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I am confused about Ohm loads on Dave Rats video and amp Bridging & Ohm's Law.

iMickey503

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The Video:

My Conundrum:
I thought when you lower the resistance, you increase the power output of transistor based amps.
Reference:
Perry Babins 12volt.com site calculators


Dave Rats Findings/ Reasoning from what I understand:
"-You get more output power/ Watts (He did use the term efficiency as well) at 16 ohms vs 10 ohms (speaker) loads".


This might make sense to me if this was a Tube amplifier with a Transformer for its output (as he did mention 70v commercial audio system) as such I understand it, that's how one of the various Tube amplifier curcuit topologies work. Generally speaking, of what I have seen or read up on. (Noob on Tubes)

However, most pro audio gear that I know of to date has shifted to Class D Designs for live sound minus the Amp stacks for the guitar. Even so, for live sound, most of the sound from these is routed to the mixer to be amplified to the mains for larger venues.



The explanation seems to violate Ohm's Law. Electrical Power = Current x Voltage as stated by © Geoffthegreygeek.com 2020, as well as my own background & education in electrical power delivery systems and other sources. But I use the formula daily with LED's I work with and Batteries.


So, am I just not understanding what Dave Rat is saying? I highly respect the man. And I don't think he would be wrong. But something does not seem right?

A great simple explanation on Ohm's law is actually on Geoff website I always like the garden hose analogy.

I myself, became very Physically familiar with how electricity BURNS when I started trying to fix my own Christmas lights and experimented with converting them over to LED. (They do explode if you get it wrong!) and trying to work them off of a regular audio amplifier to have active Christmas lights instead of sequencing them with something like a controller or a dedicated circuit.

About the speaker wire and the 1 ohm calculation


I know it was just done to make the math simple, but 1 ohm speaker cables... That's a lot.
Rodger Russells site goes in depth about speaker wires and connections, and cable lengths.
1631516743387.png

(The webpage is a good read about how wire resistance and capacitance effects of signal, highly recommended as it shows the results on a O-scope)


Also, Rodger, from what I can recall worked at McIntosh and made some FANTASTIC SPEAKERS DESIGNS! So I know he knows his stuff. I wanted to include this from his site as a reference. And I encourage you to read the article he published on his website.

Here is the Conductivity rating in the industry standard unit of ICAS. More on ICAS Here
1631517372312.png


Excerpt from BlueSea Systems:
1631517510840.png


Notice the image depicting the physical amounts of the different materials you would need to meet the conductivity of either silver or copper.
Superconductors such as cryogenically frozen Mercury and other superconductors are not listed but again the chances of you using one of these materials either in your equipment is pretty much nonexistent. Still, would be fun to see what something like Carbon Nanotubes would be rated at if they could make wires out o those.


The thing that shocked me is that, Gold has a lower ICAS then Copper? Why do they use it on semiconductors instead of cheaper silver or copper? I know it does not corrode, but if we are talking about speaker wire resistance? I'm going to have to look into my own now as much of the
interconex I have purchased as of late actually include these gold-plated connectors. Which in itself is usually or using some type of solder to make the contact surface from what I understand.
Something to ponder about.


Anyways, would really appreciate if any of you could see where I made the error to Dave Rats findings. Thank you.
 

samsa

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If each amp is rated for a minimum speaker impedance of 4Ω, then the minimum speaker impedance for the bridged amp is 8Ω. If you hook up a 4Ω speaker to the bridged amp, bad things will happen.

I thought when you lower the resistance, you increase the power output of transistor based amps.

Only up to a point (i.e., up to the minimum rated impedance). Beyond that point, ... well ... you can try out a 0Ω load (ie, a straight piece of wire), and see what happens.

[Please don't actually try this.]
 

Doodski

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I thought when you lower the resistance, you increase the power output of transistor based amps.
This is where it gets interesting. The amplifier and it's current supply has a internal resistance that current limits and causes a voltage drop thru reducing the current output and also reducing the power output at low load impedances. There's a couple to a few things going on simultaneously. The meat and potatoes of the matter is that amplifiers don't output linear power unless they have very low internal resistance and a large current supply.
 

kchap

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It's a bit confusing but the mathematics is correct:
An amp in bridge mode gives 4 times the power of a single channel for the same speaker load
2 channels in bridge + 1 Speaker = 4 x P in Watts (1 x channel + 1 speaker)
I know, not really a formula!

With 2 speakers in series the power of is reduced by 0.5
Putting the amp in bridge mode still gives you 4 times the power but it's 4 x 0.5 P as the speakers are in series i.e. 2 times the power.

