LeSkid

it's actually the other way around.

A 2 ohm speaker has 1/2 the impedance of a 4 ohm speaker (generally speaking). No big deal until you turn the volume up and the amp's output exceeds what it would be putting out with the 'correct' impedance speaker.

A 50W / 4ohm amp with 4 ohm speaker = 50W output
the same 50W / 4ohm amp with a 2 ohm speaker = 100W output, hence the question / issue. Is your amp stable in this configuration....




Next, you can play with the impedance (or load), on your amp by installing speakers in parallel or series. Think of it like drinking a milkshake through a straw. By putting speakers in parallel, you're doubling the size of the straw so you'll double the output of the amp (more flow of milkshake, or power)

If you wire them in series, you're using one straw but making it twice as long, INCREASING the impedance and therefore dropping the power output. (much harder to get a sip of milkshake, or LESS flow of power)

parallel means less impednace / more power output


Series means more impedance and less power output



On big systems with many speakers, you can combine pairs of spkrs in parallel and than GROUP those pairs of speakers into the amp in series, so in the end, you have a speaker 'system' that arrive at the desired impedance.

And since I'm making car audio 101 diagrams, this should help visualize why it's important to make sure speakers are installed in phase (no wires crossed)



Yeah, I used to install car audio car audio in a past life. In the 90s, Rockford Fosgate amps were a preferred choice because you could continue to tie-in speakers in parallel into a little "Punch 40" (40 watts at 4 ohms) until you were running as low as .5 ohms!!. So this little 40 watt amp was actually putting out almost 8X it's rated 40 watts (@ 1/8th the impedance). These amps would be 'stable', meaning low distortion under these circumstances, without frying the amp itself.

7T1vette

A Fosgate amp is just a little bit different than the [very basic] amp system on the early C3 cars.

mrvette

OH'Rel'y???

there is a limit what you can pull out of old transistors and their small matching transformers....which means the parts WILL FAIL if you push them....sure you CAN hook up 4 ohm speakers, and if you push the amp at all ....say at speed with top down.....windows down even.....you can easy blow it up.....

used to fix them things back when they were new.....

7T1 is about closest to the facts here.....

being a home audio hobby man for some decades, owning McIntosh high powered stuff with FOUR 12" woofers, PER CHANNEL....

any mere silly little car system is just that.....

Kilroy1024

A common misinterpretation of the maximum power transfer theorem.

The best impedance for an amplifier is 0 ohms. Not 4.

http://en.wikipedia.org/wiki/Maximum_power_theorem

7T1vette

Kilroy...

The next time you find a "0" output impedence amplifier, let me know. You obviously have been trained in "theory", but have limited practical knowledge.

P.S. The only reason for discussing 4 ohm speakers in this thread is that almost all of the available car audio speakers are 4 ohms. You can find high-end 2 ohm speakers; but using them for a sound system inside a C3 is certainly foolish.

Kilroy1024

Another common misunderstanding. Yes, you will have cancellation nodes resulting from a two source interference pattern. You will also have points where the waves add up and you have double volume. It doesn't matter if your in phase or out of phase.

This is easy to detect in an sound deadened room with only two speakers outputting a specific frequency. I challenge you to find a node in a car, filled with randomized reflecting surfaces and playing your favorite song.

http://zonalandeducation.com/mstm/ph...rference1.html

BBCorv70

0 ohms? You mean a piece of wire?

My background is EE, pardon the boring theory for many who may not care...

So far as matching impedances goes, for maximum power transfer impedances should be matched. The basic theory. In reality we're talking about music, not sine waves. There is a broad spectrum of frequencies which make up what you hear. Impedance of a speaker has a frequency sensitive component and a static component (resistance).
There is no fixed impedance which is seen by the amplifier.

RE: Power.. it takes a 10 times increase in wattage for a human to perceive twice the volume. A doubling of power isn't going to sound that much different anyway.

RE: Phasing.. again, in theory the waves will cancel if they're out of pahse but since we're talking about a car with anything but regular surfaces reflecting sound I doubt the phasing would be noticed.

