Impedance question

[Steve A.]

Here's how such a cable would be wired:[ATTACH=CONFIG]22671[/ATTACH].
Such a cable should be marked so that it does not get accidentally used as a regular "parallel" y-cord.

Another way of accomplishing this would be to put the "Y" inside a small box with a cable to plug into the amp and two jacks for the speakers. You might even wire up a switch to run the two speakers in series or in parallel. If you use a metal enclosure you would need to use insulated jacks.
With the box I think that there is less of a chance of hooking it up wrong...

Steve Ahola

P.S. A pair of speakers respond differently when connected in series or in parallel so a switchbox would also be useful using a pair of 8 ohm speakers with an amp with 4/8/16 ohm outputs.

[Mango Moon]

Awesome! Thank you all, gentlemen! I think I'm beginning to see what's going on here and the principles involved, and I should have no trouble making such a cable. Your instructions have been great and I've got what's needed to do it sitting right here. I've got a perfectly good set of super-duty (super heavy) 1/4" phone plugs, some really stout speaker cable (8-gauge or some such thing?), a good soldering iron, and some suitably-sized heat shrink tubing, so its all-good. This means that I can now purchase the amp that I was considering right off the shelf without concern or need for custom ordering of a switchable 4/8/16 impedance feature which was an up-charge and would have involved a significant wait time. Very cool! I'm glad I asked. Thanks again to everyone.

[Mango Moon]

Another way of accomplishing this would be to put the "Y" inside a small box with a cable to plug into the amp and two jacks for the speakers. You might even wire up a switch to run the two speakers in series or in parallel. If you use a metal enclosure you would need to use insulated jacks.
With the box I think that there is less of a chance of hooking it up wrong...

Steve Ahola

P.S. A pair of speakers respond differently when connected in series or in parallel so a switchbox would also be useful using a pair of 8 ohm speakers with an amp with 4/8/16 ohm outputs.

Steve, I suppose you're right - doing so within a sealed and insulated box of some sort would be the best all-around way to do this, and I'd have no problem constructing and wiring such a rig, but in all candor, I think incorporating a series/parallel switch-ability to it (presumably with an SPST or DPDT mini-toggle of some sort?), would be getting up over my current pay-grade. However, that would indeed be very versatile, functional and cool.

[bob p]

Regarding that comment in the diagram that says not to mix an 8R and a 4R cab in series -- what's the problem with that?

We've already established that a 100% mismatch is OK, and the result of mixing an 8R cab and a 4R cab in series would result in a 12R load, which is roughly a 50% mismatch-up on the 8R tap and a lesser mismatch-down on the 16R tap. That's a pretty good match. A 12R load would work pretty well on either the 8R tap or the 16R tap, and would give you some flexibility when it comes to which operating point sounds better. The only thing that you'd need to worry about is whether each cabinet could safely dissipate it's share of the applied power.

[Enzo]

Just an opinion here. COnsidering bob p's special Y cord, May I suggest makinbg a small box with three Cliff jacks in it, or similar. Two jacks for speaker cabs and one for amp. And the inner wiring would be the same as the special Y cord, puts the two speaker jacks in series. IN fact the jack cutout contacts could be usede to allow one cab to be unplugged and the remaining one will still work. But by using the box, you don't need a special cord. Any three speaker cords can be used, so you can move the two cabs farther apart if you want, or if one plug breaks, just use a different cord.

Having been on the road enough, I just hate having special cords.

[bob p]

^^^ What he said. If you went with a specialized Y-cable then you'd need to carry a spare specialized Y-cable. If you put the connections in a box, then you could just use regular speaker cables with it, and spare regular speaker cables are easy to find.

[bob p]

When it comes to making something that's fault tolerant, here are some extra things to think about:

* The idea of putting the series connections into a box instead of a cable is a good idea, because if the cable were to break it could be inconvenient to replace. Having the special part of the wiring in a little box that uses regular cables is a good idea because it moves the special wiring into a box that's not likely to fail. The part that is likely to fail (the cables) remain a generic item that's easy to replace. I like this idea because it moves the fault location away from a customized part that's hard to replace and puts it in a generic part that's easy to replace.

* Using a junction box has a downside -- it places you at risk that a speaker cabinet (or cabinets) could become unplugged if/when somebody trips over them. One potential downside to the junction box idea is that you now have more risk for connections being unplugged inadvertently. Maybe it would be better to make these connections with locking connectors instead of 1/4" phone jacks that can easily be ripped out. The last thing that you'd want would be to have somebody trip over the cable, disconnect the amp's feed from the speaker load, and have your amp go to a no-load condition while you're driving it. That would be a disaster.

* It might be worth considering the addition of a properly sized high value resistor across the OT secondary leads to protect you from the inadvertent disconnect.

[Mango Moon]

Interesting - very interesting.

