Quick question about matching impedance.

Gavin · Post by **Gavin** » Tue Jun 29, 2010 7:45 pm

Ok so the back of my cab says 8Ω and the back of my Amp says 4Ω. So, because the cab is higher than the amp, does this mean it's ok to use, or is it the other way round?

BACK OF THE CAB

BACK OF THE AMP

Haze · Post by **Haze** » Tue Jun 29, 2010 7:53 pm

Higher can feed lower but it won't be as efficient
If lower tries to feed higher it strains the output transformer, not a cheap replacement part
in short, dont

finding a 4ohm cab is a bitch

serfx · Post by **serfx** » Tue Jun 29, 2010 7:57 pm

i have no idea what haze just said
seriously i wish i did
because i have the exact opposite issue (8 ohm head and 4 ohm cab)

Ankhanu · Post by **Ankhanu** » Tue Jun 29, 2010 8:00 pm

serfx wrote:i have no idea what haze just said
seriously i wish i did
because i have the exact opposite issue (8 ohm head and 4 ohm cab)

\

You'd burn out our amp... he'd have reduced apparent volume.

Viljami · Post by **Viljami** » Tue Jun 29, 2010 8:03 pm

I understood it so that 8ohm amp into a lower cab is OK and the not the other way around.
edit: Or maybe not.

Indeed as I have found out, there really isn't that many options in 4 ohm cabs. Not in the budget range, at least.

What kind of an cab do you have there, Gavin?

serfx · Post by **serfx** » Tue Jun 29, 2010 8:05 pm

Ankhanu wrote:
serfx wrote:i have no idea what haze just said
seriously i wish i did
because i have the exact opposite issue (8 ohm head and 4 ohm cab)
\

You'd burn out our amp... he'd have reduced apparent volume.

thats probably why it was burned out in 3 months (that and being a floor model when i bought it..) either way warranty took care of that for me..

Gavin · Post by **Gavin** » Tue Jun 29, 2010 8:10 pm

[youtube][/youtube]

Bill Oakley · Post by **Bill Oakley** » Tue Jun 29, 2010 8:10 pm

serfx wrote:i have no idea what haze just said
seriously i wish i did
because i have the exact opposite issue (8 ohm head and 4 ohm cab)

Haze is correct. You want the amp to be lower ohms than the cabinet. Well, you ideally want it to be the same ohms.

Think of it as pushing spitwads through a straw and the straw is the ohms of the cabinet and the spitwads the ohms of the amp. In the first case we have a straw with the diameter of 8 (we'll use mm just for reference) and spitwads with the diameter of 4mm. They will go through just fine but there is air going around them and it won't be very efficient. Therefore you will get less output.

In Serfx's case, we have a 4mm straw with 8mm spitwads. They won't work. The spitwads are too big and just going to sit in the amp. Boom! to your transformer.

damienblair17 · Post by **damienblair17** » Tue Jun 29, 2010 8:13 pm

The short of it: If your amp HEAD has a SMALLER number than your CAB, you're fine. A 4 ohm head can power a 4, 8, or 16 ohm cab.

The long of it:

ohms is the amount of resistance. 8 ohms is more resistance than 4, so you're 4 ohm head is being resisted more with your 8 ohm cab. Less power will be sent from the head to the cab, but that just means your head isn't working as hard.

If your cab is 4 ohms and your head is 8 ohms, that means your head is working harder to power the cab, and could likely break or blow up or something. Not recommended.

Anything I missed or got wrong feel free to correct.

EDIT: I type too slow. Bill got there first.

Ankhanu · Post by **Ankhanu** » Tue Jun 29, 2010 8:14 pm

colabonham wrote:I understood it so that 8ohm amp into a lower cab is OK and the not the other way around.
edit: Or maybe not.

Indeed as I have found out, there really isn't that many options in 4 ohm cabs. Not in the budget range, at least.

What kind of an cab do you have there, Gavin?

Yeah 4Ohm cabs are a bitch to get. My bass amp can run 2, 4 or 8 Ohm cabs (500W, 350W and 200W respectively), and I want to run it at 4Ohm... been a bitch finding a cab for it that meet my other criteria too

Gavin · Post by **Gavin** » Tue Jun 29, 2010 8:14 pm

Ok, so I am fine then? Just with lower apparent volume? Which I don't think will be a problem, since it's more for frequency range than volume.

Ankhanu · Post by **Ankhanu** » Tue Jun 29, 2010 8:17 pm

Send 'er my way!

Bill Oakley · Post by **Bill Oakley** » Tue Jun 29, 2010 8:18 pm

Gavin, you're ok. You've got a 4 ohm amp going into an 8 ohm cabinet. It will work fine.

You can also open it up and see what ohm speakers are in there and possibly rewire it to be 8 ohms if you have the right ohm speakers in there.

Gavin · Post by **Gavin** » Tue Jun 29, 2010 8:38 pm

Ok, cool. Thanks.

Bacchus · Post by **Bacchus** » Tue Jun 29, 2010 8:40 pm

There are people in this thread who have a lot more experience and know how with amps and electronics than me (it's some manner of black magic, is electricity, I've worked out that much), but:

I managed to blow up a Fender Champion 600 by using it with an 8 ohm Marshall cab. I was using a saltbooster in front of the amp. I smelt burning and it stopped working.

I got it replaced and won't use that cab with it again.

Haze · Post by **Haze** » Tue Jun 29, 2010 8:46 pm

I'll bet it sounded good right before it blew up

You can always switch some wires around and convert it to 4 ohms if the speakers allow it

Bill Oakley · Post by **Bill Oakley** » Tue Jun 29, 2010 8:49 pm

BacchusPaul wrote:There are people in this thread who have a lot more experience and know how with amps and electronics than me (it's some manner of black magic, is electricity, I've worked out that much), but:

I managed to blow up a Fender Champion 600 by using it with an 8 ohm Marshall cab. I was using a saltbooster in front of the amp. I smelt burning and it stopped working.

I got it replaced and won't use that cab with it again.

In theory the 4 ohm load of the 600 should be fine with the 8 ohm cabinet but mother nature always has the last word!

Justin J · Post by **Justin J** » Tue Jun 29, 2010 9:01 pm

the back of the amp (and the manual) say minimum of 4 ohms. solid state is much more forgiving than tubes in impedance matching situations. i don't even know where you'd find a cab with less than 4 ohms.

stewart · Post by **stewart** » Tue Jun 29, 2010 9:06 pm

strange that an ashdown extension cab wouldn't be designed specifically to work with their bass amps... you'd think they'd at least have a switchable output on the amp if they didn't want to make their cabs 4 ohm.

Post by **Doog** » Tue Jun 29, 2010 9:16 pm

stewart wrote:strange that an ashdown extension cab wouldn't be designed specifically to work with their bass amps... you'd think they'd at least have a switchable output on the amp if they didn't want to make their cabs 4 ohm.

It's very common in amps without an impedance switch or different impedance outputs; that way you can use one or two of their 8 ohm cabinets (resulting in a 4 ohm total load when connected in parallel like that) without any real problems aside from maybe a slight loss of efficiency.

I guess it's just cheaper not having to tap the transformer for different loads.