May 8, 2017

High Impedance Installations with HD Amplifiers

MAY 8, 2017

High Impedance Installations (70/100V Lines) with HD Amplifiers

Did you know that HD amplifiers are suitable for high impedance installations (70V / 100V lines)?

High impedance installations, also known as 70V or 100V lines, are commonly used in applications with the following requirements:

  • There is a long distance between the amplifiers and the loudspeakers.
  • Each loudspeaker does not need to handle a high amount of power.
  • A high number of loudspeakers are required; where each amplifier typically needs to drive more than four loudspeakers per channel.

In this kind of installations, a transformer is attached to the amplifier output so that the voltage output is up-converted either to 70 or 100V. At the loudspeakers, another transformer down-converts the 70 or 100V voltage to a voltage that can be handled by the loudspeaker at low impedance.

Fig. 1: Typical high impedance configuration with step-up and step-down transformers

The transformers are passive components with a high inductive behaviour which influences the general frequency response of the system. The more power a transformer must handle, the more difficult is to maintain a flat frequency response within the audio spectrum.

Using amplifiers without a step-up transformer

The requirement for a step-up (or output) transformer at the amplifier output can be avoided, when the amplifier has an output voltage swing exactly in the range of 0 to 70V, or 0 to 100V. In that case the signal will be received directly by the transformers at the loudspeakers. This brings a number of advantages to the installation, most importantly:

  • Lower cost (with no requirement for a large output transformer)
  • Better frequency response.
  • Lower complexity.

Fig. 2: High impedance configuration with HD Series (without step-up transformer)

The performance specifications of some Amate Audio HD Amplifiers enable them to be used in 70 and 100V lines without an output transformer. The specific models are:

Model 70V Line 100V Line
HD1200 (Bridge Mode)
HD2000 (Stereo Mode)
HD3200 (Stereo Mode) 100V @ -1dB

Table 1: Suitable HD Amplifiers for high impedance installations

Configuring the installation

To ascertain the number of loudspeakers that can be used with each amplifier, either the input impedance, or the rated power of the loudspeaker, operating with the step-down transformer, must be known.

In Amate Audio’s range of public address speakers, there are two models available with an integrated transformer: The B5/T and B6/T. The B8 and CS6FR can be also installed in high impedance lines, using the TF-100 transformer (sold separately).

With all of these models, the transformer has selectable operating modes to limit the output power of the loudspeaker; the lower the power that is required by each loudspeaker, the higher the number of loudspeakers that can be connected to the same amplifier channel.

On the B5/T and B6/T, power selection is made with a rear mounted selector switch. With the TF-100, power is selected by connecting the cables in the appropriate socket:

Fig. 3: TF-100 (left) and selector in B5/T and B6/T (right)

Max. Power @100V
Selector position(*)
Max. Power @70V Load Impedance
40W 20W 250Ω
20W 10W 500Ω
10W 5W 1000Ω
5W 2.5W 2000Ω

Table 2: Relationship between load impedance and power in 70V and 100V lines

(*) Selector position is that of the selector switch on the B5/T or B6/T, or the cable sockets on the TF-100.

With the data in Table 2, it is possible to know how many loudspeakers can be connected to one amplifier. Two methods of calculation can be used based on either impedance load or power rating.

Method 1: Impedance

Best practice recommends installing all loudspeakers in parallel, so the resulting impedance load the amplifier will drive can be calculated as follows:


If all impedances R1, R2, R3 are equal, this can be simplified to


R is the resulting impedance of the loudspeaker line
RL is the input impedance of each transformer
n is the number of loudspeakers

The resulting impedance R must be always higher than the rated impedance of the amplifier for high impedance installations (see Table 3 for HD amplifiers).

Method 2: Power

A more straightforward method is to just add the rated power of each loudspeaker.


When the rated power of all the loudspeakers is equal, this can be simplified to


P is the total power of the loudspeaker line
PL is the rated power of each loudspeaker
n is the number of loudspeakers

The power summation must be below the rated amplifier power (see Table 3 for HD amplifiers).

It should be taken into account that the rated power is different if the loudspeaker is used in a 100V or in a 70V line. If the loudspeaker is rated at 40W in a 100V line, the rating for 70V will be half of the power, i.e. 20W.

Model Configuration Minimum Impedance Maximum power Max. loudspeakers (250 Ω)
HD1200 Bridge 6.5Ω 1500W 38 @ 40W (100V)
HD2000 Stereo 1000 + 1000 W 50 + 50 @ 20W (70V)
HD3200 Stereo 1800 + 1800 W 45 + 45 @ 40W (100V)

Table 3: Impedance and power of HD amplifiers in high impedance installations

Application examples

Example 1: 100V line with HD1200 (bridge mode)

20 loudspeakers with transformer: n = 20
Selector position: 20W.


The result is below the rated power of 1500W specified in Table 3, so the amplifier is perfectly capable.

Example 2: 70V line with HD2000 (for one channel)

12 loudspeakers with transformer: n= 12
Selector position: 40W (will be equivalent to 20W at 70Volt).


This is also below than the maximum rated power of one amplifier channel, 1000W (shown in Table 3).
The types of loudspeaker can be mixed; for example, one channel may be used to drive 12x B5/T and the other channel can be used for 12x CS6FR with TF-100.

Example 3: 100V line with HD3200 (for one channel)

50 loudspeakers with transformer: n = 50
Selector position: 40W.


This is above the maximum power supported by the amplifier (1800W, shown in Table 3). In this case, there are two options possible: either reduce the loudspeaker count or change the selector position to 20W:

With selector at 20W:


With 40 loudspeakers:


In both cases the amplifier is now capable of driving the loudspeakers.

Final notes

Although high impedance installations bring many advantages where there is a requirement for a high loudspeaker count, it is highly recommended to first assess whether a low impedance installation can be used instead. Transformers add complexity, and cause loss of power and audio quality in an installation, so should be avoided whenever possible.

Amate Audio manufactures a number of loudspeakers with 16 Ω impedance to help avoid the use of 100V lines in certain cases. The B6, B8 and CS6FR are available as low impedance models (16 Ω), so one amplifier can drive up to eight of each without use of transformers.

If a high impedance installation is required, using amplifiers without an output transformer (such as the HD amplifiers referred to in this guide) reduces the cost and improves the audio quality of the installation.

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