The important question is whether the OEM radio can safely drive a 4 ohm load for a long period of time. I can't honestly answer that question without knowing a bit about how the output stage was designed. May work fine, may heat up if played at high volume for a long period and fail.

Kilroy1024

This is the part I took issue with: "in order to maximize output efficiency". Efficiency on a matched system is 50%. I started to write a hole bunch of stuff, but thought Wikipedia said it better.

I said the "best", as in "Ideal". An ideal amplifier has an output impedance of 0 ohms. I didn't say you could achieve that. You can get close, But not 0.

A modern FET amplifier is close, so you can put 2,4,8 ohms on it without a lot of concern. However, a mid 60's BJT amp? If you increase the load on that, your forcing a lot power into the output stage. I doubt it has any kind of over current or thermal shutdown built in. Burn Baby Burn!

I'd use two 4 ohm speakers.

Apologies if I edited this while you were responding.....

MotorHead

I spent a decade in the high tech world and 4 years learning this stuff, It is not easy to understand for people who have actually gone to school to study it especially when it come to Max power transfer, impedance ( not resistance ) , reactance and numerous other 50 cent words

If you can understand this statement below ( taken from the Wikipedia site referred to above ) then you can make some suggestions to the OP if not then it's a long 4 years as any of the the EE's or Electronics Technologists with tell you "

"The theorem can be extended to AC circuits which include reactance, and states that maximum power transfer occurs when the load impedance is equal to the complex conjugate of the source impedance"

If you don't have a fricken clue as to what the above statement means then it's time to put an newer amplifier behind the passenger seat that will accept 4 or 8 ohm speakers as well as having a must more robust input circuitry as well as the out put stages. This can be hooked up to ( I am assuming here, because I never had an original stereo ) your original in dash stereo and everything will look stock.

I would even go out on limb here and say that you would not find any problems hooking up any speaker to a new ( Not Chinese) amplifier as you learn in the world of engineering you have to cover all the bases including a kid sticking a fork into the amp, you basically have to make it so there is no way a chimpanzee to f it up

I have 8ohm Bose 301 house speakers in the back of my Vette hooked up to the same amp that powers 4 more 4 ohm car speakers and everything works fine. Max power transfer ?, who cares the amp will blow a hole in you ear drums the way it is setup now

Kilroy1024

I'm not sure who your referring to, but I think it might be me!
I actually agree with most of what's been said here by 7T1Vette, LeSkid and yourself. Just pointing out a few things so that myths do not get passed on.


I'll close with the following:

Consider the following two circuits:



Circuit A: Choose ZL in order to deliver maximum power transfer to the load.
Answer: 4 ohms. Efficiency = 50%

Circuit B: Choose Zo in order to deliver maximum power transfer to the load.
Answer: Well, its not 4 ohms.

7T1vette

All an interesting discussion. Of course, the OP said he had an original AM/FM Delco radio, wanted to replace dash speakers, and didn't have much money to drop on upgrading it. He asked if he should put 4 ohm resistors in the circuit with 4 ohm speakers [in order to get a decent impedence match with the amp]. Since the Delco amp only puts out about 16 watts RMS, sticking a 4 ohm resistor in with the speaker didn't sound like a very good option to me, since he could buy a couple of CHEAP 4 ohm speakers and stick them in the kickpanels for about the same price as buying two 4 ohm, 20 watt resistors. Seems like a 'no brainer' to me, given the conditions stated.

pauldana

....... and this is why this real EE, that had sold and installed 1000's of car stereo systems in the 80's while going to collage, just did not want to fight the good fight on this one..........


reminds me of a PCV discussions thread

I believe a lot of the new speakers are actually 6 ohm, and there are many 8 ohm out there if you are really worried...... this is not rocket science....


and yes you can 100% tell the difference in bass in a GOOD system when phasing is in or out. if you cant your system is junk, or stock.... ether way, same thing...... jmho..... buti do understand thoes that are going for the 100% stock thing, and thats also cool!!! ...

johnmb

You could do this, but it's not needed.

Put in the 4 ohm speakers and you'll be fine. No other adjustment/modification is needed.

