Why 2.5 ohms impedance?

[John10b]

I have been restoring a couple of Bush AC34 Radios recently.
One Loudspeaker was u/s, Celestion oval 10x6, so I obtained a couple from from eBay.
Here is my question:
Why is the impedance 2.5 ohms?
What is the criteria when designing a set? Performance,manufacturing cost etc!
But I still wonder why 2.5 ohms? Does the speaker cost that much less with just a few less turns on the voice coil? Does it a requie a much less costly output transformer to match the speaker impedance?
Cheers
John

[ionburn]

One obvious benefit might be less susceptible to hum pickup in a similar way to low impedance microphones. There are other reasons but they escape me at the moment.

[ukcol]

I'm not sure exactly what question you are asking, it could be 1 of 2.

Question 1 - Why is my speaker 2.5 ohms when the service data says 3 ohms?

The vast majority of moving coil loudspeakers in vintage radios are 3 ohm types. 3 ohms is the impedance measured I believe at 1kHz. If you measure the DC resistance with an ohm meter it will be slightly less than 3 ohms because you are not measuring the reactice part of the coil (the inductance).

Question 2 - Why do moving coil loudspeakers invariably have a relatively low impedance speech coil?

I think it this to keep the mass of the moving parts of the speaker as low as possible (the more turns of wire the greater the mass of the coil). The heavier you make the coil the more energy is required to move it.

[G8HQP Dave]

'Impedance' 2.5 ohms sounds like it is actually 2.5 ohms resistance, which means nominally 3 ohms impedance. This was the standard for low and mid quality speakers back then; high quality speakers were often 15 ohms.

I am going to guess that winding a 3 ohm voice coil is less work than winding a 15 ohm one. Fewer turns (so less work), thicker wire (so fewer breakages).

[G8HQP Dave]

More turns only mean greater mass if you use the same thickness, which you probably would not.

[John10b]

I didn’t intend asking two questions but it seems I did.
Firstly the circuit indicates L18 voice coil 2.5ohms.
So low impedance equates to less mass, makes sense with less turns, but why do “higher quality speaker have impedance of 15 ohms”, which would lead to higher mass?
I was originally thinking about how design engineers have to balance their designs with production costs.
Cheers
John

[mark_in_manc]

Hi -

The number of turns (and the gauge of wire) affects a few things

* DC coil resistance - as it goes up, the driver gets less sensitive
* 'Bl' - the force factor, or product of the flux density in the magnet gap with the number of coils of wire which are in that gap and producing force to move the driver. F=Bli, where i is current in voice coil. As Bl goes up, the device gets more sensitive, but
* as the wire gets heavier, then as other point out the moving mass goes up. This pulls the resonant frequency down, which improves bass response, but more seriously decreases sensitivity as more energy is required to move the mechanics of the thing, compared to that imparted to the air. If you have lots of electrical gain / power to spare, this might not matter too much. If you have less (amplifiers of old...) it matters a good deal more.

Hope that's useful.

[ukcol]

Originally Posted by John10b

So low impedance equates to less mass, makes sense with less turns, but why do “higher quality speaker have impedance of 15 ohms”, which would lead to higher mass?

Winding a 15 ohm coil as compared to a 3 ohm one probably wouldn't have much implicating for mass because as Dave pointed out above you would simply use a smaller diameter wire. However when it comes to a relatively high impedance speaker say 400 ohms you can only reduce the wire size so much and probably would end up with more mass.

High quality speaker can be 4, 8 or 16 ohms and other values. With modern solid state amplifiers where there is no output transformer other considerations come into play.

One or two of our forum members have design experience and can probably give you a much fuller response.

EDIT. It looks like someone already has.

[Boater Sam]

AC Impedance and DC resistance are not the same, that's the simple answer.

[John10b]

Thank you, as I said earlier I was only thinking about how design engineers have to weigh up their design against production cost.
Boater Sam yes I understand that but how does that answer the question, could you expand.
Cheers
John

[G8HQP Dave]

Originally Posted by John10b

Firstly the circuit indicates L18 voice coil 2.5ohms.

Circuit diagrams usually show DC resistance for wound components as this is easy to check with a multimeter. Nominal AC impedance can be quite different, but in the case of a speaker voice coil it is just a little higher than DC.

So low impedance equates to less mass

No.

why do “higher quality speaker have impedance of 15 ohms”

It costs slightly more to wind a higher impedance coil, so only worth doing for higher cost units. Higher impedance means you can cope with a longer cable, so better for component audio - again higher quality.

[Boater Sam]

The IMPEDANCE is not 2.5 ohms! That's the resistance to DC current as passed by your ohm meter.

