TPA3255: TPA3255DDVR audio frequency response issue

Subash Chandra bose M

Part Number: TPA3255

Hi,

We use TPA3255 amplifier IC in PBTL mode for 300W output power. The frequency response that we got was not upto the bandwidth of audio frequencies. We got frequency response BW from 40Hz to 30khz instead 20hz to 20khz.The higher cutoff frequency is fine and we dont have any issue in that. But at lower frequency we need to improve it to 20Hz.Here are the specifications: Pvdd(V)=35Vdc, Lout=11uH(MA5172-AE),Cout(2caps in parallel)= 1uH(MKP4-1.0/250/10) & 0.47uH(MKP4D034704D00KSSD),Load impedence = 33.3ohm. Kindly help us and reply ASAP.

over 5 years ago

0 Subash Chandra bose M over 5 years ago

Intellectual 280 points

0 Justin Bohr over 5 years ago in reply to Subash Chandra bose M

TI__Mastermind 27850 points

Hi Subash,

The low frequency cutoff will be determined by you AC input cap. A 10 uF AC cap should get you around 0.6 Hz Fc, while a 1 uF will be 6 Hz. What value are you using for your AC input cap?

Justin

0 Subash Chandra bose M over 5 years ago in reply to Justin Bohr

Intellectual 280 points

Hi Justin,

Thanks for your reply but I'm confused bit.Are you talking about dc blocking cap at the inputs(inputA and Input B)? If yes, then since we have been using differential input we have connected 1uf (GRM155R61E105KA12D) at each pin(inputA and Input B) with input C and input D grounded.

0 Subash Chandra bose M over 5 years ago in reply to Subash Chandra bose M

Intellectual 280 points

And what was the lower corner frequency you have achieved for TPA3255EVM?

0 Subash Chandra bose M over 5 years ago in reply to Subash Chandra bose M

Intellectual 280 points

Hi Justin,
Help us out when you are back online.

0 Justin Bohr over 5 years ago in reply to Subash Chandra bose M

TI__Mastermind 27850 points

Hi Subash,

On the EVM in PBTL mode, and a 4R load, the difference between 1 kHz amplitude and 10 Hz amplitude is .24 dBV. So the corner frequency is below 10 Hz.

What is the input source to the TPA3255 inputs? Is it straight from the AP or is there a DAC in between?

Do you happen to have a DC output cap?

Justin

0 Subash Chandra bose M over 5 years ago in reply to Justin Bohr

Intellectual 280 points

Hi Justin,

We haven't taken directly from AP and we didn't use DAC either. We actually boosted the signal using Opamp circuits. And for your information we have placed 100uf/35V electrolytic cap before to that 1uf cap(C44) and also before cap(C49),which is showed on the circuit diagram on top. that is 100uf and 1uf would be in series for two inputs. Will that be a problem?

0 Justin Bohr over 5 years ago in reply to Subash Chandra bose M

TI__Mastermind 27850 points

Hi Subash,

That technically should come out to a 9 uF equivalent capacitor, but have you tested the op amp circuit response by itself to make sure it is not the limiting factor for the low end response? What is the reason for the 100 uF cap in series?

Is there a DC cap on the output of the TPA3255?

Justin

0 Subash Chandra bose M over 5 years ago in reply to Justin Bohr

Intellectual 280 points

Hi Justin,

Sorry its 10uf instead 100uf my mistake. Actually this 10uf was placed at the output of opamp to block dc component from flowing to inputs of TPA3255. And we didnt check the opamp circuit response since we were not sure whether it is because of output LC filters or not.

0 Justin Bohr over 5 years ago in reply to Subash Chandra bose M

TI__Mastermind 27850 points

Hi Subash,

I would only use one AC cap between the Op Amp circuit and the TPA3255, and take a look at the response of the op amp circuit.

There should be no low frequency attenuation from the TPA3255 output LC filter as it is a low pass filter. The expected low frequency corner on the EVM is due to the 10uF input AC cap with the 24k input resistance of the amp, giving a 0.6 Hz fc. This is what I get on the bench with the EVM.

Justin

0 Subash Chandra bose M over 5 years ago in reply to Justin Bohr

Intellectual 280 points

Hi justin,

Ok I will check the response of the opamp circuit and I hope it would help us. And Thank you

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TPA3255: TPA3255DDVR audio frequency response issue