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Old 9th April 2006, 11:15 PM   #1
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Default impedance mismatch headaches.

i donated a pair of 3 way speakers to my friend, and he agreed to buy a reciever to power them.

he ended up buying some black akai 60wpc integrated amplifier from the 80s for 25$(not a bad deal considering its condition). heres the deal though the amplifier says that it doesnt like anything more than 8 ohms on speakers a or b, or 16(!) ohms on speakers a and b. This is a problem since the dc resistance of the speakers im donating to him is 4.6ohm. I would normally just tentatively turn up the volume and see if the breaker goes, and if it does then keep it below there. BUT this amplifier has no breaker than i can see.

my idea was to hook up a 12 or 16ohm resistor in paralell with the speaker terminals, and then hook it up to the amp, but that would cut his power going into the speaker to 30w.....which should be plenty, but for a guitar player with a taste for classic rock and loud music, 3db of headroom might actually go to use.

so i got a couple questions...

- a 12 ohm resistor in paralell will bring it up to 8.6 ohms yes?
- is there anything else i can do to get that extra smidgen of power? to my knowledge no...but it cant hurt to ask.
-do you think the reciever could take a 4.6 ohm load? i know its below what it says it can do, but from my experience them 80s recievers are damn near indestructable.
-could i maybe put some pec fans flowing air over the heatsinks to make it more 4 ohm friendly?


so yeah....gimme your input.
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Old 9th April 2006, 11:54 PM   #2
dnsey is offline dnsey  United Kingdom
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Firstly, any additional resistance needs to be in series, not parallel.

Have you checked the speakers' actual impedence? With a DCR of 4.6 Ohm, I'd expect it to be somewhere around 8 Ohm anyway.
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Old 10th April 2006, 01:52 AM   #3
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i havn't run any tests on them at all...theyre a bit "thrown together"
if you will

so you think its a safe bet that it wont melt the amp? i found it weird that there werent any breakers or fuses (externally at least, havn't chekced inside)

so then i could add a 4 ohm resistor in series if the amp has trouble?
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Old 10th April 2006, 02:06 AM   #4
poobah is offline poobah  United States
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You can put the 4 Ohm in series but you will reduce your power and make heat... bad idea. Hook it up straight, and don't get nuts with the volume, and forget about it.

Dnsey is totally right... if you are measuring 4.6 Ohms than your dynamic impedance is probably close to 8 Ohms anyway...

Also, your ears should tell you if the amp is in pain... unless your listening to Metallica or something.



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Old 10th April 2006, 02:39 AM   #5
lndm is offline lndm  Australia
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Quote:
Originally posted by xstephanx

so you think its a safe bet that it wont melt the amp?
probably

Quote:
so then i could add a 4 ohm resistor in series if the amp has trouble?
If these are 6 ohm speakers instead of 8, maybe you won't need 4 ohms. Frankly though, putting a resistor in series with a speaker is not the most desirable thing to do for a numerous reasons. In your case it would be purely out of fear of damage.

I have seen 8 ohm amps blow with car speakers. In these cases the DCR of the speakers was 3.6 ohms, and the amps were operated by teenagers who's aim was to damage the house foundations with their bass control.
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Old 10th April 2006, 03:14 AM   #6
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i resent the metallica statement!

if anything youd know more eaisly if the amp is under stress with metal than with say jazz. Double bass drum isnt the easiest thing and lets yo know if youre stressing your amp or spakers VERY quickly, metal is also constantly demanding unlike say orchestral pieces whcih have massive dynamic range whcih gves the amp a "break" i guess.

and the thing is, its operated by my friend who is, for lack of better terms, a hard ******* rocker(if dave tenney reads this....sorry for stealing your term). it will be in an apartment, but his tenants dont care, and knowing him will probably played fairly loud every once and a while.

maybe add a breaker into the signal path jsut to be safe? woudl that be a better idea than a resistor? i know breakers arent perfect but are usually good enough to prevent exessive damage.

sorry if my typing is iffy...this laptop has a druved keyboard

also thanks for the input so far guys!
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Old 10th April 2006, 03:47 AM   #7
lndm is offline lndm  Australia
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I once had a Druved keyboard after I accidentally spilled it on the floor.

Have you looked at polyswitches?
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Old 10th April 2006, 04:06 AM   #8
clem_o is offline clem_o  Philippines
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"and the thing is, its operated by my friend who is, for lack of better terms, a hard ******* rocker("


Well then, either the amp blows or it doesn't. It really doesn't make sense to add a resistor. 2 ohms would probably bring up the impedance enough to save the amp from blowing, but that would mean 20% would be going into the resistor doing nothing; not to mention you'd have to find resistors that could handle the power... and the loss in damping factor may make a sonic difference...
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Old 10th April 2006, 04:12 AM   #9
jp88 is offline jp88  United States
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have you thought about an impedance matching transformer or speaker volume control?
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Old 10th April 2006, 10:58 AM   #10
sreten is offline sreten  United Kingdom
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Hi,

any series resistance is a very bad idea, it will completely destroy bass damping, and
interacting with the impedance of the speaker affect the frequency response severely.

As long as the speaker does not have a punishing impedance charateristic,
which can happen with amateur designs, the amplifier will be fine driving it.

/sreten.
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