Impedance question

[Mango Moon]

Hello folks,

I'm thinking about purchasing a new 50w (boutique) amp head that has two output jacks at 8/16 ohm, but after researching things here and elsewhere, I'm concerned about whether I can safely run two 8-ohm cabs with it? Can anybody help with this question?

As I understand it (via Ohm's law), running two 8-ohm cabs connected to a pair of output jacks listed as 8/16 in this way would amount to running at 4-ohms, and some say that although doing so yields less power, it is perfectly safe, because its "lesser" (4-ohms), not "greater" (16-ohms) impedance, but I'd like to be "certain" of this assertion. I own (and can use, if necessary) a decent attenuator with impedance control for just such applications, but I don't really NEED an attenuator in this application, so if possible, I'd like NOT to use it strictly for its impedance regulation function.

Can anyone shed some light on this subject for me? Can I plug my two 8-ohm cabs into the two 8/16 output jacks on this amp head and run at 4-ohms without any fear?

Thank you!

[Bob D]

you would run a cable from one cab to the other cab (for 4 ohms) and run from ONE of the cabs to the 8ohm jack on your amp. You do not plug a cab into each of the jacks on your cab. With that said, there are lots of different opinions on whether this is safe for your amp. I always match impedances fwiw.

[Mango Moon]

Thanks Bob, but no, I'm not interested in modifying the two 8-ohm cabs to run in series. They are factory-issued 8-ohm Mesa-Boogie EMV-12L's in vintage (and expensive) road-ready ported (Thiele-type) cabs, meant to be run as stereo cabs, so if it won't work, the amp that I'm considering will go away LONG before the cabs ever will.

[loudthud]

Your question is best directed to the manufacturer of the amp. Ask if the amp is safe with a 4 ohm total load, or if it can be modified to be such.

[Tom Phillips]

Another option is to make a custom "Y" speaker cable that configures your two 8 Ohm speaker cabs in series. Then plug it in to your amp's 16 Ohm output jack.
No mods to your speakers or amp required.

This assumes that by "...two output jacks at 8/16 ohm..." you mean that one jack is connected to the 8 Ohm OT tap and the other is connected to the 16 Ohm OT tap.

Regards,
Tom

[bob p]

I'll start off by saying that I really like your choice of EVM-12L in a TL-806 type of ported enclosure. Their sound is HUGE. If I had a pair of them and an amp that offered only 8/16 taps then I would definitely run the two 8R cabs in series on the 16R tap to get the most power out of the amp, or I'd run just one of them on the 8R tap. A single 12L will have no problem handling a 50W amp.

I would follow LT's advice and get the answer straight from the horse's mouth -- ask the amp manufacturer what they recommend, or if they can supply the amp with a 4 ohm tap. In addition to the questions that LT recommended, I'd specifically ask if running the 16/8 amp into a 4R load would void their warranty.

Having said that, you could probably get by running two 8R cabinets (4R load) on the 8 ohm tap, but doing that is going to shift the operating point for your tubes such that they're likely to produce less power than they would when optimally loaded. That may not be what you want.

@Tom -- I like your y-cable idea.

[Brad1]

Hello folks,
50w (boutique) amp head that has two output jacks at 8/16 ohm

More info would be useful.

Two outputs...at 8/16 ohms?

Are you saying it's switchable on them? Or, either or both outputs can safely run either a 16 or 8 ohm single or combined load? Like, you can use either a single 16 or 8, or combine two 16's for 8 in parallel? One for 8 only, or one for 16 only? See where this is heading?

Maybe mentioning the model of the amp would be more helpful, and if it's not something easily found (specs) online, asking the maker?

It's kind of an open-ended question, when nobody knows how they concocted the thing.

Brad1

[Enzo]

I think part of the problem is people hear rules of thumb for tube and solid state amps and confuse them.

If you drop from 8 ohms to 4 ohms on a solid state amp, you double the current draw from the output. Tube amps don;t work that way.

Certainly wise to ask the amp maker, but in general, if you are o0ff one notch, it won;t hurt th amp. By one notch I mean in the 4-8-16 scheme of things, pick one.

[Mango Moon]

Another option is to make a custom "Y" speaker cable that configures your two 8 Ohm speaker cabs in series. Then plug it in to your amp's 16 Ohm output jack.
No mods to your speakers or amp required.

This assumes that by "...two output jacks at 8/16 ohm..." you mean that one jack is connected to the 8 Ohm OT tap and the other is connected to the 16 Ohm OT tap.

Regards,
Tom

Thanks Tom.

Yes, the amp in-question has two separate (unswitchable) jacks, and its my understanding that one is an 8-ohm and the other a 16-ohm tap. Also, I appreciate the "Y" cable suggestion, although I'm not sure that I follow you. I'm admittedly a bit of an amateur when it comes to electronics, but in using the term "Y Cable", are you suggesting the following?:

Amp ---> Plug #1 ---> Plug #2 (series)
vs.
Plug #1 <--- Amp ---> Plug #2 (parallel)

Is that what you mean by a "Y"? If so, I'm not quite sure how I would go about preparing (wiring) such a cable using standard 1/4" plugs.

[Mango Moon]

I'll start off by saying that I really like your choice of EVM-12L in a TL-806 type of ported enclosure. Their sound is HUGE. If I had a pair of them and an amp that offered only 8/16 taps then I would definitely run the two 8R cabs in series on the 16R tap to get the most power out of the amp, or I'd run just one of them on the 8R tap. A single 12L will have no problem handling a 50W amp.

I would follow LT's advice and get the answer straight from the horse's mouth -- ask the amp manufacturer what they recommend, or if they can supply the amp with a 4 ohm tap. In addition to the questions that LT recommended, I'd specifically ask if running the 16/8 amp into a 4R load would void their warranty.

