HK Linear 5 Sub 1200's not firing.

[JC_Tech]

Hey eveyone,

It's my first time posting here, I've always seen some great topics being discussed and really helpful suggestions on the forum, so thought I'd post about an issue I'm experiencing.

At work, we're running the following signal chain for our FOH and subs for the hall:

Allen & Heath SQ7> 2x Lab Gruppen IPD 2400 (1 amp dedicated to the FOH tops and another amp dedicated to the subs) > 4x HK Linear 5 112F FOH Tops and 2x HK Linear 5 1200 Subs

The tops are all working as expected, however I'm struggling to get the subs to fire.

We had a refurbishment of the hall this year following the removal of RAAC concrete in the roof and this is the first chance I've had to properly set up the FOH and subs since starting in the role.

Here's what I've tried so far:

- Double checking routing and IO patches on the SQ7 (both amps are definitely receiving healthy signal levels and the IO meters on the amps show a healthy output signal)
- Swapping the speakon runs (the FOH tops work in both amps and all amp outputs and the subs don't fire from any output on either amp)
- Trying different speakon cables from the outputs of both amps
- I even moved one of the subs into our Control Room and went directly out of the amps into the sub to see if the speakon install in the hall was at fault (the sub doesn't fire even with a new cable out of the amp going directly into the sub around 1 metre away)
- Double checking that the amps aren't muted (again, the FOH tops work through every output on both amps)
- Looking through the menu on the amps to double check there aren't any strange settings impeding the output (haven't noticed anything untoward here)
- Mixing the outputs on the amps (e.g having the FOH L tops and one of the subs plugged into the first amp and FOH R tops and the other sub being fed from the other amp). In this scenario, only the FOH tops are working.

This is a really confusing issue and I'm at a loss to know what to try next. I'm not even getting a tiny bit of signal combined with a noise floor, the subs don't output anything whatsoever, absolutely dead.

I should also add that all the FOH speakers and subs are only ~ 5 years old and have largely been stored away during my time in this role.

Thanks for your help with this.

Joe

[Hugh Robjohns]

You don't mention crossovers anywhere. It would be very unusual to feed full range signals into both tops and subs. Normally tops recieve a high-pass filtered signal, and subs a low-pass filtered signal. Cross patching these us likely to result in nothing from the subs and very little from the tops!

Normally, the filtering is performed by a dedicated crossover unit, or a PA controller (like a DriveRack), but it can be performed in some DSP-based Amps, which might be the case for you. Worth checking that's all set up correctly.

You seem confident there's signal reaching the amps, and that the amps are delivering power out. And you've swapped cables which should rule out cable-failures — although I'd use a digital voltmeter (AC) to make sure amplifier volts really are reaching the cabinets.

The next check I'd make would be to measure the impedance at the input connectors to the subs (with the amp cables disconnected) to see if the drive units have gone open circuit. Seems unlikely all units would have failed... but maybe the previous users over drove them.

[JC_Tech]

Hi Hugh, thanks for the advice and for the suggestions for the next steps to take.

That is a very valid point in regards to the crossovers. I'll menu dive through the amps again in case there is a high pass being applied in any way/shape/form. The amps do have various PEQ options, delay and phase adjustments so I'll take a more detailed look to see if that's where the problem may lie.

Forgive my ignorance, I haven't really had to troubleshoot with a voltmeter for an installed system before. How would I measure the output voltage from the amps? Would I use the probes on the voltmeter and just contact the speakon sockets to be able to measure this?

What's also the best method of measuring the impedance of the input connectors on the subs themselves as well?

Thanks for the suggestions and your wisdom Hugh, that's really appreciated.

[AlecSp]

I suspect it's a simple cabling issue.

The subs receive their input on 2+/- of the 4 pin Speakon (https://hkaudio.com/products/linear-sub-1200/)

You'll very likely be feeding them with 2 core speaker cables connected to pins 1+/-

Take the Speakon cable feeding the sub and, at the speaker end, swap the cables to pins 2+/- in the connector, and I'm sure you'll be golden.

This slightly confusing setup is to allow a single 4 core cable run from amps to feed both sub and top. Helpful if you have the right cabling and, even more, if you have an amp that outputs both signals on one Speakon. But confusing and problematic when using 2 core speaker cables.

If you do end up using a non standard Speakon cable (connected from 1+/- to 2+/-), please ensure it's clearly labelled as such, or this will come back and burn you, or someone else, in future.

As ever, it's always worth reading the user manual, or checking specs, where this becomes clear.

[Hugh Robjohns]

Genius! Yes, it'll be the standard cable wiring! Good call.

The tops are wired on 1+/- while the subs are on 2+/-.

Haven't used passive speakers for so long now I'd forgotten about speakon wiring conventions!

