Some notes on using preamps with an LMS.

*Note, this applies to ALL preamps, not just the 3u and 5u and applies if you plan to do the final crossover in a DSP amplifier instead of an LMS. There’s nothing inherently wrong with using DSP amps with preamps, most users do now, it’s just a question of routing. With DSP amps you can either use the individual outputs from the preamp as normal (recommended) or use the Mix/AES out and do the crossover in the amp, but the compromises detailed below will apply.

How much any of this affects you I cannot say, some people just want anything with kill switches they can throw on the system and will be happy regardless whereas others simply cannot live with even a 1dB deviation in frequency response, where you fall on that spectrum will determine how much any of the below text matters to you.

Many of the customer support enquiries we get are from people thinking they can improve their preamp configuration with the addition of an LMS, but are disappointed to learn it’s not so simple; getting one crossover right is hard enough, getting two right is harder. That’s not to say you shouldn’t use an LMS with a preamp but it can be a compromise and there are some things you should be aware of, if you want the best results. These are detailed below.

Just briefly though…It is our opinion that the 5u preamp used alone offers the best overall solution for most people. It’s generally the more cost effective option (compared to preamp+LMS), likely easier to set up and it avoids the complications detailed below. Adding an LMS to the 5u is unlikely to offer anything of value and in practice we tend to observe it makes things worse. Don’t fall in to the trap of thinking you need all the processing under the sun, just because you can use a hundred EQs, doesn’t mean you should!

There are 3 main aspects to consider;

1: A crossover that is designed to sum flat electronically (the Mix/Tape output), as you would want when feeding an LMS, isn't necessarily the best arrangement for summing acoustically (feeding amps/drivers directly). Real loudspeakers introduce their own frequency response, phase shifts and delays, so the crossover needs to be designed with acoustic summation in mind. The 3u and 5u are primarily designed to feed amps/drivers directly, which means their combined electrical response won't necessarily be flat.

2: Filters are not brick wall steep, they have a gradual roll off. Therefore, with the preamp+LMS setup when you mute one frequency band on the preamp, you will continue to hear the adjacent band through your speakers, even if the filters between the two crossovers match up perfectly. For example, you kill the kick channel on the preamp but through your kick speakers you still hear the low end of the low mids. This is because, lets assume you have a 24dB L-R crossover at 150Hz, at 100Hz the low mids channel on the preamp will still have content at around -15dB and this is naturally passed through to the LMS which will play it through the kick speakers.

3: Unless wired appropriately, any sub bass processing will be heard through the kick bins and potentially low mids.

-Point 3 is easy to resolve, both the 3u and 5u can remove the sub channel from the overall mix output, you can then use a separate cable to feed this to your LMS. This means the processed sub sound never meets the kick and mid speakers.

-Point 2 is impossible to completely resolve when feeding an LMS from a single combined Mix output, that's just physics. The way around it is to not use an LMS with the preamp. The 5u has a more than capable LMS inside it and will not have this issue when used alone. Again, whether this matters to you I can’t say, I only mention it because some users have been disappointed to find this is the case. And again, this applies to ALL preamps when using the combined output.

-Point 1 is potentially very problematic, depending on how discerning you are. The main issue comes from the narrow kick region, typically 90-150Hz. Using such a narrow band with the typical 24dB/oct filters creates dips in the overall frequency response when summed electronically. This is partially due to phase cancellations and partially due to the reduced volume these closely spaced filters create. However, when feeding amps/drivers directly, this is likely the crossover you want.

See simulation chart below which shows the result of the common 90-150-1500-7000 crossover with 24dB/oct L-R filters. Sub channel not included as recommended in point 3.

You’ll note the reduced level of the kick, the 3u adds more gain to the kick channel to compensate, the following illustrates how additional gain can help:

But that’s still not optimal. You can improve this by moving the kick to low mid crossover much higher, 300Hz for example:

So yeah, it’s not like you can’t get good results but there are some things to be aware of. Care should be taken to set the crossover appropriately if you want a flat response going in to the LMS.

The new 1u Music Channel uses a specially designed crossover that preserves a flat response perfect for feeding an LMS, so why not use that crossover in the preamps? Because the requirements are different. In order to keep a crossover that sums flat electronically over a variety of frequency settings, it uses filters in a configuration that, when feeding amps/drivers directly, would give very unpredictable results across a variety of different systems… and likely not good results.