IFFT Synthesis from an empty spectrum I'm trying to generate a sawtooth wave using the IFFT. I can do this successfully by first performing an FFT and then putting that analysed spectrum into the IFFT, however I would really like to be able to write a UI that enables me to draw the bin magnitudes and generate a full spectrum ready for IFFT from that. I have been partially successful, however the waveforms don't look/sound quite right even though the spectrum input is per the formula for synthesising a sawtooth wave.  

My question is how do I generate a spectrum array of complex values for an IFFT when all I have are the magnitude values?

I know that bin 0 and 1 are zero valued. I've been trying to render the wave by only calculating the magnitudes and phases for the significant bins (bins with magnitude > 0). 
Am I able to put white noise in the remaining magnitude bins and then calculate the accompanying phases at the centre frequency or alternatively calculate these non significant bins  as random phase?  my start phase is -90, bin size 1024, complex fft.   Thanks in advance for your contribution.
I believe that if you will keep the phases and not convert it into magnitude (power spectrum) the IFFT will work. When you use the center frequency, the phase is 0, in other words, the real part is also the magnitude. As you move from the center frequency the error in magnitude growth as the phase growth. As result your error in recovery by means of IFFT growth.