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Series Resistor in Place of Line Distribution Transformer

epsolutions

Sep 7, 2019
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I have a 60W PA amp that specifies 166 Ohm impedance at 70V out. I know that a matching transformer is normally used at every speaker. But if I want to drive a single speaker (at distance) would it be possible for test purposes to simply insert a 200R 100W resistor in series for current limiting? If so, how would this affect the sound quality and wellbeing of the amp?
 

Delta Prime

Jul 29, 2020
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Where did you get that idea?
Does your Public Address( PA) amplifier have one or two output channels?
I know that a matching transformer is normally used at every speaker
Amplifiers do not necessarily need keyword" impedance" matching transformers,maybe yours to do..but I have no idea.Make, model of your amplifier?
Impedance and resistance are different animals. It can be confusing. AC impedance is specified in Ohm's and DC Resistance is also specified in Ohm's.
 
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kellys_eye

Jun 25, 2010
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Instead of getting high voltage/low current 'transformed' into low voltage higher current (using the correct impedance matching transformer) you will get low voltage at even lower current if you use a series resistor into a low ohm speaker.

It'll sound crap.
 

Ylli

Jun 19, 2018
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I have a 60W PA amp that specifies 166 Ohm impedance at 70V out. I know that a matching transformer is normally used at every speaker. But if I want to drive a single speaker (at distance) would it be possible for test purposes to simply insert a 200R 100W resistor in series for current limiting? If so, how would this affect the sound quality and wellbeing of the amp?
That amplifier has a 4 ohm output - why not just use that. It will efficiently drive a 4 ohm, 8 ohm or even a 16 ohm speaker.
 

epsolutions

Sep 7, 2019
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That is true. But for experiments with driving speakers at a distance I wanted to apply the higher voltage output.
 

epsolutions

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Sounds like it would be best if I just tried it to see what happens. I was worried about damaging the amp, but I will keep a keen eye on the oscilloscope. Thanks for the replies.
 

Delta Prime

Jul 29, 2020
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What is the least series resistance I can have in-line with a single 8 Ohm speaker at 70V that will not cause a 60W rated amp to overload?
Here you go kiddo!
The photos I posted or screenshots are for reference only I can point you in the right direction. Research is a very big part of conducting experiments.
Screenshot_20231019_174312.jpgScreenshot_20231019_174329.jpg
 

kellys_eye

Jun 25, 2010
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No, that part won't work.

The purpose of the LINE output is to develop a high voltage to overcome the relatively high cable resistance between amplifier and speaker. For a PA system with speakers located many hundreds of feet away, the resistance of the cable would be a significant part of the expected load and the power would be dissipated in the cable rather than the speaker (assuming an 8 ohm speaker and hundreds of feet of cable).

Transforming the audio to a 100V (or 70V) line voltage is done for the same reason we transform power station voltages from 240/120 to many 100's of thousands of volts i.e. to reduce the line losses.

You won't damage an amplifier without having a load connected as the amplifier usually has its own transformer to 'up' the voltage to the required line voltage level in the first place. Leaving the transformer secondary 'disconnected' has little effect on the amplifier operation.

You cannot drive an 8 ohm speaker (using the LINE output) without a transformer. You CAN just dissipate the line voltage in a resistor though. Ohms' Law will tell you what resistor you need to dissipate x Watts from a 70V source.
 

epsolutions

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Instead of using a transformer at each speaker to drop the 70V, can this not be done using series resistance? IOW what is the practical advantage of each speaker having its own step-down transformer? I am sure there is one, but I would just like to understand. Thanks.
 

crutschow

May 7, 2021
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I have a 60W PA amp that specifies 166 Ohm impedance at 70V out.
That calculates to only 30W into a 166 ohm impedance.
The spec says the output impedance is 83 ohms for the 60W amp at 70V out.
1697823348889.png
But if I want to drive a single speaker (at distance) would it be possible for test purposes to simply insert a 200R 100W resistor in series for current limiting? If so, how would this affect the sound quality and wellbeing of the amp?
No, that will not be a valid test, since the resistor will give a much different line current for the same speaker power, than the transformer.
You need to use the transformer.
Instead of using a transformer at each speaker to drop the 70V, can this not be done using series resistance? IOW what is the practical advantage of each speaker having its own step-down transformer?
That won't work.
You will be wasting most of the power in the resistors, leaving little for the speakers.
In contrast, a transformer has high efficiency (likely 80-90%), so most of the power will go to the speakers.
The transformer converts the high-volage, low line current, into the low-volage, high-current the speaker requires.

If you don't want to use a transformer, than use the amp low impedance output to directly drive the speaker (but you will still need a matching transformer volume control if more than two 8 ohm speakers are connected in parallel to the 4 ohm output).

The reason a 70V output is used for long distances, is to reduce the line current, giving lower power waste for a given load power and wire size.
It's similar to why power lines have a much higher operating voltage than your house voltage.
Power is equal to volts times amps.
 

Ylli

Jun 19, 2018
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Instead of using a transformer at each speaker to drop the 70V, can this not be done using series resistance? IOW what is the practical advantage of each speaker having its own step-down transformer? I am sure there is one, but I would just like to understand. Thanks.
NO. It cannot. Using a series resistor will introduce more attenuation than you would get from a long cable run connected to the 4 ohm output port. If you want to use the 70V or the 100V ports, then get yourself an appropriate transformer.
 

davenn

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Sep 5, 2009
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Amplifiers do not necessarily need keyword" impedance" matching transformers,maybe yours to do..but I have no idea.

you missed the important points .... it's running a 70V line ... it is a PA ( public address amplifier)
These commonly are 70V or 100V line outputs and each speaker has a impedance matching transformer on it to
drop the impedance down to 8 Ohm.

what is the practical advantage of each speaker having its own step-down transformer? I am sure there is one, but I would just like to understand. Thanks

the advantage is that you run higher voltage out to each speaker on the network, thereby minimising losses.
it means you can have many more speakers and over longer distances that what is achievable with a 8 Ohm feedline

I worked for many years on these Amp/Speaker systems in shops, shopping malls, school PA systems etc
 

Delta Prime

Jul 29, 2020
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you missed the important points .... it's running a 70V line ... it is a PA ( public address amplifier)
These commonly are 70V or 100V line outputs and each speaker has a impedance matching transformer on it to
drop the impedance down to 8 Ohm
I miss nothing.
model of your amplifier?
Chronologically asking for more information to make my decision to provide guidance. Should have quoted the entirety of my statement. Any further discussion would be superfluous.
Unless of course you got cookies!
:)
 
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