Labgruppen C684 Power Amplifier -

Labgruppen C684 Power Amplifier

Here in the states, Lab.gruppen (a Swedish amplifier manufacturer) existed in relative obscurity until the late 1990s. Today, their products appear on many high profile tours and their fP series amps are known as a benchmark for quality and competence. Recently, the folks at Lab.gruppen have released the C Series of amps - designed specifically for use in permanent installations.
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(click thumbnail)Fast FactsApplications: Installation, live sound

Key Features: Four-channel; Voltage Peak Limiter; Current Peak Limiter; Power Average Limiter; Class TD; compatible with NomadLink

Price: $4,895

Contact: Lab.gruppen/TC Electronic at 818-665-4900, I will admit that, as recently as a few years ago, I was one of those archaic souls who thought of power amplifiers as relatively simplistic devices. To me, it seemed that either an amp worked or it didn't. Turn on the switch, and juice comes out the back. In the years since my personal "dim-bulb era", amplifiers have gone from lumbering beasts to lean, thrifty, reliable, dynamos. I will also admit to being amazed at how something that you can lift with two fingers can move so much air. Nowadays, as if power-to-weight ratios weren't enough, amp manufacturers are creating amps that have on-board diagnostics and extensive protection and remote control capabilities.

Here in the states, Lab.gruppen (a Swedish amplifier manufacturer) existed in relative obscurity until the late 1990s. Today, their products appear on many high profile tours and their fP series amps are known as a benchmark for quality and competence. Recently, the folks at Lab.gruppen have released the C Series of amps - designed specifically for use in permanent installations.


There are four models in the C series line and they are all of the two-space, four-channel variety. The model I received for review was the C68:4, the flagship. The amp has a claimed power output of 1700 watts per channel (remember there's four) into 4 ohms (with the amp's Voltage Peak Limiter set to maximum - more on this in a moment), representing a massive power density. As would be expected from any designated installation amp, the C68:4 is comfortable driving both low-impedance loads and 70V/100V, distributed systems.

The C68:4, like the fP series amps from Lab.gruppen, has a Class TD topology. Purportedly, the patented TD process combines the efficiency of a Class D amp with the sonics of a Class AB amp. The amp features a regulated switch-mode power supply that, according to convention, should yield greater stability in the face of voltage drops or fluctuations.

The amp has an adjustable global input gain of +23 dB to +44 dB in 3 dB steps, meaning it should be possible to achieve a good signal-to-noise ratio regardless of incoming signal level (within reason of course). There are three dedicated DIP switches on the rear panel that, depending on the combination of their settings, can globally compensate for the level of incoming signal. After that, each channel has its own front panel attenuator that will dip from 0 dB to –Inf.. Appropriately, those volume controls are hidden behind a security panel on the front of the amp.

Other prominent features of the amp include a Current Peak Limiter (CPL) and a Voltage Peak Limiter (VPL). The Current Peak Limiter is an internal limiter that ensures that the amp will not try to deliver more current to the outputs than what is physically possible for the transistors. It does this by comparing the output voltage and the output current - maintaining operation in what Lab.gruppen calls the Safe Operating Area (SOA). The Voltage Peak Limiter (VPL) is a process that matches the amplifier's output to the connected speaker load. This is achieved through another group of DIP switches on the back of the amp. There are eight separate settings for Voltage Peak Limiter ranging from 42 to 141. Fortunately, this function, unlike the previously mentioned CPL, can be individually applied to each of the amp's channels. That means that, with four channels, you could drive both a distributed sound system (for paging perhaps) and a low impedance system (for performance), all from one C68:4.

The C68:4 also has an extensive system of fault warnings and protections. First among them is the Very High Frequency Protection system. This system detects continuous ultrasonic program material (starting at 10 kHz) at full power and mutes the output channel if the oscillation lasts for six seconds in an effort to protect high frequency speaker components. This feature is based on the assumption that full-scale VHF signals do not appear in natural sources (like music or speech)- especially in a non-transient fashion. The VHF protection system has a corresponding LED on the front panel that indicates when it is functioning.

