As per our design requirement we need to drive PA (public addressing ) speakers whose power rating is only 6W but impedance is 1.3k/3.3k.

So what should be the amplifier rating ?

I want use an audio power amplifier rated for 6W but most of the datasheets for such ICs show this wattage with only 4 Ohm or 8 Ohm speakers.

No datasheet mentions high impedance speakers.

PA speakers are generally mentioned in connection with 100V systems.

Will an amplifier rated for 6W at 8 Ohm or 4 Ohm drive a high impedance speaker?

Length of cable might be 20meters .

Can i use a 70 v, 10w line matching transformer and connect secondary to audio ampifier and primery to pa speakers (in reverse direction ) whose impendence is high. In line matching transformer seconary is named as 4ohm and 8ohm .So if i choose a power audio amplifier for 4 ohm or 8 ohm and connect it to secondary then at primary itself will come at 70v rms and then it may drive our PA speakers whose impendence is high and made for 100v line ?

plz make me correct if i,m missing somthing.

  • \$\begingroup\$ You should use a 70V or 100V distribution transformer on the output of a regular amplifier, or just buy a constant-voltage amplifier. \$\endgroup\$
    – uint128_t
    Mar 8, 2016 at 7:29
  • \$\begingroup\$ Note that it takes 141 V to push 6 W into 3.3 kOhms. \$\endgroup\$ Mar 8, 2016 at 12:57
  • \$\begingroup\$ Or build a vacuum tube based amplifier. \$\endgroup\$
    – Pentium100
    Mar 10, 2016 at 7:21

2 Answers 2


You need a step-up transformer to drive a high impedance speaker with a common PA amplifier. The advantage of using high impedance speakers is the possibility of paralleling them, and use of smaller gauge cable. In your case, having a single speaker at a distance of 20 m from a 6W amplifier, it is not clear to me why a high impedance speaker was required by design.

  • \$\begingroup\$ if we use low impendance ,current requirement wil increase i think and if we increase length from 20mtr to 30 mtr.(in future) then power drop will increase in wires. \$\endgroup\$ Mar 8, 2016 at 9:15
  • \$\begingroup\$ can low impendence speaker provides same SPL in comperison to high impendence speakers ? \$\endgroup\$ Mar 8, 2016 at 9:20
  • \$\begingroup\$ @sandeepsingh - Yes, longer wires will drop more power, but the PA speakers don't draw much current in the first place (they are 1.3 k, remember?). For the same power, a higher impedance requires a higher voltage, which is why the PA uses 100 V lines. It is exactly analogous to AC power transmission, which uses high voltage for long-distance lines, then steps the voltage down (and the current up) at the user end. You need to actually calculate the losses for different wire resistances/currents to appreciate whether or not an extra 10m of wire will be a problem. \$\endgroup\$ Mar 8, 2016 at 16:19

You will easily find step-down transformers designed to connect 4 or 8 ohm speakers to a 100V PA line. Simply use one of these transformers (6W rating or higher) in reverse.


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