Check my ohm calcs

clinton

Hi all

Just want to check if I calced this right.

I have a poweramp that can be bridged/mono at 8ohms to give me 1600watts rms.
I would like to bridge the amp and then connect 2 x 400watt 18" bass bins rated at 8 ohms each.
If the subs have 2 sockets at the back to facilitate daisy chaining, will the result be in parralel or in series. I ask because I would obviously want parrallel so the total of my 2 8 ohm subs will give me 16ohm/2=8ohms (which is the amps bridged value)

OR

Another option I have is to purchase 1 x 800watt 18" bass bin at 8 ohms and run it alone on the bridged amp. Bass response should be omni-directional so I'm not sure if 1 x 800watt bin would be better/worse than 2 seperate 400watt 18" bass bins.

What u guys say?

AlanRatcliffe

clinton wrote: If the subs have 2 sockets at the back to facilitate daisy chaining, will the result be in parralel or in series.

Parallel.

I ask because I would obviously want parrallel so the total of my 2 8 ohm subs will give me 16ohm/2=8ohms (which is the amps bridged value)

Where do you get 16 Ohms from? Two 8 Ohm speakers in parallel is 8/2 = 4 Ohms

clinton

MATHS IS HARD......

So basically I cant do the 2 subs on my bridged 4ohm amp.

So what are the pros/cons of the other option with 1 800watt 8 ohm sub on the bridged amp?

AlanRatcliffe

You have a few options without buying anything else:

OK given similar speaker sensitivity, two speakers will usually be twice the volume of one and two speakers at 400W will be louder than one at 800W. So you can use the amp in two channel mode with an 8 Ohm speaker on each (and feed both the sub signal). The amp will put out 400W into each. You could also parallel the two on one channel of the amp to use it's full power, which then leaves you a spare channel for something else (monitors, mids, another sub, whatever). You're still underdriving your speakers in either case, but as long as the amp isn't struggling and distorting and it's loud enough for the venue, that's fine (you could always hire in another pair of 8 Ohm speakers when you have the larger gigs).

clinton

Thanks for the advise Alan.

I think I will go that route. The amp can actually push 500watts at 8 ohms so I will get 2 x 400w 8 ohm subs and run on the 2 seperate channels.
This is the way i kinda wanted to do it but was worried that the 100watts headroom on each channel wouldnt be enough.

TomCat

Amplifiers in bridge mode and subs do not go together well.

The reason being is that although the output power doubles for a particular load (8 ohms), the damping factor also halves as the output impedance of the amplifier doubles.

A higher damping factor means that the amplifier can deliver more current to the loudspeaker/s when the loudspeaker demands that current of the amplifier.
This is particularly important at Bass frequencies as the loudspeaker's impedance becomes very unstable and can easily drop below it's nominal impedance.

When the loudspeaker is pushed hard, a lower damping factor (i.e. low instantaneous current delivery) can result in mechanical failure of the loudspeaker as it over excurts past it's design limits. This is also the reason why small diameter loudspeaker cables and long cable runs are a no-no for Bass bins.

As Alan has said....Better to run two smaller subs (each 8 ohm or 2 x 8ohm=4ohm) with one sub on each channel than a larger sub running off the amp in bridge mode. You have more control over the sub loudspeakers and they will end up sounding louder and punchier as a result.

BTW nearly all Dual 18" subs on the market are 4ohm impedance's so you would probably run adual 18 on each channel at 4 ohms.

Oh....and never run a power amplifier in bridge mode lower than it's recommended load impedance....The amp will overheat and either go into protection or get damaged as a result.

clinton

Thank Tom

So do u think that the 100watt headroom is enought on each channel of the amp.

TomCat

Generally speaking.....no problem.

Loudspeaker power specifications are a complex and confusing minefield to walk through.
It all depends on the manufacturer and how they measure their loudspeakers.

One of the main standards is RS426A, which specifies both the continuous average power and the peak power with the peak power usually being 4x the average power.

Say the loudspeaker is rated at 400w RS426A then the peak power capability will be 1600W peak. I'm not sure how you loudspeakers are rated so it will be hard to say what the correct power amplifier power rating should be. I know for RS426A one tends to double the amplifier power rating over that of the loudspeaker. In my example above the ideal amplifier would be between 600 to 800w.

The idea with RS426A is to both specify the thermal rating of the loudspeaker (400W) and the mechanical limit (1600W).
Some manufacturers rate their loudspeaker at 2 hours and other at 100 hours.

In your case I would say that having the amplifier more powerful than the loudspeaker is a safe bet. 100W is not much but all should be good. If the amplifier Clip lights are coming on all the time...back off to prevent damage.

I hope this is not too confusing...... ???

clinton

prepare to be 'thanked'

AlanRatcliffe

TomCat wrote: Amplifiers in bridge mode and subs do not go together well.
The reason being is that although the output power doubles for a particular load (8 ohms), the damping factor also halves as the output impedance of the amplifier doubles.

Thanks! I knew about damping factor, but I didn't know that a bridged amp has half the DF. My new fact for the day... Yet another thing to add to the "things the amp manufacturers leave out of the specs" list...

TomCat

I tend to avoid bridging amplifiers wherever possible. I'd much rather add another amplifier and raise load impedance's (i.e. higher impedance)...More "Real Power" and a louder, more reliable system.....especially at sub frequencies. Your Bass is louder, tighter and punchier, considering other factors as crossover frequencies, crossover overlap and time alignment are all correct)

A story for another day.....

Neps

+1 to Tom!!! ? Ive seen faaaarrrrrr too many blown drivers and amps due to Impedance miss matching..