Having said that, you could probably get by running two 8R cabinets (4R load) on the 8 ohm tap, but doing that is going to shift the operating point for your tubes such that they're likely to produce less power than they would when optimally loaded. That may not be what you want.

@Tom -- I like your y-cable idea.

Bob,

Thanks for detailed response. Yes, I truly do like the EVM-12Ls. I've owned them for 25-years now and used them much of that time with a Mesa Studio Preamp and 50:50 stereo power amp. Its been a great setup and your characterization of them as "huge" sounding is spot-on. It amazes (or puzzles) me in fact, that so many people will report not liking them. Clean or dirty, they've provided me with everything I've ever needed.

Anyway, regarding your comment about . . . "I would definitely run the two 8R cabs in series on the 16R tap to get the most power out of the amp". Could you be more specific about how I would go about doing this? In other words, with sealed mica cabs like these, how would I run the pair in series? Are you referring to the same approach as Tom did above (i.e., a "Y" cable of some sort)?

[Mango Moon]

More info would be useful.

Two outputs...at 8/16 ohms?

Are you saying it's switchable on them? Or, either or both outputs can safely run either a 16 or 8 ohm single or combined load? Like, you can use either a single 16 or 8, or combine two 16's for 8 in parallel? One for 8 only, or one for 16 only? See where this is heading?

Maybe mentioning the model of the amp would be more helpful, and if it's not something easily found (specs) online, asking the maker?

It's kind of an open-ended question, when nobody knows how they concocted the thing.

Brad1

Thanks Brad.

Hopefully, I've answered your questions in my replies to other above, but yes, its my understanding that its non-switchable with one jack at 8-ohm and the other at 16-ohm, so presumably, the intent by-design was to run either one, but not both.

[Mango Moon]

I think part of the problem is people hear rules of thumb for tube and solid state amps and confuse them.

If you drop from 8 ohms to 4 ohms on a solid state amp, you double the current draw from the output. Tube amps don;t work that way.

Certainly wise to ask the amp maker, but in general, if you are o0ff one notch, it won;t hurt th amp. By one notch I mean in the 4-8-16 scheme of things, pick one.

Enzo,

As always, many thanks for your input! Interestingly enough, I have another (smaller or lower-powered) amp head that is configured similarly, with a pair of 8-ohm jacks, and I asked the builder of that amp three years ago (when I bought it) if I could run it at 4-ohms with a pair of 8-ohm cabs, and he said no problem. As a matter, to my ear, that particular amp sounds notably 'better' (i.e., thicker, fatter, richer sounding) under a 4-ohm load than it does at 8-ohm, and its not from the additional speaker or choice of speakers.

[bob p]

The "Y" cable to hook up two cabs in series would be made this way:

First, you need three 1/4" phone plugs with a suitable length of speaker wire attached to them. Strip off the wires at each end and label the plug/wire combinations as Cables A, B and C. Cable A will be the one that plugs into your amp's 16R jack, Cables B and C will plug into the two 8R cabinets. We'll hook up the wire ends of A, B and C so that the speakers are wired in series and in phase.

To do this, identify the wires on each cable that go to the tip connection on the phone plug. We'll call those the "+" wire. The other wire on each cable will be called the "-" wire.

Hook up the "+" wire on cable "A" to the "+" wire on Cable B. Hook up the "-" wire on Cable A to the "-" wire on Cable C. Hook up the remaining two connectors to each other. If you've done everything right then the two leftovers would be the "-" wire on cable B and the "+" wire on Cable C.

The signal path then, would flow from A+ to B+ to B- to C+ to C- to A-.

Plug the A cable into the amp, B into one of the 8R cabinets and C into the other 8R cabinet and you've got the two 8R cabinets wired in series, which presents an matched 16R load to the amp.

Another way to accomplish the same result while using 3 standard speaker cables would be to build a junction box that has the jacks for the speaker load wired in series. To do that you'd follow the same signal path as described above.

Rock On!

bob

[g1]

Amp ---> Plug #1 ---> Plug #2 (series)
vs.
Plug #1 <--- Amp ---> Plug #2 (parallel)

Is that what you mean by a "Y"? If so, I'm not quite sure how I would go about preparing (wiring) such a cable using standard 1/4" plugs.

The first example is also parallel, not series. It is also called "daisy chaining" where you connect multiple cabs together and one cable connects back to the amp. The load impedance is still calculated like parallel speakers.
The reason for using only one of your amp ouput jacks is that they are not parallel, they are using different impedance taps, and are not meant to both be used at the same time.
So your two 8 ohm cabs connected together would be a 4 ohm load, which would be a one step mismatch if you plugged into the amp's 8 ohm output jack.
What Tom has suggested is a custom made y-cable that is not wired like most y-cables, but rather would put the 2 cabs in series for a 16ohm total load.
Here's how such a cable would be wired:.
The plug tht connects to the amp should be marked and always connected to the amp when used. Otherwise the cabinets will be out of phase. Such a cable should also be marked as "series" so that it does not get accidentally used as a regular "parallel" y-cord.

[Steve A.]

If you drop from 8 ohms to 4 ohms on a solid state amp, you double the current draw from the output. Tube amps don't work that way...
Certainly wise to ask the amp maker, but in general, if you are off one notch, it won't hurt the amp. By one notch I mean in the 4-8-16 scheme of things, pick one.

That's what I always figured as a basic rule of thumb. I think that many people got spooked out by impedance mismatches when there were a lot of OT's toasted in some of the Marshall heads but I think that the OT failures were usually caused by the impedance selector switch failing, not by just having it set to the wrong impedance. Or so I gathered- correct me if I'm wrong!

Steve Ahola