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 -
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 -
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
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...
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.
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.
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?
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?
there are very few chipamps that can work with supply rails @ ~ +-45Vdc.
the lm3886 is limited to <= +-42Vdc absolute maximum when signal is present.
Your mains voltage can rise as high as 254Vac and the PSU will then output ~+-48Vdc. That is what your amplifier must survive !
You will have to use a discrete component amplifier, or one of the range of driver chips with a discrete output stage. Aim for 60W to 80W into 8ohms.
the lm3886 is limited to <= +-42Vdc absolute maximum when signal is present.
Your mains voltage can rise as high as 254Vac and the PSU will then output ~+-48Vdc. That is what your amplifier must survive !
You will have to use a discrete component amplifier, or one of the range of driver chips with a discrete output stage. Aim for 60W to 80W into 8ohms.
Hmmmm - two quality 10,000uF electrolytics will probably run to more than the cost of a new 225VA toroidal...
I've also read that the TDA 7294 is not suitable for a 4 Ohm load when bridges - is this absolutely true?
The 7294 will see 2r0 when driving a 4r0 test load in bridged arrangement.
Fit a 4ohms speaker and each 7294 sees the equivalent to a 2ohms speaker load.
That is much more severe than a 2r0 test load. It is worse than a 1r0 test load.
Can the 7294 drive a 1r0 test load with the supply rails you plan to use?
If it can that could be a good clue to carry on with your proposal. If it can't then stop and re-evaluate.
Fit a 4ohms speaker and each 7294 sees the equivalent to a 2ohms speaker load.
That is much more severe than a 2r0 test load. It is worse than a 1r0 test load.
Can the 7294 drive a 1r0 test load with the supply rails you plan to use?
If it can that could be a good clue to carry on with your proposal. If it can't then stop and re-evaluate.
It is the transformer from the original subwoofer amplifier. This was rated at 175W.
However, +-45V is 90V in total and a 1R0 linear load would draw 90A!
The numbers don't add up. Please you shed some light on this for me as I don't have audio design experience?
Is this 45Vdc after the caps or 45Vac from the secondaries of the transformer?
Cheap and DIY don't always go together. I think this class d amp would be good.
http://connexelectronic.com/product...ucts_id/143?osCsid=qd7h1p7chlsgn8eg7ckk7rhqi6
Powerful, cheap, good on supply rails and you don't have to worry about heatsinking.
Cheap and DIY don't always go together. I think this class d amp would be good.
http://connexelectronic.com/product...ucts_id/143?osCsid=qd7h1p7chlsgn8eg7ckk7rhqi6
Powerful, cheap, good on supply rails and you don't have to worry about heatsinking.
It's 45VAC each side of zero - I thought I'd only lose about 0.7 of a volt per rail through a normal bridge and the whole point of the big caps was to maintain that?
Definitely not the +-62 to 68V specified for that class D amp.
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