70 Volt Addition

[Grimey]

I have a 70 Volt system with a single amplifier powering several speakers, with different wall volume controls. The system requires one zone (2 speakers) which need more power. I am looking for an inline amplifier which can accept a 70 volt input signal, and amplify that. I have seen similar systems in 8 Ohm versions, however cannot find 70 Volt solutions, short of rewiring and sending the line level signal into a seperate amplifier, and running completely new speaker wire from the home run.

[wg_ski]

The 'proper' way to do it is to use a higher power tap (or new higher power trafo) on the 70V speakers you wish to upgrade. You may need a bigger 70V amp if it's maxed or close to maxed out.

If you want to use another local amplifier just for these speakers, take the 8 (or 4) ohm output of a 70V trafo and feed it into the line input of the separate amp. Use the lowest available wattage tap (0.3W, which puts about 1.5V at full signal on the 8 ohm output). You may be tempted to use a resistor pad in place of the trafo, but don't because it's needed for isolation.

[Tweaker]

Hi Grimey,
You shouldn't do it that way.

The way 70volt distributed audio systems work is to take a low voltage current driver (the amp ouput) and send it through a step-up transformer to create a high impedance voltage driver. At the speaker, another transformer steps down the voltage back to a current driver to provide the desired power (wattage) to drive the speaker at the desired level.

The correct approach, is to use different power taps on the xformer located at the speaker. Add all of the load together, then you'll know how big an amp you need.
If you want, I'd be happy to help you with this.

1st, are these ceiling speakers? The reason I ask, is 1/2 space calculations are generally quite easy for ceiling speakers and the comments that follow are applicable to a ceiling installation. If these are wall mounted, things get a bit different, but generally, the same rules apply.

The following is GREATLY simplified, but the most critical factors are:
Sound pressure level (SPL) of ambient noise. You'd typically like to be able to provide about 10dB above that at the listener's ears.

Distance from the listener to the speaker.
Speakers are usually rated at a given SPL at 1Watt-1meter. This indicates that the speaker will output a certain level (SPL in dB) 1 meter away from the center line of the speaker with 1 watt driving the speaker. The further away from the speaker, the less SPL the listener will hear from the speaker.

If you know the output capability at the speaker, the distance from the speaker to the listener, and the required SPL at the listener; you can then determine how much gain is required to achieve that SPL and how many watts it will take to produce that level at the listener.

After you figure all the speakers in a given run, or zone, add up the wattage taps you've selected and you now know how big of an amp you require for that zone.

The last thing to consider is the length of the run. The reason is, copper wire, as good of a conductor as it is-will introduce resistance to the circuit and lower the voltage/power supplied to the speaker.

A good rule of thumb is that for runs below 100' 18 guage wire is probably OK, for runs over 100' you should step up to 16 guage and over 200' 14 to 12 guage wire is likely to be required but with the price of copper nowadays it's best to do the math to be sure.

I typically specifiy Belden 6200UE 16 Ga or Belden 6100UE 14 Ga for most of my system designs.

So, to your original question, if you don't have an amp with enough power to drive the additional load, or if your wiring is too thin for the extra load, you will likely need to tap the line level and send it to another amp. I'd first suggest you check the existing wiring, if it's robust enough then just buy a single bigger amp to drive the whole system. In your case, you will also need to account for the volume controls. They introduce additional loss that needs to be considered in the overall design.

As I see I've probably spewed forth too much information, I would just about bet, if you were to go look at the wattage tap on the 70V xformer for the space in question, you may even find that it has a larger tap available. When I design a system, I try to make sure I've got 10-20% of additional head room. If your designer did the same you may find you can just retap the transformer for more volume.....
Just make sure you don't exceed the capability of the amp. Again, add all of the taps together to see the total load you've tied to the amp's output.


I hope this helps,
Best regards,
Greg D. "Tweaker"