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Understanding Basic Electricity of your Amp and Speakers

The name plate ratings on the back of your amplifier says something like [100 watts at 16 ohms. 200 watts at 8 ohms. 400 watts at 4 ohms. 800 watts at 2 ohms]. If you are confused by the markings on your amp and speakers you are not alone. Many people do not understand what ohms and watts are. In order to understand ohms and watts you must understand volts and amperes too.

First let me explain it in terms that everyone can understand. Think of your amplifier as a water faucet or hose bib on the outside of your house. You connect the garden hose to the faucet. This is the wire going to your speaker. You then connect the end of the hose to one of those spinning water sprinklers. This is your speaker. Now. The amount of water PRESSURE is the VOLTAGE. The amount of water FLOWING is the AMPERES. If you have one of those little spinning water sprinklers for the lawn it does not take much to make it move but if it is a heavy industrial sprinkler for a large field it takes more water pressure and flow to make it move. The pressure and the flow overcome the resistance and the RESISTANCE is the OHMS. The total amount of land getting water is work, and the WORK BEING DONE is the WATTS. Also, the size of the hose will effect the amount of flow so whenever you do wiring on your speakers be sure the wire size is large enough to handle the flow.

Parallel Example
What if you have two 200 watt, 8 ohm speaker in parallel?

Watts in parallel add so 200 + 200 = 400 watts. The wattage marking on your speaker is the handling capacity, in other words, how much power the speaker can handle, not how much power you will get if you hook it up.

Ohms in parallel can be calculated by: 1 / r1 + 1 / r2 + 1 / etc. = rt
1 / rt = ohms

Formula for 2- 8 ohm speakers connected in parallel.
1 / r1 + 1 / r2 = rt
1 / rt = ohms

1 / r1 = 1 / 8 ohms = .125
1 / r2 = 1 / 8 ohms = .125
add them: .125 + .125 = .25 rt
1 / rt or 1 / .25 = 4 ohms


Now you know your two speakers can handle 400 watts at 4 ohms which is exactly what we started with but, if one of your 200 watt speakers were to blow out than you would be back to 8 ohms and 200 watts of power. You can safely oversize the wattage of your speakers. If you used two 500 watt speakers it would be fine but, the sound may be a little weak. There are only certain speaker wattages available for purchace. The practicality can be lost in the price of the speaker. Imagine the cost of two 1000 watt speakers to handle a 1000 watt amplifier compared to two 500 watt speakers. The common manufactured speaker wattage values available will allow you to make a choice between cost and capacity. Sure, you are going to go for the ones that cost you less. Just make sure the total wattage of your speakers is not less than what the amplifier specifies. Also pay special attention to the polarity markings on your speakers. If you get them crossed the sound coming from one speaker will be 180 degrees out of phase with the other. This will cause a canceling effect and the resultant sound will be dead. You can check the polarity of an unmarked speaker by connecting a "D" cell battery to the speaker. If the cone moves outward than you have the positive of the battery to the positive of the speaker but, if the cone moves inward than you have the positive of the battery to the negative of the speaker.

Now we will calculate the voltage of the amplifier and the current flow in the speakers. The name plate rating of your amp says 400 watts at 4 ohms. This is a constant and can not change so we can find the voltage of the system by calculating: Voltage = SQR of watts x ohms. So 400 watts x 4 ohms = 1600 take the square root = 40. This is the voltage of your system. Also we can take 400 watts / 4 ohms = 100 take the square root = 10. This is the amperes of your system. Watts is equal to volts times amperes so 40 x 10 = 400 watts true but, if one speaker winding is blown then the amplifier sees 8 ohms of load and only 200 watts of power is produced.

Series Example
What if you have two 100 watt 4 ohm speakers in series?

Watts in series add just like in parallel so 100 + 100 = 200 watts handling capacity.

Ohms in series just add. That is r1 + r2 = rt. So 4 ohms + 4 ohms = 8 ohms.

Series Parallel Combination
What if you have four 8 ohm speakers. Two hooked in parallel and the other two hooked in parallel but the two parallel branches are hooked in series? Calculate the parallel circuits first, then the series circuit.

Each parallel branch is 4 ohms but, the two 4 ohm branches hooked in series brings the total back to 8 ohms.

