A Little help with the M250 power amp construction ! - diyAudio
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Old 9th December 2005, 08:58 PM   #1
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Talking A Little help with the M250 power amp construction !

Hi everyone !

So, I REALY want to build this amp:

http://www.astro.uu.se/~marcus/private/m250.html

But, there's no PCB! Can you help me to design this PCB ?

What program should I use ? Anyone here have this PCB done?

Another thing ! The output stage transistors, here are very expensive... The transistors used in this project are MJ15024, that is 16A 200V.

Why 200V if the amp works with 55+55v ?

So, can I replace them with TIP3055 or TIP35? The first is 15A and the second 25A, they are 100v.

If I replace with 3055, I'll add more one pair, because it's 15A, and the original is 16A...

The output stage is a half PNP and a half NPN, and if the supply is 55 + 55v, each stage works with 55v right? So, there's no problem using 100v transistors right?


Thaaaaankksssss !!!!!!!
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Old 9th December 2005, 09:33 PM   #2
CBS240 is offline CBS240  United States
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Default Re: A Little help with the M250 power amp construction !

Quote:
Originally posted by -_nando-_


Another thing ! The output stage transistors, here are very expensive... The transistors used in this project are MJ15024, that is 16A 200V.

Why 200V if the amp works with 55+55v ?

So, can I replace them with TIP3055 or TIP35? The first is 15A and the second 25A, they are 100v.

If I replace with 3055, I'll add more one pair, because it's 15A, and the original is 16A...

The output stage is a half PNP and a half NPN, and if the supply is 55 + 55v, each stage works with 55v right? So, there's no problem using 100v transistors right?


Thaaaaankksssss !!!!!!!
The output devices have to operate from rail to rail, 55 + 55 = 110V!. So you may be pushing it here. You also need to have similar Hfe and be mindful of safe operating area. SOA is the parameter that determines how many devices you need, not current rating. TIP35 is a rugged device, but I think the Hfe is lower.
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Old 10th December 2005, 01:09 AM   #3
quasi is offline quasi  Australia
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TIP35C has a Vce of 100v max so this too could expire. You can use almost any transistors for the output provided that;

they have a Vce of at least 120v
they have a gain of at least 20 or so
you can get them in P and N
they have a combined power rating of at least 800 w per rail
are the right package (or one that you can use)
are not a darlington (for simplicity)

But you know what, Ive had a look at the cct and there are better amps out there, so if you don't have a board yet and haven't started, why don't you look for alternatives, plenty around here you can use.

Cheers
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Old 10th December 2005, 08:17 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
what current can the Tip take when dropping 55V across CE?
That is the output device current limit when driving a speaker (reactive) load.
Using an 8ohm speaker with a 6r minimum resistance as an example.
The peak current from your amp will be (55-5)/6 =8.3Apk.
The derating factor for your Tip will depend on the transistor casing temperature. This in turn depends on the heatsink size and airflow and how much power the amp has recently been pumping out (both quiescent+output).
If the Tc = 50degC then derating is likely to be 0.8
Tip current = SOAR current at 55V * 0.8.
number of devices = output current/ tip current = 8.3/SOAR/0.8 = a lot of devices.
If you use a higher voltage rating device then usually the SOAR goes up dramatically and you can then reduce the number of parallel devices. MJL15024 has a max 4.5A at 55V. derated = 3.6A. Number of devices = 8.3/3.6 = 2.3 therefore use 3pairs for an 8 ohms load with any phase angle and 5 pairs for 4 ohm load at any phase angle. With accurate V & I analysis you may be able to trim off 1 pair from each of the example calculations resulting in a robust 2pair 8ohm amp or 4 pair 4ohm amplifier.

Using Tips is probably going to be a non starter. Try working out the numbers. Unfortunately design involves maths, change the design and you change the arithmetical results. Avoiding the maths may result in a blown amp.
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Old 10th December 2005, 08:40 AM   #5
quasi is offline quasi  Australia
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Andrew is right. My advice of transistors rated at 120v is misguided. You are likely to require devices rated over 200 volts because as Andrew has stated the SOAR generally improves with higher voltage transistors.


Cheers
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Old 10th December 2005, 09:06 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi Quasi,
don't give in just on a whim.

100Vce0 and 120Vce0 could just about do the job but he would need an awfull lot of them. Let him do the arithmetic and he'll learn from the experience. Lots of cheap components or fewer more expensive components are, in this case, equivalent.

A better solution to allow the low voltage transistors to work is use lower voltage PSU rails, try +-40Vdc or +-45Vdc. Then repeat the arithmetic.
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Old 10th December 2005, 09:48 AM   #7
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MJ15015/16 are 120V, 15A, 180W with a full 3A SOA at 60V, and about 1/2 the cost of MJ15024/25.

Crown and Carver used these in several models. Crown got 240W with only two pair, but I would recommend more at that power level. A different model Crown with four pair of these did 335W 20~20Khz, 400W 1Khz.

An amp I did with three pair of these on 57V has been running at over 200W into 4 ohms for over 20 years. This amp was fan cooled for small size. I used the Pass A40 as the driver board, and wired the drivers and outputs point-to-point on their sockets.

The A40 driver board is still available for only $6 from AudioXpress. The outputs should be available for less than $2 each.

I give alternate transistor numbers for this project, and a link to the board source, at the Parts Express tech forum. Search on A40 and djk.
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Old 10th December 2005, 09:09 PM   #8
jaycee is offline jaycee  United Kingdom
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Check marcus' page carefully and you will see that there is a PCB for the amp. It's rather custom, but you could use it.

MJ15003/4 should work in this amp also. 2955/3055's will just blow as they cannot take the rail to rail voltage.
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Old 10th December 2005, 11:09 PM   #9
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But as the problem related by them, if I use a lower voltage transistor (at least 120v), I'll need more pairs...

ML15004/3 are 20A 140v, I think it work well in this amp, but as I don't know the maths, I'll not try it without any math...

I found the original transistors with a "low" price, but I found 2SC 3281 for the HALF of 15024...

2SC 3281 is a 200v 15A 150W transistor, thermal resistence 0,83 C/W, Hfe:

lc = 8.0A Vce = 5.0v min 35 max 160

MJ15004 is a 250v 16A 250W transistor, thermal resistence 0,70 C/W, hfe:

lc = 8.0A Vce = 4.0v min 15 max 60

Wow, not easy...

Can I replace with 3281 ? Do I need to add more pairs because the lower current and watts ? How many?


I've found this amp (c200):

http://www.ampslab.com/c200.htm

Is it good? M250 is better right? And about sound quality, M250 is very better right?

As you see, I want to build M250...
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Old 11th December 2005, 07:29 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
3281 would be an excellent hi speed replacement.
But only if they are genuine TOSHIBA parts.
They are now obsolete, because, I believe, fakes came on to the market.

The low power and hi speed would demand some changes in both device numbers and component values.
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