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Old 27th August 2003, 12:33 PM   #1
@fundum is offline @fundum  Belgium
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Default searching for an power amp design

Hello folks,

For some years now I wanted to build an high-power amp, but haven't been able to do so, due to my lack of money and knowledge. Now my education has given me the knowledge and I found some of the expencive peices of an amp.
I have 2 trafos, +/-35V, 550VA, some big caps (10,000F) and big heatsinks.
Could anyone of you help me with an amp desing (also a bit hifi if possible), about 2x300W?
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Old 27th August 2003, 10:33 PM   #2
Jeff R is offline Jeff R  United States
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The Leach Super Amp at http://users.ece.gatech.edu/~mleach/superamp/ can give you 300 Watts, but Leach says you need 93 volt rails. This is a good design.

I don't know what you mean by your transformer rating - do you get +/- 35 VDC out of it after rectification, or is is a center-tapped transformer that gives two 35 VAC outputs, and will thus give you 70 VAC when connected in series? If this is the case, then that 70 VAC will convert to 98 volts, which is probably ok to use with the Leach Amp and will clearly get you your 300 W with this design.

I should probably recommend that you build a simpler amp first. A 300 W amp is a huge project, even for an EE (if that is what you are), and the voltages you are dealing with can be dangerous. It might be best to first build a regular Leach Amp, say, and then upgrade if you find it necessary.

I am sure others will recommend other designs - I just happen to be partial to the Leach designs.

Good luck!
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Old 28th August 2003, 08:26 AM   #3
@fundum is offline @fundum  Belgium
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Thanks for the reply,

The transformers are indeed centertapped ones with two windings of 35VAC.
I've been looking around the internet some time now and found the M250 at p://www.astro.uu.se/~marcus/private/m250.html which has been discussed here also a while ago. Is that one of good quality?
I've made a lot of integrated amps already but am rather new to full discrete desing. Other replies are very welcome.
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Old 29th August 2003, 04:18 AM   #4
Diode is offline Diode  United States
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Hey Sander,

Is that +/- 35V DC? or is that AC volts? If that is AC volts, after rectification and filtering, you'll get around 43 Volt rails in which you can get about 160W if your transformers don't drop too much voltage at full load. I built a couple 500W amps and didn't have the full rated power supply and didn't get the rated power output either. I'm only getting about a 118 Volt PK-PK into 4 ohms and that is only about 380W or so. With those rails, you will have a great 125-180W amp unless you drive a 2 ohm load!!!!

Good luck,
Chris
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Old 29th August 2003, 10:32 AM   #5
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Default Here's a thought

If you're new to amp building, start by making a stereo amp using one of your transformers, running off +/-50Vdc rails (they're 35-0-35V AC, right?). I did just this, see
Avondale NCC200 module pics; or you could look for a MOSFET design as they're usually more resistant to thermal abuse than bipolars.

Once you're happy it's not going to overheat or explode, you can turn it from a 2x180W/4ohm amp into a 1x360W/8ohm amp by bridging it.

Build another identical one with your other transformer and you'll have two >300W monoblocks, no spare transformers, and hopefully no fried underpants...

Cheers
IH
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Old 29th August 2003, 12:42 PM   #6
Jeff R is offline Jeff R  United States
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The M250 is more like the regular Leach Amp, power wise anyway. The Leach Amp uses +/-56 volt rails and is rated at about 120 Watts into 8 Ohms. Marcus rates the M250 at 250 Watts into 4 Ohms. Into 8 Ohms, it would be 125 Watts. I cannot comment on the sonic differences, but I like my old Leach Amp and have spent the last three years considering upgrading it from a Version 3 (but with a Version 1 power supply, so I only get about 50 Watts) to 4.5. Someday......

With 35 VAC transformers, you can get out either about 49 or 98 VDC rails. Both amps would probably work ok at 49 volts and give on the order of 50 Watts into 8 Ohms, but only the Leach Super Amp would survive with 98 volts, and is the only one of the two that would give you your desired 300 Watts into 8 Ohms.

Again, a high powered amp is a huge project (and dangerous) and unless you have experience, I think starting with a less ambitious design would be best. You will learn all kinds of things while building, so why not learn on a cheap amp than on your dream amp? But whatever you decide, have fun.
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Old 10th October 2003, 06:10 PM   #7
darkm4n is offline darkm4n  Romania
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Default Just do it :))

Hi ,

I just finish building the M250 Amplifier ... )
GOOD DESIGN !!! VERY GOOD DESIGN !!!

I don't know what to say .. if a 15 year's old kid do it .. why you couldn't do it ?? No kidding .. i have only 15 years and i do it very good ...

Be carefull with the bias current ... !!


I have some problem's with one channel .. it have a little distorsion on the 60 - 120 HZ Band and the board with the distorsion it's getting hotter that the other ..

Look at my post on the M250 AMP .. maybe someone will help ..

[QUOTE]I changed some transistors on the board , all the capacitors , and the distorsion it is still there ...

I notice that the board with the distorsion in getting hotter ( not the output transitors , the MJ15030 and MJ15031 , MJE 340 - bias ) that the board with cristal sound witch is allmost cold ...

The bias is set at maximum resistance on all the board's . I used the same tipe of output transistors , the same resistor's the same all .. it's not diference between the board's ..

The distorsion it's on the 80 - 120 HZ band ...

And of course .. it's not normaly that one board it's getting very hot .. and the other ( no distorsion .. ) no even hot ...
It's getting hot in 2 Minutes since i open the amp .. at half volume .

The bias is set at 850 Ohm on all the boards .

Maybe a resistor it's damaged ? Maybe a diode ? Maybe output transistors ?

Thank's ! Mihai .
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Old 10th October 2003, 06:57 PM   #8
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Default Suggestion

Just a suggestion
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Old 10th October 2003, 07:00 PM   #9
darkm4n is offline darkm4n  Romania
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Default Nobody ??

Nobody can help me with my problem ???
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Old 10th October 2003, 07:06 PM   #10
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Default darkm4n

Maybe you have a oscillation problem on that board......
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