8mm Forum: Confused about my OHMS

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Posted by Melvin England (Member # 5270) on January 19, 2017, 01:22 PM:

Not being electronically minded, but very aware that electricity and resistance has to be respected, can someone please guide me as to which way round it is safe to plug in an external speaker with a different resistance?

If the sound output resistance is,for example 8 ohms,is it okay to plug in a 4 ohm speaker, or a 16 ohm speaker (assuming an 8 ohm ,of course, is unavailable.) A basic question,I know, but if one doesn't know.....!

Posted by Phil Murat (Member # 5148) on January 19, 2017, 01:45 PM:

Hi Melvin,

If your Amplifier is designed to work with 8 Ohms (Loud speaker impedance) avoid to use lower "Impedance" (as 4 Ohms) there is a high probability quality of sound diminish and then damage occurs to Power Amplifier if high power is used. A higher Impedance leads to a loss of power (normaly).......But has no damaging effect on Power Amp

To confirm

Posted by Brian Fretwell (Member # 4302) on January 19, 2017, 01:56 PM:

If you are good at wiring putting two 4 ohm speakers in series will show a load of 8 ohms to the amplifier, as would two 16 ohm speakers in parallel.

If you aren't experienced best not to try it to prevent damage.

Posted by Andrew Woodcock (Member # 3260) on January 19, 2017, 01:56 PM:

The higher the speaker impedance, the harder it is to drive the load for the amplifier.

Think of impedance as resistance, although a 4ohm load is easier to drive than an 8 ohm one, it also means that your amplifier will be subjected to a heavier current demand than its designed to handle.
This can cause damage to your amplifier.

So while harder to drive, always ensure you match or exceed your amplifiers rated impedance with your loudspeaker matches.

I.e. 8 ohm rated amp = 8 or 16 ohm speakers. Not 4ohm.

[ January 20, 2017, 07:37 AM: Message edited by: Andrew Woodcock ]

Posted by Paul Browning (Member # 2715) on January 19, 2017, 02:00 PM:

A 16 ohm speaker would be fine with a 4 or 8 ohm output but you would get reduced power from the speaker, and 8 ohm would ok with the 4 ohm output, again reduced power. A 4 ohm speaker would not be a good match for an 8 or 16 ohm output, would eventually damage the output transistors with prolonged use.

Posted by Melvin England (Member # 5270) on January 19, 2017, 02:02 PM:

Thank you everybody. That has made it much clearer. Would hate to blow my projector due to my ignorance!

Posted by Maurice Leakey (Member # 916) on January 19, 2017, 02:02 PM:

Note that two speakers connected together, either in series or parallel must be phased correctly, i.e., their cones must go in and out together.

Posted by Steve Klare (Member # 12) on January 19, 2017, 02:17 PM:

Well...

When I learned it we were told to think of it in terms of water, because a lifetime of standing at the bathroom sink every morning gives you an intuitive understanding for that. Sane folk can't see electric fields and moving electrons. (Yet, some people I've worked with....)

Voltage is like pressure. When you stand there with the faucet turned off, there is no water flowing but there is still pressure. Pressure is not power, neither is voltage.

Current is like flow rate. It's how much of the good stuff (either electric charge or gallons) flows past you in a unit of time. Flow rate is not power, neither is current.

Resistance (impedance) is like the opening inside the faucet. A given opening will allow a certain flow rate at a known pressure. A certain resistance (Ohms) will allow a certain current (amps) at a known voltage. (Amps=Volts/Ohms)

-these aren't power either!

So what (...or "Watt") is power? Its the flow times the pressure, and it's the voltage times the current.

-that's how much good stuff is flowing out per second times the energy applied to each gallon or coulomb gushing forth: Energy/Second=Power

What has this got to do with projectors and speakers? An amplifier is set up to produce so and so much voltage, and when you apply a certain speaker impedance you get this much current into it and exactly as much power as everything is set up for appears: not like magic, but like mathematics!

What this means is if you have a speaker lower in impedance than it should be, more current will flow than there should be, and therefore more power will be produced than should be, things will get hotter than should be and just maybe something expensive and important will overheat and die. Taken to the extreme (Zero Ohms), this is what a short circuit actually is.

Too high in speaker impedance: too little current, not enough power. No harm done really: more of a waste of potential.

