Rewiring and fusing a speaker cab

[Bass Awkward]

So..

I've been gigging a lot of informal jazz and latin stuff recently, and the 700w Hartke and 4.5xl are simply too big to cart about for these little, regular gigs. I've gradually bought progressively smaller amps, and have latterly found my SWR black beauty to be just a little too big and unwieldy.

So, after some fairly in-depth reviewing of the class-D amps about town, I settled on an Orange Bass Terror (500w at 4 or 8 ohms), largely because it makes a sort of sound - and features a fun sort of eq. and gain - that none of my other amps do.

But the only speaker enclosure I can find that's light enough to justify it is an Eden ex110, rated 250w at 4ohms. I have settled on this as I don't like the other lightweight alternatives, and even were I to jump for another speaker, I'd like to know about the electronics aspect of rewiring this.

I know that there's absolutely nothing I can do to really improve the power handling, unless I want to rewind and heat-sink the voice coils. And I know that it doesn't necessarily break the thing to run it in these conditions, so long as I don't run it too hard.

But I've blown speakers on stage before. And I don't like it when equipment fails.

So I'd like to rewire the cab up to 8 ohms (so I can use it with my old p2p valve amp) possibly with a switch for 8/4 ohm resistance.

And I'd like to fuse it at about 200w, so that I'm not going to blow the speaker onstage, with a little sled, so I can replace the fuse.

All pretty straightforward, I think - I'll add a 5 amp (50v) fuse on a little sled.

And I'll add four (for reliability) 250w 1ohm resistors in series with the speaker load. These will be shorted by the switch, if I bother to add it.


Questions -
1> Do I really have to use these expensive power resistor thingies?

2> I use an AC fuse, I assume? Is a circuit breaker a reasonable alternative (more costly, I know)?

3> Could anyone post links to appropriate parts (Maplin, RS electronics, etc.)?

4> Will the fuse (or circuit breaker) really offer effective protection?

5> Will switching 4 to 8 ohms alter the sound in any way?

6> Should I wire the resistors in after the speaker, so that any inherent capacitance in the resistor load will not affect the tone of the speaker output? Or is that bunk theorising?

7> Are there any notable sources of power resistors in electronic junk, if they are necessary here?

Thanks in advance!

[Stewart]

My advice:

1 If you must fit them (rather than keep an eye on the volume control and your ear out for any nasty sounds) - Yes
2 Don't use a fuse or a circuit breaker
3 N/A
4 Not really....
5 Not a lot
6 Makes no difference
7 Large stepper motor drive circuits perhaps

[flyfisher]

[quote name='Bass Awkward' post='1034058' date='Nov 23 2010, 04:13 PM']6> Should I wire the resistors in after the speaker, so that any inherent capacitance in the resistor load will not affect the tone of the speaker output? Or is that bunk theorising?[/quote]
Many types of high power resistors are wirewound and can have significant inductance. Whether this is a problem at audio frequencies might depend on your Bunk-Theory-Ometer. If you want inductance-free resistors then look for thick-film types - but they're not cheap.

[Bass Awkward]

Cheers for all the replies - it's always nice to learn, thanks.

...Even if it is a bit disappointing that I can't kit the thing out with 40p worth of resistors and cheap switches!

The induction issue for power resistors is interesting - I guess this sort of thing comes up in amp design, etc. I'll have a think about it when I feel a bit more like doing some maths.

Anyhoo, why shouldn't I use a fuse/circuit breaker (or why wouldn't it be of benefit)?

Surely a 5A (50v) AC fuse would limit any power in the cab to 200w RMS, and save most risks of blowing the speaker?

Or should I use a fuse related to peak voltage?

And surely power amps [s]take[/s] [b]afford[/b] ample time to power off (a couple of minutes (at least))[b] before they[/b] cook when faced with an infinite resistance?

Thanks again for sating my curiosity and tolerating my ignorance.

Edited November 23, 2010 by Bass Awkward

[Stewart]

Four of these would probably be fine:

[url="http://www.rapidonline.com/sku.aspx?tier1=Electronic+Components&tier2=Resistors+%26+Potentiometer&tier3=Power+Resistors&tier4=Aluminium+clad+wirewound+resistors&moduleno65217&catref=62-8140"]50W resistor[/url]

As to why you might not want a fuse in line with the speaker, just think about why you might want some nice heavy duty cables to drive your speaker/s...?

Edit: Add amp comment:

Solid-state amps will drive a high resistance all day - no bother

Edited November 23, 2010 by Stewart

[Bass Awkward]

[quote]As to why you might not want a fuse in line with the speaker, just think about why you might want some nice heavy duty cables to drive your speaker/s...? [/quote]

D'oh.

