m250

To answer your question about smoothing caps - no. For an amplifier like this you want at least 10,000uF capacitance PER RAIL, and they must be able to withstand the rail voltage so you'll need 63V capacitors.

Generally, the more capacitance you have, the smoother the voltage and the more energy reserve the amp has which leads to better bass. AS you can see Marcus uses 2x22,000uF per rail, this could be a bit expensive. Check prices on Farnell and you'll see that these sort of high capacity high voltage caps are expensive. As an example 10,000uF 63V capacitors from Panasonic (Farnell part code 652179) are £6.31 each. I would suggest looking on eBay for filter capacitors. I have sent you a private message with a link to an eBay auction you may be interested in.
 
i think i've found the solution to the smoothing caps- an old computer monitor which has been sat in my garage roof for a couple of months. i'll stick a large rod into the ground and use some fairly thick wire to drain what little charge will be left in them now. of coure, i don't know the value and voltage, but i know that they are usually pretty large and that will give me a really smooth supply


would there be any problem with using these caps apart from the need to completely drain the caps before desoldering?
 

TonyTecson

diyAudio Moderator Emeritus
2003-05-29 2:57 am
Maybunga, Pasig City
ldanto said:
i think i've found the solution to the smoothing caps- an old computer monitor which has been sat in my garage roof for a couple of months. i'll stick a large rod into the ground and use some fairly thick wire to drain what little charge will be left in them now. of coure, i don't know the value and voltage, but i know that they are usually pretty large and that will give me a really smooth supply


would there be any problem with using these caps apart from the need to completely drain the caps before desoldering?


depends on what type of montor you have, most likely autovolt from90 vac to 240vac, in which case the filter caps inside can be a 150ufd to 470ufd 400wvdc, which is not of any use in our case! tube amps can use that kind of caps however......
 
The major cost with building an amp is:
* Case
* Heatsinks
* Capacitors for PSU
* Transformer
* Power Transistors (although if you're lucky you can take advantage of OnSemi's sample program).

A good idea might be to visit a junk electronics store, and see if they have an old amplifier/reciever in that's busted. Chances are the power supply components will be fine, and if you're lucky the casing can be reused.

The other major source for these parts on the cheap is eBay, you just have to keep watching.
 
hi guys, i'm going to build that M250 power amp but i'm just wondering has anyone used power transistors other than Motorola ones? I've found here in Bulgaria some MJ15024/25 pretty cheap (about $2 each) but they are not made by Motorola. However there are also Motorola ones for about $6 each which will be too expensive for me. thanks
 
The non-motorola ones are probably fakes. Although, these transistors are now branded "ON" from On Semiconductor.

At $2 each you can bet they are 2955/3055 rebranded, ie fakes. They would just blow when you try and use them, as the Vceo is not good enough. Read http://sound.westhost.com/counterfeit.htm for more details

Maybe you should see if the On Semiconductor samples program on their website will deliver to your country.
 
jaycee said:
The non-motorola ones are probably fakes. Although, these transistors are now branded "ON" from On Semiconductor.

At $2 each you can bet they are 2955/3055 rebranded, ie fakes. They would just blow when you try and use them, as the Vceo is not good enough. Read http://sound.westhost.com/counterfeit.htm for more details

Maybe you should see if the On Semiconductor samples program on their website will deliver to your country.
thank you, this info was very helpful. i'm gonna buy few of these cheap ones and i'll make some tests with them.
 
The 2$ ones are 100% fake. :D Trust me! ;) I have experience with a lot of the cheap ones.
Just tell me where did you bought them.

I rather think Bulgaria is the place with the most counterfeit components.
Sorry but this is the crude reality :bawling:

I have a plastic bag with suspicious components (transistors mostly) wich I have broked to see whether they are counterfeit or real :smash: :smash: :smash: :smash:
Learn yourself to broke a sample when you are in doubt(too cheap something) before buying more.
This way you'll have to pay through the nose for it, but very soon you will start to identify the fakes very easily(even without breaking anything).
 
lachko973 - I do agree with "Bulgaria is the place with the most counterfeit components."
i've bought samples from those cheap ones. they are absolutely fake although they have the Motorola logo printed on them. actually i opened 2 of them with a kitchen knife !?! I bought them from Pulsator and Electroninvest in Sofia. surprisingly I found ONSEMI originals MJ15024 at a good price (they are indeed originals - it was very hard to open one of them and internally their die is totally different and massive like it supposed to be for a 250W transistor) so now I only need MJ15025s. I don't have a good digital camera at the moment to take pictures of those fake ones so everybody could be warned of buying them, but I'm working on it.