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Author Topic: External speaker impedance  (Read 6730 times)
KD0TLI
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Posts: 42




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« on: March 16, 2013, 03:22:46 PM »

My rig states external speaker should be be between 4-8 ohms.
I have a nice 3 ohm speaker that I'd like to try, but I am wondering what the effects would be.
Now, obviously its not smart to go deviate from specs (and I have not tried it) so I want some advice.
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K2OWK
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Posts: 1037




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« Reply #1 on: March 16, 2013, 03:53:16 PM »

Would work OK. This speaker is most likely 3.2 ohms. I would keep the volume less then full, so as not to overload the amplifier output.

73s

K2OWK
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K1CJS
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Posts: 5871




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« Reply #2 on: March 17, 2013, 06:20:03 AM »

It would work.  If you're worried about the slight difference, put two of those speakers in series.  6 ohms--and a speaker for each ear!   Grin   Roll Eyes
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KD0TLI
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Posts: 42




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« Reply #3 on: March 17, 2013, 03:45:45 PM »

The obvious choice is to go with mfg specs, I'm more curious why.
For the cost, it really is the obvious answer to get the right speaker, but again
I'm more concerned with trying to understand the difference.

Thanks for everyone's input.
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AA4PB
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Posts: 12672




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« Reply #4 on: March 17, 2013, 04:29:25 PM »

It's an Ohms law thing. For a given output voltage a lower resistance will draw more current. A 3 Ohm speaker will draw a little more current from the audio output amplifier than a 4 ohm speaker but it isn't enough to worry about. First off, unless you crank the volume wide open the amp won't be attempting to send maximum power to the speaker anyway. Secondly, a speaker voice coil is inductive (turns of wire) so the impedance is really 3 Ohms at only one frequency (I think 1000Hz is the standard) so the actual load varies with the applied frequency.

The bottom line is that if you have the speaker and it meets your needs I wouldn't worry a bit about damaging the receiver output amplifier with it. It would be different if you had a 300W amp in a music system designed for a 16 Ohm load and you connected a 3 Ohm speaker load and tried to max out the volume.

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K1DA
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Posts: 473




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« Reply #5 on: April 03, 2013, 08:22:10 AM »

The better quality radios today have good audio output chips that  can stand a bit of "abuse".  Put a long cord on the speaker, at least an ohm's worth.  Smiley
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KD8IIC
Member

Posts: 149




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« Reply #6 on: April 04, 2013, 09:48:49 PM »

Speakers: Cheap, Plentiful. Radio Repair of you rig's audio output circuit: Expensive, Time Consuming.
Why would anyone chance it? Put the radio in a closet for a month for the full effect OM.No Fun. 73.
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N8YQX
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Posts: 66




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« Reply #7 on: May 06, 2013, 01:38:36 PM »

Based on my personal and professional (automotive audio system) experience, I don't think anything dramatic will happen, but there are those rare cases, so YMMV.  Typically, the worst thing to happen from hooking up a low impedance speaker is more distortion and poor dampening.

Most audio amplifier circuits have multiple protection schemes, and I suspect your radio's audio amp has at least one of these: thermal shutdown, short circuit check, and/or impedance check.  Furthermore, a speaker rated for 4 Ohms is not 4 Ohms throughout the audio spectrum, and the actual impedance will vary as  a function of the input frequency.  So, any good audio amplifier is designed to tolerate lower impedance than advertised.

So, based on my experience, here's the list of what I think would happen, with the first item being mostly likely:
1. Works just fine
2. No audio after a while (thermal shutdown)
3. No audio (short circuit/impedance fail-safe)
4. Component failure
5. Thermal incident (AKA fire)

Ultimately, it's your rig, so you have to asses the risk, but if it was me, I would 't think twice.
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73,
N8YQX
AD9DX
Member

Posts: 1464




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« Reply #8 on: May 06, 2013, 03:12:01 PM »

You could always just put a resister in series with the speaker.
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EX, KC9TRM, KB9IRZ
N8YQX
Member

Posts: 66




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« Reply #9 on: May 06, 2013, 04:13:11 PM »

You could always just put a resister in series with the speaker.

Yep, that's the proper way to go.  Just make sure the resister is big enough (enough wattage).  Depending on the amp's capability, you may end up smoking your typical 1/4 watt resister.  (Yes, I learned the hard way)
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73,
N8YQX
KD0REQ
Member

Posts: 854




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« Reply #10 on: May 07, 2013, 01:05:52 PM »

the speaker is 3.2 ohms AT WHAT FREQUENCY?  speakers are basically all over the yard depending on what frequency you are measuring at.

bottom line is if a speaker impedance from DC to daylight varies from 1.6 ohms to 60 ohms (if you wanted to, and had the response chart, you can work backwards from it), rated impedance is somewhat of a joke for casual usage.

short answer is... don't worry about it unless you run into a nutcase voice coil wound for 600 ohms at 400 hertz.  that's enough mismatch that your amp will only deliver a fraction of its power into the speaker.  this is not +- 1/4 dB response to fill an opera hall on 2 watts you are designing for.

now, a 3.2 ohm load on a 600 ohm output, that has the potential of heating things up some inside the radio cabinet.  that's for testing only.  better to get a 70.7 line transformer for that speaker, for that's the impedance of the line side of a 5 watt 70.7 transformer.
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AA4PB
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Posts: 12672




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« Reply #11 on: May 07, 2013, 02:09:09 PM »

Exactly. When a mfg specifies the speaker impedance it is the "nominal impedance". That is generally an average of all the impedances measured over the specified bandwidth of the speaker. The actual impedance of a 4 ohm speaker can vary from probably under 3 ohms to 15 ohms or so over the audio range. Not only that, but the impedance at any particular frequency will change with changes in the enclosure, position of the speaker in the room, and relative humidity of the surrounding air. Given all the variables, the difference between a 3.2 Ohm and a 4 Ohm speaker is negligible.
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