Film-Tech Forum ARCHIVE: SC2150 LF bridge or not ?

This is topic SC2150 LF bridge or not ? in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Anhtu Vu (Member # 7689) on 09-09-2015, 09:06 AM:

I'm about to install the QSC SC-2150 with the DPA 4.2Q (4 channel amplifier) and QSC recommended not to bridge the amp for the LF. Anyone care to chime in, pros and cons ?

Posted by Dave Macaulay (Member # 813) on 09-09-2015, 09:54 AM:

This is normally a question of the amp's ability to power low impedance loads. If each channel is rated for an 8 ohm load, bridging two channels for a 4 ohm load is allowed. The SC2150 shows a 4 ohm LF impedance. It looks like this amp would be OK for that using two channels bridged. I can't find anything in the DPA series spec sheet or available documentation that says channels can be bridged, though.
However: the LF cabinet is rated for 500W amp power and the amp is rated at well over that for 4 ohms. These amps are a bit different in that total amp chassis power output is shared among the four channels, so each channel's max power output depends on the simultaneous output level of the other 3.
I don't know what you plan: four channel amps are not commonly used by us because of their various limitations. I would tend to avoid more than one LF cabinet on one 4 channel amp because of the shared power issue but it looks like the chassis has enough total power to handle it. How many of these amps do you plan on using, and what's your plan for channel assignment?
You can use two channels for one screen speaker, having one amp for L and R LF/HF and half of one one for C LF/HF. The problem comes with the extra two channels on that amp - you don't want to use them for the sub or surrounds which typically have very high power requirements. In a small house with few surrounds, maybe... but don't share a screen channel amp with the sub.
These amps have some good features but are overpriced if you don't use those: two channel ISA or DCA amps (the 4 channel DCAs have poor low impedance ratings) cost less and give more flexibility if you're using them in a simple sound rack with direct signal connections rather than using Qsys distribution.

Posted by Brad Miller (Member # 2) on 09-09-2015, 12:30 PM:

Anhtu - do not post your questions in multiple forums. It makes a mess of the forum and also makes it harder for people to have a discussion when all of the comments are split up in different threads.

Please read the forum rules before resuming posting. Thank you.

Posted by Anhtu Vu (Member # 7689) on 09-10-2015, 05:34 PM:

Just checked with the cinema guru at QSC and there's no need to bridge, basically one amp for L-R and a second for center (amp channel A-B) and LFE (channel C), channel D goes to a mono PA, the third amp takes care of all the surrounds.

Posted by Marco Giustini (Member # 4544) on 09-11-2015, 01:46 PM:

that doesn't tell us why you shouldn't bridge a DPA. Maybe the document you have suggests the power on a single channel is plenty for that type of speaker and there is no need to bridge another one?

Posted by Dave Macaulay (Member # 813) on 09-11-2015, 02:14 PM:

I'm not sure you can bridge DPA outputs. The documentation available is, well, effectively nonexistent.
They are similar to Crown DSi amps with a digital signal processor on board to handle crossover, delay, and some EQ. One might expect a user or installation/setup manual... maybe you get them if you buy one? Maybe you have to take an expensive course at QSC World HQ to learn "the knowledge"?

Posted by Marco Giustini (Member # 4544) on 09-12-2015, 04:58 AM:

You can have the amp in 4,3,2 and 1 channel configuration.

Posted by Carsten Kurz (Member # 5396) on 09-12-2015, 05:28 AM:

With this amps FAST feature that allows one channel to operate close to the maximum power rating of all 4 amps, there is probably no need for bridging.

Yet, the spec sheet says:

'Flexible Amplifier Summing TechnologyTM (FAST) actively distributes total amplifier power across one, two, three or all four outputs, enabling amp channels to be paralleled or bridged for maximum current and voltage output.'

Wether that means 'bridging' in the conventional sense, I don't know.

I find all these QSC smart amps impressive from a technical point of view, and I would probably choose a full QSC system for larger operations, but I would never use them for smaller cinemas. I wouldn't want to be dependent on a proprietary technology. If a standard amp fails, I just dig up any replacement, push it in or rewire.
With these QSCs, you are more or less forced to store a replacement unit locally. Plus, I hate not being able to download proper manuals/documentation for devices I install. QSC knowledge seems to be only available to trained QSC installers.

I do understand these amps solve a lot of issues for very large installations like multi-screen multiplexes and ATMOS.

- Carsten

Posted by Marco Giustini (Member # 4544) on 09-12-2015, 05:47 AM:

So are QSC saying that because the power is shared between channels, bridging won't give you any advantage?

So what's the point of having the bridge feature at all?

If all the available power is share between channels, I could have one channel running a sub and one channel running a stage speaker. The other two channels idle but does not really matter as their power is then diverted to the other channels.

That said, the DPA are indeed great for Atmos and for Qsys. Besides that a good old DCA or even an ISA would suffice.