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Old 7th November 2012, 10:54 AM   #1
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Default Active sub replacement amp

This is my first post so, to dispense with the formalities, Hello everybody!

I have a Mordaunt Short Ms309i subwoofer (175W) that has given good service for about 6 years, but recently the amplifier has given up the ghost. Although I am compentent with a multimeter and soldering iron and have a reasonable level of electronics knowledge, repairing the digital amplifier is beyond my capabilites.

Rather than spend my hard-earned beer tokens on a replacement, I've done a little research (much of it here on diyaudio.com) and believe that I can replace the amp with a bridged TDA7294 power amp like this from eBay:

2x | eBay

I can happily live without the notch filter on my original sub, although that doesn't mean that I won't consider adding one in the future

I do have some questions before proceeding though, so here goes:

I don't know the impedance of the driver in my box - I guess it will be around 4-8 ohms, but I can't be sure - can I check with just a multimeter?

I believe that I can probably use the toroidal transformer from the sub - can I get a

good indication of its output voltage by measuring the resistance of the primary and secondary coils, or do I have to test it under load?

Can the above referenced amplifier module be bridged as it is, or do I have to make a modification / order a different one?

As the module stands, the heatsink would be inside the sub if the gain pot was mounted on the panel. This is obviously not best practice, so am I correct in assuming that I can mount the gain control on the panel via a cable and have the heatsink external to the box?

I do appreciate that I'm asking a lot for a first post, but I would really appreciate any feedback on my questions and anything else that I've omitted. - Maybe I'm about to beging a new hobby -
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Old 7th November 2012, 11:44 AM   #2
DUG is offline DUG  Canada
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How about one of these...designed for subs:

The Madisound Speaker Store
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Old 7th November 2012, 12:18 PM   #3
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Quote:
Originally Posted by DUG View Post
How about one of these...designed for subs:

The Madisound Speaker Store
Hi, and thanks for the help, but I'm looking to do things a bit more DIY and cheaper - I could get a second hand replacement sub for that price - that's why I asked those specific questions.

I'm quite competent to wire the module up, I just don't have experience of audio electronics...

...Yet
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Old 7th November 2012, 12:56 PM   #4
AndrewT is offline AndrewT  Scotland
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Measure the transformer output.
That determines what amp is suitable for the duty.
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Old 7th November 2012, 01:02 PM   #5
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Quote:
Originally Posted by AndrewT View Post
Measure the transformer output.
That determines what amp is suitable for the duty.
Do you mean the voltage? - I don't have any dummy loads to test the power rating (yet). The original sub was rated at 175W, but:
1. These were the manufacturers figures and;
2. An all-digital amp is likely to be more efficient than a TDA7294 design, right?

I'm at work atm, I'll have to get the screwdrivers and multimeter out tonight...
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Old 7th November 2012, 01:11 PM   #6
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For the driver's impedance: just measure the dc resistance of the coil.
Around 3 ohms: Z = 4 Ohms
Around 6.5 ohms: Z = 8 ohms
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Old 7th November 2012, 01:16 PM   #7
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Quote:
Originally Posted by bobodioulasso View Post
For the driver's impedance: just measure the dc resistance of the coil.
Around 3 ohms: Z = 4 Ohms
Around 6.5 ohms: Z = 8 ohms

Will do!

Thanks.
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Old 7th November 2012, 02:56 PM   #8
AndrewT is offline AndrewT  Scotland
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Never mind load resistors. Just measure the output voltage with your typical mains voltage.
We can estimate the regulation once we know the size and/or weight of the transformer.
From there we can tell you if the chip or amplifier you choose will work with that transformer. Then we can tell you if the heatsink is usable.

Case + transformer+ heatsink is the biggest proportion of cost, by far, of a built up amplifier.
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Old 7th November 2012, 05:07 PM   #9
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Yes mind the load impedance as this parameter, conjugated with rails voltage will tell how much power will be dissipated.
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Old 7th November 2012, 07:26 PM   #10
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The transformer is +-45V off-load and weighs in at about 2.2Kg. Is that too high for these off the shelf reference designs?

The driver measures a tad over 3 Ohms with a multimeter, so I'm guessing it's a 4 Ohm unit.

The heatsink is not big enough for anything useful as far as I can tell...

...What are my options, bearing in mind that I'm getting more adventurous as I read more?
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