A speaker coil is a wound component and therefore is an inductor. It has inductance.
An inductance has an impedance to passing an AC current like the sound output of the radio. This varies with the frequency of the AC and the waveform.
At zero frequency it is DC and is resistance.

[John10b]

Thank you all for your informative replies, Loud Speakers have always interesting me.
I just wonder how designers from way back went about working out production costs, did they strike a deal with Speaker Manufactures and work around what they were offered or did they specify what they wanted?
Cheers
John

[ms660]

Chapter 20, sections 1&2 in RDH4 might be worth reading, section 9 gives the RMA spec for receiver loudspeakers in 1947....guess what the impedance given is.

Lawrence.

[John10b]

Thank you for that but I don’t know what publication you are referring to, could you elaborate please.
Cheers
John

[ms660]

Originally Posted by John10b

Thank you for that but I don’t know what publication you are referring to, could you elaborate please.
Cheers
John

My apologies, RDH4 is the Radiotron Designers Handbook 4th Edition, link to chapter 20 here:

http://www.paleoelectronics.com/RDH4/CHAPTR20.PDF

Link to contents of the book here:

http://www.paleoelectronics.com/RDH4/

It's worth keeping the last link in favourites, a very good reference source amongst others.

Lawrence.

[John10b]

Lawrence thank you very much, this is an excellent source of information, i shall enjoy reading it, it’s a long time ago since I studied Loudspeakers.
Yes I noted it was 3.2 ohms impedance!
If you read my first post I was only really wondering how Radio designers selected Loudspeakers, whether it was a trade off between performance and manufacturing cost, I didn’t make that very clear in my post.
Cheers
John

[mhennessy]

Impedance is always nominal at best.

When you measure impedance, you find it varies with frequency. At resonance, there is a great big spike. Either side of that you normally find low spots. As you go down in frequency you head towards the DC resistance; as you go up, the inductance of the voice coil means that the impedance rises.

Manufacturers plot impedances vs. frequency on a plot, and you take a nominal value based on that. Beyond that, it's not worth worrying about; amplifiers can drive a range of impedances.

I also wonder if the trend towards lower impedance speakers was about saving money on the output transformer. In the scheme of things, winding the voice coil was only 1 step in the process of making a speaker, so I doubt there were huge savings there.

Modern amplifiers are voltage sources; speaker designers assume as much. Things were a bit more empirical in those days, but I imagine Bush tried a range of speakers before settling on the one chosen; the choice being based on how things sounded to a range of people...

[John10b]

Thank you Mark, I did wonder about the output transformer, it is very small and I suppose quite cheap to manufacture, reducing the price of the final product.
Cheers
John
Ps I did pm you about separate issue.

[Radio Wrangler]

There are many interacting factors.

In the days of all amplifiers having output transformers the choice of impedance could have been arbitrary with the winding in the speaker and the secondary of the transformer made to suit any chosen number.

One limitation is a desire to have the speaker voice coil a single layer coil.
Another desire, especially for economical equipment, is efficiency, and that means a narrow gap in the magnetic circuit. This sets an upper limit on the wir gauge which will fit with reasonable clearance. The length of the winding affects the available throw of the speaker and the linearity of its drive, so high quality speakers are pushed towards longer voice coils giving higher impedances. Usually higher power and a greater tolerance of inefficiency militates towards thicker wire but the voice coil length wins.

The impedance is going to be at least that of the resistance of the wire in the coil, and people have pointed out that coils are inductive. But the pole pieces are not laminated, so this inductance will get lossy with increasing frequency. AND a loudspeaker is an electromechanical beastie. There are mechanical resonances which include not only its own structure but also those of the cabinet and the air inside it. So the speaker might have a resistance as a single number, but the impedance is a 3-dimensional graph plotting real and imaginary components against frequency, and the plot is spiky!

Thats per driver. So for multi-way high quality jobs, you get all the resonances of all the different types of drivers as modified by the crossover networks.

Pragmatically, you design an amplifier to handle the lowest impedance dip and leave the rest to look after themselves.

Transistor amplifiers are OK with this, but valve amplifiers rely on the load impedance to define the maximum voltage swings at the output transformer primaries and the valve anodes. High impedance swings can cause trouble.

Transistor amplifiers with their constant voltage source type output are bothered by out of control current maxima, so low impedance dips need to be controlled by speaker designers. Some famously dip a little below 2 Ohms, yet are supposed to be 8 Ohm speakers. 'Nominal' really does mean 'in name only'

Nowadays, the constant voltage source drive is exploited. Current flows until the cone reaches a velocity that the generated reverse EMF opposes the amplifier output, so volts out translates into cone velocity and cone velocity translates into pressure. So in the full system the signal voltage translates into pressure. Keeping the loop impedance low makes this relationship more controlled. Hence the interest in amplifier damping factor - at least up to a point.

David