400 watts rms 8ohms audio amp design...help

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I only got a 38-0-38 @ 8ampere transformer available,,,but the designed I got from the web required for about 50-0-50 @10 ampere can any one help me on the design to acquire the 400 watts rms.... if there is can anyone provide me also a PCB layout.....


thankz and more power to all diyaudio member....
 

taj

diyAudio Member
Joined 2005
I would expect you to get 250-300 watts with a 4-ohm load using that transformer.

Just a hunch (without the arithmetic), but I don't think a 2-ohm load is advisable with that transformer, but it would probably provide 400 watts before it burst into flames.

..Todd
 
I would take about an hour to burst into flames. You'd notice the 'heat smell' long before that. And if there's a thermal cutout device installed it would trip before things got out of hand.

It's still a bigger trafo than what they usually get away with in store-bought amps.
 
darkwind_j said:
I only got a 38-0-38 @ 8ampere transformer available,,,but the designed I got from the web required for about 50-0-50 @10 ampere can any one help me on the design to acquire the 400 watts rms.... if there is can anyone provide me also a PCB layout.....


thankz and more power to all diyaudio member....

These are mere specs. you do not know the capability of the transformer until you measure it....

for starters, you can give the dimensions of your 30-0-38@8amps transformer, the wieght, what size core does it use and then we can judge...

what about dc resistance of the primary and secondary windings? can you measure them and report here?

these numbers that i mention will tell if what you have is sufficient or not...

from experience, there is not a world of difference between an 8 and a 10, for all we know, your 8amps traffo would probably suffice....
 
darkwind_j said:
how about if I'll use the LM4702 together with a darlington pair output transformer using a 35-0-35 @ 8 amp. transformer how can I get the 400 watts output..Is it possible to attained the 400 watts rms @ 8 oms

100watts 8ohms, 200watts 4ohms, 400watts2ohms with that kind of transformer....assuming your rails can be kept constant...

the psu matters a lot, if you want the figures at 2ohms, you better have a power transformer that is robust and able to supply that power....
 
how about if I'll use the LM4702 together with a darlington pair output transformer using a 35-0-35 @ 8 amp. transformer how can I get the 400 watts output..Is it possible to attained the 400 watts rms @ 8 oms

darkwind_j
Please sit down and reconsider your projekt.........

From your posts here, I assume that you are not very experienced (Cheer up. We all had to start somewhere).

Building a power plant amplifier delivering 400W into 8ohms is not an easy task- even if you have a working circuit schematic and perhaps also the PCBs for it. There are so many things that can go wrong, and it could easily end up as a big :hot:

Your transformer will NEVER give you 400W into 8ohms!
For that you need +/-80VDC rail Voltages. Your transformer (one place you state 2x35VAC and next 2x38VAC) will only give you app. +/-50VDC rail Voltage = 150W into 8ohms.
You can reach 300W by making 2 identical amps and bridge them. But then you need two trafos + lots of uF in the PSU.
 
Hi Jan, First let me say I agree with most of what you say. This is not a project for beginners.

Where I disagree is on the power output that can be obtained from his transformer. Two 100W channels (8 ohm) that produce 200W at 4 ohm can be bridged to give 400W at 8 ohms for music reproduction. It will not do that power very long with high power steady state signals while testing.
 
560VA should be able to support one 400W output assuming 1/3 average power program material (which is quite heavily compressed stuff).

The voltage seems to be just on the edge for getting 400W/8 ohms bridge. It might not make it all the way to 400W, depending on transformer regulation and such, but it will be close.

edit: I was referring to the 35-0-35V 8A, and didn't notice there were several different transformers being discussed. The 38-0-38V should give enough voltage for 400W 8 ohm bridge.
 
I already start building from 50 watts to a 200 watts amplifier... due to limited parts of my project that why I seeking for help if I could used alternative spare part.... I found a lot of diagram but the problem is the transformer which limited only to that 38-0-38 @8 amp. max to my country...
 
Steve Dunlap said:
............... This is not a project for beginners.

...................Two 100W channels (8 ohm) that produce 200W at 4 ohm can be bridged to give 400W at 8 ohms for music reproduction.
theoretically this is correct.
Your suggestion requires twice as many amplifiers and each has to have 40% more current capability to achieve that 400W into 8ohm target.
That is simply a waste of resources due to trying to use the wrong transformer for the "invented" target.
 
The only extra parts needed are one opamp, a pair of predriver transistors, a pair of driver transistors and some resistors compared to a non-bridged amplifier. You will only need one power supply capacitor with half the energy storage capability and you might be able to use fewer output devices, depending on type, thanks to the better SOA at lower voltage.

For an amplifier with many channels it requires separate power supplies for each one though. Still Crown (MA series and similar) chose the topology as it let them use fewer/cheaper output devices and power supply capacitors. The extra driver transistors don't cost much compared to the savings in power supply and output devices.
 
darkwind_j said:
I only got a 38-0-38 @ 8ampere transformer available,,,but the designed I got from the web required for about 50-0-50 @10 ampere can any one help me on the design to acquire the 400 watts rms.... if there is can anyone provide me also a PCB layout.....


thankz and more power to all diyaudio member....


I have a PCB for a 450WRMS MOSFET amp if your interested.

I also have a PCB for a 450WRMS tube/MOSFET hybrid amp.
 
Jan Dupont said:
by making 2 identical amps and bridge them. But then you need two trafos + lots of uF in the PSU.


Jan Dupont said:

Lack of experience, I think ;)


megajocke said:
The only extra parts needed are one opamp, a pair of predriver transistors, a pair of driver transistors and some resistors compared to a non-bridged amplifier. You will only need one power supply capacitor with half the energy storage capability
that's where we disagree.
 
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