Good point - and a fuse in parallel with the speaker signal to control a switch over the speaker signal.... means running the speaker in-line to a switch and a couple more resistors. That's not much better, I suppose - or is it?

Eyes and ears seem more and more attractive as you suggest.

So, to ask another stupid question: why would a circuit breaker be problematic?

[amnesia]

[quote name='Stewart' post='1034179' date='Nov 23 2010, 06:18 PM']Four of these would probably be fine:

[url="http://www.rapidonline.com/sku.aspx?tier1=Electronic+Components&tier2=Resistors+%26+Potentiometer&tier3=Power+Resistors&tier4=Aluminium+clad+wirewound+resistors&moduleno65217&catref=62-8140"]50W resistor[/url][/quote]

Sheet 3 of the datasheet says that a HS50 running at 50W on a 'standard' heatsink will have a surface temperature of 150degrees. Standard heatsink is defined on sheet 2 as 535cm2, 1mm thick.

As an electronics engineer, I would not specify a component rated at xW, to operate at xW in its normal duty. If temperature rise wasn't an issue in terms of personal contact (ie burns) then for example 50W load I would use a 75W or 100W rated component. If temperature was critical, and good heatsink cooling could not be acheived then I would rate a much larger component.


The Marshall Power Brake - [url="http://www.marshallamps.com/downloads/files/PB100%20hbk.pdf"]http://www.marshallamps.com/downloads/files/PB100%20hbk.pdf[/url]
Fitted with fan cooling to dissipate heat from the load so you don't burn your fingers!

Thats my 2p.

Edited November 24, 2010 by amnesia

[Bass Awkward]

Cheers for checking up on that Amnesia.

An L-Pad was somewhere on my list of projects. Maybe I'll move it forward, though I'm not overly confident in my ability to successfully implement it.

I'd also say; is it not the case that speaker cables are thick for the purpose of reducing resistance (not EMI, which is negligible on this level of signal), and aren't fuses pretty negligible in terms of resistance?

Ampeg seem to use some sort of fusing system on the horns in their cabs, with lightbulbs. I guess it's because they're non-ohmic resistors, and they give an obvious feedback when you're pushing too hard - the trouble would be in finding the right lightbulbs, and whether there is a particular issue preventing their use in fusing bass frequencies.

Again, I'd like to restate that I'm really happy to be told I'm wrong - especially when I can be told [i]why[/i].

I'd far rather learn of my ignorance than implement a crap solution on an otherwise serviceable - though not ideal - setup.

[Bloodaxe]

[quote name='Bass Awkward' post='1035234' date='Nov 24 2010, 04:17 PM']I'd also say; is it not the case that speaker cables are thick for the purpose of reducing resistance (not EMI, which is negligible on this level of signal), and aren't fuses pretty negligible in terms of resistance?[/quote]
Speaker cables use higher gauge wire to handle increased current, not to reduce resistance. Resistance over a cable run of less than 1m will be so small as to be unmeasurable by any reasonable means, even if you used the cheapest twin-core mains cable (or even zip/bell wire) that you could find.

[quote name='Bass Awkward' post='1035234' date='Nov 24 2010, 04:17 PM']Ampeg seem to use some sort of fusing system on the horns in their cabs, with lightbulbs. I guess it's because they're non-ohmic resistors, and they give an obvious feedback when you're pushing too hard - the trouble would be in finding the right lightbulbs, and whether there is a particular issue preventing their use in fusing bass frequencies.[/quote]
Riddle me this... If the lightbulb is such a panacea, why doesn't everybody use it? I've only heard of it in relation to Ampeg... no-one else. If the Eden has a piezo tweeter, then it's kind of self-crossovering (you can tell I'm not an audio engineer ), so shouldn't need any protection as it's intrinsically out of harm's way.

[quote name='Bass Awkward' post='1035234' date='Nov 24 2010, 04:17 PM']Again, I'd like to restate that I'm really happy to be told I'm wrong - especially when I can be told [i]why[/i].

I'd far rather learn of my ignorance than implement a crap solution on an otherwise serviceable - though not ideal - setup.[/quote]
Forgive me for saying this, but it seems to me that you're worrying over nothing here. Since you've downsized your rig to suit the needs of the gig, if a single 1 x 10 isn't providing enough SPL, then you'd probably be better off just adding another. By the time you've cranked the Orange up to the Eden's danger level, you won't have a tone that Jazz/Latin audiences would regard as appropriate anyway - sort of like Chris Squire playing 'Watermelon Man' .

Just my 2p.

Pete.

[Bass Awkward]

Had I a money tree, or no interest in protecting my speakers, I'd agree completely, Bloodaxe - and I think I can see what you're getting at: I can use this rig without re-wiring it.

But I like re-wiring. And I [i][b]hate[/i][/b] things dying onstage.

