HELP! Testing my amp? It looses sound on strong bass

[DigiSoft]

Ok so I have 300W car amplifier that had burned MOSFETS. Now I repaired the amp and decide to test it in house environment. I set up big 300W toroid transformer that has 12V output. Then I used diodes to convert the AC to DC and on the end I used big capacitors to make it fine.
I turned on the amplifier and it sounds great but when I crank it louder SPECIALLY when the song has strong bass, the protection LED stops and starts the supply as the song goes. So when there is strong bass the amp stops and then continues. It is like the torrid is week?

Can you tell me what could be the problem. Should I test it on car battery? Is it possible that 300W torrid has no enough power?

Thank you.

[Perry Babin]

Unless the 300 watt transformer is under rated, it isn't likely capable of supplying enough power to the amp.

When the output drops, what is the DC voltage measured directly across the B+ and ground terminals of the amp?

[DigiSoft]

It stays at 16V and it doesn't drop.
I've tried with smaller torrid of about 100W and the EXACTLY same problem occurs but this time on very lower volume. It is very strange.

The 300W toroid is REALLY big so I don't think that could be underrated.

[chris661]

What value were the capacitors you used? You'll need several 10,000uF caps to give the power you're after.

What speakers are you feeding?

[DigiSoft]

Originally Posted by chris661

What value were the capacitors you used? You'll need several 10,000uF caps to give the power you're after.

What speakers are you feeding?

I used one cap from 6400 uF and I use 8 Ohm 15 inch woofer.
Without this capacitor is much worse because it is turning off even on lower volume. And this always happens on strong bass beats. After the beat it turns back on and the sound is little distorted.

[Perry Babin]

Measure the AC voltage across the B+ and ground terminals when driving the amp hard. Is it more than a fraction of a volt?

[event horizon]

Originally Posted by DigiSoft

I used one cap from 6400 uF

See below

Originally Posted by Perry Babin

Measure the AC voltage across the B+ and ground terminals when driving the amp hard. Is it more than a fraction of a volt?

Digisoft, a 10,000UF cap will have about 1Volt of Ripple with a 1Amp load, as you are using only a 6400UF cap this will be worse. A 300W car amp is likely to draw somewhere near 30A so effectively you are going to need at least 100,000UF of capacitance so as to keep ripple voltage below 3V. Of if you like so that the rail voltage doesn't drop below 13V when the capacitor isn't being charged

You don't have anywhere near enough capacitance, simple.

[percival007]

Why not test with a car battery? (A nice big Capacitor!!!!)

If not the one from your car then a second hand one? Get one from a scrap yard maybe, if no good for starting a car it may still have enough 'Ooomph' to test your amp, even if only for a few minutes.

Do make sure you fuse it correctly, even work your way up the fuses untill you are sure all is ok and feel safe enough to fit the 'correct' value.

And do be carefull, it's ''the volts that jolts, but the mills that kills''!!!!!!!

Percy.

[DigiSoft]

Oh thank you. I though the amp is bad.

Anyway I measured the AC voltage at full load it is 12V CONSTANT.
Then I measured the DC voltage and it is dropping from 15V to 14V when there is strong bass beat.

Yes I'll try it tomorrow on my car because it is late now. But I really wanted to hear its sound and when I crank it and I heard of/on sound I was disappointed. Then I put one capacitor from 6400uF and it wss better and I could turn it louder but then again at louder level started turning off. I gues I need more capacitors?

I have one question. I have made Class-D amplifier with TDA and it is 150W. This amp is for home use and it is packed with 300W transformer.. I used one 6400uF capacitor and no matter how loud I turn it doesn't turn off This is puzzling me.

And this amp is somehow hungry for capacitors.

[percival007]

Originally Posted by DigiSoft

.................I have one question. I have made Class-D amplifier with TDA and it is 150W. This amp is for home use and it is packed with 300W transformer.. I used one 6400uF capacitor and no matter how loud I turn it doesn't turn off This is puzzling me......................

Class D amps require much less current.

Courtesy of Wikipedia:-

''..............The main advantage of a class D amplifier is power efficiency. Because the output pulses have a fixed amplitude, the switching elements (usually MOSFETs, but valves and bipolar transistors were once used) are switched either completely on or completely off, rather than operated in linear mode. A MOSFET operates with the lowest resistance when fully-on and thus has the lowest power dissipation when in that condition, except when fully off. (When operated in a linear mode the MOSFET has variable amounts of resistance that vary linearly with the input voltage and the resistance is something other than the minimum possible, therefore more electrical energy is dissipated as heat.) Compared to class A/B operation, class D's lower losses permit the use of a smaller heat sink for the MOSFETS while also reducing the amount of AC power supply power required. Thus, Class D amplifiers do not need as large or as heavy power supply transformers or heatsinks, so they are smaller and more compact in size than an equivalent Class AB amplifier............''