If you are setting up for an outdoor party, the speakers are some distance from the amps, the cable is a bit light and you just happen to have a warehouse full of amps and speakers, putting speakers in series would give you some benefit. Less overall power than standard bridge but less power is wasted in the cables.
 

sandymc

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a) Amplifier ratings are complex. E.g., many amplifiers are thermally limited. If you increase the speaker impedance, more voltage across the speaker, less voltage across the amplifier output devices. So less heating, and the amp can actually deliver more power to the load rather than dissipating it in the output devices.

b) Gold is often used because it doesn't corrode rather than its conductivity.
 

Vini darko

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A simple rule of thumb with bridged amps is take the minimum impedance of your speaker and half it. That's what your amplifier will see. So if your speaker dips to 3.5Ω the amp will see 1.75Ω. Most amps won't like that and may die.
 

DanielT

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I apologize. A little silly I know but it itched too much in the old copy and past finger :)

About speaker cable made of copper. The resistivity of copper is 0.017 Ohm per meter at the area of 1 mm². If the cable is single-square and a total of 6 meters of conductor (2x3 m), the resistance will be about 0.1 Ohm

Edit:
This from another forum. I ran the text through google translate, so the text might look a little strange:

"First and foremost - an output stage is voltage-controlled, which means that it is primarily the maximum voltage swing that sets the maximum possible effect if the amplifier is reasonably properly designed. This is why the output power is twice as high (almost) at 4ohm compared to 8ohm load.

Why then "almost" in the paragraph above? Well, several "limitations" in the construction means that the effect is not twice as great.
1. the mains part that supplies the output stage is usually unregulated, which means that the supply voltage drops
2. at higher loads, the loss voltage across the final transistors increases minimally.
3. in the output stage there are small power resistors that are needed to stabilize the operating point of the final transistors and thus give voltage losses that increase at high load.

The first point above (unregulated supply voltage) is the one that also results in higher "music power" than RMS power.

As I said, it is the size of the voltage swing that the amplifier can deliver that determines the output power as long as no protection circuit beats tlll, (eg too high heat or too high current).

If you want "a hum" about an amplifier's output power in RMS, you can measure the voltage to the output stage at idle, subtract about 10% because the power supply is unregulated and subtract about 3-4 volts for the transistors and the small power resistors. You still have the maximum possible voltage swing. Note that it is peak value, then use Ohm's law and do not forget to convert to RMS.
This applies to class AB amplifiers with semiconductors, pipe fittings become more complex due to output transformers and generally lower feedback rates. Class D can also not be estimated as easily.

Important! It is not the RMS effect that limits how high you can play without the maximum voltage swing. The impedance of the speaker varies greatly and the effect can in practice be very low at certain frequencies."
 
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tomtoo

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The resistance of a metall conductor depends mainly on two variabels(temperatur we forget for now) and a material constant.

The length of the conductor and the area of the conductor and that material constant. In a contact the length is very small micrometer , and the contact area big. So the material constant is not so importend. And the anti-corosive propertys of gold are very importend couse metalloxyds or metallsulfids are very bad conductors. In a cable that is long, things are different. The longer it gets the more importend is the material constand to keep resistance low. You can use alu instead of copper, but to keep the resistance low you need bigger area. Thats why contacts are made of gold, not silver or copper. The resistance difference with such a small length is not importend.
 

DVDdoug

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The explanation seems to violate Ohm's Law.
I didn't watch the video...

But as you know, when you double the voltage you also double the current. That might be too much current so you might have to use a higher impedance load (and live with only twice the power).
 

Blumlein 88

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8 ohm load connected to an amp with 24 volt max output.
24/8 ohms will flow 3 amps.
3 amps x 24 volts is 72 watts.

Bridge the amp, and now it has 48 volts max output.
48/8 ohms will flow 6 amps.
6 amps x 48 volts is 288 watts.

Now the question is can the amp push 6 amps. You've increased the voltage output by bridging, but bridging this way does not increase the max amperage the amp can flow. If the amp were rated for 4 ohms you'll probably get that or close. If it was only rated for 8 ohm loads prior to bridging you'll probably not get the full 4x power increase as you'll get current limiting at a lower total power level. Possibly at only twice the power of one amp or possibly no power increase if the amp could only put out 3 amps max.

Of course few speakers are simply resistive and you get into a bit more complex things with reactive loads.

Now with transformer coupled tube or SS amps you can bridge in the way that works like the above or bridge a different way that doubles amperage, but does not double voltage. To extract extra power you can use a lower impedance speaker to take advantage of the higher current available. With the same speaker connected with 8 ohms this kind of bridging would result in no extra power.
 
