|
|
Post by andy on Sept 10, 2009 4:45:16 GMT -5
I am planning to buy a new amplifier head and want to built my current speakers into some cabs to run it through.
The Celestion Vintage 10 is simple- there's only one of 'em!
My decision is with a pair of Eminence Legend 125s (Jensen-ish things) which are 8 ohms each. Ideally I would like to build two open backed 1x12"s, using one for recording and lower volume use, but being able to daisy chain them both for the extra spread and volume if needed. As the proposed amp only has one output which can handle either 8 or 16 Ohms, is it possible to link two 8 ohm cabs via a standard speaker cable to an impedance of 16 ohms?
The ins and outs of impedance continue to baffle me, and I will have to deal with the specifics just before the time of wiring, but just knowing whether it is possible, and therefore worth the research and head scratching, would be a good thing.
Otherwise I can just relax and make a 2x12!
Thanks chaps!
|
|
|
|
Post by newey on Sept 10, 2009 5:43:16 GMT -5
Hey, Andy, good to hear from you again!
I don't know how relaxed you'll be, since the same impedance issue will arise whether it's in a single cab or in 2 cabs.
If you wire the 2 8Ω speakers in series (whether inter- or intra-cab), you'll have 16Ω total, which you indicate is acceptable.
This can be done inter-cab with a standard cable. But, once you do it in series, the 2 cabs would have to be hooked together to work, neither cab would work individually.
To hook either up individually, giving the 8&Omega, you could use a switch to switch the wiring- but this runs the risk that if you connect both cabs together with the switch in the wrong position, you'll have too little impedance and you could damage the amp. Better, I think, to use separate jacks for 8 and 16Ω, appropriately labelled.
But there may be an easier way- I'll let others weigh in . .
EDIT: It occurs to me that it would be easier if one of the 2 cabs was designated as "series only", to be used only as the second cab in the chain of 2, never as a single cab. It would have a single jack for "series in".
The first cab would be the one you would use for a single cab and as the first in the chain, if 2 are used. This cab would have both "series in" and "series out" jacks. To use it as a single cab, you would use a "dummy plug" in the out jack, with both connections of the plug simply wired together.
At least, I think that would work . . .
|
|
|
|
Post by newey on Sept 10, 2009 6:37:48 GMT -5
I should also mention that connecting the 2 in series will reduce the output somewhat. That may not matter if the amp in question has enough power to cover both situations, but in a smaller-wattage amp it might be an issue.
|
|
|
|
Post by andy on Sept 10, 2009 10:50:33 GMT -5
Thanks Newey, it's nice to be back with something technical to discuss, rather than my usual idle chit-chat!  Thanks for putting the thought into the problem. Your answers have brought up exactly what I was hoping wasn't the case- it's just not as simple as wiring the thing in and sticking a jack plug in between. I can trust myself to check and recheck a diagram and the pos/neg tabs on the speakers, but doing anything more than that and being confident that the amp is seeing the correct load is not really me! A 2x12" will do me fine, and save a bit of wood in the process, not to mention a few hairs on my head. I think I'll be keeping it simple after all. Photos will be posted once it all comes together.  |
|
|
|
Post by ChrisK on Sept 10, 2009 14:42:58 GMT -5
How does it handle this, through broad output load specifications or an impedance selection switch? What is a "standard speaker cable"? If it involves 1/4" plugs, using one of these magical 9-lug Stereo Jacks, enables one to have what you want. The second (last) cabinet in the chain would have a mono input jack. The first cabinet in the chain requires two jacks. Let me know if 1/4" jacks are used with a "standard speaker cable"? If so, I'll explain the mystical madness of the solution. Why, it's just load impedance. The smaller the impedance, the greater the load presented (there is less to impede current flow). |
|
|
|
Post by newey on Sept 10, 2009 14:44:17 GMT -5
Andy- Just remember, you'll still need to wire the 2 in series to get the 16 Ohm load. If your only concern is that the speaker wiring be foolproof, so that there is no chance whatsoever of mis-matching the impedance, I believe there is a solution to that as well. JohnH and I exchanged PMs about this a while back, and he came up with a way to use a switch and a TRS jack to ensure that the wrong impedance was never presented. If you're still interested, I'll have to go dig that out of the PM archives. But if a 2X12 suits you, have at it! And post your results, too!  |
|
|
|
Post by ChrisK on Sept 10, 2009 15:00:24 GMT -5
Which the 9-pin jack will enable.
