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Class D Switching Power Amplifiers and Power D/A conversion

TDA8954TH - XH-M252 board - anyone?
TDA8954TH - XH-M252 board - anyone?
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Old 8th December 2019, 09:35 AM   #51
FauxFrench is online now FauxFrench  France
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The speaker protection circuit is designed with a simple relay that only connects or disconnects the speakers from the amplifier. No amplifier load when the relay disconnects the amplifier. That, I do not find good when a class D amplifier is started up without a load.
I short-circuited the relay contacts such that the speakers are also connected to the amplifier at start-up and shut-down. Then, the amplifier functioned well and all I hear in the speakers during start-up is a small "click". This type of protection circuits with a relay, I find mainly belongs to Class AB amplifiers without integrated diagnostic and protection functions. For this class D amplifier, I will probably disable the protection circuit.
Now the amplifier plays well with +/-24Vdc supply but even when the amplifier funtions well, the relay coil is not activated. I have a slightly high DC offset at one output (around 85mV) so I will try to make a compensation circuit later. 85mV should not influence the protection circuit.

Last edited by FauxFrench; 8th December 2019 at 09:37 AM.
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Old 8th December 2019, 09:28 PM   #52
dzwer is offline dzwer  Bulgaria
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No, 85mV is too small DC offset to activate the protection.
There should be a 1k-1.5k resistor directly connected at the output filter, before the relay.
Examine the protection chip, if it is OK or not.
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Old Yesterday, 08:27 AM   #53
FauxFrench is online now FauxFrench  France
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You are right that as an engineer I should investigate and repair the problem. However as a pragmatic, I should realize it is a useless feature anyway and just bypass it. I chose half-way with a quick investigation.

The design values shown in the NEC datasheet are used without further thinking. That means for pin 8, 15K to Vcc and 56K from pin 8 to pin 7. 15K to pin 8 is for 45V positive rail supply. However, as the rail capacitors are rated to only 35V and Fig.6 of the datasheet is disregarded, 15K is a poor choice. With +/-18Vdc supply, I measure 2.8V on pin 8 of the C1237HA and it has to be minimum 3.0V. The Vcc-ON detection is invalid for lower range supply voltages which the TDA8954 can else operate with.
I stopped my investigation here but now you know where to look if also you have problems. I for a start continue with my relay by-pass and the amplifier part itself seems to work well (luckily). Later on, the C1237HA will be removed and perhaps also the relay, if not too much of a hassle.

If anyone is looking for an amplifier board that needs no modification, this board is not it. With some modification, I believe it can be turned into a very well sounding amplifier. If you aim for high power as promised for this board (more hundreds of Watt), I would rather go for an IRS2092 based board as the cooling abilities are better with external power FETs.

Last edited by FauxFrench; Yesterday at 08:32 AM.
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