I have a tda7498e amp board that has a high noise floor and the only was I can describe the noise is as a staticy hiss. This is coming from the amp board itself because it happens whether the inputs are connected or not. I am using a 36v 10a SMPS to power it so to rule out noise from the power supply I tried it with a tpa3255 amplifier board and its quiet on that board. The tda7498e has noise with every power supply I tried including 2 different laptop 19v psu's and an unregulated ring transformer. I think I narrowed it down to the 50k volume pot on the board, or at least the pot is contributing or interacting somehow. When the pot/volume is turned all the way down the noise floor is extremely quiet/almost non-exsistant but as soon as you start turning the volume up the noise gets louder and louder. I then tried removing the pot entirely and placed a jumper on the board connecting the pots pins 1 & 2 and the noise is still there! So now I am stumped as to what to do next. The circuit is based on the datasheet layout except for the addition of the 50k pot.
The pot is in between the 1uf and 1nf caps.
The pot is in between the 1uf and 1nf caps.
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Well I removed it entirely now and the noise is still there as if the pot was turned up high. The gain is configured at the lowest setting around 25db if I remember correctly. At this point I don't know if there's something wrong with the IC itself or there's something causing it that I'm doing.
I'll try and ground them as well as pins 32&33 because they're also used it's a 2 channel design. Not sure I'll be successful because the pins on the chip are so small.
Shorting c3 and c13 will short left and right positive inputs to ground, should I also short the negative inputs to ground or will that not matter since it's already connected to ground?
Well it got a little better when shorting the inputs after the pot. I ended up installing a ring terminal around the shaft of the pot and connected the other end to the audio input ground. It's as good as it's going to get at this point I'm afraid. Doing a little more reading online seems like the tda7498e chips are known to be noisy. I guess another tpa3255 is on my future to do list.
This is why:
400 µV is pretty darn noisy. And that's at the lowest gain setting. It likely only gets worse at the higher gain settings. It looks like the voltage on the GAIN pin (pin 30) controls the gain:
I would look at how pin 30 is connected in your circuit. If there's a resistive divider there I'd look at changing it such that the GAIN pin is connected to ground. That'll reduce the gain to 23.8 dB, which is more than plenty in 99% of cases.
Figures screen grabbed from the data sheet: https://www.st.com/resource/en/datasheet/tda7498e.pdf
Tom
400 µV is pretty darn noisy. And that's at the lowest gain setting. It likely only gets worse at the higher gain settings. It looks like the voltage on the GAIN pin (pin 30) controls the gain:
I would look at how pin 30 is connected in your circuit. If there's a resistive divider there I'd look at changing it such that the GAIN pin is connected to ground. That'll reduce the gain to 23.8 dB, which is more than plenty in 99% of cases.
Figures screen grabbed from the data sheet: https://www.st.com/resource/en/datasheet/tda7498e.pdf
Tom
Look what I found (cleaned up for clarity)...
So try this:
1. Pull any jumper at J12 (if applicable)
2. Add jumper on J9
That'll set the gain to 23.8 dB.
Tom
So try this:
1. Pull any jumper at J12 (if applicable)
2. Add jumper on J9
That'll set the gain to 23.8 dB.
Tom
Wow thank you so much Tom!! Thats by far the most information I've seen about the gain on the tda7498e. The board I have has dip switches to adjust the gain between the 4 settings in the datasheet so pin 30 may already be grounded with the dip switches off but I will pull a the heatsink and verify. Thanks
Edit: I have some high sensitivity drivers hooked up to this amp which makes the noise floor worse obviously. Although I tried it with less sensitive drivers and could still hear noise only marginally quieter.
Edit: I have some high sensitivity drivers hooked up to this amp which makes the noise floor worse obviously. Although I tried it with less sensitive drivers and could still hear noise only marginally quieter.
Well it's hard to know when most information on the Internet is contradicting. When I decided to buy the 2 boards I have all I could find was good reviews on the board and I liked the power specs. There isn't many mainstream options for amp boards that use 32-36vdc and put out decent power.
Certainly opinions on the internet vary widely. Better read the datasheet that reveals the flaws or limitations
and look at TPA3255 noise specs.
You will not consider crappy ST any more...
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This is simply too much. Resistor thermal noise, equivalent of 12.5k resistor when driven from very low impedance, worse if from higher impedance.
It is simply too much.
Just because noise is far above thermal resistor noise.
Besides the abnormous input voltage noise this chip seem to show high input current noise, too.
Just because noise is far above thermal resistor noise.
Besides the abnormous input voltage noise this chip seem to show high input current noise, too.
400 µV is very noisy. That will be audible even with a medium efficiency bookshelf speaker; possibly even audible from the listening position. My power amps measure below 20 µV (14.1 µV, A-weighted). The Class D amps by Purifi and Hypex are very quiet as well.
I don't see the attraction either. Same goes for the TDA7294.
Tom
When I started designing class-D amps with chips more than 10 yrs ago,
I compared datasheets of MPS, ST and TI.
My two key parameters were Rdson and noise.
Following these I ended with TI.
Never looked back to ST who btw seem(ed?) to be premium supplier of Bose.
I compared datasheets of MPS, ST and TI.
My two key parameters were Rdson and noise.
Following these I ended with TI.
Never looked back to ST who btw seem(ed?) to be premium supplier of Bose.
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