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Old 19th April 2014, 08:55 AM   #1
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Default Crossover impedance

Hi,

I'm newbie on this forum, bass player in seek for building a 15" extension cab with boomer and tweeter.

My amp requires 8 ohm for an external cab which paired with internal speaker/tweeter loads 4 ohm to give 400W vs 250W for 8 ohms.

Problem : I plan to use in my future cab an Eminence Basslite c2515 (4 ohm) with a Monacor HT30 tweeter (8 ohm).

Custom crossover design is OK regarding frequencies , my concern is about the load, how to get the 8 ohm my amp requires ?

Thanks for any advice.
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Old 19th April 2014, 09:10 AM   #2
system7 is offline system7  United Kingdom
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There is no sensible way to convert a 4 ohm bass driver to 8 ohms load except to wire two in series.

If your amp genuinely can't drive 4 ohms without going outside its output transistors safe operating area, you are just stuffed here.

At the design stage with an amplifier, you'd usually double up the output transistors in an 8 ohm design to give it enough current drive to take a 4 ohm load.
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Old 19th April 2014, 10:33 AM   #3
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Quote:
Originally Posted by tourettes View Post
Hi,

I'm newbie on this forum, bass player in seek for building a 15" extension cab with boomer and tweeter.

My amp requires 8 ohm for an external cab which paired with internal speaker/tweeter loads 4 ohm to give 400W vs 250W for 8 ohms.

Problem : I plan to use in my future cab an Eminence Basslite c2515 (4 ohm) with a Monacor HT30 tweeter (8 ohm).

Custom crossover design is OK regarding frequencies , my concern is about the load, how to get the 8 ohm my amp requires ?

Thanks for any advice.
Forget about these numbers. They mean nothing to your amp. You are talking about active resistance. When you introduce crossover, your speaker will become complex system with reactive resistance. That reactive resistance will be the load for your amplifier. So, say you have a speaker with 4 Om DC resistance and poorly designed crossover with reactive resistance ( in between 20 Hz and 20 KHz ) , say 2 Om, than total resistance become even less than 2 Om.

Just an example.

Last edited by Devilsdance7775; 19th April 2014 at 10:38 AM.
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Old 19th April 2014, 11:36 AM   #4
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Unless you're doing something really silly (like putting a 4ohm resistor across a 4ohm driver), I can't see how you'd ever get below the 4ohm resistance of the driver. Got a schematic?


OP, have you already bought the Eminence woofer?
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Old 19th April 2014, 11:43 AM   #5
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Quote:
Originally Posted by chris661 View Post
Unless you're doing something really silly (like putting a 4ohm resistor across a 4ohm driver), I can't see how you'd ever get below the 4ohm resistance of the driver. Got a schematic?


OP, have you already bought the Eminence woofer?
Easy. I know that we cant trust no one nowadays but anyway, here is example:

804S Floor-Standing

— Design: Three-Way Ported

— Drivers: One 1" Aluminum Dome Tweeter,
One 6" Kevlar Midrange, Two 6-1/2"
Woofers
— MFR: 38 Hz - 22 kHz ± 3 dB

— Crossover: 350 Hz, 4 kHz
— Sensitivity: 90 dB/2.83V @ 1m
— Impedance: 8 Ohms (Nominal)
— Dimensions: 40.2" H x 9.4" W x 13.8" D
— Weight: 62 Pounds/Each

— Available Finishes: Cherry, Rosenut, Black
Ash

— MSRP: $4,000/Pair USA


Click the image to open in full size.

Click the image to open in full size.

Producer says Impedance is 8 Ohms (Nominal), but from the pictures above it is clear that reactive resistance is close to 3 Om, at one particular spot. Everything is possible

Last edited by Devilsdance7775; 19th April 2014 at 11:45 AM.
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Old 19th April 2014, 03:49 PM   #6
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No no no.

That's literally just the manufacturer misrepresenting. Those are a pair of 8ohm (nominal, =~6ohm minimum) woofers in parallel. Nothing more.

The phase angle just tells you the time relationship between current and voltage. Not irrelevant, but definitely different.

Rod Elliott has done some great articles on this sort of thing.

Chris
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Old 19th April 2014, 05:24 PM   #7
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I haveńt got any speaker yet, thanks for your answers but I'm a bit lost in those resistance considerations . Does a specific crossover design allow to give this 8 ohm load with a 4/8 ohm speaker pair ?
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Old 19th April 2014, 06:28 PM   #8
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Quote:
Originally Posted by tourettes View Post
I haveńt got any speaker yet, thanks for your answers but I'm a bit lost in those resistance considerations . Does a specific crossover design allow to give this 8 ohm load with a 4/8 ohm speaker pair ?
No, better to use the (2x) drivers in series for 8 ohms so that you can play with your amp set.
Or go with the other equivalent driver 8 ohm to your specification reference above.
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Old 19th April 2014, 06:53 PM   #9
system7 is offline system7  United Kingdom
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It's worth explaining something about PA drivers. They are usually stacked in vertical pairs or quads. This is because a vertical array has great advantages in a hall. The acoustic power falls off at a slower rate with distance. The lobe is strongly horizontal too.

If you were using a pair of PA drivers, you'd probably want 2X 4 ohms in series for 8 ohm load.

With 4 drivers stacked vertically, you'd use two pairs of 8 ohm drivers wired in series/parallel.

Eminence Speaker is proud to offer unparalleled customer support | Eminence Speaker

FWIW, Eminence do some standard crossovers designed for their drivers with horn tweeters. It's quite a deep business actually. I expect they would advise you.
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Last edited by system7; 19th April 2014 at 06:55 PM.
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