[JC_Tech]

The subs receive their input on 2+/- of the 4 pin Speakon (https://hkaudio.com/products/linear-sub-1200/)

You'll very likely be feeding them with 2 core speaker cables connected to pins 1+/-

Take the Speakon cable feeding the sub and, at the speaker end, swap the cables to pins 2+/- in the connector, and I'm sure you'll be golden.

Yes! It was exactly this. I've just rewired one end of the speaker cable to the 2+/2- as suggested and lo and behold I have a mighty rumble being delivered from the sub I just tested.

Thanks for the suggestion, I was suspecting the solution to be something deceivingly simple. This will go in the memory palace for troubleshooting issues next time!

[AlecSp]

Glad to hear it. And Hugh's right, it's too easy to forget about the intricacies of speaker cabling with the prevalence of active PAs.

And, taking a look at your Lab Gruppen amps, see how channel one's speakon outputs both ch 1 & ch 2 across the Speakon's 4 terminals.

The idea is that you can use one amp per side, each with ch 1 feeding the top and ch 2 feeding the top. This would let you run a 4 core Speakon cable from ch 1's Speakon output to the sub, and another short 4 core Speakon cable from sub to top.

Nothing wrong with what you're doing, though. You just need the appropriate (labelled) cables. And maybe a reminder label on the rear of the subs!

[Hugh Robjohns]

I love an easy fix! Well done all.

[JC_Tech]

The idea is that you can use one amp per side, each with ch 1 feeding the top and ch 2 feeding the top. This would let you run a 4 core Speakon cable from ch 1's Speakon output to the sub, and another short 4 core Speakon cable from sub to top.

Nothing wrong with what you're doing, though. You just need the appropriate (labelled) cables. And maybe a reminder label on the rear of the subs!

Ah that makes sense by using all the 4 conductors within the cabling and at the connector itself. I wonder how the active crossover functions in the amps when used this way?

We do a lot of Musicals and Theatre based events at work, so for our use case it makes more sense to me to have control of the subs on a dedicated amp and control over the FOH tops on another.

Cheers Alec, always good to get an idea of what is possible with gear and why this is the case.

[Hugh Robjohns]

I wonder how the active crossover functions in the amps when used this way?

Nothing changes. The amp has two channels, and each can be filtered as required.

The (filtered) amp outputs are present on 4mm binding posts and 4-pole Speakons.... but if you read the manual (or just read the markings on the amp) you'll see that they are wired differently.

Pins 1+/1- on both Speakons are wired to their respective amp channel.

However, pins 2+/2- are wired differently on each side to maximise versatility.

On Channel 2, the amp output is wired to both 1+/1- AND 2+/2-.

Thus, using a four-core cable delivers the amp output to both pins 1 and both pins 2 — so it would work with a sub that expects its input on pin2+/-.

On Channel 1's Speakon the 2+/2- terminals carry the output from Channel 2.

This arrangement allows a single cable to be used when bi-amping a two-way speaker, or if working a sub in Bridge mode.

... so for our use case it makes more sense to me to have control of the subs on a dedicated amp and control over the FOH tops on another.

For every use case, surely? The sub amp needs low-pass filtering and the tops need high-pass filtering, to form a sensible crossover and protect the drivers from damage.

[JC_Tech]

I wonder how the active crossover functions in the amps when used this way?

Nothing changes. The amp has two channels, and each can be filtered as required.

The (filtered) amp outputs are present on 4mm binding posts and 4-pole Speakons.... but if you read the manual (or just read the markings on the amp) you'll see that they are wired differently.

Pins 1+/1- on both Speakons are wired to their respective amp channel.

However, pins 2+/2- are wired differently on each side to maximise versatility.

On Channel 2, the amp output is wired to both 1+/1- AND 2+/2-.

Thus, using a four-core cable delivers the amp output to both pins 1 and both pins 2 — so it would work with a sub that expects its input on pin2+/-.

On Channel 1's Speakon the 2+/2- terminals carry the output from Channel 2.

This arrangement allows a single cable to be used when bi-amping a two-way speaker, or if working a sub in Bridge mode.

... so for our use case it makes more sense to me to have control of the subs on a dedicated amp and control over the FOH tops on another.

For every use case, surely? The sub amp needs low-pass filtering and the tops need high-pass filtering, to form a sensible crossover and protect the drivers from damage.

Thanks for the detailed explanation here Hugh, that makes a lot of sense when using a 4 core speaker cable that makes use of all 4 conductors within the NL4 connectors. Certainly is a clever way to minimise on cable runs while maintaining the same functionality as individually outputting different channels via individual speaker cables.

Yeah, that's not wrong either with regards to the use case. Horses for courses though in the way that you achieve that I guess. It's always good to know of different methods and this has been quite a learning experience for me with regards to 4-pole Speakons.