PA amp - why 100 Volt - UK Vintage Radio Repair and Restoration Discussion Forum

NANETTE · 15th Sep 2017, 6:53 pm

Can someone please explain to me the 100V output on a PA amp - in addition to its speaker socket. Is it safe/possible to use with just one speaker? David

G6Tanuki · 15th Sep 2017, 7:05 pm

100V line-output is the standard used for distributing audio-power over relatively long distances [hundreds of yards] for PA applications - by using a relatively high-impedance/high-voltage system the power loss in the wiring is lessened.

[if you used hundreds of yards of wire to connect up the output of an amplifier to usual 3-Ohm/8-Ohm speakers the resistance of the wiring would dissipate quite a significant amount of the audio power].

See https://en.wikipedia.org/wiki/Consta...speaker_system

Yes you can use it with one speaker: you will need an appropriate matching transformer between the 100V line and the speaker's coil.

These transformers are available quite cheaply: http://cpc.farnell.com/unbranded/p03...ormers-LS01708

Lucien Nunes · 15th Sep 2017, 7:10 pm

The 100V distribution system was used to allow multiple speakers spread around a building to be connected to a common circuit, with the capability to select how much power would be absorbed by each speaker. Doing this with the low impedances of typical speaker voice coils would result in an impractically low impedance for the amplifier to drive, and excessive losses in wiring and matching / attenuating circuits. The convention is to drive the amplifier to full output swing all the time, and then select a suitable tap on each speaker's matching transformer for the volume required.

Yes you can use a single speaker, up to the maximum power of the amplifier, given a suitable transformer. However, unless you have great distances to cover or multiple speakers, it is better to connect to the low-impedance output as the 100V line matching transformer introduces extra distortion and tends to impair low-frequency response. An amp with a 100V line output may be transformer-coupled even to the low-Z output anyway, as they are often designed for speech and background music in which the response is not so important.

Oops post crossed with above.

NANETTE · 15th Sep 2017, 11:06 pm

Thank you very much. Not sure I can deal with that - another for the tip! David

Clydeuk · 15th Sep 2017, 11:13 pm

Surely if you've got a normal speaker socket as well as the 100V line, you can just connect a single speaker to the normal socket and use it that way?

Seems a waste to throw something like this on the tip. Why not stick it on eBay?

Skywave · 16th Sep 2017, 11:55 am

Quote:

Originally Posted by Clydeuk

Surely if you've got a normal speaker socket as well as the 100V line, you can just connect a single speaker to the normal socket and use it that way?

Extract from the opening post: " in addition to its speaker socket". Therefore there is no "if" applicable here.

Al.

Kala_12 · 16th Sep 2017, 5:46 pm

David,
If you are still not sure, could you post a photo of the back of the amp and or the make and model.
Stuart.

15th Sep 2017, 6:53 pm	#1
NANETTE Triode Join Date: Oct 2013 Location: Cambridge, Cambs. UK. Posts: 49	PA amp - why 100 Volt Can someone please explain to me the 100V output on a PA amp - in addition to its speaker socket. Is it safe/possible to use with just one speaker? David

15th Sep 2017, 7:05 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: PA amp - why 100 Volt 100V line-output is the standard used for distributing audio-power over relatively long distances [hundreds of yards] for PA applications - by using a relatively high-impedance/high-voltage system the power loss in the wiring is lessened. [if you used hundreds of yards of wire to connect up the output of an amplifier to usual 3-Ohm/8-Ohm speakers the resistance of the wiring would dissipate quite a significant amount of the audio power]. See https://en.wikipedia.org/wiki/Consta...speaker_system Yes you can use it with one speaker: you will need an appropriate matching transformer between the 100V line and the speaker's coil. These transformers are available quite cheaply: http://cpc.farnell.com/unbranded/p03...ormers-LS01708

15th Sep 2017, 7:10 pm	#3
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: PA amp - why 100 Volt The 100V distribution system was used to allow multiple speakers spread around a building to be connected to a common circuit, with the capability to select how much power would be absorbed by each speaker. Doing this with the low impedances of typical speaker voice coils would result in an impractically low impedance for the amplifier to drive, and excessive losses in wiring and matching / attenuating circuits. The convention is to drive the amplifier to full output swing all the time, and then select a suitable tap on each speaker's matching transformer for the volume required. Yes you can use a single speaker, up to the maximum power of the amplifier, given a suitable transformer. However, unless you have great distances to cover or multiple speakers, it is better to connect to the low-impedance output as the 100V line matching transformer introduces extra distortion and tends to impair low-frequency response. An amp with a 100V line output may be transformer-coupled even to the low-Z output anyway, as they are often designed for speech and background music in which the response is not so important. Oops post crossed with above.

15th Sep 2017, 11:06 pm	#4
NANETTE Triode Join Date: Oct 2013 Location: Cambridge, Cambs. UK. Posts: 49	Re: PA amp - why 100 Volt Thank you very much. Not sure I can deal with that - another for the tip! David

15th Sep 2017, 11:13 pm	#5
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: PA amp - why 100 Volt Surely if you've got a normal speaker socket as well as the 100V line, you can just connect a single speaker to the normal socket and use it that way? Seems a waste to throw something like this on the tip. Why not stick it on eBay?

16th Sep 2017, 5:46 pm	#7
Kala_12 Hexode Join Date: Dec 2012 Location: Carmarthen, Carmarthenshire, UK. Posts: 268	Re: PA amp - why 100 Volt David, If you are still not sure, could you post a photo of the back of the amp and or the make and model. Stuart. __________________ Half my stuff is junk - luckily, my wife doesn't know which half!