4 or 8 ohms???

Dag Wolf · Post by **Dag Wolf** » 9 Mar 2001 4:46 am

A friend are switching speakers from 15 inch 4ohms BW to 12 inch BW cabinets. Should he still stays at 4 ohms?

Thanks, Dag

Jack Stoner · Post by **Jack Stoner** » 9 Mar 2001 6:57 am

If the 4 ohm speakers worked OK and that is what the amplifier wants to see then the 12" speakers should be 4 ohms too.

I assume it is a 1 for 1 swap. Replacing a 15" with a 12".

Dag Wolf · Post by **Dag Wolf** » 9 Mar 2001 8:17 am

Thanks Jack, Yes it`s a Mosvalve 500 amp. Anyone knows the the number it should say, BW 1203-4 or something?

Jack Stoner · Post by **Jack Stoner** » 9 Mar 2001 11:27 am

I have a MosValve 500 power amp. The amp wants to see an 8 ohm load. On the back of the amp there is a statement that use 8 ohm load for best performance.

I've talked to the MosValve factory tech and 4 ohms is OK but the 4 ohms is MINIMUM recommended load.

I have Peavey 1203-8 BW speakers that I'm using with my MosValve 500. The 1203 is the same speaker that Peavey used in the Nashville 112E enclosure, except they were 4 ohms.

Bobby Lee · Post by **Bobby Lee** » 9 Mar 2001 11:51 am

I'd be nervous about using an amp that had a 4 ohm minimum. My EV speakers are "8 ohm", but I've heard that at some frequencies they are considerably less - sometimes as low as 6.5 ohms. If you wire two in parallel to get 4 ohms, you could get as little as 3.25 ohms impedence on some transients.

My Webb has a 2 ohm minimum load. Mesa/Boogie amps are designed to handle 3 ohms comfortably on the 4 ohm transformer tap. Since I sometimes run two speakers per channel, I'll steer clear of the MosValve. Thanks for the warning.

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C Dixon · Post by **C Dixon** » 9 Mar 2001 1:08 pm

b0b,

The impedance of ANY speaker lowers as the frequency decreases. And rises as the frequency increases.

When a manufacturer rates a speaker, or amp, etc. it is understood it is at 1khz. The standard reference frequency for all audio related gear.

For tube amps, it is important that the total speaker's impedance (at 1KHZ) not go below the amp's output impedance. Failure to observe this could burn up components in the power section of the amp, IF, the amp were driven to its maximum power. Which in some cases is rarely if ever done.

Solid state amps have another problem. There is NO fixed ouput impedance. But to hold the power transistor's current to a predetemined amount, most manufaturer's say don't go below 2 ohms total speaker impedance. But again, the 2 ohms assumes 1Khz. At 100HZ, it will be less than 1 ohm. At 10Hz, approaching Zero ohms.

carl

Al Marcus · Post by **Al Marcus** » 9 Mar 2001 5:18 pm

Now that we are talking ohms. You experts tell me which is best when you can use either on your amp.? Which is more powerful, or gets the most out of the amp without damgage, etc, etc......al

Bill Crook · Post by **Bill Crook** » 9 Mar 2001 11:31 pm

I would say :

4 ohms.......

Like others have stated, below that you run the risk of damage to the amp, wheather it be tube or solid-state.

As you go up in the impedience, (of the speakers) the tone begins to get a bit mellow, of course, you also split the output approx in half. This isn't really so noticeable unless your playing at a very loud volumne, in which case, it begins to get distorted a wee bit.

Most Manufactors suggest : XXX power into a 4 ohm speaker(s).

Again, like others have stated, I think 4 ohms would be the mimunum choice.[This message was edited by Bill Crook on 09 March 2001 at 11:34 PM.]

Jack Stoner · Post by **Jack Stoner** » 10 Mar 2001 5:17 am

Optimum performance is when the speaker impedence matches the desired amplifier load impedence. At the optimum match you are getting the maximum power transfer and maximum frequency response.

How much power or frequency is lost in a mismatch can vary depending on the speaker and the amp. Some amps can tolerate a wider impedence load than others. And as Carl stated, Impedence is an AC measurement and only referenced to a particular frequency. The speaker impedence and the amplifier impedence will vary at any given frequency and the speaker can have a different impedence reaction to frequency than the amplifier.

Lyle Bradford · Post by **Lyle Bradford** » 10 Mar 2001 8:14 am

Jack are you saying that an 8 ohm speaker will work in a 4 ohm amp if it is never run at full capicity. I know this is not a wise thing to do but am just trying to understand some of this.

Jack Stoner · Post by **Jack Stoner** » 10 Mar 2001 3:06 pm

Lyle, no. There are many variables as to what will work safely with any particular speaker.

Dag Wolf · Post by **Dag Wolf** » 14 Mar 2001 2:47 am

Jack, on the back of my Mosvalve 500 it says 4ohms minimum load so does the manual. Are you running 8 ohms load to be on the safe side? I figured running at minimum load would make it all sound optimal.

Thanks, Dag.

Jack Stoner · Post by **Jack Stoner** » 14 Mar 2001 3:03 am

Dag, mine says use 8 ohm load for best performance.

The minimum 4 ohms is not printed on the back of my amp, but as I said before that is what the Genz-Benz (MosValve) tech told me.

My MosValve 500 may be older (or newer) than yours. The one I have belonged to Joan Herndon Cox and I understand she bought it used.

Dag Wolf · Post by **Dag Wolf** » 15 Mar 2001 6:00 am

Jack, thanks for all the help. Anyone know why they changed the ohms on these amps?

This is a great forum!!

Dag