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Old 23rd June 2015, 08:03 AM   #1
TroelsM is offline TroelsM  Denmark
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Default TDA7498 ebay strange output levels

Hi

I've been building an active speaker with a TDA7498 amp from ebay:

El-cheapo active LOUDspeaker

The amp-module is the cheapest one from ebay:
100W High Power 100W Class D Amplifier Board DC20V to DC36V TDA7498 | eBay

The amp works, but measuring the output levels of the bridges I got some odd results.

PSU was approx 32V (lab psu, stable). One half of each bridge did approx 25Vpp into 8Ohm without visible distortion on a scope, but the other half does only approx 15Vpp at the same time.

This goes for both channels on the board and for two boards purchased 6weeks apart.

Is this normal operation for a TDA7498?
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Old 4th July 2015, 02:55 PM   #2
Dinithm is offline Dinithm  Sri Lanka
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Quote:
Originally Posted by TroelsM View Post
Hi

I've been building an active speaker with a TDA7498 amp from ebay:

El-cheapo active LOUDspeaker

The amp-module is the cheapest one from ebay:
100W High Power 100W Class D Amplifier Board DC20V to DC36V TDA7498 | eBay

The amp works, but measuring the output levels of the bridges I got some odd results.

PSU was approx 32V (lab psu, stable). One half of each bridge did approx 25Vpp into 8Ohm without visible distortion on a scope, but the other half does only approx 15Vpp at the same time.

This goes for both channels on the board and for two boards purchased 6weeks apart.

Is this normal operation for a TDA7498?
yes it is,,,,
just google full bridge configuration(full bridge amplifier,H bridge)
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Old 4th July 2015, 03:10 PM   #3
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A lot of Bridged class D amplifiers use virtual ground. That will confuse most readings!
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Old 6th July 2015, 07:37 AM   #4
TroelsM is offline TroelsM  Denmark
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Hi.

Thanks for the answers.

I understand how a bridge-amp works and the amp-board in question do indeed use a virtual GND. Input is referred to Vss.

But that stilll dosnt explain my findings. No matter what reference-point we use, the two halves of a bridge should output about the same level PP. The two signal are 180 deg out of phase, but approx the same level pp. Otherwise one half will clip before the other and output-level becomes lower than expected.
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Old 6th July 2015, 11:08 PM   #5
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You may check for the bootstrap voltages.
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Old 6th July 2015, 11:43 PM   #6
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I once saw a sign that said..."Our work is Good, Fast and Cheap, you may choose any two; Good and Fast won't be Cheap, Good and Cheap won't be Fast, and Cheap and Fast won't be Good." Aparently you chose Fast and Cheap.

Mike
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Old 7th July 2015, 01:24 AM   #7
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Might be worth measuring virtual ground to each rail 0v and 35v.
If that is not the same you will get same error on output.
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Old 7th July 2015, 08:04 AM   #8
TroelsM is offline TroelsM  Denmark
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@Doctormord: If the bootstrap-voltages was sagging, then both parts of the bridge should be equally effected yes?

@Mike: Cheap; yes. Fast? Not so much. I'm not blaming the seller. Just being curious and trying to improve. Part of the Ebay-thing is getting the parts to actually work :-).
Do you have any input on the OT?

@Nigel: If the virtual GND was not at the correct potential, then both parts of the bridge should be affected?

Kind regards TroelsM
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Old 7th July 2015, 10:28 AM   #9
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Quote:
Originally Posted by TroelsM View Post
@Doctormord: If the bootstrap-voltages was sagging, then both parts of the bridge should be equally effected yes?
Would guess, theres something wrong with the caps at volume level. Have a measure for the caps in the working brigde compared to the not working one and see if they capacitance differs.

Quick and easy, change all bs-caps to known working ones with the right capacitance.
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Old 8th July 2015, 04:22 PM   #10
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Quote:
Originally Posted by TroelsM View Post
@Doctormord: If the bootstrap-voltages was sagging, then both parts of the bridge should be equally effected yes?

@Mike: Cheap; yes. Fast? Not so much. I'm not blaming the seller. Just being curious and trying to improve. Part of the Ebay-thing is getting the parts to actually work :-).
Do you have any input on the OT?

@Nigel: If the virtual GND was not at the correct potential, then both parts of the bridge should be affected?

Kind regards TroelsM

OK, My point is that if you buy something that is TOO inexpensive for what it claims to be, either it is of inferior quality, design or was derived from slave labor. I dislike all three possibilities.

Mike
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