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How to calculate Limiter ADAU1701 VRMS

Category: Software
Software Version: Sigmastudio

Hi everyone, I need your help with my project.
I want to create a simple DSP with Adua1701, but I don't understand how I can calculate the components for the limiter, let me explain better:
I have a digital amplifier from which I can detect the output through the MONITOR_OUT PIN (with a 4.5Kohm resistor).
I want to inject this AC voltage into my DSP and through the limiter function, attenuate the signal so that it does not exceed 400W with an 8ohm load.
How can I calculate this? and in Sgmastudio how do I manage this function?.
I haven't found the formula to execute and I'm stuck.
Thanks for the support.

PS. The audio input into my DSP via XLR is up to 4V

  • Hello dstitlab,

    I had to read this several times and I think I understand now what you are doing but I am still not 100 certain.

    I assume the audio is coming into the audio ADC inputs that can be up to 4V rms? It is RMS? 

    Then you will be feeding the output to an amplifier that you want to limit to 400W of output?

    Then you mention a Monitor Out Pin. Is this an output from the amp as a voltage feedback of the output? Is it audio or a DC signal? 

    How many channels? If this is in one speaker then I guess one ADC input can be the audio in and the other ADC input can be the voltage feedback? 

    You will need to know the gain of the amp in dB. However, if you have a dummy load I would simply work on setting the limiter using the dummy load and an Audio Precision to measure the output power. 

    You will want to split this up into two problems. The first is from the DSP to the Amp. Use an internal oscillator as a test oscillator. These will output at exactly 0dBFS so it is a full scale output. You cannot go louder than that or the serial port data will be clipped. Sending this out of the DSP you will need to check if the amp is putting out greater than 400W. This also is subject to what you are doing with a DAC filter/buffer. 

    Then insert a limiter to limit the audio to be 1 or 2 dB less than the level that produces a 400W output. 

    Then focus on the input. You will need to add a "DC block" filter block. Then you will have to pad down the input and/or change the input resistor because 4Vrms is too high. I would suggest you put in an Opamp for a filter/gain stage rather than increase the input resistor. The reason is that the switched capacitor needs to have time to charge and if you have too high of an impedance it will take too long and be way off the true level. 

    I can supply some more details and a starter project when I have more info.

    Dave T