# Lowering speaker resistance?

Discussion in 'The Projects Forum' started by GRNDPNDR, Jun 17, 2012.

1. ### GRNDPNDR Thread Starter Member

Mar 1, 2012
449
7
Another quick question.

I have 2 12" speakers that measure 7.5 ohms each.

If I run them in parallel I would get 3.75 ohms correct?

So would I be able to lower that even further by adding a resistor of some kind in parallel tricking an amplifier into "seeing" another speaker?

I'm guessing if it were possible I would probably need some ridiculously sized, possibly non-existant resistor due to the power output being 600W @ 2 ohms.

2. ### #12 Expert

Nov 30, 2010
16,704
7,354
Resistors in parallel have less resistance than any one of the resistors.

1/R1 + 1/R2 + 1/R3 = 1/ Rtotal

3. ### MrChips Moderator

Oct 2, 2009
12,646
3,458
What would you achieve by lowering the load seen by the amplifier?
You will just be wasting power in the extra resistor.

4. ### wmodavis Well-Known Member

Oct 23, 2010
737
150
And are you talking just speaker RESISTANCE or do you care about the IMPEDANCE over the appropriate frequency range? DC resistance is only part of the equation. What actually are you trying to achieve?

5. ### #12 Expert

Nov 30, 2010
16,704
7,354
I think he's trying to test an amplifier to see if it meets specs.

6. ### wmodavis Well-Known Member

Oct 23, 2010
737
150
Well it would certainly help if people were clear and descriptive in their posts. Other wise answers are based on guessing by reading between the lines and may be way off base from what the OP was wanting to know. Since most people only have the ability and/or knowhow to measure a speakers DC resistance and he said resistance not impedance I certainly made some of those assumptions. Poor communicators beg for no or SA answers!

7. ### #12 Expert

Nov 30, 2010
16,704
7,354
True. Some days it seems like half of this job is imagining what the question really is.

8. ### GRNDPNDR Thread Starter Member

Mar 1, 2012
449
7
It was a quick random thought about whether or not you could make the speakers "louder" by getting more power out of the amp by running it closer to 2 ohms without buying 2x4ohm speakers.

I'm not actually doing anything, it was literally just a thought that I became curious about. I'm sure there is a reason it hasn't been done otherwise I'm sure car audio experts the world over would already be doing it.

can someone refresh my memory as to the difference between resistance and impedance? I know impedance takes into account more than just the resistance of a single component, but the actual definition eludes me at the moment.

9. ### MrChips Moderator

Oct 2, 2009
12,646
3,458
As I said in post #3 you cannot make the loudspeaker louder by adding resistance anywhere in the circuit. You will only be wasting more power in the resistor.

Resistance refers to the impedance to a DC current.
A resistor has pure resistance and impedes both DC and AC currents in the same manner.

A circuit with C or L has reactance.
A circuit with R and any combination with C and L will have both resistance and reactance. The combined effect of resistance and reactance is called impedance.

This allows us to determine how the circuit will impede both DC and AC currents.

10. ### wayneh Expert

Sep 9, 2010
12,394
3,246
More power out of the amp doesn't mean more sound power. For that, you need power driving the cones. Power taken by the coil doesn't perfectly translate to cone movement since some power is converted to heat. The coil's response to incoming power is related to its impedance, and there's little you can do about that from the outside. Matching the amp's impedance to the speaker will maximize power transfer.

It's like riding a bicycle in the wrong gear - you may be putting out huge power but it's not going to moving the bicycle. Shift gears (or match impedances) and things improve.

Oct 23, 2010
737
150

Sep 9, 2010
12,394
3,246

13. ### wmodavis Well-Known Member

Oct 23, 2010
737
150
And for decent fidelity you would not drive the amplifier anywhere near the max PO because THD rises exponentially as PO increases.

Some don't care about fidelity - only LOUD!

14. ### Audioguru New Member

Dec 20, 2007
9,411
896
No.
The output of modern amplifiers NEVER match impedances like old antique vacuum tube amplifiers did.
The output of a modern amplifier has an extremely low output impedance so that it damps speaker rersonances. The amplifier has a minimum rated load impedance. If the load impedance is too low then the amplifier blows up or limits the output voltage.

15. ### Audioguru New Member

Dec 20, 2007
9,411
896
No.
Max PO is simply double the real output power so the amplifier will be clipping like mad.
The output rating of a half-decent amplifier is when it begins to clip so its distortion is extremely low.
Only crazy and deaf people drive an amplifier hard into clipping (good-bye tweeters).

16. ### wmodavis Well-Known Member

Oct 23, 2010
737
150
When clipping is just beginning the THD is already increasing at a very rapid rate. If you measure and plot THD vs PO you will see distortion rise in an exponential fashion well before clipping is visible or audible. At clipping it is very high.
An example of PO vs THD can be found here on page 10 fig 6.

17. ### Audioguru New Member

Dec 20, 2007
9,411
896
No.
The distortion is just beginning to increase at clipping.
This is a puny-power amplifier anyway.

File size:
33.5 KB
Views:
17
18. ### wayneh Expert

Sep 9, 2010
12,394
3,246
Yes, I've been corrected twice on that now. So much for writing without thinking. Anyway, got it!