|
|
|
|
Post by andy on Sept 11, 2009 5:56:26 GMT -5
Good point. There isn't a switch, so it must have a wide tolerance. It is a handbuilt-blah-blah-blah amp so I'd assume that the components would be sturdy enough. There are in fact two outputs but only one can be used at a time, one as a 'straight' out and one via an attenuator. Far better than my trying to explain it, it is a Matamp MiniMat II... www.matamp.co.uk/amps.htmand here is a PDF of the manual for the previous MiniMat model... [a href="http://www.matamp.co.uk/Matamp%20MiniMat%20-%20User%20Guide%20-%20Version%203[1].0.pdf"]http://www.matamp.co.uk/Matamp%20MiniMat%20-%20User%20Guide%20-%20Version%203[1].0.pdf[/a] Just one that I can pick up at any old music store. Nothing fancy, 1/4" jack -jack. Maybe I just expect it to be more complicated than that! I still get thrown by how the ways of hooking it up seem to change things all the time. I've looked at it enough that I'm sure one day the penny will drop and I'll wonder what all the fuss was about.  Newey, don't worry about rummaging through the archives just yet, but thanks for the offer! I'll probably end up going for the unadventurous option, but I'll bring it up again if I'm feeling brave. |
|
|
|
Post by ashcatlt on Sept 11, 2009 11:56:35 GMT -5
Does this help?The parallel thing looks complicated with all the "1/x" and whatnot. Would it help if it was "x -1"? Probably not... Some interesting things to consider: 1) The total resistance of any two components in parallel can never be greater than the smallest individual resistance. As their values approach equal, the total approaches 1/2 of either. As one gets larger than the other, it becomes less significant, the total gets biggger, and approaches the value of smaller. 2) When you've got more than two parallel resistances, you can figure them two at a time: a||b||c||d = (a||b)||(c||d) = [(a||b)||c]||d
|
|
|
|
Post by sumgai on Sept 11, 2009 12:04:15 GMT -5
As it happens, I was recently doing some research for an answer to another question when I stumbled across this page: www.jumbosunshade.com/misc_docs/swd03.htm I found it to be right on the money, and I think it will answer all of your questions, Andy. HTH sumgai |
|
|
|
Post by newey on Sept 11, 2009 19:03:44 GMT -5
Andy- I fixed your link to the manual, as it was non-functional. SG- Thanks for the link, I've added it to the links page as it is very helpful. I'll give you a +1 for it, but only if you haven't already deleted the last karma I gave you!  EDIT:You hadn't, so I did exalt accordingly. Now leave your flippin' karma alone . . . |
|
|
|
Post by newey on Sept 11, 2009 19:43:07 GMT -5
I did find JohnH's PM to me (from March) on a related subject. I was going to use a 1X10" 8Ω cab as an extension with my 10" 8Ω SS combo amp. In 2-speaker mode, series wiring would be used to give 16Ω. Our discussions centered around how to make it "newey-proof" such that there was no chance of running the amp unloaded. I thought his solution was an elegant one. Now that the bass project is done, and pending finishing the 4Caster, I may actually get around to implementing it! Since John's PM wasn't really "personal", I'm sure he wouldn't mind sharing it for the purposes of this discussion. I quote his PM in full: Note that this uses one 'o them thar fancy switchin' jacks like Sumgai linked to. (And Mouser has 'em ) And my thanks to JohnH for his help. +1. |
|
|
|
Post by ChrisK on Sept 11, 2009 21:32:15 GMT -5
This infers that both speaker cabinets must be in inter-cabinet series, and not intra-cabinet series.
JohnH's design does this, but since inter-cabinet series is in play, the operator has the exacting responsibility to ensure that a tube amp output is never unloaded (by forgetting to plug in the other end of the cable to the second cabinet).
About the only way to do this is to hardwire the other end of the second cabinet's cable to its speaker.
|
|
|
|
Post by JohnH on Sept 11, 2009 23:26:03 GMT -5
No problem newey, and thanks for the +, although that is not my best drawing quality!
One issue that is worth raising, following on from Chris'watchit:
Neweys amp, I believe, is a solid state design. With those, the worst thing that can happen is to short the output, causing transistorised barbeque, and minising risk of that is the focus of making it safe. In a tube amp however, the open circuit condition is a bigger risk, and so care is needed to avoid such.
