Noob Amp/Sub wiring questions

[Tonestire]

I am hoping someone can throw me a little help.
What is the best way to wire up setup listed below.

2x JBL Jtx215 Speakers
1x JBL 218S Subwoofer
1x Rockville RPA12 Amp (For Sub)
1x Rockville RPA14 Amp (For PA Speakers)
1x Rockville Crossover
Behringer Wifi Mixer

I was hoping to have the RPA12 Amp drive the JBL Sub but it only has one speakon input along with a speakon loop. I have wire and connectors to make my own cable if needed but I am unsure if this is possible to use both outputs of the amp to drive the sub with one input.

Thank you for any help you can give me.

[Hugh Robjohns]

Normally, yes.

You would select the BRIDGE mode on the amp and then a single speaker cab is connected between the positive output terminals of both amps. (With no connection to the negative terminals at all)

Looking at the manual for the RPA12, though, while it confirms the arrangement above using its binding posts it doesn't provide the Speakon connector wiring, or even mention using Speakon connections in the context of bridge mode...

However, there are standard conventions followed by most amp manufacturers...

For normal (two channel) operation, each amp output appears on its respective speakon terminals 1+ and 1-, and the speaker cabs are wired using the same 1+/1- terminals.

For Bridge mode, the positive terminal of the B channel is normally wired to the 2+ terminal of the A channel speakon.

So to connect a sub to the amp in bridge mode, the cable should still be wired 1+/1- at the sub end, but 1+/2+ at the amp end. Obviously such a cable needs to be marked at each end of the cable accordingly since it won't work if connected the wrong way around. Commercial Bridge connection cables often have a red connector for the amp end.

NB. In bridge mode make sure the sub impedance is above the minimum for the amp in bridge mode. Also do not connect anything to the second speakon connector! Both amp channels are in use feeding the one cabinet!

USE THE INFORMATION ABOVE AT YOUR OWN RISK.... While this is the usual arrangement, I've not been able to confirm the Rockville amp is wired according to this convention....

If you have a multimeter you can check the amp wiring for yourself. With the Amp turned off and disconnected from the mains for at least 10 minutes, set the meter to continuity or resistance mode and place a probe on the channel A positive binding post. Use the other probe to confirm connection to channel A speakon terminal 1+. Repeat for channel B, then check whether channel B's positive terminal is also connected to channel A's speakon 2+.

If it is, wire up as described above. If its not, check whether channel A's positive terminal is wired to channel B's speakon 2+. If so continue as above but use the B output to the sub rather than the A output.

[Wonks]

Hi and welcome.

Assuming the sub is the JBL JRX218S (to give it its full name), thenthe sub hs a 4 ohm speaker rating and a 350W power handling capacity (I'd ignore the peak rating, it just makes it 'look' better in adverts).

The spec sheet goes with the standard rule of thumb recommendation for a power amp with a 350W-700W rating (one to two times the rated continuous power handling of the speaker).

The Rockville RPA12 has a quoted continuous power output of 700W per channel into 4 ohms.

So you only need to use one channel of the RPA12 to feed the 218S. It can't handle any more power and you'll get as much volume from it as it's designed to provide. Any more power and you'll be in danger of burning out the speaker voice coil.

Whilst the Rockville RPA12 can run in 'bridged' mode to use both outputs, that has to be into a minimum load of 8 ohms (or the amp will run into over-current protection and shut down) and its 1400W into 8 ohms is more than the JBL can handle.

So a single channel is all you need. Get another 218S and you can feed it from the currently unused channel.

For the sub, set the amp mode to 'stereo' and crossover mode to 'bypass'. The crossover is in the crossover unit, and you don't want two low-pass filters in series.

If you use channel A, then simply turn the output level for channel B to zero and you'll only have an input from the low frequency output on the crossover going to

The crossover frequency should be set to around 80Hz. 18" subs tend to be run at lower crossover frequencies than 15" subs, though you could try running the sub at anywhere up to 125hz. But with a single sub, 80Hz avoids having any frequencies that can be considered 'directional', so if you are playing back stereo music, you get slightly better stereo positioning in the low frequencies. It's not a big difference, but it is noticeable if you are listening hard.

