OT - speaker impedance

[Paul Marossy]

If I have a speaker, but don't know if it's a 4 ohm, 8 ohm or whatever ohms, how can you determine what it is?

[aron]

Put a meter on the terminals and measure using your multimeter (ohms)

[petemoore]

Iv'e heard this is actually innaccurate but I can tell whether its say above or below [4 8 and 16 ohms being the most common spkr ohmage]
 using just an ohmmeter. I take a reading from a speaker of known ohmage and use this as a comparator to the reading I get from the unknown speakers ohmage reading.
 You can hear a slight difference when 'running' between a 15ohm and a 16 ohm speaker [I had a Celestion 15 ohmer once never heard of one before or since] but since most are 2,4,8,16,32ohm devices///when powered and running the smaller ohmed speaker [say one 8 and one 4ohm] will be hogging a lions share of the power the higher ohmage [twice the ohms] will be basically not doing anything worth bothering and will sound very faint.

[Jim Jones]

Hey Paul,

Just use you meter.  The DC reading will be slightly less than it's value, i.e., an 8 ohm speaker will measure in the neighbourhood of 6 ohms.

Hope this helps!

Jim

[Paul Marossy]

I was thinking that I could use my DMM, but that seemed a little too simple...
Thanks for the second opinion.  

[Paul Marossy]

OK, next dumb question.

I have determined that this 12" speaker I have is an 8 ohm speaker. I would like to put it into a stand-alone speaker cabinet for use with my guitar amp. The external speaker jack on my amp indicates that it needs to be 4 ohms. But I would like to use this 8 ohm speaker. Is there a way to make it appear to be a 4 ohm speaker to the output transformer without using another 8 ohm speaker wired in parallel to achieve a 4 ohm load? And, would it wreck the frequency response of the speaker?

Anyone ever try to do this?

[Nasse]

:oops: Hmm dont know about such tricks, but I have always believed that you can put larger impedance speakers to solid state amp ouput, but tube amps are different animals, perhaps. My Marshall 50 watter did not blow up immediately when I switched to "wrong" impedance (someone told to me that it does not matter, just use the setting that sounds best. I remember have read that Marshall recommends output impedance set just right. Maybe it depends how loud you play.

My friend´s band bass player had a Fender Bantam Bass amplifier, and both voltage selector and speaker impedance selector were in wrong position, and it sounded like shit. One fuse was replaced with piece of steel.

I did answer, because I remember something trick with tube amps with four output tubes. With VOX AC30 you can remove two tubes and play with less power (not recommended long), but impedance selector must be adjusted...dunno remeber how but must check my archives :oops: . Maybe Fender and Marshall can do the same trick. But not very practical and reliable trick.

[Aharon]

If we are talking tube amps,it depends on the amp,what type of output transformer and other factors.
Usually it CAN be done.Also a quick and dirty solution is to put a large ceramic resistor in parallel with the speaker to bring the total impedance to 4 ohms.Depending on the power output of the amp choose the resistor wattage acordingly.
Aharon

[Paul Marossy]

Oh, I guess I should have mentioned more about the amp...
I can't rember if it's a 60 or an 80 watt tube amp, but it's a Fender Hot Rod DeVille. It has two 12" speakers in it, and according to the schematic, they are wired in parallel. The output transformer on the schematic shows two 8 ohm speaker jacks, one for the speakers mounted in the amp cabinet and one for an external speaker. But the manual says the external speaker should be a 4 ohm cabinet... I guess it might have to do with the fact that they have a 2x12 and a 4x10 version of the amp.

I just want to make sure that I don't damage the output transformer.

I also had a thought to use a ceramic resistor in parallel with the speaker, too. That would change the DC resistance, but I am not sure about the impedance part of it, though. I mean would it affect the impedance as well?

[Aharon]

Not unless the resistor is inductive.It has beed done,it's just a patch.
Aharon

[Nasse]

:oops: Checked the tube removal trick, but it was told to turn impedance selector to half value with same speakers after removing two tubes, and it was for 100 W Marshall. I just thought what if you not adjust impedance selector but put different impedance speaker instead...