The amp also has a protection functions for very low-frequency DC output (also featuring muting and an LED indicator), "Hi" impedance to indicate an open load (LED indicator), and a "Lo" impedance protection system that indicates a short circuit or excessive load (muting the output). The amp also has a high temperature detection system that measures temps at the power supply and each amp output. When a moderate overage occurs, it will illuminate a front panel LED (flashing) and send info to the Nomad Link network. If the temperature reaches a dangerous level, the amp will mute outputs and the LED will be solidly illuminated.
NomadLinkThe NomadLink Bridge 60E ($1,245) is the control counterpart to the C series amps. Bridging between the NomadLink daisy-chained network and standard Ethernet/LAN, it provides utilization of any standard Ethernet architecture when connecting a PC running the DeviceControl software. It has the ability to manage up to 60 units within the confines of cable length restrictions. It is a one-rack space unit that has a modest display and user interface. When used with a PC, it is a formidable tool for controlling a large group of amps. In addition to the previously mentioned power sequencing and monitoring capabilities, the system has some nice security features and I like how it has Ethernet connections in the front as well as the rear - facilitating easy hookup for periodic maintenance.
One of my favorite protections on the C68:4 is the Power Average Limiter (PAL). This controls performance between the power supply and the mains inlet, limiting the maximum average power consumption according to the power supply capabilities. On larger models like the C68, it can be a show saving feature as it could prevent the amp from tripping the mains breaker. Whenever any protection circuit or limiters engage in the amplifier, front panel indication will notify the user with exact feedback on what limiter or protection circuit is engaged.

The C68:4, like so many modern amps, has a soft start sequence for power up to prevent tripping a mains breaker. Also, if many amps are coming on at the same time, you can use the Nomad Link network to establish a power up sequence.

In Use

After installing the amp in my rack, I found the C68:4 to be an impressive design. For example, since it draws air from the front to the back, there are two large foam air intake filters that can be accessed for cleaning by removing the front panel cover (which also yields access to the channel volume controls). There are a slew of LEDs on the front of the amp- most of them I've already mentioned. There are also LED indicators for level (-4 dB, -10 dB) and Bridge mode. The front panel is also home to a power switch and a remote switch (to transfer power on/off commands to NomadLink network control). The rear panel has a large bank of DIP switches that address some of the previously mentioned functions as well as bridge mode (A+B and C+D), a mode function for the VPL (hard or soft) and a switch labeled "Option Active" for future upgrades (thinking ahead, I like it!). Input signals are

ushered in through Phoenix connectors and outputs are via screw terminals. Also, there are in and out connections for the network (Ethercon-housed RJ45) and a mains AC cord (30a).

At one facility I wired in the C68:4 to power stage monitors and a subwoofer. Like the fP series amps from Lab.gruppen, the C68:4 is powerful and very clean sounding. In my shop, I tested some of the protection features and it responded flawlessly- especially when presented with a dead short. The only feature missing is high-pass filtering. Yes, it protects against DC but it would be nice to be able to utilize some type of rolloff in the 30 Hz – 80 Hz range, further protecting loudspeakers.


This amp is one of the, if not the most, comprehensive amplifier I have ever used. It has a remarkable set of features designed to protect itself and other components in a sound system (including the user, who will benefit from lower maintenance costs and improved system performance). It has a massive power density and it is incredibly lightweight considering the amp's power output capability. The C68:4 also has enormous potential for remote control when used with Lab.gruppen's NomadLink network. At $4,895.00 it is not cheap, but remember it delivers four channels of high output power. This is not a budget piece, it is intended for high quality systems where big power and excellent control are required.

Review Setup

Midas Venice 320, Allen & Heath GL2400 consoles; JBL SRX700 series cabinets; Audio-Technica, Shure, Audix and Sennheiser mics; Rane, TC, BSS, PreSonus processors.