switchable impedance?

[stevie]

I don't think it's one of his brightest ideas, BigRed, although to be fair, he is one of the few people around who does bring new thinking to bass cab design. He's just responding to demand and I'm sure he's engineered it sensibly.

 

 

[Soledad]

1 hour ago, Dan Dare said:

would there be any point in using a dual voice coil driver

I don't think it's very good physics - as I recall, the idea is get the flux gap down as low as poss, hence flux density as high as poss, plus better heat dissipation from voice coil. Then pack the coil as close as poss (max voicecoil density). So tapping the coil is not a great idea from performance p.o.v. surely. I remember the U.S. built JBLs featuring flat ribbon wire in order to max the coil packing into the flux gap - neat indeed. And a good mate who had a Fender Twin with the optional JBLs fitted - his Strat through that... cut you in half it would ')

[Jack]

5 hours ago, Bill Fitzmaurice said:

OK, once more, but this time with feeling:

Use a dual voice coil driver.  One coil 8 ohms, two for four ohms. I don't know if that's what Hartke does, but it's one way to do it.

Yes I know that's one way to do it, I saw your first post. I asked how hartke do it. If you don't know that's fine.

[stevie]

Jack, the HyDrive HD112 manual says "The HyDrive HD112 has an innovative dual voice coil design that allows the cabinet to operate at either 4 or 8 ohms impedance." So it looks like dual-coil drivers.

 

[Soledad]

8 hours ago, stevie said:

The HyDrive HD112 has an innovative dual voice coil design

Well spotted - didn't see that.

[Dan Dare]

On ‎21‎/‎02‎/‎2019 at 20:10, Bill Fitzmaurice said:

Not that I can think of. The original reason some sub drivers used dual coils was to sum the L/R channel low frequency content, back in the days before receivers had LFE outputs and subs went from being mainly passive to being mainly self powered. They're becoming less and less common as there's no real need for them anymore.

Thanks. That makes sense. I wondered what the point was. So the best advice is keep it simple.

[Bill Fitzmaurice]

JBL does make some unusual dual coil drivers, which they call Differential Drive. Instead of the usual one coil over another on the same former they have a front coil and a rear coil, along with front and rear neo magnets that don't use a pole piece, front plate or back plate, because they're small enough to fit inside the coils. I don't think they have the ability to choose between series and parallel wiring of the coils.

[wateroftyne]

Ah, the Accuswitch. How on earth did Accugroove stay in business after that?

[Bill Fitzmaurice]

They did disappear for a while, then came back. I stopped taking them seriously when I saw an interview with the owner, whose name escapes me, where said that he got the inspiration for his speakers in a vision from God. I might have thought better of it had he said it came in a vision while doing peyote and listening to Clapton.

[three]

On 21/02/2019 at 12:51, Bill Fitzmaurice said:

Use a dual voice coil driver.  One coil 8 ohms, two for four ohms. I don't know if that's what Hartke does, but it's one way to do it.

Didn’t Epifani do this with some very expensive cabs?

[Sparky Mark]

I think that I've read that some people think their bass amp sounds better when driving a 4 ohm load? Can this be true?

Also, if you have a particularly low power amp and would like to shove its maximum output into this single cab, perhaps that might be the attraction?

[Bill Fitzmaurice]

If all is the same a 4 ohm and 8 ohm cab will sound the same. But all else is seldom the same. The usual reason why someone would be driving a 4 ohm load is they have two 8 ohm cabs. Two cabs will almost always sound better than one. For a low power amp to get a real benefit from 4 versus 8 ohms it would probably have to be rated 25% or less than the speaker power rating.

[Soledad]

38 minutes ago, Bill Fitzmaurice said:

If all is the same a 4 ohm and 8 ohm cab will sound the same. But all else is seldom the same.

If I'm wrong someone 'shout', but as I recall, amp stability and signal control reduces as load impedance reduces. The amp's damping factor is a function of load impedance and is higher (a good thing) as speaker impedance increases. It obviously takes a lot of controlled power to deal with the peaks in transient signals and the amp is working hard when it has to pull the speaker diaphragms back from the extreme travel of the big transients. (I suspect it's working even harder at the start of a transient, getting the diaphragms moving from rest.)

If my rig was powerful enough to always give required volumes, then I'd run 8 ohms in preference to 4 because I think the amp has tighter control of the speakers. Can I hear the difference? Probably not really, until the whole rig is just about maxed out and at that stage speakers are distorting anyway regardless of how well the amp is trying to control them. It reminds me though, we don't get owt for nowt - 250w into 8 becomes 125 into 4, but there has to be a price to pay.

[Bill Fitzmaurice]

Damping factor is a non-issue. It was highly touted as being important when SS was introduced, but only as a sales tool to convince people to abandon valves.

http://www.cartchunk.org/audiotopics/DampingFactor.pdf