[Elektrojänis]

If I ever understood anything correctly then with resistors it goes like this: Resistance = Impedance
(edit: Note that I meant it is like this for resistors. Caps, inductors, speakers etc. are a different matter.

However I don't know too much about output transformers of tube amps, but one thing I'm pretty sure about: If you use that parallel resistance thing then only half of the total power output by the amp will go to the speaker other half goes to the resistor.

BTW. I think that 40W resistor needs a heatsink... I would use resistor rated for full output of the amp or perhaps more if possible (at least if I wanted to crank the volume).

[Gearbuilder]

Hi,

I've always seen that it's only hazardous for the amp to connect a charge below the output's impedance.For exemple ,you could put an 8 ohms speaker on a 4 ohms output ,you're just dissipate the half of the power(Ohm Law!).If you put a 4 Ohms speaker on a 8 ohms output you may burn the transformer :this is real for tube amps because the transformer should dissipate the half of the power ,but there's some cases where you can put a 4 ohms speaker on an 8 ohms output ,some power amplifiers have outputs can shut down  only at two ohms but they are transformless.
Usualy they are P.A. amps with transistors.In the same way it because of that the transistors amps are less sensitive when you don't connect the speaker ,they don't burn, but never disconnect a speaker on a tube Amp or a output with transformer.
The question about lowering the impedance of a 100w Marshall
by runnig it with two tubes and matching your transformer with the selector is true because when you pull out 2 tubes the impedance at the plate load  is now the half then you have to divide your output impedance the same way disregarding the speaker impedance.
But i'm not right Impedance is not resistance !Impedance is for AC
resistance is for DC .
You'll have to see the schematic wiring if the external jack is parallel to
the main output.
Try with a 8 ohms in first and compare with a 4 ohms ,power and frequency.
The more power you've got the best matching you are!
Regards
Bruno

[Paul Marossy]

I thought I remembered reading that going somewhat larger would be OK, but not smaller. I think I'll just try it with it at 8 ohms and see how it works.
I haven't really messed with this sort of thing before...
Thanks for the help.  

[Peter Snowberg]

Any substitution you can make is determined by a wide set of parameters that take the transformer, the tubes, and the bias of those tubes into account. There isn't a universal "you can....".

Going from 4 to 8 ohms is a shift of 100% so don't bet on anything without knowing. That amp is speced to run 60W with two tubes that are NOT rated to pull that much power. Either the marketing people got ahold of the figures, or the amp is always running at or above yellow-line (read: it's already way too pushed). Your car engine will run fine at 4,500 RPM, but not 9,000. (unrelated analogy)

The impedance of your speakers is seen by the tubes. Give them the wrong value any you kill the tone at best, or kill the tubes and/or transformer at worst. Sorry, no free lunch.

You could add an 8 ohm 50W in parallel to get back to 4, but that will eat over 50% (depending on frequency) of the sound. You really need a either a transformer, or another speaker to pair with that one.

Good luck,
-Peter

[Paul Marossy]

Yeah, I was afraid of that. I was kind of hoping that there was some other way of doing it.  

But I have read things that go both ways, one saying it's alright to put a larger speaker on an output transformer and others saying you should only use what's it's rated for. They all seem to agree that running less than what it is rated for is bad.

[Gearbuilder]

Hi there,

There's an article from Ken Fisher about 100w/50w mods in the Aspen Tube Amp Book  vol. III.
Have a look !
Bruno

[Paul Marossy]

OK, I just added another 8 ohm speaker in parallel to make it a 4 ohm cabinet. That solves my problem.

But, out of curiosity, I was wondering is it possible to keep one 8 ohm speaker and use a 2:1 audio transformer in such a way that it looks like a 4 ohm load to the output transformer? And if that is possible, how would you calculate what would be required for the right frequency response?

Anyone?  

[Gearbuilder]

Hi Paul,
I think that you can find some articles on the Web ,(Perhaps Geofex )or on electronic forums for calculate the transformer. I  think that it's not a cheap solution because the price and the height of the transformer.
Regards
Bruno

[Paul Marossy]

Yeah, I guess it would be a fairly large transformer now that I think about it a little bit. Just a hypothetical question.