Electro-voice TEMP LIMITER V1 User Manual

Thermal Energy
Management and Protection
Limiter Technology

1

THERMAL ENERGY MANAGEMENT AND PROTECTION LIMITER TECHNOLOGY

Version 1 Electro-Voice, Burnsville, Minnesota, USA October 2010

© Bosch Security Systems Inc.

Thermal Energy
Management and
Protection Limiter
Technology

Rev. 1 / October 20, 2010

1. Introduction:

This application note discusses the use of the Electro-Voice Thermal Energy Management and
Protection (TEMP) Limiter. This document will cover the reasons for its development, the significant
advantages it provides in protecting loudspeaker transducers against thermal failures and how to
implement and use the TEMP Limiter in IRIS-Net project files and hardware.

2. The Need for the TEMP Limiter

Loudspeakers are designed for reliability. The design and testing process covers all components
and aims to provide the optimum performance, while simultaneously testing and analyzing the ability
to continue to function and be reliable even during extreme or abusive use. In the world of
transducer design there’s a lot that can be done to optimize thermal and mechanical parameters to
maximize output. Electro-Voice engineers work to constantly increase the peak voltage capacity and
the power handling capacity of our transducers. All components are selected and tested to work at
extreme temperatures.

Modern professional amplifier design has significantly increased the output power ratings and
instantaneous peak output capabilities to keep pace with sound reinforcement requirements. These
modern designs can present instantaneous peak voltages well above and beyond previous
generations. As a case in point, the TG7 is capable of producing 180-volt instantaneous peaks.

2

THERMAL ENERGY MANAGEMENT AND PROTECTION LIMITER TECHNOLOGY

While this provides tremendous dynamic range and power performance, instantaneous output
voltages in these extreme ranges present new challenges to operators and designers to ensure that
the loudspeakers are being driven with a safe RMS output level from the amplifier. In order to keep
the loudspeakers functioning reliably, tools must be provided to prevent against both mechanical
and thermal failures.

So, how is the user to know what RMS level they are sending to each speaker? How does the user
determine the temperature of their loudspeakers and how safely they’re driving their system? The
answer is that this is practically impossible for users to do, and that system techs should not have to
concern themselves with guesswork about voicecoil temperature. This is the reason that Electro­Voice has created the Thermal Energy Management and Protection (TEMP) limiter.

3. The TEMP Limiter in Action

The Thermal Energy and Management and Protection (or TEMP) limiter is based on a detailed
thermal model of the connected loudspeaker. It continuously calculates the temperature of the
loudspeaker voice coil as signal is applied and the speaker heats up and cools down. The thermal
model approach was selected because it proves to be more reliable and accurate than other
methods. A simple RMS voltage limiter would function, but it would neglect the fact that
loudspeakers can handle large RMS voltages for sufficiently short periods of time, so this approach
would potentially provide too much limiting. Another method would be a combined approach of
monitoring the voltage and current delivered by the amplifier. Unfortunately, this would make the
parameters depend upon how many speakers the user had paralleled on each channel and would
be limited to amplifiers with DSPs on board. The thermal model approach does not have such
limitations.

Sample by sample, the TEMP limiter tracks the voltage applied to the loudspeaker terminals and
uses this to calculate the instantaneous temperature of the voice coil. To minimize gain reduction
and maximize output, the temperature scale has been broken up into several regions. The first

temperature region is between

T (ambient temperature) and

Ambient

soft knee). For a voice coil temperature in this range, no gain reduction is applied. Above

T (the lower threshold of the

Knee

T gain

Knee

reduction is applied based on the RMS drive level and the voice coil temperature. The amount of
gain reduction and the rate at which it’s applied depends on the voice coil temperature. If
something goes terribly wrong, there is a

T (critical temperature) above which a lot of gain

Critical

reduction is applied quickly. These parameters are not user-adjustable. They are the result of
extensive simulation and testing under a variety of abusive conditions.

3

THERMAL ENERGY MANAGEMENT AND PROTECTION LIMITER TECHNOLOGY

)

Figure 1: An example of an abusive test with a soft knee at 120C and a critical temperature of
180C.

The TEMP limiter should not have to provide a significant amount of gain reduction under normal
operating conditions. It is designed to prevent component failures in the case where the user is
riding the fine line between pushing their system hard and pushing it to failure. The difference
between these two cases can be smaller than 1dB!

dB vs. Temperature

400

350

300

250

200

150

Coil Temperature (C

100

50

0

20 25 30 35 40

Dri ve signal (dB)

Figure 2: An example of drive level vs. voice coil temperature. The difference between a coil
temperature of 250C and 200C is 1dB. This means a 1dB level reduction can prevent component
failures!

4

THERMAL ENERGY MANAGEMENT AND PROTECTION LIMITER TECHNOLOGY

4. Important Points about the Temp Limiter

The following are details and notes about the functionality of the TEMP Limiter when used in
Electro-Voice systems.

• Bandpasses are not linked, so gain reduction can occur independently in different

pass-bands. If a system is driven very hard this can result in a change in the tonal
balance of the system. For small amounts of thermal gain reduction (<3dB) this is
not very perceptible. If there is a noticeable change in the tonal balance of the
system this can be corrected by reducing the input drive level (pre-DSP).

• The TEMP limiter algorithm assumes that the amplifier is not applying its own gain

reduction. In the case where the Circuit Breaker Protection on a TG7 is set to < 30
amps, in most cases the CBP will apply gain reduction before the TEMP Limiter will,
particularly when driving a very low impedance load.

• Since the TEMP limiter is temperature-based, the time constants involved are much

longer than for peak limiters. Attack and release times for the temp limiter are on the
order of seconds rather than milliseconds. This results in less audibly-noticeable
gain reduction and more transparent performance.

• The TEMP limiter takes a ‘soft knee’ approach to limiting, so seeing a small amount

of gain reduction is no cause for concern. If the amount of gain reduction applied is
> 2-3dB the drive level should be decreased.

• If, for some reason, power to the RCM module is lost the TEMP limiter will lose track

of the instantaneous voice coil temperature. Once the RCM module is powered up
again it will restart the temperature tracking from ambient temperature.

5

THERMAL ENERGY MANAGEMENT AND PROTECTION LIMITER TECHNOLOGY

5. Application and Use in IRIS-Net

5.1. Using the TEMP Limiter in Tour Grade Amplifiers

TEMP Limiter parameters are included in many FIR-Drive loudspeaker settings (.SPS files). These
files are available for download from the Electro-Voice website (http://www.electrovoice.com
These files will be compatible with RCM-26 modules that are operating with V.1.15.0 firmware (or
higher) and IRIS-Net V2.4.0 (or higher). Additionally, these loudspeaker settings are included in the
Speaker Settings folder of IRIS-Net V2.4.1 (or higher).

Loudspeaker settings with TEMP Limiter parameters can be imported into devices by:

o Importing the settings into an individual amplifier
o Importing the settings into a Group of amplifiers
o Importing the settings via the IRIS-Net Project Generator

Instructions for each method are listed below.

).

5.1.1. Importing Loudspeaker Settings into individual Amplifiers

1. If an existing project with TG Amplifier devices is already open, proceed to step # 3. If a

new, empty project is open, begin by dragging a Tour Grade Amplifier Object from the
Object List into the Project Window.

6