Yuanjing TDA8920B board unstable - 50KHz - Page 2 - diyAudio
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Old 14th January 2012, 02:46 PM   #11
hozone is offline hozone  Italy
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Quote:
Originally Posted by trevmar View Post
hozone, the safest way is to put a low-voltage 1KHz square wave into the input of the amplifier, not enough to drive it very hard, and look at each of the outputs with your oscilloscope. If you see "ringing" at the rising and falling edge of the square wave that means the circuit is not critically damped. A lot of oscillation means the circuit is unstable.

Better analog scopes will be able to trigger on the higher frequencies at the edge of your squarewave, and allow you to measure the period of the HF waveform, and figure out the instability frequency. Never drive your amplifier with supersonic input frequencies while there is any ringing on the edge of your squarewave.

I hope that helps
.
thank you,
let's say a 1Vpp square?
i have to scope the output of the board with a dummy load, a speaker load, or simply using the scope?
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Old 14th January 2012, 08:43 PM   #12
trevmar is offline trevmar  United States
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Quote:
Originally Posted by hozone View Post
thank you,
let's say a 1Vpp square?
i have to scope the output of the board with a dummy load, a speaker load, or simply using the scope?
1V p-p will be just fine, although you will find the stability changes with level. But that's a discovery waiting in the wings to keep life interesting

It will behave very differently into an open circuit, a resistive load, and a speaker (which is often regarded as an inductor and resistor in series). I just posted a small simulation file so you can plug in the values to MultiSim, and double-check what your scope is telling you. See the topic at:
Class-D Output Filter Designs, SURE TDA7498 as example

: Enjoy!
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Old 15th January 2012, 01:43 PM   #13
hozone is offline hozone  Italy
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thank you!
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Old 28th January 2013, 11:36 PM   #14
pix is offline pix
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Join Date: Jan 2013
Good thread!
I bought one of these - at high power the silicon bathroom sealant liquifies and leaks out between the chip and heatsink, so I too cannot recommend it as a decent thermal compound.
I also have the 50khz oscillation, it comes as standard I guess, hopefully after the recommended mods in this thread I'll have something half decent.
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