ruby speaker question - 1 ohm speakers a bad idea?

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I bought these two used Bogen PA system speakers (like the type used in a department store or other general PA system, not a music PA system) from a salvage place yesterday.

I put a meter on the terminals for the speakers and each reads about 1ohm.

I was worried that I might blow the 386 at such a low ohmage... is that something to worry about?

BTW the back story is: They came with a transformer with different wires for different voltages and wattages, but hooking up a ruby circuit to them, all of them are too quiet, so I tried bypassing the transformer and the speakers are nice and loud that way, but it reads 1ohm.

[Mark Hammer]

Well, unlikely they are actually 1ohm, even though the DC resistance reads that.  Still, a 2ohm load with a 386 begs careful use since drawing more current (which a 2ohm load would do) risks burning the chip up.

While the speakers may be sonically useful (and may have been a terrific deal), chances are pretty good you will want to a) run them in series for a higher total load, and b) power them up with something more than a half-watt chip.

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so even 2 ohm is risky business?

How can you tell the actual load of a speaker? I thought the multimeter trick worked because I have tried it on speakers labeled 8 ohms and it reads around 8, etc... I think I am not understanding what the speaker load actually refers to.

[PRR]

> speakers labeled 8 ohms and it reads around 8

Customarily, a "4 ohm" speaker has 3.2 ohms DC resistance (and "8 ohms" has 6 or 7 ohms DC resistance). So the multimeter test is close-enough.

This works because a loudspeaker is 1% efficient, or 99% in-efficient, and a large part of that inefficiency is unavoidable DC resistance.

> PA system speakers (like the type used in a department store

That's the difference. Large complicated systems with long wires favor having a transformer at each speaker. The AC/audio efficiency of a transformer may be very high, 80%-95%. Conversely, the DC resistance of a transformer may be 1/10th or 1/20th of the impedance up in the audio range.

Does it say "25V" on the back?

Does it have a "Watts" switch or tap?

Whoops: I see it does, and you discovered that the leverage is wrong.

In any case: open the box, find the actual speaker, disconnect one lead, measure DCR.

> I tried bypassing the transformer

You DIS-connect the transformer from the speaker(s)? Just feeding the speaker, with the transformer secondary still attached, will give a low DCR, and probably a very saggy bass impedance.

Whoops again. One case I know of which may use 1-2 ohm speakers. A "Column" speaker with 4, 6, or 8 drivers -might- use 1 ohm drivers wired in series to get up to a usable impedance. In that case, your best approach is to stick with the way it was designed, use the whole array of speakers. Yeah it is a little large, and yeah it sounds different every place you stand. But column PA speakers are usually loud and bright, not a bad thing for a flea-power guitar amp.

> I might blow the 386

You might. '386 is not short-protected, just too lame to hurt itself quickly. Prolonged overdrive into a low-ohms load might over-heat it, and it does not have real overheat protection. Despite all that, '386 is awful hard to kill. And if you socketed it, mighty cheap to replace.

A further issue is that power supply current will be high. If you battery-power, battery life will be short. That's of course a trade-off against "nice and loud", so you may prefer to live loud and fade-out before the end of the night.

If these are honestly low-Ohm speakers, one of the car-radio chips would be more efficient than the tame '386. Most carry a 2-ohm rating. Most are incredibly difficult to kill. However the idle current is much higher.

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Thanks for that info.

I got the 1 ohm reading by de-soldering the transformer from the actual speaker and reading the resistance across the two speaker terminals. It read about .9 ohms

I played for a little while yesterday and the chip didn't feel warm, but I had the speakers in series when I was doing that.

The volume with the connections on the transformer was just weak. I have made a lot of rubies now and with many different speakers, and this was really weak for a 8" speaker, which is why I went for bypassing the transformer.

It has many wires with a legend on the back of the speaker saying which color wire corresponds to which voltage and wattage (25v or 70v and 1/8w to 4w).

Also each speaker is in it's own little enclosure, looks just like this:


Info on the speakers is at the bottom of this page (bottom left) "WBS8T725 Assemblies, & WBS810T725" http://www.bogen.com/products/conespeakers/

[sean k]

A look at the data sheet, which convinced me to go to a 16 ohm load,
At 12V and the point where it reaches 3% THD.
4 ohms = 1.2W dissapation .3W power out
8 ohms = .8W dissapation .7W power out
16 ohms = .5W dissapation .8W power out

So I would suppose 2 ohms might be 1.4 -1.5W dissapation and 150mW output.

I think the efficiency of the speaker though is what counts.

