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Old 24th September 2008, 04:29 AM   #1
fazman is offline fazman  Canada
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Default Plate amp suggestions or other?

Hey guys,

I just bought a sub (Mirage substrata 1000) for $40.
The guy tells me that the amp is fried. The woofer is still good though.
This is one of those digital 1000 watt plate amps on the back. Stiff 12" cone in a sealed enclosure. Woofer has a massive magnet, huge rubber surround on it and requires tonnes of power.

Now... my tech will go over the plate amp but he tells me not to hold my breath with these digital plate amps.
If the amp is fried (which I'm thinking it will be) what would be my best option to continue using this sub?
I can get another plate amp from say Parts Express but the 1000 watt plate amps are expensive. Can I get away with less power? I was told these types of woofers in sealed cabs require enormous amounts of current.
My other option I guess is to use an amp and an external crossover (which I have)
The amp is a Carver TFM-25 and I have a Paradigm X-30 electronic crossover. I'm just afraid that the amp was not built to drive a sub and I don't want to ruin it.

Any suggestions would be greatly appreciated.
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Old 24th September 2008, 02:20 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Your main problem is probably any in built EQ, it is almost certain
it uses some form of bass boost, a highish Q boost / subsonic
filter or possibly a Linkwitz transform, its hard to say.

/sreten.
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Old 25th September 2008, 01:48 AM   #3
fazman is offline fazman  Canada
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Quote:
Originally posted by sreten
Hi,

Your main problem is probably any in built EQ, it is almost certain
it uses some form of bass boost, a highish Q boost / subsonic
filter or possibly a Linkwitz transform, its hard to say.

/sreten.
Thanks sreten.
Yes I think you are right. I'm sure that the original digital amp has bass boost built in and a rumble filter as to not rob the amp of all the power.
I checked out Parts Express and they have a plate amp for relatively cheap that has both of these but it is only rated for 250 watts into 4 ohms.
Now when I measured the resistance of the woofer of this Mirage sub, I get a reading of 19 ohms!
I also noticed something else that I didn't understand... Across the pos and neg terminals of the woofer I noticed a cap and a resistor... Values of 47 ohms and 2.2 uf. Why would they have done this?

Before I go buying just any plate amp I want to make sure I'm informed first.

Thanks
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Old 25th September 2008, 07:14 AM   #4
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Zobel cicuit to provide a flat impedance curve ?? but that is a fairly big resistor sure it wasn't 4R7?

Did you read resistance across the tabs of the driver?
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Old 25th September 2008, 09:21 AM   #5
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Low resistances can be tricky to measure, especially when comparable to the resistance of the test leads, are you certain of that measurement or was it a cheap multimeter job?

If the latter, get a larger resistor, say 100ohm, and measure that with your multimeter, then measure the driver with the 100ohm resistor in series and subtract the resistors measured value from this, you should probably get something less than 19ohms.

Ofcourse you'll need to have that resistor - capacitor (zobel) disconnected from the driver too while measuring.
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Old 25th September 2008, 10:09 AM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Pikefish

Ofcourse you'll need to have that resistor - capacitor
(zobel) disconnected from the driver too while measuring.
Hi, Why ? /sreten.
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Old 25th September 2008, 10:19 AM   #7
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Because the resistance meter does its job by providing a voltage and measuring the current that flows, so when you hook up the ohmmeter aka 'dc voltage supply' it will first see the cap as a short and thus show you the value of the resistor in the zobel, in parallel with the driver Rs, and as the cap charges it's 'resistance' will appear to increase, and after quite a long time it will show the actual drivers resistance, provided the meter is capable of such a low measurement accurately.

Better to just skip the capactor loading sequence.
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Old 25th September 2008, 02:01 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

You do not need to have it disconnected,
just wait for the reading to settle ......

Where 19 ohms comes from I'm not sure,
I cannot think of a failure mode that would
cause it, or that it is the correct value.

It is most likely a 4 ohm driver with a 4.7R zobel.

/sreten.
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Old 26th September 2008, 09:22 AM   #9
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I'll bet that the Mirage amplifier is a direct line switcher. It doesn't have a high voltage transformer to step down the 120VAC. It switches directly from the AC outlet. Since there is such high voltage available this way, the woofer impedance used in these designs is usually very high to limit the power output to a reasonable value.

Tell your tech to be very careful working on this amplifier.
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Old 26th September 2008, 01:02 PM   #10
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Jack Hidley

I'll bet that the Mirage amplifier is a direct line switcher.
It doesn't have a high voltage transformer to step down the 120VAC.
It switches directly from the AC outlet. Since there is such high voltage
available this way, the woofer impedance used in these designs is
usually very high to limit the power output to a reasonable value.

Tell your tech to be very careful working on this amplifier.
Hi,

That does make sense, but I've never seen a ~ 20 ohm driver.
If it is the case then 47R for the zobel is also very reasonable.

If it is true he is completely stuffed, I've never seen a subamplifier
for ~ 20 ohms either. A very high power amplifier for 8 ohms
bridged is the only option I can think of.

/sreten.
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