Wednesday, November 12, 2014

For this installment, I’d like to hand the mic over to Matt Skogmo. He is one of the amp gurus at QSC, and his article on multi-channel amplifiers is a tour de force of knowledge. Take it away, Matt:

With all of the new installation power amplifiers coming out on in the market—each with its own unique feature set—how can an integrator make a wise choice in order to maximize profit and minimize system complexity, customer callbacks, and overhead?

Pressures to reduce environmental footprint, the dwindling of rack spaces, the inclusion of IT gear, the mix of high performance audio with background and paging systems, interest in the total long term costs of ownership have driven some substantial changes in the market.

What are some of the trends system integrators and equipment manufacturers should be paying attention to?

Space is money. Any more, it seems that the audio system is given less and less space in integrator racks. With the necessary IT, video, and building automation demands on rack space, this is no surprise. Few if any installs have enough room for racks and racks of power amplification.

It is unlikely that this trend will stop any time soon. In the realm of audio amplification the trend is driving manufacturers of audio equipment to increase both power and channel densities. More channels, more power, less space.

Heat is money. Everyone knows it – heat kills electronics. In the old days, no one batted an eye about adding an extra ton or two of AC to an equipment closet in order to keep the power amplifiers cool. These days, there are many factors that the integrator has to balance. Green building standards, soaring costs of electricity, limited space, LEED credits, and the total cost of ownership (amortized cost of the product plus all of the long term electrical and HVAC cooling costs).

The industry has had to respond to this trend. Amplifiers in particular have had to dramatically change internally over the years. Linear power supplies and linear amplifiers gave way to switch mode power supplies and linear amplifiers, which have recently given way in many cases to switch mode power supplies and amplifiers – also known as class-D amplification.

A more detailed example can be very eye opening. Let’s compare the heat losses of two power amplifiers, both from QSC. The first, a CX1102, is a high-power installation amplifier that utilizes a switch mode power supply followed by a linear amplifier.

The second, a CXD4.3, is one of the company’s newest models, offering both a switch mode power supply and a class-D output. Both amplifiers have approximately the same output power per channel.

Let’s start with idle losses (as published by QSC) – how much heat the amp puts out when it is simply on and not passing audio. The CX1102 has an idle heat loss of about 195 BTU/hr whereas CXD4.3 has an idle heat loss of 286 BTU/hr. Now, at first blush you may think that the CXD4.3 is worse – until you take into account that there are four amplifier channels versus the CX1102’s two.

If we compare apples to apples, we would have to double the number of CX1102’s in the system. This equates to about 390 BTU/hr for two CX1102’s – an additional idle loss of 104 BTU/hr.

Things get even more interesting when the system is operating. At 1/8th power into 8 ohms, that same pair of CX1102 amplifiers will generate 2390 BTU/hr compared to a single CXD4.3 which generates 811 BTU/hr – a full 1579 BTU/hr less. That means for every CXD installed you can spec 0.13 tons less of refrigeration. That can add up quickly in infrastructure and total cost of ownership.

Time is money – voltage options. As the systems integration business has expanded, many contractors are now bidding international jobs, or have locations and partnerships that span the globe. In the “old days,” the contractor had to pour over a catalog and try to make sure that the correct mains voltage units were purchased.

As more manufacturers have migrated toward switch mode power supplies, it has become much more common to see “universal” or “one world” power supplies. These power supplies are designed in such a way as to allow the integrator to buy one model and be able to use that one model anywhere in the world without having to fuss with switches or other such interventions.

At the least, this reduces the number of products an integrator has to know by three times! No longer do you need 100-volt, 120-volt, and 240-volt models to cover the globe.

Time is money – direct drive. Broadly speaking, up until about 10 years ago, a distributed system was most often serviced by a low impedance amplifier coupled with a large output transformer.

Since then, in order to save space and weight, some manufacturers have offered special “high voltage” amplifier versions that are specifically designed to directly drive a 70-volt or sometimes 100-volt line without the use of an output transformer.

Although it may not be instantly obvious, any amplifier that can drive at least 625 watts at 8 ohms can also directly drive a 70-volt line – and if you could bridge that amp you could most likely drive the 100-volt case as well.

While it’s not common to find a true “direct drive” amplifier that delivers low impedance, 70-volt and 100-volt drive from a single channel or perhaps from a bridged pair of channels, true direct drive products—that is, having a single amplifier that can do the job without math, without transformers, and without exception—can significantly reduce the number of product choices required, often by as much as 50 percent.

Additionally, amplification products with true direct drive offer much more flexibility when on site. No longer does a change order that includes one more zone of distributed speaker mean another specialized amplifier…just an unused direct drive channel.

Time is money – flexibility. As we have shown, the industry trends and manufacturers are moving toward fewer and fewer different SKUs. This tends to make both the integrators and the manufacturers’ jobs easier.

That said, there is one axis of flexibility that has always been fixed across the amplifier marketplace: power. Power has generally be available in finite, discrete steps. Thus it was up to the integrator/system specifier/designer to know a product line and a potential job well enough to know which amplifier would most appropriately deliver the required power.

The only flexibility within a given amplifier power point would be to bridge a pair of channels in order to gain more voltage swing – and power delivered into higher impedance loads.

There are two manufacturers who have taken this issue on. One manufacturer (Bose Professional) does allow for a more flexible way of configuring a given power amplifier, but effectively restricts this flexibility to within its own system line. The other manufacturer, QSC, has come up with a method of reducing the number of amplifiers needed to address a variety of power points.

