[Engine-Ear]

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.

[Bobalong]

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 ?

[Engine-Ear]

Originally Posted by Bobalong

So my 1970 Delco stereo Radio came with 10 OHM speakers. One speaker in the dash, and two in the rear package shelf ?

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

Originally Posted by Bobalong

What did fader do? What did balance do?

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.

Originally Posted by Bobalong

How did they balance OHMS with three speakers ?

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).

[tigergto]

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.

[Bill Hanlon]

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.

[Engine-Ear]

Originally Posted by Bill Hanlon

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.

Originally Posted by Greg Reid

...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?

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.

[Greg Reid]

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

Originally Posted by Engine-Ear

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.

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.

[Engine-Ear]

Originally Posted by Greg Reid

I'm assuming you meant to type need 'not' be made up?

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

Originally Posted by Greg Reid

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.

[Engine-Ear]

Originally Posted by tigergto

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.

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.

[tigergto]

Originally Posted by Engine-Ear

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.

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.

[Engine-Ear]

Originally Posted by tigergto

I should be ok the way I have it wired. They are 2 post speakers and the radio is an am /fm stereo.

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

Originally Posted by tigergto

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.

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.

[tigergto]

Originally Posted by Engine-Ear

...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.

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?

[Greg Reid]

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.

[LASJayhawk]

Originally Posted by Greg Reid

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.

[Jon Salem]

So if I want to connect a 1971 factory radio expecting 8 ohms to a newer amplifier connected to 4 ohm speakers, can I splice in a four ohm resistor in series to get the nominally correct resistance? And that’s avoid frying my old radio.

[Jon Salem]

So if I want to connect a 1971 factory radio expecting 8 ohms to a newer amplifier connected to 4 ohm speakers, can I splice in a four ohm resistor in series to get the nominally correct resistance? And that’s avoid frying my old radio.

Originally Posted by LASJayhawk

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.

[Shiny]

Originally Posted by Jon Salem

So if I want to connect a 1971 factory radio expecting 8 ohms to a newer amplifier connected to 4 ohm speakers, can I splice in a four ohm resistor in series to get the nominally correct resistance? And that’s avoid frying my old radio.

Does your amp use the radio's speaker output as input?

[LASJayhawk]

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.

[Frank R]

I have a 69 GTO and I'm installing a non-stereo am/fm that has to be used with 8-10 ohm speaker. If I'm following this correctly I can use two 4 ohm speakers hooked up in series or two 8 ohm if I use a fader switch. Is that right? If so can I use a fader from a Chevelle? They are more available that Pontiac ones.

[charles bledsoe]

Frank R-

If the picture you posted is a switch from the Chevelle I would think it would work, as it looks exactly like the fader switch out of my 69 GTO. I don't know this for sure, but if you already have it, I would try it out and see. I don't think it would hurt anything, it should either work or not.