Fun fact #2: I have a design on the bench which runs three 2164 gain cells in parallel, each driven by a 7k5 resistor. Driven off +/-15V rails in class A mode, and including the input buffer and output transimpedance converter I am measuring a THD+N of around 0.007% (audio bandwidth) with a 0dBu (775mV) sinewave input signal. Figures for 12V rails are slightly higher.guest wrote:also, fun fact, the noise on a 2164 is lower for higher input impedances. i just ran a series of tests to confirm this. the noise gets really bad, to the point of being unstable below 10k, so i wouldnt ever go lower than 30k. that resistor sets the gain of the internal feedback circuit. this feedback cancels out a lot of the noise (and i think is the main reason the 2164 is so low noise), and more gain in your feedback loop means tighter control. there isnt much improvement in noise for resistance above 100k.
What I do find makes quite a bit of difference is the mode bias current. Ideally the bias current should track the input signal amplitude. This called sliding bias and used to be used in some high-end VCA-automated mixing desks and effects units (compressors).
Also at lower input resistor values you need to adjust the input compensator as otherwise the gain cells oscillate at high input levels - you can see this in the infamous figure 6 in the SSM2164 datasheet where it causes the sharp rise at the far right of the curves (the oscillation is lumped in as noise in the measurement). However in my case because of the input matching network I keep the individual levels down and so the dominant distortion mechanism is the op-amps hitting the rails. At higher input signal levels the THD+N rises, but not too bad; for example, at 2Vrms (+8.2dBu) I'm measuring around 0.027%, rising to around 0.15% at 5Vrms (+16dBu). This is work-in-progress so take these numbers with a pinch of salt.
One thing to be careful of with these parts is the performance spread is huge! Back to the datasheet, and figures 4 and 5 show that, for example, the THD+N has a spread of about 10:1. I've seen some of the raw measurements overlaid on a single plot and the curves are all over the place.
But then this device is built using fairly standard bipolar process, not some fancy process such as THAT has. Doug did a pretty good job of designing a VCA topology that is quite immune to process variation. It's just a shame that ADI killed off both the SSM2164 and it's predecessors (I suspect the 2164 was a last-ditch cut-n-shut job to cram four VCAs into a single package resulting in chopping off all the good bits to make it all fit into a cheap package). Good to see SSI getting the old team back together again and new 2164s coming out soon (very exciting times!!!): https://www.muffwiggler.com/forum/viewt ... 33#2703533
As for the issue of mixing multiple signals into one 2164 input, yes that is perfectly possible. You need to know the characteristics of the signals you're mixing (audio, or synth, correlated or not) to determine the resistor values to avoid driving the 2164 into distortion, but if it were me I would buffer the input signals and then use typical value resistors not noisy 100k monstrosities.
Neil
