The Steel Guitar Forum

What kind of dummy load could I use with a 8 ohm speaker that would make the amp think 4 ohm ?
I feel sure Ken has the answer but some of you
other guys might have tried this.
I read a article on this one time but don't remember
the fix.

Well, you could put an 8ohm resistor (wattage dependant on the wattage of the amp) in parallel with the 8 ohm speaker. Or you could run another speaker parallel to the 8 ohm speaker you have.
Now my question to you... is it a tube or solid state amp? If it is a tube amp, are you sure you cannot change to the 4ohm tap of the transformer (different amps vary, and feel free to email me offline if you need me to research anything on that for you)...
If it is a solid state amp, you really will not gain anything by usign the dummy load-- in other words, using an 8 ohm speaker on an amp designed for 4 ohms will cut your power in half, running another 8ohm load in parallel with the speaker will make the amp put out the full power, however, only half of that will go to your speaker, in other words, there will be no change in volume. In this case, you are just better off running the 8ohm speaker by itself-- the amp will run a LOT cooler this way.
Best bet would be to either find a 4ohm speaker, or have it reconed for 4 ohms.

Thanks Gino:

You answered my question. I thought you could make a 8 ohm speaker a 4 ohm by using a resistor. Was not
sure if you would get the benefit of the extra power
from the amp.
Guess I will be looking for another 4 ohm speaker !!


Thanks

A resistor is a *BAD* idea.

First - it has to be a HUGE resistor to act as a dummy load or it'll heat up and cook in seconds. Also, a resistor is a static load, while a speaker coil is not - the impedance "flexes" a bit, and the entire unit reacts differently to signal (and thus affects the output stage) differently than a resistor. Your tone will suffer...significantly.

But without know what you're trying I can't recommend a 4 ohm speaker either. If you're working with a solid-state amp that is designed for different power ratings at different speaker loads, then yes - a 4 ohm will allow for more power - but you have to be SURE you get a speaker that handles the power.

If the amp is strictly designed for an 8-ohm load and you install a 4-ohm speaker, you could *lose* power. Less impedance does NOT mean more power except in amps specifically designed for variable impedances and power ratings (some amps take different impedance speaker but the power stays relatively the same.

What amp is it, what does the manufacturer say about speaker impedance?

I use a 8ohm ceramic 10 watt resister, I think thats the value. If interested e-mail me. I get mine fron radio shack. They are approx. 2-1/2" lg by 1/2" square.Cost is about $2.50
The amp sees the 4ohm load it looks for. Heating or tone change is no a problem.
Play your unit with the eight ohm speaker then clip the resister across the speaker connectors and play it again. Minor adjustments to the amp EQ may need to be made, just as it would if you play through another 4ohm speaker.
We have been doing this for several years, to go from 8ohms to 4ohms, or 4ohms to 8ohms. Never lost an amp or speaker because this fix.

If the amp is calling for a 4-ohm speaker and all you have is an 8-ohm, go ahead and use it. You may loose a little power, but that's only of concern if you're running the amp wide-open to begin with. There may also be a slight tone change, but that's usually adjusted with the EQ in the amp. A normal 4-ohm speaker is only 4 ohms in a certain frequency band. For all other frequencies, it's impedance will be substantially higher - at times, as much as 50 ohms!

I don't recommend resistors as impedance-matching devices because they're a terrible substitite for an inductive load. If your 4-ohm amp is damaged by an 8-ohm speaker, it wasn't a very good amp to begin with.

I'm with Donny and Jim on this no resister,
except for silent full power testing purposes.

The 8 ohm won't do too much except want a little more juice.

What dummy load can you use to may an 8ohm speaker show the amp a 4ohm load.
We don't know what kind of amp he is using or if he is even playing with this dummy load.

Curtis Alford wrote:What dummy load can you use to may an 8ohm speaker show the amp a 4ohm load.
We don't know what kind of amp he is using or if he is even playing with this dummy load.

The only way to PROPERLY do that is with
another same type speaker ideally same ohm
and make / model.

You can for instance also us two 4 ohm speakers in series,
those two in parallel
with the 8 ohm unit.
4+4 || 8 = 4
But miss mating speakers means one set might be
a more efficient part of the whole. Therefor louder.
That could be intentional or inadvertant.

You can put a slightly higher load on the amp with little penalty.
running 8ohm one a 4 ohm output,
they other way round is harder on the amp.

Tube amps like a load at or a bit above if you must.
Transisters don't care quite as much
but you can get away with being on either side of ideal.

Just don't try a 4-16 jump or 8-2 jump etc.

But as Jim noted adding a large resister,
like a 8 ohm 100w in parallel will just
give off heat, and waste power
but NOT balance the load better.
Especially over the frequency range of
actual music playing.

Just run the one 8ohm, even if 4 ohm is 'the ideal'..

