Determining PC speaker impedence

[Andrew]

Hi

My 'temporary' main tv since July 2012 has been a Humax HD-FOX-T2 stb connected to my Hazro 24 inch IPS monitor and Philips MMS 321 speaker system.

This is fine but the 3.5 connector on the Philips amp is crap and there is an intermittent crackling that does not cease when I press mute on the Humax remote. The Philips amp has a cheap transformer that hums quite noticibly too, but back in 2006 the MMS321 was given a good rating in various PC mags.

Plan is to buy a Mini-1 amp from

and remove and recable the Philips speakers.

How could I measure the impedence of these speakers ?. It is 50 years since I did physics in school - things are a bit rusty now.

Alternatively I could use the 40 year old Wharfedale Linton 2 speakers that have been in my loft for the last 10. Thats 10 cycles of 40 centigrade in summer and at least 3 winters where it has got really cold. Again, before using them how do I do a check on their impedence.

Andrew

[Andrew Gabriel]

For small speakers, it's usually the same as the DC resistance. For higher power speakers, the impedance will be higher than the DC resistance.

The impedance is extremely non-constant across the operating frequency, and the cabinet design has a massive influence on it (particularly something like a bass reflex cabinet).

When I was still at school, I made my speaker cabinets (still have them). Dad borrowed a signal generator and oscilloscope from work, and I used these to measure the impedance across the audio range, both of the bare speakers, and once installed in their cabinets. Did some nice graphs, but I don't have them now.

If your stored speakers had foam supports which were popular

40 years ago, you may find the cones no longer have any support. You can apparently get replacement supports, but I imagine fitting them could be quite a challenge, without ending up with the voice coil rubbing on the magnet.

[Dave Plowman (News)]

The DC resistance as measured with a DVM won't be a million miles different from the nominal impedance. And pretty well all half decent speakers are near enough the same impedance anyway.

[whisky-dave]

I thought yuo could just measure it with a standard DMM providied you don;t need accurate result.

measured a small 2.5" (0.3W) 64R speaker came to 55R

measured a small 2" (0.25W) 8R speaker came to 7.7R

[The Natural Philosopher]

impedance is pretty close to 'resistance'

I.e an 8 ohm nominal speaker generally looks like 5-7 ohms.

I am running our TV through an old Sony 'mini HiFi' system from 15 years ago. The sort of thing that cost 50p at a car boot sale.

[meow2222]

1. Look at what it says n the box or speaker magnet.
2. Measure with a multimeter. IME an 8 ohm speaker has typically around 5 ohm resistance.

as above. Lets hope the suspension survived. If not it can be replaced, but you need to get the positioning right.

NT

[Dave Liquorice]

That's what happened to my rather nice Acoustic Research AR58's from about 1985. B-(

Driven by home built Quad 405, they could make a good sound. But they where rather big, and "too loud" for SWMBO'd B-((

One day I will get a decent system again, quite impressed by the Genelec monitors that many trucks have, small boxes, lots of clean poke and the matched sub could probably blow the door off. B-)

[Dave Liquorice]

It's got DC on it, unplug and have probe about with a high impedance meter you should be able to find where it's coming from. Shove in a couple of blocking capacitors, 10 uF 16V will probably do. Or just wire the connector out of circuit.

Hums? As in acoustic or ripple on the supply rail(s)? If acoustic then replacement wall wart or power brick can probably be obtained. If ripple check the smoothing C's perhaps add a bit more to counter the "engineered to a price" factor.

Nice little box but as it (presumably) runs off a wall wart no great power output, 5 10 W? I can't see any spec on the site. Ah, white on white very clever... but not very useful "20W MAX" so that'll be "peak music power" rather than RMS and it doesn't relate the THD to output.

At those power levels and modern kit I doubt it really matters if the speakers are anominal 4 or 8 ohms.

[robert]

Agree unless you are pushing the amp to its limits , especially at low frequencies, any 4,8, or even 15 ohm speaker will be okay. Impedance matching was more of an issue when using transformer outputs and low power amps when every watt mattered.

re Foam or rubber suspension, I see quite a lot of old speakers when checking items for sale in my local charity shop : some makes of "Vintage" speakers seem very prone to disintegration with time, others are never affected ( Wharfedale). Repair kits are often available for the sought after (expensive) models.

[Johny B Good]

Just measure their voice coil resistance and add 5% on. That should be close enough for you to determine which of 4, 8 and 15/16 ohm impedances they're most likely _supposed_ to be.

If you get a 5.5 ohm reading, assume it's 6 ohms impedance (I'm not sure, but I think that's also an oddball in betweeny impedance that's sometimes used with oddball drive units).

[Johny B Good]

The quoted impedance for that looks to be a little on the high side (I trust your DMM reading more than the manufacturer's labelling). Perhaps the extra turns makes the inductance add a larger contribution?

Yup, that's just about what you can expect (Z= R x 1.05).

The fact that it's so simple as taking a DC resistance measurement to determine the driver impedance to a very close approximation is a handy consequence of the very low efficiency of voice coil driven cone speakers (typically 1 to 2 percent) when fitted into an IB enclosure.

Presumably, the resistance test on a horn loaded pressure driver will be only a tenuous guide at best (48% efficiency when loaded with a single horn - the other 48% normally goes to waste unless it's a dual horn loaded PA unit as seen on poles at outdoor venues).

In this case at a guess, I'd say the resistance would be around 5 or

10% of a horn loaded pressure driver's specified impedance.

[Brian Gaff]

Why do you want to know? Most modern amps cope with most speakers naand of course if the speakers use inductive crossovers, as the Lintons certainly do, the actual impedence will vary with frequency, as will thereactiveness of the load. Brian

[meow2222]

1 way moving iron speakers are fun impedance-wise. Z covers multiple orders of magnitude in the working range, making matching a futile task - but still necessary, as far as its doable. I suppose basing anything on resistance is also futile :)

NT

[Andy Burns]

My mission 707's suffered that fate, their cardboard boxes out-lived them ...

I didn't attempt it.

[Geo]

I just checked my Linton 2(s) with a DVM and got about 6 ohms - so say

8 ohm nominal. They have been in use every day for years.

[Johny B Good]

If thats for each speaker (I'm assuming a stereo pair) it quite possibly is a 6 ohm impedance speaker. If, otoh, that's on just a single unit, it does beg the question as to whether there is a short across part of the VC winding.

I had a Baker 'Group 25' 12 inch bass/mid range drive unit go almost fully short circuit on me (about half an ohm) 40 years ago. I sent it back to Baker for a recone. The new cone and voice coil raised the rating from its original 25W to 30W. After some use with a 100W RMS per 8 ohm per channel amp, the voice coil started to rub on the magnet centre pole

Aparently, the resin used in the coil former had started to soften and bubble up. Careful application of a flat screwdriver blade soon scraped the excess away and the speaker has worked flawlessly ever since (no soft foam outer suspension to break down with age, just corrugation of the outer rim of the paper cone).

[Geo]

You are correct - both speakers checked - google found a specification stating 6 ohms. Sorry for any generated confusion...