[Mark Hammer]

Those wall/ceiling-mounted enclosures are too often overlooked as the basis for a viable practice amp.  The chief shortcoming is that the cabinet is often too thick to simply pop a couple of holes into and stick in some pots.  More typically, one would need to cut out a larger hole and insert a metal plate, to which jacks, toggles, and pots coud be installed.  Alternatively, what I've used to mount controls/jacks to in some instances is simply a piece of copper-clad board, with the fibreglass side facing "out" and the copper side facing "in".  If you want to get fancy, you can even go so far as to etch contacts onto the board for whatever purpose you wish (securing pots, mounting battery holders, etc.).

The thing with the 386 chip is that there are several different "386 chips", some of which can tolerate passing more current than others.  The NatSem ("LM") and similar chips have suffixes that go from 386-1 to 386-4.  The "-4" version is able to handle more current without burning up.  Alternatively, if you are perfing it, consider fashioning some sort of heatsink for it out of a piece of aluminum.  I realize you said that the chip "didn't feel warm", but you yourself noted that you had increased the impedance of the load for that bout and hadn't really played all that much.  The precaution I suggest is anticipation of when you crank it up and feed it a compressed (via a compressor or distortion pedal) signal that imposes the demand to supply lots of current for much of the time.

Finally, if you plan to run it off batteries, consider what the optimum supply voltage is and take the opportunity afforded by the size of the enclosure to use a battery clip with C or even D-cells for long battery life.

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I have built in these kind of thicker enclosures before Mark and did exactly what you mention - cut holes in the wood and mount pots and jacks on a sheet of aluminum which is then mounted to the inside of the cabinet with wood screws.

I will probably not be using these cabs for that purpose, however. Ultimately they will be coming out and put into cabs I am building for a larger, multi-channel, multi-amp setup. I tried this previously with three speakers in one cab - each powered by it's own ruby. That one triple amp gets pretty loud even in clean settings.

Also, they will be powered by 12-15v DC power supplies and I have purposely been ordering the "386-4" chips.

Is there any way to increase the load on the speakers? I could put them in series with a small 8ohm speaker, but then I am guessing I will loose some of the clean volume I can get out of these. I don't plan on pushing the amps into distortion territory (though there will be compressed guitar and synthesizer going into them).

[sean k]

Just an idea. Theres stuff floating about on the net about making an 8 ohm driver out of what is normally a guitar pickup. What if you were to make one of these and then add another winding at 2.8/1 ratio to define number of windings? These little string drivers are sustainers and use an LM386 so you may very well be in the same territory. Though instead of magnets you could use a bar of silicon steel.

Alternatively, and most probably a far better idea is to get some small transformer to do the job. 2.8/1 voltage ratio might be hard to find though. But also you could unwind something and make your own.

[head_spaz]

Why not just bridge mode the LM386 to double the drive current?
Then you could attenuate the output. It can be loud when you need it, and raunchy if you want at lower volumes.

[Mark Hammer]

Probably more fruitful to simply opt for a higher wattage chip, rather than go through backflips to get a 386 to under-deliver.  I've had very good fortune with the LM380, the NJM2073 (stereo but run in bridged mode) and the TBA820.  Indeed, many of the cheesy $10 plastic computer speaker pairs you find discarded often come with a nice little chip that can be run in bridged mode for a couple of watts, not to mention a pack of nice "greenie" caps.  Feel free to cannibalize them.

[PRR]

The best match between a 4VRMS amp and this transformer is 4W 25V, which is a 156 ohm load. The '386 will be running light, power will be near 0.1 Watts.

If you clear out the warehouse, 10-20 of these is a 16-8 ohm load, will extract a large part of a Watt from the 12V chip, and the large total cone area (more than a four-Ten cab) will give stunning midbass efficiency.

However, eight raw drivers in series gives you your 8 ohms, ample cone-area, without transformer loss and weight.

> I got the 1 ohm reading by de-soldering the transformer

I'm gabberflasted. They can save a part-cent that way, but they also sell an 8-ohm version, and you'd think the savings would vanish in the extra cost of inventorying two parts.

> Those wall/ceiling-mounted enclosures are too often overlooked

Yes.

I was in WallMart, looked up, saw one, and wanted to buy it off the wall.

These systems are the direct descendants of the Good Home Radio, indeed of the first Domestic Loudspeaker worth having. A LIGHT-weight 6"-8" cone in a 24"-12" open-back box. These are the direct ancestors of the first guitar amps, which guided the development of guitar pickups and our notions of tone.

"Hi-Fi" drivers have largely killed this type speaker. They persist in guitar world, but mostly as HIGH-power versions with robust heavy coils.

The low-bid small PA speakers often suck, but are always worth trying.