifacts are minimal in that application. But also like I said, if I ever get to build my ultimate room, each speaker will have it's own amp channel. [Big Grin]

And in surrounds, where deep bass is not needed, the loss of damping caused by the series connections is not really an issue.

As for dual voice coils, in general they still are NOT intended to be connected in series! For example, in car subs, the dual coils are used to drive a single speaker element from the 2 stereo channels. A really dumb idea in any event, in those (rare) cases where the bass frequencies/levels/phase are NOT identical will result in some very odd cone movement and excessive heating of the coil(s).

And in regards to your earlier post where you suggested you just become a lurker...don't. You are certainly welcome to voice your opinion or suggestions, but just be ready to hear about it if you're wrong. [Smile]

In the case of technical subjects, if you are not really sure, just preface your suggestions with "could you try..." or "would this work?....." that way it doesn't sound like an "expert" opinion. [Big Grin]

Posted by Sean McKinnon (Member # 612) on 03-20-2013, 11:09 AM:

Thank you tony between your post and Steve's post above it makes a lot more sense now. I appreciate you guys taking the time to educate.

I think your right about properly qualifying ones posts such as "I have this idea that I have never tried in practice so make sure you validate it before trying it" or some such is a very good idea. I was just upset at myself for giving bad advice and for making myself look stupid in front of people that I respect.

I also have been bothered at the fact that a lot of skills I had learned are becoming obsolete and even though I have moved on to different things and had great success so far I hate feeling like all my prior experience has become wasted and worthless. My training and skills are obsolete. I know I am not the only person who must feel this way and I don't know why I even care but I think I took it a little harder than normal because I was already feeling that way when my ignorance was called out.

However, I don't have the time to Check up here much anyways because I spend most of my time running my business or with my family but if am here and think I can add something useful I will. I will just make sure to qualify my posts appropriately in the future.