[sdiy] Banana vs. 1/4" question

[Grant Richter]

> As a practical requirement one should be able to feed 16bit (real 16bit!)
> audio into the modular system, process it, and feed it back into the
> 16bit system without too much signal degradation: unwanted nonlinear
> distortion (sometimes we want some), hissing noise (Johnson, junction)
> and picked up RF and low frequency interference.

For setting up the Wiard modules I use a 24 bit sound card (M-Audio Delta
66) feeding a real time 64K point FFT with graphical display. Frequency bins
are around 1 Hz wide, bandwith 20Hz to 22Khz.

With nothing connected, the baseline noise floor floats between -130 dB and
-140 dB as would be predicted by the math (6 x 24 = 144, but you never get
it).

When I plug a TL074 output into the analyzer, the noise floor does not come
up at all. Whether the input is set for +4 dBv or -10 dBv.

Using a 24 bit, 1Khz sine wave (a utility of the software, direct loop back
gives a THD of 0.001%) second harmonic distortion is around -70 dBv refered
to a 0 dBv sine wave (0.02% THD as specified in the data sheet). And the
"Signal to Noise Ratio" utility reports a SNR of -70 dB (it simply
calculates the different between the peak signal and next lower signal
peak). Putting 8 TL074s in series (inverting configuration 100K carbon film
resistors) increases the THD to 0.05% with a SNR of -62 dBv.

But the white noise floor is still below -130 dBv, the inherent self noise
of the converter. So I am STILL not able to measure the white noise floor of
even a TL074 with modern equipment. Bear in mind the noise floor is not the
same as SNR. The second harmonic distortion is considered "noise" and is the
worst case signal. And there is no circuit gain, as modules are generally
unity gain.

As far as far as I can tell, analog electronics is VERY quiet in this
configuration. Quieter than the 24 bit A/D it's feeding. It seems that the
term "analog" has been confused with analog tape recording, which typically
have actual white noise floors of -50 dBv.

I know it's heresy, but as far as I can tell, the digital systems are ADDING
noise TO the analog signal. Not being less noisy.