David,
We all know the best answer is to put in the rated speaker the amp is designed for. How ever, I don't know where you are generating this heating problem. If you are reading 4ohms from the speaker wire where you connect to the amp, that amp sees a 4ohm load, and I disagree on the heat generating.
Again the question is what dummy load can be used to make an 8 ohm load be a 4 ohm load as the amp sees it. If you have means, please measure the amount of heat you are detecting and post it. I need to find it for future reference.

Bill Moran wrote:What kind of dummy load could I use with a 8 ohm speaker that would make the amp think 4 ohm ?

I got it from this original question about
adding a dummy load to match impedances.

Well the MEASURED heat will be different for :

Every amp power rating if running flat out and
every resister power rating relative to resistance.

Hence I can't give a definitive number.
It is a flux in any case.

But the basic function of a resister is to dissipate energy
or resist a potential of energy.

I do know in many cases the heat put off
if the resister is under rated for the job
is enough for second and third degree burns,
or to burn down you house, if it's mounted poorly.

The bigger the wattage rating the safer.
The more the heat sink for smaller wattage the safer.

A good rule of thumb is
wattage rating 1.5 times the worst case max amperage/ voltage spike.

But the point being also :
He doesn't need to add this resister.

IDEALLY replace the speaker, practically why bother.

Curtis - what David and Donny said (thanks Donny, for expanding and more clearly explaining the fact an inductive load isn't static).

We don't know what kind of amp he is using or if he is even playing with this dummy load.

Errr - then how could you recommend a "10 watt" resistor? You have NO idea how much power that resistor needs to soak up - and I daresay *most* steel amps are rated for a little more than 20 watts (your 10-watt resistor and a 10-watt speaker would come up a tad short, as an example of a doubtful combination).

I still would be helpful if Bill could clarify what amp he's talking about and what he thinks this means:

Was not sure if you would get the benefit of the extra power
from the amp.

Use that resistor in a Twin reverb turned up to "2" and you won't have a problem (just lousy tone, probably) - dime the amp and watch things go boom. You can find all sorts of instructions and/or kits on the 'net from professional amp techs for building dummy loads - none with less than a 50-watt resistor, and many with at least 200-watt. Here's a link for an inexpensive 200-watt dummy load resistor....I'd suggest buying one if you want to use a resistor as a dummy load. I'd take that 10-watt resistor and throw it away, it's probably close to being a small BBQ if you've been using it.

http://www.amazon.com/200W-Non-Inductiv ... B0002KRE3E

However, Ted Weber has an even better system. It uses a VC from an actual speaker as a dummy load - FAR better since it will react like what you are trying to emulate - a speaker. His MASS products are very popular speaker attenuators and dummy loads, as they react like a speaker - because they ARE a speaker - with no cone. He also clearly explains the wattage issue:

http://www.tedweber.com/atten.htm

Also, there's this clip from just one of MANY websites dealing with dummy loads and heat:

Dummy loads dissipate energy by generating heat. Heat generated in a small space translates to temperature rise, and temperatures can be hot enough (under the right circumstances) to burn people and ignite adjacent materials. Because of the thermal mass of the dummy load and its enclosure, that heat can stay around for a long time. Always locate your dummy load in a safe place, where there is no chance that it will burn people or catch something on fire.

For safety's sake I will be very blunt - anyone who thinks a dummy load will not generate heat does not know what they are doing, and following their instructions could be extremely dangerous to your equipment AND start a fire.

I did not recommend any resistor, stated what we have used. I went to the shop to check the valvue of what I use, I am using a 20 watt resistor, the same type used by Peavey in their amps.
Your opinions is welcome, I only stated I have not had any of the problems you refer to in my amps or speakers.
I don't pretend to be an electrical Eng. I am only a mechanical Eng. I have designed and built electrical controls for use in HVAC and manufacturing process. My last two projects were National Semi-Conductor in S. Portland,MA a $150M project, and Rio Grande Regional Hospital in McAllen, Tex $57M.
I do not despute your opinions, I only know what logic and my projects have been

Jim the resistors I use are the same as Weber shows on the referenced website. His is 25 watt mine are 20 watt.
I think you made my point.

Curtis your 1st post said 10w, so that set off flags.

The only benifit I see to using a resister in parallel to the speaker is if you want a quieter amp.

Without the resister the amp gives
all it's available power to the speaker at hand

With the resister 50% or so of the power
goes into the resister and emerges as heat,
so the amp will likely break up a tad sooner.

But as Jim mentioned it is a fixed load,
not a load reactive to frequency in the same way
a real speaker does, so it will change your sound also.

I like Ted's speaker coil idea. Coil without cone,
and you get an exact duplicate of the magno/electro-dynamics at play.