-it's like showing up at a drag race pulling a trailer! You can't ever go as fast as you want until you reduce your load.

So mind your Ohms, and for heaven's sake clean out the sink when you are done with it!

Posted by Andrew Woodcock (Member # 3260) on January 19, 2017, 03:05 PM:

You lost your way Steve. You should have been a physics teacher! 😂😂😂

Joking aside though mate, over here we were taught that very same analogy between the flow of water vs the flow of electricity.

Posted by Graham Ritchie (Member # 559) on January 19, 2017, 03:21 PM:

Melvin

Have a read on ...ohm's law...Its really simple to understand and very usefull to know. I went through all this stuff as part of being a motor mechanic a million years ago [Big Grin]

This sums it up [Wink]

Posted by Steve Klare (Member # 12) on January 19, 2017, 03:23 PM:

About ten years ago I was out of work and considered teaching as a career change...

-just as well!

If ONE teenage kid gives me gray hair, a couple of dozen might just kill me!

A teacher of mine once said: "All of electronics flows from understanding this, everything else is just details!"

Nice, Graham! It's 75% of what I was trying to say in one picture!

Posted by Graham Ritchie (Member # 559) on January 19, 2017, 03:39 PM:

I think you could have made a good teacher Steve...its all down to how you approach your subject and students.

I had a teacher at the Watt Memorial College during my apprentiship years. He was X army tough as nails and a X army boxer, but he was brilliant at teaching. He once held up, with one hand a college tech guy employed there, who was pushing his luck with him, so he hung him on the coat hanger in the front of the class...saying one more word from you and I will ram my fist down your throat...much to the amusement of the class [Big Grin]

....he once looked at me and said..you dont understand this...I replied no...he came up to me and spent time approaching the subject but this time from a different angle until I got it...he really was very good teacher. [Smile]

Posted by Winbert Hutahaean (Member # 58) on January 19, 2017, 07:12 PM:

Seeing the title, I thought I was going to read a discussion about "On her majesty's secret service"..... apparantly it was not the case.... [Big Grin]

Posted by Brian Fretwell (Member # 4302) on January 20, 2017, 03:02 AM:

What gets me angry in films is people saying things like they "Put 1000 volts through it". Volts go across things only amps go through them.

Posted by Steve Klare (Member # 12) on January 20, 2017, 03:48 AM:

These are strange words. They are so important to modern life that they have become conversational, even among people that really don't understand what they mean. (This is why I said "Voltage is not power", for example.)

My first electronics job was testing high voltage power supplies (I was 18 years old...). Now I was running 15,000 Volts, 20,000 volts. (It was an uneasy feeling at first!). They actually weren't dangerous, since they were capable of only about 10 watts. Getting a hit off one of these was not that much worse than scuffing across a really long carpet and grabbing a doorknob.

-yet a 13.5V car battery can electrocute you just fine under the right (or is it "wrong"?) conditions.

Once again: Voltage, Current, Resistance, Power: related, but not the same.

I've learned to avoid other people's professional lingo: I'm from a family with a lot of medical people (mostly nurses). When they want to talk about their various "-itis"es and long gibberishy abbreviations, I tend to fade into the background: I haven't a clue and I know it! (Sometimes I think they are just toying with me!)

(Never get them talking shop over dinner: you are one emergency amputation away from losing your appetite! -Sometimes I add a second stanza of Grace when they do this: "Oh, GOD!")

Posted by Alan Rik (Member # 73) on January 20, 2017, 05:52 AM:

I keep thinking this thread is about "On Her Majesty's Secret Service"!!

Posted by Graham Ritchie (Member # 559) on January 20, 2017, 04:09 PM:

As an X motor mechanic I was always getting a belt from a car ignition system "by mistake I should add" and thats something around the 8000 volts plus you are getting hit with. Apart from a jolt I am stll here, the reason is simple, you are only getting hit with a short burst of volts, high it might be, there is not the amps behind it to flow to do any real harm apart from hitting your head on the bonnet/hood from getting zapped.

If a cylinder was not firing properly you simply shorted out each spark plug while the engine was running with a screw driver across the block to see which cylinder made the difference.

Although the mains here might be only 220 volts its the amps flowing through your body that "will" kill you.

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