Perhaps a better title for the thread would be [size=4][b]Why don't speaker cabs have intrinsic circuits to protect speakers from blowing out?[/b] [/size]

So... I play punk gigs too, where the gain will be up if I use the Orange.

I don't want to make things as loud as they possibly can be, I just want to avoid the irritation and cost of blowing a speaker and a gig at once.

And I shan't carry another speaker for the same reason that I won't just take my SWR, Selmer or Hartke along: constraints of space and the effort of lifting them.

[quote]Riddle me this... If the lightbulb is such a panacea, why doesn't everybody use it? I've only heard of it in relation to Ampeg... no-one else[/quote]

And as regards Ampeg's system - well, I have heard of fused hifi speakers, and I can certainly think of a few gimmicks, or unique selling points that apply to other manufacturer's products. Pretty much nobody but Hartke uses aluminium cones, nobody but MarkBass use VPFs post eq. - few folk but Warwick use a tiny power tube with output load prior to a solid state power amp, and so on. It doesn't make them bad features.

[quote]Speaker cables use higher gauge wire to handle increased current, not to reduce resistance.[/quote]
It's pretty easy to work out that you only have 6A or so in a 250w 4 ohm cab, at most. Voltage is, what, up to 92v?
From what (minuscule amount) I know of such things, 2mm cable can be rated to carry 1000v at 6A without overheating, but because of the application, the sound coming out is affected by tiny changes in resistance. Hence the big thick cable, as it's cheaper to use more conducting core than purify it excessively.

What I really want is some sort of advice on the electronics involved - why a fuse (which would only limit the current to a level toward the limits of the voice coils) would affect the tone - especially if, as can relatively easily be managed, it were in parallel, controlling a switch to cut the speaker signal.

If I have a fuse in parallel, I need a few more resistors, but I'd like to wire something to protect my speaker and I'd like to know if there are any really decent reasons that it ought not to happen.

[Mr. Foxen]

What you actually need to protect your speakers is a correctly set limiter, I think it is a voltage limiter, but it is something that comes up in PA, you'd also need to know the appropriate point to set it to, because it isn't the 200w thermal limit that is the limiting factor on where your speakers break.

[Stewart]

I wouldn't be too concerned about the rating of the resistors (if you *must* use them at all) - you're never going to be running at anything like the rated power for any appreciable period of time.

As to the question:

Why don't speaker cabs have intrinsic circuits to protect speakers from blowing out?

Given the variable nature of virtually everything concerned (power handling at specific frequencies, impedance at specific frequencies, compliance, maximum excursion blah blah...) you'd need something like a preprogrammed 'smart' circuit breaker to suit the model of driver in the cab concerned.

Even if such a device existed it would likely cost more than every other component.

It is much easier to do at line signal levels, and many active PA cabs use a DSP to practically achieve what you're after - but it was an intrinsic part of the design from the outset - and includes the amplifier/s in the equation.

[flyfisher]

[quote name='Stewart' post='1035888' date='Nov 25 2010, 08:53 AM']As to the question:

Why don't speaker cabs have intrinsic circuits to protect speakers from blowing out?

Given the variable nature of virtually everything concerned (power handling at specific frequencies, impedance at specific frequencies, compliance, maximum excursion blah blah...) you'd need something like a preprogrammed 'smart' circuit breaker to suit the model of driver in the cab concerned.

Even if such a device existed it would likely cost more than every other component.[/quote]
Yep, that would be my take on the matter as well.

Speakers are simple, cheap and robust. Size them correctly for the system being used and they'll be fine.

Under-spec them and yes they'll be at risk of dying on stage (certainly a bad thing). Under-spec them but protect them and they'll still 'die' on stage because blowing a fuse or electronically tripping out is the same thing as far as the audience is concerned.

[orangepeelneil]

[size=3]Hi there,

Have you considered a positive temperature coefficient (PTC) resistor or 'POLYSWITCH' ??

There is quite a bit of info on the internet about them.

Basically you choose the device based on current rating and I guess parallel them up for increased current rating.
They look okay and cheap, I have no experience of them but have used something similar. They will open circuit when the rated current is exceeded and reset when the current is reduced back to a normal level.


[url="http://www.jaycar.com.au/images_uploaded/polyswit.pdf"]http://www.jaycar.com.au/images_uploaded/polyswit.pdf[/url]

[url="http://www.circuitprotection.com/catalog/app/C12_loud_(121-124).pdf"]http://www.circuitprotection.com/catalog/a...d_(121-124).pdf[/url]

[url="http://www.diyaudio.com/forums/multi-way/151272-loudspeaker-protection.html"]http://www.diyaudio.com/forums/multi-way/1...protection.html[/url]

Cheers Neil[/size]

Edited November 26, 2010 by orangepeelneil