m250

[ldanto]

ok, so i know there are loads of threads on Marcus's M250, but i'm doing this because i like the m250

His website is here

[jaycee]

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.

[ldanto]

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?

[jaycee]

They will be only 1000uF (at the most), at 400V+, so not really enough capacitance. Also, if they've been in an old monitor they are likely worn due to the heat.

[Tony]

Originally posted by ldanto
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......

[jaycee]

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.

[ldanto]

well the caps are sealed in a unit which needs to be cut open so i don't think i'll bother....

[stj2000online]

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

[jaycee]

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.

[stj2000online]

Originally posted by jaycee
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.