Wiring Four 8-ohm Speakers for 3.2 ohms

[cjlandry]

I have a Fender Vibro Champ, which only has a 4 ohm speaker output, and I just got a deal on a 4x12 cabinet with four 8-ohm speakers.

 

The output transformer's secondary is actually rated at 3.2 ohms.

 

So two 8-ohm speakers in series gives me 16 ohms, and two 8-ohm speakers paralleled gives me 4 ohms. Paralleling the 16-ohm pair with the 4-ohm pair comes to 3.2 ohms.

 

 

I understand that it's best to match the output as closely as possible, so it seems 3.2 ohms would be most efficient. However, due to tolerances, I wonder if I should just run two speakers in parallel for a 4-ohm impedance, just for the added safety of a little "head-room", or if it's safe to drive it all the way down to the 3.2 ohms rating of the output transformer.

 

Any thoughts on this?

[badbluesplayer]

Yup - you got it. You'll need to go with two of the eight ohm speakers wired in parallel. There's no way to wire (4) eight ohmers to get 4 ohms. If you wire it like you're proposing, you'll end up with uneven loads on the speakers. The resistance of the two speakers wired together will probably be more like 3.5 to 4 ohms if you test them.

 

By the way, I have a Vibrochamp XD that I run thru (2) eight ohm 12" cabs in parallel and it sounds great. Two twelves is plenty.

[cjlandry]

Yup - you got it. You'll need to go with two of the eight ohm speakers wired in parallel. There's no way to wire (4) eight ohmers to get 4 ohms. If you wire it like you're proposing, you'll end up with uneven loads on the speakers. The resistance of the two speakers wired together will probably be more like 3.5 to 4 ohms if you test them.

 

By the way, I have a Vibrochamp XD that I run thru (2) eight ohm 12" cabs in parallel and it sounds great. Two twelves is plenty.

 

Thanks for the reply.

 

I'll likely split the cabinet, then, since my G-DEC 30 has an 8-ohm speaker out. I can wire it to a single 12" until I get a pair of 16's for the 2x12 option for that amp.

 

I don't need 4 12" speakers, by any means, but I was about to pay $109 for an empty 2x12 cabinet, then buy my own speakers, when I ran across a scratch and dent 4x12 on eBay. I got it for $152.49, including shipping costs.

 

Seismic Audio. Their speakers don't seem great, but I was planning to buy speakers anyway. This way, I can at least give them a chance before I shell out more cash on Eminence speakers. They're nearly certain to at least give me more sound than the "8-inch Special Design Speaker".

 

How do you like the hybrid amp? I've played nothing but solid-state until now, and I won't get to try out the VC XD, or the new speakers, until I return home in two weeks.

[cjlandry]

I forgot to mention that there's another option, which is a Hammond 1760c output transformer. It has coil taps for 4, 8, and 16 ohm outputs. The 1750c is the direct replacement, with only a single 3.2 ohm secondary tap.

 

I don't think it's gonna be necessary, though. I'll just run the pair from that amp.

[badbluesplayer]

I love the hybrid amp. It's very "tubey".

[NeoConMan]

If you're trying to decide between one wiring set up over another, consider this;

When you're gonna err one way or another, a higher ohm load is the way to go.

 

You're looking for, say, 3.2 ohms? I would say forget the decimal point after you finish the math.

Too many variables to be that accurate.

Input voltage varies a bit, transformer isn't gonna be exact, etc.

By the time you get through the amp circuitry, your math might not match reality.

 

If you wind up closer to 2 ohms, the current will rise dramatically - more heat and possible component damage.

If your load is 4 ohms or more, the only real difference is less output power due to the load.

No harm done.

 

The difference in volume would probably not be noticed, and the tone isn't gonna change much.

I don't mind ohms a bit on the high side for a cushion in reliability/durability for the amp.

 

Of course, the alternative I would consider may be too far that direction.

Two 8-ohm speakers paralleled yields 4 ohms.

Do that twice, then run one pair in series after the other.

4 ohms + 4 ohms in series = 8-ohm load.

 

Too much maybe?

I dunno.

 

I'll ask my amp guy next week.