David, I qualified my statement by stating I thought 10 watts that was the value and, I would check if he was interested in that avenue of fix. I assume he does not wish to spend the cost of a new speaker. I agree Ted has some great products but, for the price, may as well by a new speaker. Sorry if I misled any one.
Somewhere in my storage I still have my first amp(1952) built by converting an old jukebox system to vocal and instrument useable. It still worked the last time, about 5 years ago when I turned it on. All kind of won't work will blow it up caps and resistors. Kicked out about 150 clean watts. Ran it with all kinds of speakers 4,6,8,11,16 Ohm speakers never tripped out one time in my memory.

That resistor would have to handle a lot of power. Why don't you try a Peavey Automatch transformer. It can fool the amp into thinking that it is connected to the correct load, when actually it isn't. Look here;
http://www.peavey.com/media/pdf/manuals/automatch2.pdf

or at www.peavey.com and perform a site search for the operating guide. If you have further questions, please feel free to contact me here at Peavey by calling toll free at 1-877-732-8391, ext. 1180 and I'll be glad to assist.

Thanks for using our products.

Mike Brown
Peavey USA

Mike.
Interesting box it solves several problems
typically seen over the years.

Mike,
That is very useful transformer. Is this not for a PA system more than for a combo amp? I assume(do not assume), this is what his question is about. Adding 9+ lbs to the weight of the amp might be overboard for him to handle.
Again the MSRP Peavey indicates is very near the price of a new speaker.
Oh well I,m out of here, Mike thanks for the info you post on this forum it is a service to us all.
Curtis

Curtis Alford wrote: Somewhere in my storage I still have my first amp(1952) built by converting an old jukebox system to vocal and instrument useable. It still worked the last time, about 5 years ago when I turned it on. All kind of won't work will blow it up caps and resistors. Kicked out about 150 clean watts.
Ran it with all kinds of speakers 4,6,8,11,16 Ohm speakers never tripped out one time in my memory.

Basically this says it all.
A good amp will run with something close for indefinite periods,
like 1952 to 2008 even if it isn't the IDEAL theoretical set up.
No need to put in the big resister if you are happy with the sound now.

David,
I am finding the power amps are very selective in their load requirements. They over heat, and trip out regularly. We have 5 power amps and they want to see the correct load or they trip out even a half out put levels.This not one brands units its different brands. The same condition holds true. Adding fans was not the answer either. The out put rating on most power amps are over stated IMHO. This leads persons to believe they can use a less expensive unit than is need for their purpose.Some are listing the primary input watts to sound like it is a powerful unit when it 100 watt RMS out put.

Curtis Alford wrote:David,
I am finding the power amps are very selective in their load requirements. They over heat, and trip out regularly.

That usually happens when you go LOWER in impedance (or resistance, as some might call it) than the manufacturer suggests. Most people fry their amps by adding a lot of speakers, not realizing that each one they add lowers the impedance the amp sees. Add too many, and the amp sees almost a dead short, and we ALL know what a dead short does.

Of course, asking an amp (or any device) to do something it wasn't designed to do usually spells trouble. So, you have to have some knowledge before you go willy-nilly connecting things. Using an 8-ohm speaker on a 4-ohm amp is not necessarily bad, and I wouldn't hesitate to do that. Using two or three 4-ohm speakers on an amp designed for 4 ohms, however, can be disastrous...especially if it's a solid state amp. Solid state amps have protection circuits and other such devices because they're not as forgiving as most tube amps, and it's important you realize that.

An amp that "trips out" regularly is either defective, or it's telling you "something is screwed up". And 99% of the time, it's the latter.

I thought we were talking guitar/steel amps,
not extended multi speaker p.a. systems,
a TOTALLY different animal. And MUCH more finicky.

Also he said he though "Ken" would have an answer, implying Ken Fox and thus old tube amps.
So I procceded from those clues.

Miss matching a 8ohm vs 4ohm speaker on most guitar amps is not the same
as running 35, 6 inch Public Address units all off one power amp.

Apples and Oranges.

I do build and spec. PA systems,
less of the multi-point systems you do,
but I am no stranger to that drill.
This week I am spec'ing a 15,000w watt system
for a music festival that I am mixing next Feb.
for the 3rd time. So I guess I don't blow up much.
This year I also will be providing several tube amps for the players too.

David L. Donald wrote:I thought we were talking guitar/steel amps,
not extended multi speaker p.a. systems,
a TOTALLY different animal. And MUCH more finicky.

Apples and Oranges.

Not really. Whether it's a 30,000-watt PA system, a musical instrument amp, a home stereo system, or a pocket radio - the load must be within acceptable parameters. Amps don't count speakers, they don't even care if there are speakers. They just look at the load presented them (or lack thereof), and react accordingly.

I don't pretend to be an electrical Eng.

Yet you're providing advice trying to sound like an experienced amp tech when it's now obvious you don't know the basics. The recommendations given can wreck equipment.

Please explain your reference to the Peavey 20-watt resistor; EXACTLY how is it wired, in what amps, and what purpose does it serve? It's certainly not a simple dummy load, which is your claim.

These types of recommendations can cost people money. If you KNOW you don't have the experience, why make the recommendations? You're not helping anyone.