Maker Pro
Maker Pro

Series Resistor in Place of Line Distribution Transformer

epsolutions

Sep 7, 2019
106
Joined
Sep 7, 2019
Messages
106
I was thinking that the internally transformed 70/100V outputs would equate to more signal gain than the 4/8 ohm outputs. Conventional wisdom and raw driving power aside, I still do not understand why this added voltage cannot be controlled with series resistance at any point in the output circuit. For example, if I wanted to drive a special speaker, perhaps even an electrostatic one, with more voltage and less current. But rather than further overstay my welcome, I will just try it with expendable parts and learn what happens. Thanks to everyone for their constructive input.
 

Delta Prime

Jul 29, 2020
1,793
Joined
Jul 29, 2020
Messages
1,793
Your response clearly showed that you didnt understand what the OP was talking about.....



cheers :)
This site is a pediatric intensive Care unit and I am adjusting. I am no longer taking the information provided by a thread starters at face value.
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
The statement of every speaker leads me to believe more than one although PA do have outputs for more than one I needed to confirm the amplifier itself. At that point you quoted me I did not know! I need documentation so everyone can be on the same page.
And it seems to me I'm the only one who admits to making mistakes around here yourself included for shame.
Now if you would be so kind please guide me to the lollipop guild so I may plead my case to the munchkins in the Land of Oz.
Cheers back at y'a
:)
You can leave the plate the cookies with my sister.you can you can find her easily a house fell on top of her.
 
Last edited:

Ylli

Jun 19, 2018
400
Joined
Jun 19, 2018
Messages
400
I still do not understand why this added voltage cannot be controlled with series resistance at any point in the output circuit. For example, if I wanted to drive a special speaker, perhaps even an electrostatic one, with more voltage and less current. But rather than further overstay my welcome, I will just try it with expendable parts and learn what happens. Thanks to everyone for their constructive input.
Let's look at some numbers. The AMC+60 can deliver 60 Watts into a 4 ohm load connected to the 4 ohm output port.

It is also spec'd to deliver 70 Vrms into a minimum impedance of 83 ohms. Let's put a 75 ohm resistor in series with an 8 ohm speaker and see how much power the amp will be delivering. Voltage across the 8 ohm speaker will be 70*8/83 = 6.75 volts. Power = E^2/R = 5.7 watts. The resistor will have 70*75/83 = 63.25 volts across it and will dissipate 53 watts.

The 100 volt output is spec'd for a minimum load of 166 ohms. Using an 8 ohm speaker and a 158 ohm series resistor.. Voltage across the speaker 100*8/166 = 4.82 volts, power to speaker E^2/R = 2.9 watts. The 158 ohm resistor will have 95.18 volts across it and dissipate 57.3 watts.

So yes, you could just use a series resistor, but you will lose most of the amplifiers power to that resistor.

Now if you are using a high impedance speaker (such as an electrostatic one), the story changes completely.
 

crutschow

May 7, 2021
833
Joined
May 7, 2021
Messages
833
I still do not understand why this added voltage cannot be controlled with series resistance at any point in the output circuit.
It can be.
But for the umpteenth time, you will lose most of the power in the resistors.
Is that difficult to understand?
:rolleyes:
 

Delta Prime

Jul 29, 2020
1,793
Joined
Jul 29, 2020
Messages
1,793
I do not have a sedentary lifestyle it's called speech to text I was walking my four legged companions as I normally do sometimes it's a blessing cuz it leaves out the expletives!
In general I'm a horrible speller anyway.
@epsolutions use the L pad attenuator, turn the audio level
Up slowly, you're going to feel the heat build up on the L pad. Use gloves welding gloves if you got them or ceramic knob put your oscilloscope on it as well I am hoping you know how to use the oscilloscope proficiently as not to damage it. use 10x probes
And I only say this cuz I have a feeling you're going to do it anyway so be careful. It's it's your show brother! Good luck to you.
 
Last edited:

AnalogKid

Jun 10, 2015
2,860
Joined
Jun 10, 2015
Messages
2,860
I still do not understand why this added voltage cannot be controlled with series resistance at any point in the output circuit.
It can if you are careful. Try this -

Most people are familiar with a home stereo power amplifier. This is an almost pure voltage source, and has a very low output impedance, almost equal to a theoretically perfect 0 ohms. It is so far removed from the vacuum tube amplifiers that were the standard when the physics of impedance matching were worked out that those concepts do not apply. The voltage across all speakers, 4, 8, 16, 32, 60 ohms, whatever, is the same. Thus, as the speaker impedance goes up, the maximum power delivered *at the speaker* goes down.

PA systems are the last remnants of the empty-state device era. A tube amplifier is an imperfect current source, not a nearly perfect voltage source. To get the maximum signal power out of a tube amp, the Maximum Power Transfer Theorem (MPTT) applies. The tube output stage has a relatively high output impedance, and a speaker has an inherently low impedance. To match these two efficiently for max power transfer, the impedance of the load must match the impedance of the source. Thus the 4-, 8-, and 16-ohm taps on the secondary of the output transformer.

A PA system has other constraints, chief among them the fact that they often are "designed" by people with no formal EE training. This is wny the speaker transformers are rated in watts. The idea is that if you have two rooms of different sizes, and experience or calculation indicates that for approximately the same perceived audio volume in the two rooms one needs 5 W at full volume and the other needs 10 W, then use transformers with those two ratings with *any* PA amp 70 V output, and the volumes will balance. All PA amps make the same 70 Vrms at full volume. The only difference among them is the amount of available current, but having transformers with inputs rated in watts and output taps for various speaker impedances eliminates all of the usual load-matching math.

Yes, you can connect an 8 ohm speaker without a transformer - IF - the series resistor is such that the minimum resistance seen the amp output is greater than 83 ohms. For example, an 8 ohm speaker with a 75 ohm resistor will not damage the amp. However, for every watt of audio power in the speaker, the resistor will be dissipating approx. 9 W. When the amp is making 60 W, and almost certainly overheating, the speaker will receive only 5.8 W.

ak
 
Top