Radio Speaker Basics: Series and Parallel Connections
 

I answered a question in another thread that might be useful for more people than who read that thread.

We get a lot of calls and e-mail from people who (for various reasons) want to add more speakers to their factory radios in their old cars.

There are some dos and don'ts but before DOING anything, it important that you understand a few fundamentals concepts. To help out, I created the attached tutorial.

Hope it helps! Any questions, please post a reply. I'll do my best to clarify.

If interested, I will continue to add more information about this topic from time to time.

So my 1970 Delco stereo Radio came with 10 OHM speakers. One speaker in the dash, and two in the rear package shelf ? What did fader do ? What did balance do ? I've had it disconnected for so long I can't remember (hid a stereo in the glove box). How did they balance OHMS with three speakers ?

Hope this helps...
 

Did GM install TWO speakers in the back? The cars I own/know have at most one front, one back.

To me, BALANCE gives you Left - Right control and FADER gives you Front and Rear control. When you have fewer than four speakers the definitions tend to overlap.

I don't know for sure, but I suspect that a 3-speaker configuration is similar to a 2-speaker (one front, one rear) configuration in that the two at the rear run in parallel.


Going back to my sketch from the original post, 10 ohms in parallel with 10 ohms "looks like" 5 ohms to the Front-Rear fader switch. Then the front 10 ohm speaker and the 5-ohm equivalent impedance at the rear are what connect to the fader switch. Inside the switch are electrical components that match the as-seen speaker impedances (depending on what switch position is chosen).

Engine-Ear I have a question. My 71 came with an aftermarket radio and 2 rear aftermarket speakers. I installed an original AM/FM radio and replaced the old speakers with 10 OHM speakers. The front speaker is disconnected. Should I have the 2 rear speakers wired in parallel or series? I currently have them wired in series.

To stay on the safe side with your OEM radio your 10 ohm speakers should be wired in series.

Some speakers only have one wire connecting to them and use the metal frame of the speaker to attach to the metal frame of the car for the ground circuit. If your speakers are this type you cannot make them wired in series without modifying the speaker itself.

Timely post Matt. I've been wondering about this for a while. I have several good 4ohm auto speakers around but 10ohm speakers seem a little less available.
I was wondering what the result would be if I wired the two 4 ohm speakers in series and used them with my factory AM unit? How could I make up the other two ohms?
I don't believe a resistor would be acceptable as ohms and impedance aren't exactly the same... Maybe an inductor or coil but I don't know how to calculate the impedance.

You are correct, DC resistance and impedance are not the same thing. When they say a speaker has say 4 ohms impedance, that is at a specific frequency, say 1,000 Hz. At 100 Hz it could be 20 ohms, or 2 ohms for that matter. What the Delco mono radios care about more than anything is the DC resistance of the speaker. But in general, lower impedance speakers have lower DC resistance.

The older Delco radios are DC coupled, and put out 1.5 volts DC to the speaker, even with no volume (assuming they are aligned correctly) so if you put a 4 ohm speaker, that is only 1.5 ohms DC on it, you have 1 amp of current flowing through it. That forces the amp in the radio to dissipate 13 watts or so without making any sound. It the bias is high the power goes up and the radio cooks itself.

I would make sure the radio has the front/rear speaker fader and run a 10 (or 8) ohm speaker in front and one of the 10 ohm speakers in the rear.

Be sure to read and understand both Bill Hanlon's and LASJayhawk's posts above - from their posts, the single-wire speaker connection and the always-flowing current are important things to remember.

I will also add (I missed it if someone posted it already) that the radio will tolerate an 8-ohm speaker where a 10-ohm unit first existed. That is a 20% change.

Assuming that the speakers' minus sides are isolated from the speaker frames, you could connect them in series. The other two ohms need be made up. Again, the "20% Rule" applies.

One other thing that might not be obvious is that the early/mid 60's AM/FM radios will tolerate a low resistance speaker better than their AM counterparts, even though they have the same amplifier.

Why? The output transistor is on the outside of the radio in the am/fm units, but it is on the inside of the am radios. So it dissipates heat better on the am/fm radios. Think of cooking a turkey. It won't cook on the stove top, but will in the oven. :)

I should be ok the way I have it wired. They are 2 post speakers and the radio is an am /fm stereo. The front speaker is disconnected so having 2 back speakers is the same as 1 front and 1 back. I keep the fader in the middle to get equal output from each speaker. If this is not correct please let me know.

Bill, Jayhawk and Matt, thanks. As I mentioned, I have been wondering about this for a while now...

I'm assuming you meant to type need 'not' be made up?
Making that assumption I gather that my two 4 ohm, twin post speakers could be wired in series with the first post as the input and the final post grounded.

Just make sure...
 

...with an ohm-meter that neither speaker wire terminal has continuity to the speaker frame.


Yes, IF you run wire that would normally connect to the FRONT speaker to one of the rear ones, and then the rear speaker wire to the other rear speaker.

I always reserve the right...
 

...to be WRONG! Uh, YEAH... definitely NOT.

Just make sure with an ohm-meter that neither speaker wire terminal has continuity to the speaker frame.

In other words, make sure they don't wind up in parallel....and if so, isolate them from the body of the car. Understood.

This may be a stupid question but I want to make sure I'm doing it right. Do I test it by putting 1 end of the meter lead on the radio post and the other lead on the speaker frame?

There are no stupid questions!
 

Sorry for the delayed reply.

Yes. Have a look at the attached pics. (Saves me thousands of words! ;) ;) )

BTW, the Fluke Model 12 (or 10 or 11) are really good meters for the money.

i'll throw this out there too. i only have the one hole in my 62 buick right in front of the passenger and am trying to answer the above question as far as location myself. i'm thinking maybe forget the dash speaker all together and just go kick panels and package tray, but still. some of you look like this might be an answer for you, or at least an option. you could just put one speaker in there too i spoze.: http://www.hotrodders.com/journal_ph...1143113560.jpg and http://www.hotrodders.com/journal_ph...1143113561.jpg and http://www.hotrodders.com/journal_ph...1143113562.jpg

oh yeah, in case you are wondering this stock speaker is out of 72 camaro and stock looked like this: http://www.hotrodders.com/journal_ph...1143110182.jpg sorry about the 8 month delay. this thread was linked into a newer one @ radidios and., i dont remember wtf the radio was, if it was stock or not. sorry. i think it was, but i'm not sure. i prolly have a pic. if anybody cares pm me. ill look then

I have a quick question to. I have in my 73 GTO 2 rear speaker now , but originally it was 1 front 1 rear. I have a later model AM/FM stereo now that doesn't use the convector. I have some after market speakers in the rear hooked up to the speaker plugs in in the back of the radio, & don't use the front speaker right now, but it does have a spot for it with the 2 rear speakers. It is a 4 speaker radio from a similar car. The thing I have to ask is about the volume. When I turn it up it starts to distort at a little less then half volume. What can I do to increase the volume as running down the road with the windows down I can just barely hear it . They are just some cheap speaker from I think walmart but the ones that came with it which were the small 5" sound very good in my other car. Is it the radio or ??? Any help would be appreciated.

Interesting idea. Thanks for sharing the pics!