John (Also an EE) :-)

MotorHead

Wasn't referring to anyone in particular. Just trying to get across that there is much more to it than meets the eye and you could take a couple of semesters at school trying to understand exactly how this type of equipment works ( there's allot of mathmatics involved ).

I also wanted to get across that modern amps have built in (at least the one's worth buying ) protection in both input and output stages so they are pretty much foolproof these days, even going back 15 years or more.

Kilroy1024

I agree with all of this.

I'm simply trying to point out that any discussion of impedance matching in an audio amp is a bit of a red herring. We don't have a choice of 2,4 and 8 ohm amplifiers to go with our speakers.

Its not a matched system. Zo is made as close to 0 as possible so that it will work with a variety of loads.

Kilroy1024

I am starting to question my thoughts on the phasing. I could see how it would make a difference if you had two big thumpers pointing up at the back window.

jnb5101

since i started this whole argument, i'd like to finish it with this: i read a warning (shop manual,radio schematic, or?) not to operate the radio with the speakers or amp disconnected. the guys that designed these units must have known something about the robustness of the 1960's transistors that we don't. and considering the cost and limited availability of repairs, i'd sleep better knowing that the load impedance matched the radio's specs as closely as possible.
let's all put this puppy to sleep.

Richard454

Wow....

OK...A speaker does more than act as a load- It actually makes power. Don't believe me? Take a speaker and hook some wire up and put them on your tongue-then pop the speaker- the larger the better. Coil of wire moving through a magnet- humm- sort of sounds like an alternator...

It's called a "MHO" (reciprocal of an ohm) and causes a problem w/ the amp when it can't handle negative feedback. (an amp's "damping factor")

And you guys keep talking about "matching the amp to the speaker" making it "more efficient"- In doing so the amplifiers "headroom" ( ability to make the sounds jump at you -more dynamic peaks) is diminished.
It'll sound louder - but the peaks in sound will not be as defined. Just like a car- there are always trade-offs...


As far as power- Double the power to get noticeable increase in SPL (sound pressure level) or about 3 db. You can do the same thing by adding an identical speaker w/ same amount of power and you'll just get a 3 db increase.... Speakers are rated for efficiency (in dbs)- 1 watt at 1 meter- So running a stock - low power -you'll want a speaker that is efficient- which is a trade off for sound quality- In the mid 90's (db/ 1 w/ 1 m) would be a good number to look for.

And as far as phasing- your ears hear (can discriminate) the difference between the original sound and a reflection....Ask Dr Bose on that one.....
Your ears are only about 10"s apart -thus you can not hear stereo below 300hz since the wavelength is only 10 inches or so (unless you are a pinhead- not naming names here). That's why phasing is more noticeable in the lower frequencies... A 20 hz ( low base ) wavelength is about 28 ft long- that's why you hear the car booming down the road so far a away...

I also went to school (Georgia Tech ) as an EE and worked in the summers putting in stereos (and I did a lot of my engineering buddies who had no clue about what all was involved...)...Then spent almost 20 years in the business as a Tech specialist/Trainer for several different manufactures ...

I too hate myths passed on because "it was on the internet- it must be true..."

The OP should simply add a second set of speakers and the amplifier/radio w/ live a lot longer...

Richard

MotorHead

Older stereos and amps were not as sophisticated as they are today. When I say older, I am talking 50's and 60's here and obviously older ones than that. With these old stereos you had to have the correct load ( speaker ) or you would be in trouble.

When designing something these days an engineer ( if he is worth his salt ) will put in a safety factor and speaking strictly about a stereo amplifier designing in an output stage that would accept 4, 6, 8 etc ohm speakers is a no brainer, he's going to do it. Now you may have power loss or other undesirable characteristics if you use the wrong speaker but it will work

Kilroy1024

Well, you got 5 or 6 engineers putting in their 2 bits.
This is what I hear:

Option 1) Put a single 4 ohm speaker on each side. Your amp will probably be fine with that. I'd give it a 95%+ chance of success.

Option 2) Put two 4 ohm speakers in series, or an 8 ohm speaker. Probability of success: 100%.