Audio distortion using IIR filters for an audio equalizer

I am building an audio equalizer. I have created my filters using the ellip() function. Unfortunately I am still hearing distortion when I test the equalizer with music. I am currently using the sosfilt() functions to filter the audio. I tried filter() but there was way too much distortion and fitfilt() did not really help. Below is my filter function code for the low pass section. This filter has n = 6 coefficients. Any ideas how to improve my filtering so as to reduce distortion in the audio?

function filtered_audio = lpFilter(input_audio, Fs)
    % Desired specifications
    % pb_ripple
    Rp = 1; % in dB
    
    % stopband_attenuation
    Rs = 50; % in dB
    
    % passband frequencies
    Wp = 500 / (Fs / 2); % remember to divide by (Fs / 2) to normalize
    
    % stopband frequencies
    Ws = 600 / (Fs / 2); 
    
    % Get minimum number of poles required
    [n, Wn] = ellipord(Wp, Ws, Rp, Rs);
    
    % Get zeros, poles and gain of filter with the specs
    [z,p,k] = ellip(n, Rp, Rs, Wp);

    % Convert to SOS matrix
    [sos, gain] = zp2sos(z,p,k);

    filtered_audio = sosfilt(sos, input_audio); 

%     maxVal = max(abs(filtered_audio));
%     if maxVal > 1
%         filtered_audio = filtered_audio / maxVal;
%     end

end

Answer 1

[Mathieu NOE]

hello

simple correction

% Convert to SOS matrix
[sos] = zp2sos(z,p,k); 
filtered_audio = sosfilt(sos, input_audio); 

and you get :

sos = Columns 1 through 5

0.0042342 -0.0029254 0.0042342 1 -1.7705

1 -1.6899 1 1 -1.8021

1 -1.787 1 1 -1.8323

Column 6

0.79926

0.90188

0.97547

when coding your way :

[sos, gain] = zp2sos(z,p,k);

sos = Columns 1 through 3

1 -0.6909 1

1 -1.6899 1

1 -1.787 1

Columns 4 through 6

1 -1.7705 0.79926

1 -1.8021 0.90188

1 -1.8323 0.97547

you get a SOS filter with a gain in the passband frequency range of +46 dB (roughly) instead of 0 dB

and then you get a huge clipping of the filtered output as it will exceed +/-1 range

[Mathieu NOE]

hello @Samuel Witt

problem solved ?

[Samuel Witt]

Yes. Thanks! It was more to do with filter states when I used the filter() function but got it thanks.