Sub resist load for speaker?

No. It's *impedance*.

Typically 5 ohms for an 8 ohm speaker.

Now try measuring the ac impedance at audio frequencies.

Graham

As far as I remember, the impedance of the coils is measured at 1kHz. Since the peaking in case of resonance is not that high, the accoustical power output appears to add a significant resistive component.

Rene

"Robert Moore" schreef in bericht news: snipped-for-privacy@4ax.com...

Not that much lower. I'd use an 8 ohm resistor. But audio amplifiers are made for music, better not run the amplifier full power for long times. Things might get hot, too hot perhaps. Check the temperature during your first experiments.

May even be 400Hz.

Exactly. The DC resistance takes no account of motional impedance nor the power conversion.

Graham

I want to drive a resistive load with a commercial 50W audio amp.

What resistance do I need to use to prevent damage to the amp if it replaces an 8 ohm speaker.

I understand the 8 ohm represents inductance. The actual speaker coil resistance is much lower.

Thanks for any advice.

Robert Moore

I read in sci.electronics.design that Robert Moore wrote (in ) about 'Sub resist load for speaker?', on Tue, 4 Oct 2005:

An 8 ohm 50 W resistor.

No, or not much. In fact, the voice-coil inductance resonates with the motional capacitance around 300 Hz, and the amp then sees the naked voice-coil resistance. If the driver is designed properly, the resistance won't be less than 6.4 ohms.

No, not much lower; see above. Not lower than 80% of the rated impedance.

What exactly ? I suggest that in vaccum, the resistive part may become smaller, while you appear to deny that.

Rene

No you're telling the same thing, but...

Speakers efficiency is rather poor: typically under 1%. This can't change much the real part of the impedance. I guess the losses in the spider and outer suspension accounts for the difference.

That word "commercial" bothers me. IHFM ratings are (or used to be) based on a 3msec 1KHz tone burst. Very few commercial "50W" amps can meet even that except in some marketer's dreams. You might want to consider building your own or buying a 400 or 500 W amp.

Ted

We are.

Although horn loading can increase this enormously.

It must do I guess. It probably takes a heft bit of power to actually move the cone regardless of radiation.

Graham

Modern sound reinforcement speakers may have a DC R as low as 5 ohms IME ( for 8 ohm nominal impedance ).

Your 80% figure is likely more true of hi-fi units.

Graham

"Pooh Bear" schreef in bericht news: snipped-for-privacy@hotmail.com...

( for 8

I bet cheap Chinese speakers can go as low 3 ohm, being actually

"Pooh Bear" a écrit dans le message de news: snipped-for-privacy@hotmail.com...

Yes, but that's not the usual situation. That's why I said typically.

No. The mass translate to inductance, suspension elasticity to capacitance. This, in itself, doesn't dissipate power. Regardless of radiation, it is not the cone movement, but the suspension movement, and also the voice coil resistance that requires power.

"John Woodgate"

The AC resistance value, measured at the mid frequency impedance minima, is about 10 to 20 % higher than the DC value. The additional resistance with AC is due mainly to resistive losses in the spider & surround plus radiated sound energy.

Most speaker makers quote this AC resistance value as the "nominal impedance".

........ Phil

I once saw a pair of speakers that a guy had built, back in the days when people built their own "bass reflex" or "folded horn" speaker cabinets. Each one had just one speaker - the kind with the "whizzer cone" to make it "full-range" or whatever they were calling that back then. His amp sat on top - on a piece of perfboard about 3" by 4". His output trannies were the latest [ca. 1972] RCA whiz-bang complementary NPN/PNPs in TO-5 cans, with those little flag heat sinks on them. He claimed it could output almost a whole watt RMS per channel. ;-) And when he cranked it up, it got uncomfortably loud. And when he played it at a reasonable level, it sounded quite good.

With good speakers, it doesn't take all that much. :-)

Cheers! Rich

A solid state amp needs no load. A noninductive 8 ohm resistor would be used to test the amp for power rating tests. A real load would of course provide impedance. A sub has varying impedance which is around 8 ohm or much higher depending on a test frequency. A real "sub' would not usually go higher than 500 Hz. An 8 ohm speaker will measure DC at about 6-7 ohms, or even a bit higher at times, depending on the driver. What are you doing exactly?

greg

When I said 'the cone' I meant it as simply shorthand for the whole mechanical system.

The suspension is also lossy btw so it will absorb power. When the cone moves there is also some deformation of its shape which will additionally appear as a loss.

There will also be some eddy current losses in the motor to consider as well.

Graham

I read in sci.electronics.design that Rich Grise wrote (in ) about 'Sub resist load for speaker?', on Wed, 5 Oct 2005:

By 'good', you mean 'sensitive'. The pressure to extend the low-frequency response limit and the low cost of amplifier power these days means that most current loudspeakers are significantly less sensitive than in the past.

In the more distant past, sensitivities of some products were very much higher, due to the use of very high permanent magnetic inductions (around 1.5 T). Someone already mentioned the Voight products; the British Lowther products were similarly sensitive; about 50 mW was enough for quite a large room.

I read in sci.electronics.design that Pooh Bear wrote (in ) about 'Sub resist load for speaker?', on Wed, 5 Oct 2005:

Well, don't buy them, then.