The JRX215 is an 8 ohm speaker with a 250W rating. The RPA14 provides 700W per channel continuous into 8 ohms and is definitely overpowered for the 215s. Whilst you can keep the amp volume knob turned down, that doesn't stop a strong signal from the mixer, or someone turning the crossover volume knob up, from still getting the amp to put out too much power and burning out the speakers. Another RPA12 would be a much better choice for the JRX215s.

Assuming you already own the two amps, and given the higher power handling of the sub, I'd use the RPA12 for the tops and one channel of the RPA14 for the sub, but make sure you never turn the power output of the RPA14 all the way up and never past the 3 o'clock position, and better if you can keep it at 12 o'clock.

But I would really look at exchanging the RPA14 for another RPA12.

[Wonks]

Normally, yes.

You would select the BRIDGE mode on the amp and then a single speaker cab is connected between the positive output terminals of both amps.

Looking at the manual for the RPA12 though, it doesn't provide the Speakon connector wiring, or even mention its use in the context of bridge mode...

However, there are conventions followed by most amp manufacturers...

For normal (two channel) operation, each amp output appears on its respective speakon terminals 1+ and 1-, and the speaker cabs are wired using the same 1+/1- terminals.

For Bridge mode, the positive terminal of the B channel is normally wired to the 2+ terminal of the A channel speakon.

So to connect a sub to the amp in bridge mode, the cable should be wired 1+/1- at the sub end, and 1+/2+ at the amp end. Obviously you'll need to mark each end of the cable accordingly since it won't work if connected the wrong way around.

USE THE INFORMATION AT YOUR OWN RISK.... While this is the usual arrangement, I've not been able to confirm the Rockville amp is wired according to this convention....

Bridge mode appears only to be on the terminals, and as I've already mentioned in my post above, 8 ohm (and above) bridge mode won't work with this particular 4-ohm sub and also provides far too much power.

[Wonks]

Looking at the crossover manual (the web page doesn't give this detail), the sub crossover is adjustable between 30 -200Hz, so an 80hz crossover for the sub is achievable. You can try higher frequencies, but don't go above 125Hz.

Operate the crossover in stereo mode, which gives you a 2-way crossover. Mono mode gives you a 3-way crossover, which you'd only use if you had separate sub, mid and high frequency speakers. The 215 has combined mid and high frequency speakers with an internal crossover, so it needs to be fed all the frequencies above the crossover frequency.

[Hugh Robjohns]

Bridge mode appears only to be on the terminals...

The manual seems to lack any info on the Speakon wiring at all. I'd be surprised if they aren't wired for standard bridge operation, but it is possible. Rockville is not a brand I've used.

....bridge mode won't work with this particular 4-ohm sub and also provides far too much power.

Thanks, Wonks. I didn't check the speaker specs as I assumed it was supplied as a suitably matched system. But you're quite right. Bridge mode is not appropriate with that gear.

Perhaps the sub amp was supplied in anticipation of getting a second sub later.

So a standard speakon cable running from one channel with the amp in stereo mode will be fine.

[Wonks]

The manuals for the RPA12 and RPA14 do mention in their bridged mode description that the signal is sent to the positive binding post terminals of Channel A and Channel B.

Nothing mentioned for the speakon outputs.

They are very low-cost amps for the power output, ($275 $325 respectively), and so you can't expect all the bells and whistles a more expensive amp would have.

[Tonestire]

Wow, you guys are awesome! I'll try out the suggestions when I get home and let you know how I made out. I really appreciate all your feedback and help.

[Hugh Robjohns]

They are very low-cost amps for the power output, ($275 $325 respectively), and so you can't expect all the bells and whistles a more expensive amp would have.

We are talking one extra wire or pcb track. Hardly an expensive bell or whistle...

But the absence of any speakon wiring details in the manual is certainly poor.