The company calls this method (or technology) FAST, an acronym for Flexible Amplifier Summing Technology. Effectively this technology allows amplifiers channels to be configured in both bridge mode and in parallel mode. For example a traditional 4-channel amplifier might allow you to bridge channels 1 and 2 and/or bridge channels 3 and 4.

FAST technology opens up a wider range of options. For example, channels can be bridged (doubling the available voltage), or placed in parallel (doubling the available current). These options are not limited to a pair of channels. This means that amplifiers equipped with FAST technology can be deployed as a 4-channel, 3-channel, 2-channel, or even a mono-block amplifier. Basically this means that no matter the load impedance, from 100-volt distributed all the way down to 1 ohm, a single amplifier can be configured to deliver maximum power.

Let’s examine a possible configuration to better understand this process. Based on the technical data supplied by QSC, an amplifier with FAST could be configured such that the first three channels (A,B,C) are placed in parallel with each other and connected to a 2-ohm concert subwoofer array and the remaining channel is connected to a distributed 100-volt line. That subwoofer will get about three-quarters (3/4) of the total output power, while the distributed line still has over 1000 watts of power available.

Although confusing at first blush, there is a method to the madness. Rather than offering dozens of power points, QSC only offers three power points. Now the option to configure each amplifier in the most appropriate way is accessible with the only trade-off being the available number of channels. The theory is that the integrator only has to reach for a small handful of tools and can configure each to drive whatever power/impedance combination necessary.

Security is money. Obviously callbacks cost time and therefore money. You have spent hours tuning and commissioning a system, measuring and modeling – only to have the client call because “something sounds funny.” On site, you’ve discovered that the local “talent” has gone into the system and completely modified everything.

In the old days, lockout plates, locked racks, and locked doors tried to thwart intervention, but were rarely effective. Certainly as more and more processing has moved from the analog domain to digital, many manufacturers are offering software based lockout features on their products.

The range of protection available is really broad. In some cases the password is a simple 4-digit pin that when correctly entered will unlock the full system. In other cases the security can rival that of some banking systems featuring multiple layers of password protection with variable length passwords.

Systems can be set up that offer minor control to the end customer, moderate control to an onsite resource, and only full control to the integrator. Tight security at an affordable price, can certainly result in a much more stable install, fewer damaged devices, happier customers—and ultimately—fewer callbacks.

Time is money – signal processing. System commissioning is one of the most time consuming and specialized portions of any project. Managing tuning, gain staging, levels, etc. are where the science turns into an art.

The primary tool for this phase of a project is some sort of digitally based signal processing unit. Currently, this capability has migrated into power amplification products that also integrate the majority of the necessary functionality in one unit. Although useful in certain circumstances, industry feedback shows that the difficulty in actually accessing the signal processing functions through a simple intuitive interface has driven many users back to stand alone processing hardware/software.

The most recent trend, driven largely by the prevalence of inexpensive graphic display tablets (and smart phones) and the desire for elegant visual interfaces, has begun to integrate sophisticated signal processing, easy to use graphical interfaces, and power amplification into one package.

Clearly, for enterprise-scale jobs, some sort of centralized processing will more likely be specified rather than using DSP in an amp, but for the smaller jobs, having a well-integrated, visual, and easy to configure DSP could well spell the true market acceptance of the signal processing integrated amplifier. If the required processing can be quickly configured without having to reach for a discrete solution, savings can be accumulated in configuration time, rack space, and wiring costs, more time doing the fun part of the job.

Many of these integrated solutions offer voicing for both manufacturer specific, as well as alternative loudspeakers. Thus, in summary, less time needs to be devoted to figuring out crossover and limiter parameters for a given loudspeaker more time devoted to optimizing system performance.

Time is money – service costs. One of the major time consuming portions of any system design is trying to figure out what type and how many spare products to keep in inventory or onsite at a given installation. Even the most reliable systems can fail at some time and users and suppliers must be prepared because nothing trumps maximum system up time.

Unfortunately, there is no scientific method to determine who is going to produce the most reliable product or products. In our small industry, generally long-term manufacturer reputation speaks loudest, followed by peer reviews and recommendations.

Really, at the end of the day, the only valid assessment that an integrator can make is based on how easily it is to repair or replace any given portion of an install. Are there multiple amplifiers or processor that can fill the spot of the broken unit? Can settings be transferred from one unit to another or from a backup to a new unit? Is the customer support from the manufacturer solid? What do your peers say? Hopefully the trend toward more and more reliability and excellence in customer support will continue industry wide.

In summary, the integrator needs to provide a solution that is affordable to the customer, meets the continually evolving needs of the marketplace, and puts money in his or her pocket. So here’s a short list of the key points to consider:

• Channel density – how many channels per rack space?
• Rack density – how many rack spaces per amp?
• Power density – how much power per rack space/channel?
• Thermal output – how many tons of AC will a given solution need – at idle and while running?
• Flexibility – how configurable is a given product – can one product meet many needs?
• Simplicity – how easy is a product to deploy, how deep is the catalog line?
• Expandability – is a product line able to adapt as system needs shrink or grow?
• Reliability – is the manufacturer known for building reliable gear? How do they treat you if there’s a problem?

A self-professed geek, Matt Skogmo is the director of hardware engineering at QSC where he oversees the development of all electronic hardware. An audio products designer at QSC for more than 17 years, he’s still thrilled to wake up each morning and go to work doing what he loves best.