[dataway]

So my wonderful new reproduction "10" ohm rear speaker is actually 6.9 - 7 OHM .... what to do? Add a 3 Ohm resister inline?

Another product that just isn't what it's advertised to be.

[DavesGTO]

I sent mine to Jackson speaker , if you are interested in original but don’t have one he sells re-coned ones of all kinds on eBay under rcm3921.


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[Shiny]

As I remember, the amplifier designs push a lot of DC current and low-impedance speakers will add a lot of stress to the power transistor. So you are wise to get it right.

Adding a 3-ohm resistor in series will not hurt. If you do, it needs to be rated for high wattage. Figure 12v/7 ohms= 1.7A. Round up to 2A, and wattage will be 3 ohms x 2A x 2A = 12 W. So look for a 15 or 20W resistor. The EEs on here may correct me but that's my 2 cents.

[Stuart]

From the Electro-Tech website, at http://www.turnswitch.com/speakers.htm :

Note: Many replacement speakers now sold by the discount stores, auto parts stores and mail order restoration parts houses are labeled as "4 to 8 ohm compatible" and are in fact 4 ohm speakers. These speakers can damage the older transistor radios. 4 ohm speakers will measure around 3.2 ohms with a DC ohmmeter. An 8 - 10 ohm speaker will measure around 7.4 ohms with a DC ohmmeter. The DC resistance is the critical number for the Delco radios as the output transistor is in series with the speaker. A properly operating Delco radio should have 1.5 volts DC across the speaker voice coil. A 4 ohm speaker will draw twice the current of an 8 ohm and cause the amplifier transistor (the half dollar size device mounted to the black finned heat sink on the back of a Delco radio) to run very hot. The speaker cone should be displaced in an upward (away from the magnet) direction about 1/8" when a Delco radio is turned on. If the cone moves down toward the magnet, the speaker leads need to be reversed.

I think the impedance of speaker you have is close enough that you will be okay, but if you do want to use a resistor in series it doesn't need to be that big since the voltage is 1.5V at the speaker output and not 12V. Volts / ohms = amps, 1.5 volts / 7 ohms = .21 amps. Amps x amps x ohms = watts, .21 x .21 x 3 = .13 watts.

[padgett]

ps the dash (only) speaker back in the day was a 10 ohm speaker.

[Shiny]

Thank you Stuart!

[mrennie]

Impedance and resistance are not the same. Resistance is a DC function and what you measure with an ohmmeter.

Impedance is an AC function and dependent on frequency. Not uncommon for the DC resistance to be lower than the rated impedance.

Having said that, I do not know what is a "typical" resistance for factory speakers.

[dataway]

If it's any help these speakers are what Ames is selling as replacement 10 ohm front and rear speakers.

So at 7 ohm on the meter I'm about 0.4 ohms off the norm. When I measured the resistance it was through a long (like 10 feet) lead that was coiled up ... that shouldn't make any difference in the reading in a DC circuit right? Other than any additional resistance due to length.

Measured with two different meters by the way.

I've got about 10,000 different resistors lying around, should be able to find one that will do.

[w72]

You're not really reading a DC circuit, since a speaker is an AC device in simple terms. 8 ohm speakers typically show a DC resistance of six-ish ohms. I wouldn't hesitate to use the replacement you have. If you do want to add a resistor, remember that it must be rated for the amount of power it will be subjected to. If you try to use a little 1/4 watt 3 ohm resistor in series, prepare for it to burn up.

[dataway]

Thanks folks, sounds like I can go with the speaker as is ... not that anything much comes in on my AM radio

[Geoff]

Do NOT add a resistor. It would reduce max volume available. This is about the maximum power transfer theorem.

Using a 7 ohm speaker with an amplifier designed for 10 ohm speaker will cause two veeeeeery minor problems: max volume available will be slightly reduced. You could get some slight sound distortion.

Given the less than ideal audio environment of a car cabin, plus cheap speakers, neither of these 'problems' would be worth losing sleep over.

[w72]

The o.p. doesn't have a "7 ohm" speaker. Speakers are rated by AC impedance, not DC resistance as measured with a common multimeter.

[bob d]

When I changed my front speaker which was sort of a pain on my 1965 GTO I used the Speaker from AMES. It worked out well. Then added a rear speaker. No problem with impedance. Bob.

[dataway]

Yep, I'm going to assume the Ames speakers will work. Thought about getting an impedance meter ... but figured I have no other use for one.

[Bill Hanlon]

Good thing you didn't get an "impedance meter" because if you followed it down the rat-hole you'd plot a curve that looks similar to this:

[dataway]

Chart makes sense .... the speaker impedance is 20% higher than DC resistance right in the frequency range for the human ear ... which makes sense because it's a radio speaker.

If my DC resistance is 7 ohm .. that means 7 ohm + 20%(1.4ohm) = 8.4 ohm. That's within the required 8-10 ohm stamped right into the top of the radio.