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-   -   Active sub replacement amp (http://www.diyaudio.com/forums/chip-amps/223051-active-sub-replacement-amp.html)

Pompey Monkey 7th November 2012 09:54 AM

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 - ;)

DUG 7th November 2012 10:44 AM

How about one of these...designed for subs:

The Madisound Speaker Store

Pompey Monkey 7th November 2012 11:18 AM

Quote:

Originally Posted by DUG (Post 3231906)
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 :D

AndrewT 7th November 2012 11:56 AM

Measure the transformer output.
That determines what amp is suitable for the duty.

Pompey Monkey 7th November 2012 12:02 PM

Quote:

Originally Posted by AndrewT (Post 3231968)
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 :rolleyes: 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...

bobodioulasso 7th November 2012 12:11 PM

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

Pompey Monkey 7th November 2012 12:16 PM

Quote:

Originally Posted by bobodioulasso (Post 3231987)
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.

AndrewT 7th November 2012 01:56 PM

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.

bobodioulasso 7th November 2012 04:07 PM

Yes mind the load impedance as this parameter, conjugated with rails voltage will tell how much power will be dissipated.

Pompey Monkey 7th November 2012 06:26 PM

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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