John
|
|
|
|
Post by sumgai on Sept 12, 2009 13:43:56 GMT -5
John, Your summary about opens and shorts on the outputs of solid state and tube amps is spot on. However, a great number of tube amp manufacturers use a switching jack for the output, and use the N.C. switch to simply short the output transformer. When a speaker is plugged in, the switch is opened, and the short disappears. Some few manufacturers have placed a resistor in the "shorting" position. Personally, I'm not a fan of shorting large amounts of current directly to ground, but the idea is to protect the amp for a short period of time, until the player realizes that he's not getting any sound, and corrects the problem. Usually doesn't take too long, I'm sure.  I'd prefer a 1-watt 32Ω unit myself, but if Fender can get away with a true short-circuit for over 6 decades, then I guess there's no real reason to use a resistor. Might as well save the shekels.  sumgai |
|
|
|
Post by ChrisK on Sept 12, 2009 19:50:21 GMT -5
However, there is NOTHING that can be done to prevent a second, open-ended cable from opening the speaker circuit (inter-cabinet series).
|
|
|
|
Post by sumgai on Sept 13, 2009 0:19:59 GMT -5
However, there is NOTHING that can be done to prevent a second, open-ended cable from opening the speaker circuit (inter-cabinet series). Yeah, doncha just hate it when people make things like this, and then insist that the items are good only for guitar cables. It's a cryin' shame, I tell ya.  sumgai |
|
|
|
Post by ashcatlt on Sept 13, 2009 0:21:56 GMT -5
Can't find a good link, but there used to be guitar cables available that had switch - actuated by beign pushed up against the control plate upon insertion - which was meant to silence the cable when unplugged. Since I can't find a good link I can only guess, but I would hope that this switch could be wired to short tip to sleeve on one end. Shielded guitar cables are not meant to carry speaker signals. A 1/4" TS plug is the same either way, though. Requires you plug it in the right way - switched end to extension cabinet. If we can't find the ones I'm thinking about, there's these. Requires you remember to turn it off before plugging/unplugging, and back on after. Edit - Ack ninja'd! Looks like sumgai found a link. (EDITed to fix ash's link.) |
|
|
|
Post by andy on Sept 14, 2009 7:47:24 GMT -5
Wow, thanks guys, that is plenty of information- enough to convice me that I'm going to build a 2x12 and be safe for now! My lack of confidence might be misplaced, and I'll pick it all up eventually, but what with building the cabs, finshing, hardware, and hooking them up, I want to keep it as simple as I can. A little compulsivley I spotted a Vox Night Train on ebay yesterday, which was my first choice before I saw the Matamps. It is now paid for and on the way- well, the all clips online all seem good...  This will give me separate 8 and 16 Ohm outputs, for all the difference that will make, but leaves me with one more question. To test it on arrival, does it matter what wires I use to hook it up to a jack socket? I plan to just wire a loose jack socket onto a spare speaker and fire it up for a few minutes to make sure the thing works ok. Will the solid-core copper wire I have around be ok, or do speakers require something more specific? |
|
|
|
Post by newey on Sept 14, 2009 10:12:31 GMT -5
Andy:
Should be OK for testing, if it's at least 24 gauge or larger.
|
|
|
|
Post by sumgai on Sept 14, 2009 12:53:02 GMT -5
While 24 gauge wire should be sufficient in theory, it is only that - theory. I'd make it 20 gauge at the very least, or 18 guage if I wanted to be sure the wire didn't make like a fuse...... Well, actually, the AWG Ampacity Charts posted around the net show that 24 gauge wire should be able to pass about ½ an amp without problems, and while the Matamp might be OK at 2 watts, the Vox at 15 watts will require the aforementioned 20 gauge wire. Bear in mind that most charts are a bit on the conservative side (as most engineers would like it), due to things like environment (temperature, etc.), insulation type, whether it's within a chassis or outside, all kinds of factors go into the ampacity rating. One of the better charts I've seen has a pretty good explanation of how the ratings are derived, without going into all the math details: www.interfacebus.com/Copper_Wire_AWG_SIze.htmlMe, I prefer to be about twice as conservative, given all else including the costs involved. If the chart says 20 gauge would be fine, I'll still step up to 18 gauge, just because it lets me sleep better at night. Some companies would have you believe that "more is better" no matter what the cost - don't buy that guff!  There's a practical limit on ROI - any more than twice the recommended diameter (gauge) for a given wattage, and you're just blowing good coin for no good purpose. Unless you just happen to already have the bigger stuff on hand..... ;D HTH sumgai |
|
|
|
Post by andy on Sept 14, 2009 14:31:02 GMT -5
Right, I'll dust off my digital callipers in the morning.
|
|
|
|
Post by andy on Sept 15, 2009 7:15:49 GMT -5
Ok, it seems like I've got myself some 22, one up from Newey and one down from Sumgai.
