Mismatch ohms

Eddie Freeman · From: Natchez Mississippi

Recently got two old Peavey cabinets and each cabinet had two 12 inch non powered speakers. In each cabinet, one speaker read out at 7 ohms, and the other 12 incher read 14 ohms,These readings were stand alone readings, that is, they were not connected to any thing.
My question is,with the mismatch can it hurt the PA amp, or is there some way(maybe a 7 ohm resistor in the line) to connect these up.??
,

Michael Maddex · From: Northern New Mexico, USA

Edited to say: Sorry--I misread/misunderstood the Original Post. Some good advice below. I will still say:

Enjoy your New Speakers! Cool

_________________
"For every expert, there is an equal and opposite expert." -- Arthur C. Clarke

Jim Cohen · From: Philadelphia, PA

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Forget putting resistors in speaker circuits, not a good idea. You haven't told us anything about the PA, but its probably okay to use both speakers, just don't expect them to give the same volume level, as one is going to be louder than the other. Live with it, or get matching speakers.

Lee Baucum · From: McAllen, Texas (Extreme South) The Final Frontier

Ken Pippus · From: Langford, BC, Canada

Each cabinet will end up running at 5.33 ohms, which should work fine for an amp expecting to see either a 4 or an 8 ohm load.

Pete Burak · From: Portland, OR USA

You could put the matching speakers in the same cabinets (basically have one 8-0hm and one 4-ohm Cab), then the speakers in each cab would be the same volume, and the cabinet volumes can be made equal at the board or power amp.

Georg Sørtun · From: Mandal, Agder, Norway

Ken Pippus · From: Langford, BC, Canada

Running two 16 ohm speakers in a cabinet allows you to run at 8 ohms (parallel) or 32 ohms (series, not likely to be useful.). Similarly, with two 8 ohms, your options are 4 ohms or 16 ohms. I would be inclined to leave them in symmetrical â€œ6 ohmâ€ cabinets unless there was a reason to do otherwise.

Eddie Freeman · From: Natchez Mississippi

Thanks guys, I have decided that since the speakers are so old, I will just donate them to the trash man.
I thank all for their input, again thanks.

ajm · From: Los Angeles

From the original post, you are going to use these with a PA amp.
Therefore, let's assume that you are going to use both cabinets at the same time.

So to restate what you said.........

You have two cabinets.
Each cabinet has two 12 inch speakers.
The speakers in Cabinet A read 7 ohms and 14 ohms.
Cabinet B reads the same as Cabinet A.

That means that the speakers in each cabinet are probably 8 ohms and 16 ohms.
Using those common values, each cabinet will have a total impedance of 5.3 ohms.
However, ASSUMPTION ALERT: This assumes that the speakers in the cabinets are wired in parallel.
You need to measure the total impedance of Cabinet A to see that it is in fact 5.3 ohms (roughly).
Then do the same for Cabinet B.

Connecting them to the PA will most likely mean that the cabinets are connected in parallel to the amp.
This will present a total load on the PA of 2.6 ohms.
Make sure that the PA can handle that low of a load.

Volume: The cabinets will have an equal volume assuming that the speakers in each are the same model, that they are working right, etc etc etc.
However, the speakers in each cabinet will probably not be the same volume.
The 8 ohm will be louder than the 16 ohm, theoretically. You may not notice it, but like I said, theoretically.

Now for the troubling statement: "I have decided that since the speakers are so old, I will just donate them to the trash man."
So they work, there's nothing wrong with them, but you're going to throw them out?
Why not give them to a local band?
Or local bar?
Or a school or community center?

Jim Sliff · From: Lawndale California, USA

Have somebody look at those speakers before you dump them. Many vintage speakers are very desirable and some worth a bit of money.

FWIW not only impedance needs to be considered, but also power handling. If you don't know how much power they can handle and they are Peavey speakers a qualified amp tech can find out.

A mismatch may be OK or could be harmful to the amp. Solid state systems generally handle mismatches better than tube systems, and when mismatches are marginally OK using lower impedance speakers than the amp "wants" is safer than the impedance being too high.

To explain why resistors are out - Any resistor used would need to handle a percentage of the amp's power. This would normally require a large, heavy wirewound resistor. But speakers are not resistors and you can't mix the two,

We read the DC resistance for general reference to verify the impedance specification, but the speaker's impedance spec is a bit lower than the DC resistance. However, the actual working impedance fluctuates quite a bit during use. A resistor stays fixed and doesn't "work" like a speaker and even a "safe" one will foul up the sound and create even more of an impedance mismatch during actual use.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional

Carl Mesrobian · From: Salem, Massachusetts, USA

ajm · From: Los Angeles

Jim Sliff wrote:"But speakers are not resistors and you can't mix the two, ...."

You may have meant it a different way than I interpreted it, but as a blanket statement I disagree.

Every power attenuator (which are meant for use with guitar amps, not PA systems) I've come across uses load resistors, which means that ultimately you are mixing resistors with a speaker load.
Granted, there may be some that are "reactive", but I haven't seen a schematic for one. Even the Weber MASS uses load resistors.
Also granted, there are some that don't like the sound or whatever they get with power attenuators.
However, none of these dictate that you CAN'T use load resistors in combination with speakers.

At any rate, those arguments apply to guitar amps and guitar tone. The OP asked a question relating to a PA system. I would guess that the overall "tone" would be affected much less in that application.

So then the question is: Why would you want to do it to these cabinets in this application? What benefit would there be?