Here are the basic formulas:

WATTS is POWER represented by P
P = E x I
P = R x I squared
P = E squared / R

E = R x I
E = P / I
E = square root of P x R

OHMS is RESISTANCE represented by R
R = E / I
R = E squared / P
R = P / I squared

AMPERES is INDUCTANCE represented by I
I = E / R
I = P / E
I = square root of P / R

WATTS in series and parallel add.
P1 + P2 + P3 etc.

OHMS in series add but, ohms in parallel are calculated by:
1/r1  +  1/r2  +  1/r3  +  1/etc...

Selecting Wire Size for your Speaker Cabinets
Even though the voltage to the finals of your amp fluctuates slightly under load we can say that the voltage of the system is constant. With that in mind we can calculate the amperage flowing in the speaker wires. I = square root of P / R . When we apply this formula to the name plate ratings used at the beginning we get 2.5 amperes , 5 amperes , 10 amperes and, 20 amperes. I would select the largest size necessary for the maximum ampacity. Use this chart.

Wire SizeAmpacity
18 AWG14 amps
14 AWG18 amps
16 AWG25 amps
12 AWG30 amps
10 AWG40 amps

How to switch between 4 ohms parallel and 16 ohms series using a double pole double throw switch. When you buy a double pole double throw switch make sure it is a break before make or a center off type not a make before break. If you can only find a make before break then make sure your amplifier is off before changing the switch.

When the switch is in the down position indicated by yellow jumpers you have two 8 ohm speakers in parallel which gives you 4 ohms. When the switch is in the up position indicated by green jumpers you have two 8 ohm speakers in series which gives you 16 ohms. One more thing. Even though some of you may know that a speaker winding is a coil and the true resistance of it should be measured in reluctance, it is practical to consider only the resistive value in ohms and if you use a ohmmeter to read the resistance of the winding it will always be close.

Now you should be able to understand and calculate your amp and speaker system to see if they are set up for maximum performance. Hopefully these examples and formulas were simple enough to understand and not confuse you further.

Doug Matthews is an electrician by day and a bass player by night.
Add a Comment
Inactive Member wrote:
Sep 26 2012
I have a pair of oldie but goodie Cerwin Vega 250se's and I am not sure what the specs are. I took all speakers off of one box and I am planning on building a homemade boombox that will hook up to pc/ ipod/ mp3/ etc..using a computer psu and a car amplifier. Not sure what amount of watts on the computer psu and car amplifier I should be using. Also will you let me know what size box I should build. I was planning on building it smaller then the original box. Im planning on using 3/4" MDF wood....Please Help! THANK YOU! I hope this all makes sense! :)
Paul Mariconda (40) wrote:
Apr 25 2013
Doug, thanks for the explanation. I still get a bit confused. I'm hoping you can explain further. I just built a bass cab using an 8 ohm 300 watt speaker. I want to build a 2x10 bass cabinet to use the two cabs together or separately. Do i use 2 10inch 8 ohm speakers or 2- 10 inch 4ohm speakers? How to I figure out what amp head to use. An amp that says 8 ohms or 4 ohs. I apologize in advance for my confusion. Thanks Paul
Nico Guijt (5) wrote:
Jul 1 2013
Thanks for this! I have a question though, slightly related. Recently i played a traynor YBA-1 ( i believe 90 watts at 8 ohms through a 4x12 (Plessey G12P, 30 watts 8 ohm paralell) and it sounded not like i expected. It was not loud at all, and the speakers seemed to struggle. I thought it was because of the low wattage of the speakers, because i played this head in the shop through a different speaker and sounded a lot crisper and louder. I played through the cab on a bassman 100 and sounded great. What could this be? Is it true that this configuration of amp and speakers is not too loud anyway? Cheers
Jayasankar Jj wrote:
Dec 25 2016
As you have mentioned, if the speaker is connected in parallel/ series the watts add up. My question is , My speaker RMS Watts is 325 @ 4 ohm. so if two speakers of same RMS watts is connected in parallel/series(in this situation i am assuming parallel connection) the watts add up ie 325+325= 650 watts RMS . And the amplifier output is 750 watts RMS @2 Ohm . So 350 watts will be transferred to each speaker. which is 25 watts greater than its max RMS . So , how can this be solved.
Jayasankar Jj wrote:
Dec 25 2016
My another question is , the low input which we are giving to the amp via RCA audio cable, Will the bass output has any impact if the speaker numbers increases even though the amplifier can meet those requirement. Because my concern is the audio out i am taking is from my home theater via audio cable. so if the number of speakers increases and amp can meet the requirement. will the incoming signal to amp from my head unit will reduce the overall effect.