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A quick question about impedance...

 
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Freddy V-C
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Joined: 17 May 2009
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PostPosted: Sat Apr 16, 2016 12:21 pm    Post subject: A quick question about impedance... Reply with quote

I was recently asked to play bass for Leeds' local PUNK RAWK SUPERSTARS Super Luxury, because their bass player is swanning off to have a babby. I'm dead excited because they're one of my favourite bands, but I'm essentially having to completely overhaul my bass gear because it's years since I've played bass in a band and my current setup is far from gig-ready.

I've managed to pick up a Behringer Ultrabass BVT5500H head for 60 (which seems like a steal for a 550w amp, Behringer or not). I haven't taken this thing for a spin yet, but it used to belong to David from Thin Privilege (hi Stewart) so I expect it can handle some pretty punishing playing.

I'm on the hunt for a new bass, but my OLD AS BALLS Tanglewood should be okay for now. So first on the agenda is a new cab. I'm unsure on size/speakers, but I imagine whatever's cheap-ish will fit the bill.

My concern is with power/impedance. Most cabs I've looked at run at 8 ohms, whereas this thing puts out 4 ohms. My understanding is that this won't cause any problems with a solid state head. However, I'm unsure how powerful my cab needs to be. If it puts out 550W at 4 ohms, how much power will it be putting out at 8 ohms? There must be a simple equation for this but Google has failed me.
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Doog
can i get uhhh


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PostPosted: Sat Apr 16, 2016 12:29 pm    Post subject: Reply with quote

If the head puts out 4, but the cab is 8, that'll be fine.

You're way better off matching it though (ie, a 4ohm cab), as you'll lose a lot of that power/efficiency. I remember using a 120w 4ohm solidstate head into an 8 ohm cab for years in the old days, and it never really being loud enough for me at the time.

I've since used it with a matched cab and its loud as balls.

If there's more than one speaker, you may be able to rewire the cab to yield a 4ohm load, super easy.
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Freddy V-C
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PostPosted: Sat Apr 16, 2016 12:46 pm    Post subject: Reply with quote

Aye yeah, thanks for the swift response. I understand about the volume loss. The reason I'm asking is because some of these cabs handle like 400W. If this head puts out 550W at 4ohms, will it be under 400W at 8 ohms? i.e. Could I put it through a 400W cab at 8 ohms and not blow anything up?
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Fakir Mustache
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PostPosted: Sat Apr 16, 2016 4:00 pm    Post subject: Re: A quick question about impedance... Reply with quote

Freddy V-C wrote:
I'm unsure how powerful my cab needs to be. If it puts out 550W at 4 ohms, how much power will it be putting out at 8 ohms? There must be a simple equation for this but Google has failed me.
Probably somewhere between 300 and 400 watts, if you really care check the manual. You probably won't blow the 400 watt cab if you don't turn up the amp real high.

But watts is not decibels, if you want a better spread get a second cab.
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Doog
can i get uhhh


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PostPosted: Sat Apr 16, 2016 8:55 pm    Post subject: Reply with quote

Freddy V-C wrote:
Aye yeah, thanks for the swift response. I understand about the volume loss. The reason I'm asking is because some of these cabs handle like 400W. If this head puts out 550W at 4ohms, will it be under 400W at 8 ohms? i.e. Could I put it through a 400W cab at 8 ohms and not blow anything up?


Sorry, shoulda read the whole thing 😳
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NickS
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Joined: 14 Jan 2008
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PostPosted: Sat Apr 16, 2016 9:44 pm    Post subject: Reply with quote

Theoretically you'll get exactly half the power.
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Semiconductor amplifiers are generally voltage amplifiers with high current capability. The power output P for a given maximum output voltage V depends inversely on the resistance (close enough to impedance) R. P=V*V/R. Double the R, get half the P.
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Fakir Mustache
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PostPosted: Sun Apr 17, 2016 5:33 am    Post subject: Reply with quote

Maybe theoretically, but I've seen many bass amps have in the specs something like 150W@4 ohms, 100W@8 ohms and similar.
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NickS
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PostPosted: Sun Apr 17, 2016 10:50 pm    Post subject: Reply with quote

I think that's especially true for these switching (modified class D) amps compared to old-fashioned linear semiconductor amps.
In terms of power handling, the spec for this amp into 4 ohms is 550W peak rather than RMS.
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ekwatts
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PostPosted: Mon Apr 18, 2016 6:32 am    Post subject: Reply with quote

It ONLY outputs at 4 ohms? Is that normal for bass amps?
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NickS
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PostPosted: Mon Apr 18, 2016 8:12 am    Post subject: Reply with quote

It's common with solid-state amps to quote the impedance of maximum (safe) power output, knowing it will be lower output into a higher impedance.
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ekwatts
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PostPosted: Mon Apr 18, 2016 1:24 pm    Post subject: Reply with quote

So do they never have separate outputs that adjust the ohmage to suit?

I don't have all that much experience with SS amps anymore (ooh, corksniffer).
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NickS
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PostPosted: Mon Apr 18, 2016 1:50 pm    Post subject: Reply with quote

ekwatts wrote:
So do they never have separate outputs that adjust the ohmage to suit?
Nope. That sort of facility requires an output transformer and SS is supposed to get rid of those making them cheaper and lighter.
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Freddy V-C
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PostPosted: Mon Apr 18, 2016 3:53 pm    Post subject: Reply with quote

Hmm, yeah, I suspected 550W might be the peak value. Frustratingly I can't seem to find the peak value.

Either way, I suppose what we've basically established is that I'm not gonna have any issues running it through a 400W cab at 8ohms.

Running at 4ohms seems pretty common for solid state bass heads. I almost exclusively use solid state amps these days as well because fuck an tube.
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