Impedance matching - not as noob as it sounds.

[Meelo]

I've taken to building a cab for my amp - just finished some physics studies and figured there's no better way to utilize it than to do some Thiele-Small and standing wave calculations.

That said, i'm having a bit of a problem with impedance matching -
i've scoured google, but seeing as this isn't too standard i'd rather double-check than find out the hard way;
It's an 8 ohm amp, and i want to connect it with three 8 ohm speakers.
Now, two of these actually test out @ 7 ohm each, and the third one, who came stock with the amp, tests out @ 6 ohms - meaning the closest i can make it is a 9.5 ohm impedance.

What i've read through forums suggests you can connect a higher impedance cab to a lower impedance amp, but not vice versa. I also know that the average impedance range for amps nowadays is ~3 ohms.
Thing is, if a 6 ohm speaker works with my 8 ohm amp, wouldn't that be risky?
and if it isn't, did i get something wrong reading through the forums?

Last, and most importantly - what are my chances of blowing something up connecting a 9.5 ohm set up to a 8 ohm amp?

I'd appreciate any help concerning. Thanks in advance.

[TD5_23]

Originally Posted by Meelo

That said, i'm having a bit of a problem with impedance matching -
i've scoured google, but seeing as this isn't too standard i'd rather double-check than find out the hard way;
It's an 8 ohm amp, and i want to connect it with three 8 ohm speakers.
Now, two of these actually test out @ 7 ohm each, and the third one, who came stock with the amp, tests out @ 6 ohms - meaning the closest i can make it is a 9.5 ohm impedance.

Out of curiosity, how were you planning on setting these up? Some sort of paralell-series combination? I'm to lazy to do the math on how you reached 9.5.

Originally Posted by Meelo

What i've read through forums suggests you can connect a higher impedance cab to a lower impedance amp, but not vice versa. I also know that the average impedance range for amps nowadays is ~3 ohms.
Thing is, if a 6 ohm speaker works with my 8 ohm amp, wouldn't that be risky?
and if it isn't, did i get something wrong reading through the forums?

Correct, you can always connect a higher impedance cab to a lower impedance amp. The main reason why this isn't done very often is because the output on the amp is the most efficient when you get your impedance as low as possible. The higher the impedance, the quieter the amp becomes.

Originally Posted by Meelo

Last, and most importantly - what are my chances of blowing something up connecting a 9.5 ohm set up to a 8 ohm amp?

Your chances of blowing up your amp or your speakers are less on a 9.5 ohm speaker set than they would be on an 8 ohm set. The higher the impedance, the safer you are. I ran an 8 ohm speaker on an 4 ohm-capable amp for a while. The only drawback is that I got less wattage out of the amp, and it was therefore quieter than it could have been. Since it was a bedroom amp this wasn't a big deal to me. You're only 1.5 ohms over your minimum, you'll barely notice a difference.

[garww]

if you are freewheeling down Pikes Peak in a loaded Semi, impedance(resistance) is your brakes. A static resistance measurement does not indicate what load it presents when ****ed-up and working.

I always tended to just think in terms of low - 4 ohm, medium - 8 ohm, and high 16 ohm. There are variations, but just present a load that's in the rated ballpark. You can put you meter across one disconnected plug lead to see what it reads in circuit with the amp on and working with signal.

[garww]

Oh, and your cabinet will influence resistance in a small way.

[Meelo]

Thanks a lot to both of you. I suspected as much, but i had to be 100% sure before i blow anything to pieces .

As for the math;
---7--\___6_
---7--/

6+[1/(1/7+1/7)]=9.5 ohms.

This is great news. Hopefully i'll have some cab building tips, as well as a brand new physically-deranged cabinet in a couple of weeks.

and garww; I'm aware of the impedance changing as i'm playing, especially considering there's a tube inside. I just don't know how the amp will react.
Putting the amp out on the market with a 6 ohm speaker sounds risky, at best - hence my doubts.
I like your downhill-freewheeling analogy, nonetheless. �Gives εR a whole new meaning by the time the ambulance picks you up.
(I suppose ε-Ir=medical bill?)

Again, thanks to both of you for your help!

[garww]

I think 6 ohm is quite normal. Makes the MFG's amp sound louder.

There are usually spec sheets available for drivers that can help you load your cabinet(ports).

[garww]

Here's a old school cab setup;
John Cipollina Rock n Roll Hall of Fame

[whomper]

Originally Posted by Meelo

I've taken to building a cab for my amp - just finished some physics studies and figured there's no better way to utilize it than to do some Thiele-Small and standing wave calculations.

That said, i'm having a bit of a problem with impedance matching -
i've scoured google, but seeing as this isn't too standard i'd rather double-check than find out the hard way;
It's an 8 ohm amp, and i want to connect it with three 8 ohm speakers.
Now, two of these actually test out @ 7 ohm each, and the third one, who came stock with the amp, tests out @ 6 ohms - meaning the closest i can make it is a 9.5 ohm impedance.

What i've read through forums suggests you can connect a higher impedance cab to a lower impedance amp, but not vice versa. I also know that the average impedance range for amps nowadays is ~3 ohms.
Thing is, if a 6 ohm speaker works with my 8 ohm amp, wouldn't that be risky?
and if it isn't, did i get something wrong reading through the forums?

Last, and most importantly - what are my chances of blowing something up connecting a 9.5 ohm set up to a 8 ohm amp?

I'd appreciate any help concerning. Thanks in advance.

close enough for govt work

and dont confuse the (scalar) dc resistance with the ac impedance (which is a vector function)

just keep the volume control down and all will be fine no matter what the impedances

what you dont want is a too low impedance into a high power amp when you like twisting the knobs up to 11 (on a scale fo 10)

personally i would put one speaker on each channel (left /right) and forget the third one