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About burnout amplifiers and operating frequency of the output stage
About burnout amplifiers and operating frequency of the output stage
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Old 23rd January 2019, 09:15 PM   #11
nigelwright7557 is offline nigelwright7557  United Kingdom
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Originally Posted by activik View Post
- dear friend, I have no questions to repair, I have a direct question - What should be done in order for the amplifiers to become more reliable ...
The main reason for class d amps failing is incorrect deadtime.
If it is too small or none existent you get shoot through (short circuit) which takes high currents.

In my experience when they blow they take out mosfets and driver circuit and class d IC too.
PCBCAD51 download or CDROM 2019 version out now http://www.murtonpikesystems.co.uk
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Old 23rd January 2019, 11:24 PM   #12
activik is offline activik
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Originally Posted by Achim1409 View Post
The deadtime is the time for the Fets to be switch on and off. The time between high side go off on low side switch on. If this time to short, you will possibly make a shortbreak and the current in the Fets rises until the blow. Small amount should be ok. If the deadtime is too long, you lose effectivity in a SMPS. If the deadtime in an Audiobank is to high, THD starts rising. If its too low, the Fets burning.

Originally Posted by Perry Babin View Post

It appears that the Q30 is switched on by Q24 when B+ is applied. When remote is applied, the PNP (Q24) switches off an allows CE39 to discharge through R154. When the capacitor voltage drops below the threshold voltage of Q30, Q30 releases the SD lines, allowing them to pull up and enable the 21844 driver ICs.
Click the image to open in full size.

What if we take and increase this line of inclusion by increasing the nominal value of the resistor from 4.7K to 10K or of the capacitor from 100uf to 220uf?
After switching on AMP we have a charge of capacitors and it takes time. By increasing the element ratings we get a longer run of the amplifier or am I wrong? would that make the launch stable ?


Last edited by activik; 23rd January 2019 at 11:52 PM.
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Old 24th January 2019, 08:27 AM   #13
Perry Babin is offline Perry Babin  United States
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Looking at an amp I have here, it appears that the switching from SD mode to enabled is determined by the capacitor on pin 2 of the 21844 and the internal pullup resistor (maybe, actually a constant current source) in the 21844. I can't say that the circuit above can't make a difference but I think it's unlikely to be critical.

The ICs use a fairly high value SMD capacitor between the SD pin and the negative rail. Increasing it would make switching from SD to enabled, slower.

I tried removing the cap from SD to the rail and switching the SD line very quickly (by raking a wire onto the negative rail). It survived a bit of this but the IC failed twice. The failure was simply a failure to switch on. I don't know if this failure could be a problem in larger amps that could cause catastrophic failure. With the cap in place and within tolerance, I don't think you could make the IC fail because the switching time would be much slower.
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