Regarding Cliff jacks, although I had occasionally seen them before installed in certain devices (I think a Mesa-Boogie amp maybe?), I was not fully familiar with them or their function until now, but I've since researched them on-line and can now see what they're all about. If I understand them right from the limited photos, they're fully insulated with four solder-point lugs that remain permanent, but you then establish the various connections between (or across) them) using some sort of conductive, spring-steel jumper clips? Have I got that right? In other words, you could switch between (for example) a series and parallel setting at any time (on-the-fly) by simply adjusting jumper clips? If so, that would indeed seem to be the cleanest, slickest, most ideal approach to this puzzle. And presumably (assuming he could find or purchase an empty one), a guy could use something like one of the standard, two-piece aluminum boxes that are typically used in manufacturing boutique FX (stomp box) pedals for guitar to house this entire apparatus. Very cool! Thanks Enzo, thanks Bob P! Time now to search for a suitable box.

EDITED: Doh! My bad! Looks like I might be mistaken on this business of jumper clips - after researching them further, I'm seeing all kinds of different varieties ala Neutrik, Cliff, etc.

[J M Fahey]

If I understand them right from the limited photos, they're fully insulated with four solder-point lugs that remain permanent, but you then establish the various connections between (or across) them) using some sort of conductive, spring-steel jumper clips? Have I got that right? In other words, you could switch between (for example) a series and parallel setting at any time (on-the-fly) by simply adjusting jumper clips?

Don't complicate it and don't try to add "versatility" or whetever.
Plus I don't trust internal leaf switches that much.
And having 2 jacks in series, even less so.
Go for the simplest solution which is always the safest.
Simply get that Hammond box, mount 2 jacks on one side, one in the other, wire 2 jacks in series permanently (as I said don't rely on or use the leaf contacts) , bring those 2 free contacts to corresponding ones on the other side solitary jack and call it a day.
As was suggested above.

Build 2 such boxes to always have a spare one, just in case.
And in case you don't have it available, use just 1 speaker for that night.

Although the *best* solution, from every point of view, would be to hold the GAS a little longer and get an amp with 4/8/16 outputs, or at least 4/8 ones.

As of just running both cabinets in parallel into the 8 ohms output ... yes, it *can* be done .... yes, it will work and "not change sound *that* much" ..... but just from the very beginning ordering a special head + 2 top notch cabs and use them mismatched .... not as a "one night" option but as in "the" option, scratches me the bad way.
Specially if that decision comes not from trying to get a special sound or whatever but out of impatience.

[Mango Moon]

Well, after thinking through this whole proposition of a box wired in such a way as to allow for a safe series connection between the 16R output jack on the amp to a pair of 8R speakers, it occurs to me that we've inadvertently gone full circle back to the very function served by the attenuator that I mentioned in my initial or earliest post on this thread. That attenuator has a single selectable 2/4/8/16 input jack and matched pair of output jacks, also with 2/4/8/16 variable impedance selector, which (as I understand it), is fully operational even when the attenuation mode is set to 'Bypass' or 'Variable/Load', so I think we've arrived back at square one. From the standpoint of usage or practicality, a box is a box - I see no point in building a duplicate of something I essentially already own. I don't think it gets any LESS complicated than that.





Thanks again, gentlemen!

[Tom Phillips]

I was surprised to see all the activity in this thread when I checked back. Anyway...

...the attenuator that mentioned in my initial or earliest post on this thread. That attenuator has a single selectable 2/4/8/16 input jack and matched pair of output jacks, also with 2/4/8/16 variable impedance selector, which (as I understand it), is fully operational even when the attenuation mode is set to 'bypass',...

That sounds suspicious to me. Does your attenuator box include a big transformer inside?
How about telling us the makes and models of your amp and attenuator for the record?
Thanks,
Tom

[bob p]

I have to ask about the name/model of that attenuator you're referring to. I'd also like to ask what does the inside of that attenuator look like?

I have serious doubts about any guitar amp attenuator that claims to allow you to match any Z on the input side with any Z on the output side. To make an impedance matching attenuator like that would be complicated and expensive, and that's not what I remember seeing marketed to guitar players. An attenuator that matches any Z on the primary side? I can't see that I've seen many (any?) of those offered in the musical instrument market. Maybe I'm just missing something.

[Mango Moon]

I was surprised to see all the activity in this thread when I checked back. Anyway...

That sounds suspicious to me. Does your attenuator box include a big transformer inside?
How about telling us the makes and models of your amp and attenuator for the record?
Thanks,
Tom

Tom,

My attenuator (not cheap!) is an Aracom PRX-150 (see photo of front and back panels above), while the amp in question was a 50w Carol Ann OD2, and yes, I believe the PRX-150 does indeed have an internal transformer, because I believe its rated to max 150w. I should also add that I've used this attenuator now tirelessly for over two years with multiple 100w heads (Soldano SLO-100 and Marshall DSL-100) in variable impedance mode to the same pair of 8R EVM-12L's without a hitch.

[Enzo]

If it isn't too late, Cliff is a brand of jack. A lot of people call them Marshall Jacks, since they are what Marshall has used over the years. The plastic ones with the big plastic nut.

I specified them because if you mount this in a metal box, you MUST insulate the speaker jacks. remember, in series speaker wiring, one of the speakers will not have a ground connection.

[bob p]

Wow. There's a stupidly expensive solution to a problem that shouldn't even exist.