Guitars101 - Guitar Forums banner
1 - 16 of 16 Posts

·
Registered
Joined
·
36 Posts
Hi:
what is the application?

Are you wiring a 4 x 12 ?

Or are you determining the capability of your amp. Most amp heads usually if you run two cabinets take the (2) cabinets and divide by two to set the ohms on your head. For example (2) 16 Ohm cabinets, set your head to 8 ohms. But, I have no idea what your are referring too. So, this info. may be useless.

Let us know what you need and someone here can help.

Bob
 
G

·
ohms law is pretty simple....

Voltage=Current x Resistance or V=I*R
Power=Current x Voltage or P=I*E

thats the basic formula. Resistance is measured in ohms. when you connect speakers together, you can put them in series or parallel, or in series-parallel. its hard to explain without a schematic.
but anyway as far as i know when you connect them in series you would add all their resistance together....
say 4 8ohm speakers....
R1+R2+R3+R4=?
8+8+8+8=32 ohms

for parallel its different formula...
1/R1+1/R2+1/R3+1/R4=1/?
 

·
Premium Member
Joined
·
4,955 Posts
Discussion Starter · #5 ·
thanks everyone,,,the schematic was the most help but you all helped me understand...ive always owned combos and thinking of an amp & cab and want to understand it before i blow up my new amp....


Anyone ever have an experience of a wrong ohm setting ?
 

·
Premium Member
Joined
·
4,955 Posts
Discussion Starter · #6 ·
ok...what are most cabinets wired as...series or parallel?...and whats the difference?...i really need to know this..thanks for sharing your experience and wisdom
 

·
Registered
Joined
·
473 Posts
Hey Axe;

That depends. 4 speakers can be wired series, parallel, or series parallel.

So if you have a quad of 16 ohm speakers your total impedance could be:

series - 64 ohms ..... 16 x 4 = 64
parallel - 4 ohms..... 16 ÷ 4 = 4
series-parallel - 16 ohms ...
(16 ÷ 2) + (16 ÷ 2) = 16
or
(16 x 2) ÷ (16 x 2) = 16

Using a quad of 8 ohmers would give you:

series - 32 ohms
parallel - 2 ohms
series parallel - 8 ohms

8 and 16 ohm cabinets are wired series parallel
and 4 ohm cabs are wired parallel

Hope this makes sense

Ed
 

·
Premium Member
Joined
·
4,955 Posts
Discussion Starter · #9 ·
thanks Mr crumb...your examples helped me verify other info...very complete and thanks alot.....there is no advantage in using series vs parallel ...is there?
 
G

·
yes... well as far as i know, when you connect any number of loads in series, voltage will drop and current will be the same through the entire circuit.
connec the loads in parrallel, and its the opposite...voltage will be the same through entire circuit and current will drop at each load.

the electrical load in this case is the speakers. im guessing getting more or less voltage or current gives different power characteristic...like high end volume or low end hard hitting power. correct if im wrong,.... but i guess it just depends on personal preference for how you want it to sound as to how you set up.
 

·
Registered
Joined
·
4,017 Posts
[ QUOTE ]
axeshredder said:
thanks Mr crumb...your examples helped me verify other info...very complete and thanks alot.....there is no advantage in using series vs parallel ...is there?

[/ QUOTE ]

Actually there are advantages to both. The lower the total impedance of your cabinet...the more current the amp's output stage is going to draw at a given "rail" voltage. If your amp can push tons of current you are good with a lower impedance (it should be louder). In the event that it can't...series might be a better idea. Your amp will either tell you on the back...or have a selector switch.

You should not hurt your amp with the impedance selector in the wrong position....as long as you keep the volume down. It is best to have it set correctly when cranked (or anytime really).

Here is a quick way to figure out the impedance of your speaker cabinet. Get an ohmmeter and measure the dc resistance of the speaker or speakers between teh tip and ring of the speaker cable plugged into the amp..

An 8 ohm speaker will show about 6 ohms of DC resistance on the meter. A 4 ohm speaker will show about 3...a 16 ohm load will be about 12.....etc. I could explain why....but just trust me on this one....it's a long answer.
 

·
Registered
Joined
·
473 Posts
In a solid state power amp, the lower impedance will allow more power to the speaker cab, provided the power amp is designed to run at the lower load. Here's the math

P = E² ÷ Z

P = power in watts rms
E = rms voltage to speaker
Z = speaker impedance

If you flop the equasion around you can solve for the voltage required to allow an 8 ohm cab to dissipate 100 watts.

So:

E = the square root of 100 x 8
That gives us aprox 28.3 volts rms

So let's suppose a solid state amplifier puts out 100 [email protected] 8 ohms and is designed to drive impedances as low as 2 ohms is now connected to a 4 ohm cabinet. We know it's max clean voltage is around 28.3 vrms

@ 4 ohms you get this:

P = 28.3² / 4
P = 200 watts

So a 4 ohms, if the amplifier is capable, you will double your wattage. Connect it to a 2 ohm load and you will double it again and have 400 watts to the cabinet.

--------------------------------------------------------------------------------

This doesn't really happen in a tube amplifier. If you connect a 16 ohm cabinet to a tube amplifier, you must match the cabinet to the transformers secondary winding(s). For example, a marshall tube head has a switch for a 4, 8, or 16 ohm cabinet. The transformer matches the plate impedance of the tubes to the speaker cabinet impedance. So, cabinets rated at 4, 8, and 16 ohms all recieve the same rated power, provided that they are properly matched by selecting the correct impedance on the amplifier. A 100 watt tube amp can only put out 100 watts.
 

·
Registered
Joined
·
4,017 Posts
You are on the right track Crumb......but this is not entirely correct so I will try to explain in a bit more detail. Your formula's are right...and in some cases (depending on the amp's power supply) you can actually pull enough current to actually double the output power when you put a lower load on it....so again in theory you are correct. The issue is that when you have an amp that is rated at 100W with an 8 ohm load and you put a 4 ohm load on it.......if the amp's internal power supply cannot supply a shitload more current, the rail voltage will actually get pulled down. The end result will be massive clipping. This is measured in a term called headroom and most guitar amps are so friggin loud they don't spec it.

To avoid this situation, use the load the amp is rated at (keep the switches in the right spot) and keep the volume under control.

If you are looking at PA amps (or home/car amps) it's easy to tell if they have a good power supply. The amps rating will be (as an example) 100WPC/8 ohms. If it doubles to 200 at 4 and 400 at 2ohms (at the same total harminic distortion)...that is an amp with a good power supply and lots of headroom. Another way to tell is just to look at the price tag...they will be expensive compared to other 100W amps.

This is important for several reasons...but one is that the impedance of any speaker varies a lot depending on the frequency it is producing. A typical 8 ohm speaker will actually have an impedance of 2-4 ohms at some lower frequencies. You never want to push an amp to the point where the internal rail voltages drop on peaks. Many high end amps actually have a readout or a "clipping" indicator that monitors this voltage.
 

·
Registered
Joined
·
31 Posts
Go to Mesa/Boogies website and download the owners manual for an amp. They have the most comprehensive explanation and diagrams I've seen for matching cabinets to amps. Pretty detailed, and easy to understand.
 
1 - 16 of 16 Posts
Top