The solid wire works out that way, and I also have some stranded wire from a speaker cable I butchered. The conversion charts I looked at put them at the same 'gauge' despite different diameters, but I guess the stranding makes a difference.
It won't be quite as cautious as Sumgai suggested to be, but I guess it's ok to use that speaker cable for a test hookup? I did use that cable with a 300w bass amp for some time, but I don't want to trust my decision purely on that.
[red]*EDIT*[/red] and reading back, I realise that I missed your link to the interfacebus page, Sumgai. I have it open in a tab to read right now!
|
|
|
|
Post by andy on Sept 15, 2009 17:37:59 GMT -5
After all that, the rain let up for a bit this afternoon and I popped out to pick up some wire- I could only find the one gauge, but I think it is 20, after some measurements. Thanks for all the pointers! |
|
|
|
Post by sumgai on Sept 16, 2009 3:13:47 GMT -5
andy,
If you "read somewhere" that stranded versus solid wires of the same gauge are different diameters, I'd like to see that webpage, please.
While the page I link above makes no distinction between stranded and solid wires, there are websites with charts, graphs, and other visual aids that lay out essentially what I've said all along - there's no difference in the ampacity ratings between solid and stranded wire of the same diameter. One can google for "ampacity chart" and get a wide variety of sets of figures (some even more conservative than my link above, some flagrantly dangerous, IMO), but probably not very many will make a distinction between solid and stranded.
20 gauge will suffice for testing purposes, it is rated (conservatively) to handle 1.5 amps or thereabouts, so at bedroom levels, everything should be OK, both safety and tone-wise.
HTH
sumgai
|
|
|
|
Post by andy on Sept 16, 2009 5:40:38 GMT -5
|
|
|
|
Post by newey on Sept 16, 2009 9:16:35 GMT -5
Andy-
I wouldn't obsess too much about this if it's just for a few moments of testing to see if the amp works properly. You should be fine w/ 20 AWG. For permanent use, you'll want to get a regular speaker cable, certainly.
|
|
|
|
Post by sumgai on Sept 16, 2009 12:27:30 GMT -5
andy, newey's correct of course, obsession is pointless here. For your needs 20 gauge will be more than sufficient. (But I still would have balked at 24 gauge, no matter what the test, for the Vox.... for the Matamp 2 watter, that've been OK for a short period of time.) I like the second of your two links, showing the various strandings available. As one can see, they're pretty much all of them, within a gauge, no more than 0.001 or 0.002 inches different. The resultant ampacity is unaffected by this small amount of deviation, and it's certainly in line with the chart I linked previously. (Although I had to use the resistance in Ohms per Kilometer to get there.... ) Actually, if you haven't tested your new amp by now, then you probably could have just gone out and procured some good ol' 18 ga. zip cord (or lamp cord, as it's sometimes called). That stuff's good for 150 watts of light bulb (or perhaps a bit more), without heating up, so it's certainly good enough for speaker cable on all but the highest powered amps. Cheap, too. HTH sumgai |
|
|
|
Post by andy on Sept 16, 2009 18:21:09 GMT -5
Ah, zip cord. If that is cheaper than 'dedicated' speaker wire, I might as well just pick some of that up for the finished article. Your use of the word 'cheap' helped pr  ck up my ears! I hooked the cable I have up to a jack plug and a pair of slightly-too-big grip connectors, which seemed to do the trick. I had the slightly hair brained idea of hooking it up to a jack socket, then running a speaker cable between the sockets. Whilst doing this I tested the speaker lead- a good thing, as one side wasn't showing continuity, so in the end, I had a perfectly adequate cable I could have chopped up and used! The good thing though is that I've learnt some very helpful stuff from this thread which I'll take to the actual cabinet builds. |
|
|
|
Post by ashcatlt on Sept 16, 2009 22:24:07 GMT -5
I have and do use those cheap brown extension cords (they're not always brown, but it seems everybody knows what I mean when I say "brown extension cord") for speaker cables in PA situations all the time. The little tags on those things say they're rated for somewhere over 1000W.
|
|
I'd prefer a 1-watt 32Ω unit myself, but if Fender can get away with a true short-circuit for over 6 decades, then I guess there's no real reason to use a resistor. Might as well save the shekels. 
There's a practical limit on ROI - any more than twice the recommended diameter (gauge) for a given wattage, and you're just