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tomtoo

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The video is a littel more complicatet. He liked to point out that you can have with amp bridging less cable looses. Whats for a typical hifi szenario, i would say not importend. He talks about long cable runs in PA szenario.
 

Blumlein 88

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The video is a littel more complicatet. He liked to point out that you can have with amp bridging less cable looses. Whats for a typical hifi szenario, i would say not importend. He talks about long cable runs in PA szenario.
Okay, well I didn't watch the video. And it would take quite the cable run to effect this, but in live music PA systems you can have that.
 

tomtoo

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Okay, well I didn't watch the video. And it would take quite the cable run to effect this, but in live music PA systems you can have that.

I never ever thought about cable looses in a hifi system. Not even in small PA.This is more for pros, with big installations under adverse circumstances.
 
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tomtoo

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So i think that the op youst missed the point. He not said you have more power with 16ohms. He just said that in his szenario the cable losses are less with the bridged 1kw amp and the two 8ohm speaker serial connected, than using two 500w amps non bridget and run two cables seperatly to the 8ohm speakers. But to be honest the way he talks is a littel confusing, at least for me, couse english is not my mother language and then following some jumps is not easy.
 
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tomtoo

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But i like this guy. A cool way for amp testing.:)

So i have to thank the op couse i didnt know this guy.
 

bigguyca

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I apologize. A little silly I know but it itched too much in the old copy and past finger :)

About speaker cable made of copper. The resistivity of copper is 0.017 Ohm per meter at the area of 1 mm². If the cable is single-square and a total of 6 meters of conductor (2x3 m), the resistance will be about 0.1 Ohm

Edit:
This from another forum. I ran the text through google translate, so the text might look a little strange:

"First and foremost - an output stage is voltage-controlled, which means that it is primarily the maximum voltage swing that sets the maximum possible effect if the amplifier is reasonably properly designed. This is why the output power is twice as high (almost) at 4ohm compared to 8ohm load.

Why then "almost" in the paragraph above? Well, several "limitations" in the construction means that the effect is not twice as great.
1. the mains part that supplies the output stage is usually unregulated, which means that the supply voltage drops
2. at higher loads, the loss voltage across the final transistors increases minimally.
3. in the output stage there are small power resistors that are needed to stabilize the operating point of the final transistors and thus give voltage losses that increase at high load.

The first point above (unregulated supply voltage) is the one that also results in higher "music power" than RMS power.

As I said, it is the size of the voltage swing that the amplifier can deliver that determines the output power as long as no protection circuit beats tlll, (eg too high heat or too high current).

If you want "a hum" about an amplifier's output power in RMS, you can measure the voltage to the output stage at idle, subtract about 10% because the power supply is unregulated and subtract about 3-4 volts for the transistors and the small power resistors. You still have the maximum possible voltage swing. Note that it is peak value, then use Ohm's law and do not forget to convert to RMS.
This applies to class AB amplifiers with semiconductors, pipe fittings become more complex due to output transformers and generally lower feedback rates. Class D can also not be estimated as easily.

Important! It is not the RMS effect that limits how high you can play without the maximum voltage swing. The impedance of the speaker varies greatly and the effect can in practice be very low at certain frequencies."

RMS power can be calculated, but RMS power is a meaningless quantity.

current RMS x voltage RMS = average power it DOES NOT EQUAL RMS POWER.

You need to do some basic reading on this subject.
 

DanielT

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I'm checking wikipedia now. I find this to be interesting: Attached some pictures on the subject.

On May 3, 1974, the Federal Trade Commission (FTC) instated its Amplifier Rule[12][13] to combat the unrealistic power claims made by many hi-fi amplifier manufacturers. This rule prescribes continuous power measurements performed with sine wave signals for advertising and specifications of amplifiers sold in the US. (See more in the section Standards at the end of this article). This rule was amended in 1998 to cover self-powered speakers such as are commonly used with personal computers (see examples below).

Typically, an amplifier's power specifications are calculated by measuring its RMS output voltage, with a continuous sine wave signal, at the onset of clipping—defined arbitrarily as a stated percentage of total harmonic distortion (THD), usually 1%, into specified load resistances.

Continuous power measurements do not actually describe the highly varied signals found in audio equipment (which could vary from high crest factor instrument recordings down to 0 dB crest factor square waves) but are widely regarded as a reasonable way of describing an amplifier's maximum output capability. For audio equipment, this is nearly always the nominal frequency range of human hearing, 20 Hz to 20 kHz.

https://en.wikipedia.org/wiki/Audio_power
 

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