IMO, the main reason that load resistors are not necessary in this case is that they simply aren't required and wouldn't accomplish anything useful.
I'm guessing that someone wants to add an 8 ohm or so load resistor in series with the other 8 ohm speaker in the cabinet to "match" it with the 16 ohm speaker in the same cabinet.
You can do it, but it isn't necessary, coupled with the fact that the power that would be dissipated by the 8 ohm resistor would be power/sound that is thrown away. Plus, the sound coming out of the 8 ohm speaker would be attenuated. Plus, ultimately you may not notice it anyway.

You could also add a resistor in parallel with the 16 ohm speaker to try to match it with the 8 ohm speaker. Similar results would occur, but again......Why?

There may be other reasons to add a resistor, but ultimately I don't see it as accomplishing anything useful.

ajm · From: Los Angeles

Regarding mismatching of loads, generally........

Solid state amps can handle an open load, but not a shorted load.
Tube amps are the opposite.
Tube amps can tolerate a shorted load, but do not like an open load.

However, I personally check my connections before powering up and prefer not to push my luck.

Jim Sliff · From: Lawndale California, USA

ajm - Yes, "Can't" wasn't really the right choice of words. But fixed resistors or attenuators using variable resistors do not react to changes in frequency response like impedance does, which changes significantly based on the frequencies.

Because a resistor is a fixed device (when in use even a variable resistor is left in a fixed position) and impedance is dynamic, changes in output (loudness or power) are not predictable. The resistor will do a single thing while the speaker's levels and tone change as the impedance does.

Tone is affected by mismatches in PA systems the same way it is in an amplifier - unless you are somehow claiming a PA doesn't use an amplifier to send the signal to the speaker(s) it's a function of changing impedance, not type of amplifier.

The overall sound varies, though, depending on frequencies amplified and the THD. There is not a fixed "power cut" with an antennuator of any type because the impedance of the speaker(s) isn't a constant, nor is THD. There are complicated curves used as examples of this used in acoustic engineering.

As far as the Weber MASS, you are incorrrect:

ajm · From: Los Angeles

Jim: I agree that resistors do not respond in the same way as a speaker.

Will using resistors as part of the speaker chain in a PA result in a tonal difference? That you can actually hear?
Maybe.
That can't be adjusted out with the EQ? That results in a totally objectionable, unusable sound?
Maybe, maybe not. I'll leave that to someone else to try. However, until I hear a comparison of a PA versus a guitar amp (especially an overdriven guitar amp), both being attenuated, I'd guess that the guitar amp would be much more noticeable.
Unless you were using a PA as a guitar amp (remember the old Bogen PA heads that guys used as guitar amps back in the 60's?), I don't know why anyone would want to use a power attenuator on a PA output anyway. If you want it more quiet, turn it down. (I have no doubt though, that in this world in which we live, someone is out there doing it as we speak.)

There are some people that claim that the type/composition of resistor used makes a difference in what you can hear. Does it? I dunno, that's a science project for someone else.

".....unless you are somehow claiming a PA doesn't use an amplifier......"
I am not aware that my post gave that impression, but for the record, no I don't claim that. However, if I can figure out a way to do it, I'll be a bazillionaire.

A lot of my points are interlaced with qualifiers like "practical" or "accomplish anything useful" or "what benefit" or "is it necessary". After that, it's up to the end user if they want to do it. If it doesn't cause anything to explode, then try it. Maybe you'll hear a difference that I don't. That's probably why there are a zillion clones of Tube Screamers that all use the same circuit, yet all claim to sound better than the others.

Weber Micro Mass: I found your copied photo on the interwebs. Those knobs on the front are load/power rheostats/potentiometers. In short, variable resistors. On one photo it appears from the labeling that they are 50 ohms. They and the caps inside interact with the speaker motor. I looked for a schematic for a MICRO MASS but could not find one. Therefore we don't know how the circuit really operates.
For some reason it seems that most of the other Weber attenuators do have schematics on the internet. They also use a combination of resistors and capacitors and a speaker motor.
So what effect does the speaker motor have in the way that they use it? I'm not sure, but I'm skeptical, at least that it has a huge effect. But, I have never tried one. For every brand of attenuator out there there are fans and haters.
I built my own adjustable resistive attenuator several years ago. It does what I want, it probably doesn't sound like the amp at actual cranked volume, but it won't for several reasons.
Before I actually built it I did some research. As part of a hobby/obsession I have copied various amp and attenuator schematics over the years. A lot of times there are no schematics, but someone will post a photo of the innards. You can get some idea from that of what might be inside.
Every attenuator I have ever seen photos or schematics of has had some load resistors in there somewhere.
MASS, Scholz Power Soak, Dr Z (various), Jet City, Rivera Rockcrusher, Bad Cat Leash, all have resistors.
Ones I have not seen are some of the high end ones like the Ultimate Attenuator or the Palmer Load boxes.
I have also surprisingly never seen a THD Hot Plate. I almost bought one once off of CL for cheap just to take it apart and see what's inside, but I was too late. I have, however, seen a schematic for a THD Univalve amp, which has a "built in" Hot Plate. Yep.....it has a power variable resistor.

And it just occurred to me:
Are these cabinets we're talking about in the OP stock? I'll bet not.
At any rate, why do they have two different ohmages of speakers mixed in them?
I can think of several reasonable explanations as to why.

And at the end of the day, I still wouldn't put them in the trash. Even if the speakers don't work, the wood and other parts are worth something.

Forum Index > Electronics		Next oldest topic :: Next newest topic