Tales of Buffers on the Verge of Instability
While Søren has said that the stock buffer is stable with any capacitance you can throw at it, the Brown Burr app notes suggest this is not the case, and the big cap mods to the stock buffer push firmly into the realm of "potential instability issues"...
To illustrate what this potentially means for the vref buffer, I've simulated a short 20 cycle burst of a 6mA Sine load applied to the stock Vref with 1340uF of capacitance.
First at 100Hz (38.6uV p-p ripple):
then 1KHz (247.6uV p-p ripple):
and 10KHz (249uV p-p ripple):
You can see in the above the ringing on initial transients and post ringing as the load is removed.
Consider that we don't listen to steady sine tones or white noise, and the vref will be responding to pulsed loads in response to musical elements.
The potential issue here is that non-steady state loads will provoke load dependant instability in the vref supply. White noise and steady test tones seem to allow the vref to stabilise and therefore may give us a deceptively "rosy" view of the performance.
Using "stable" values with the 1340uF cap loading - 270nF compensation cap, 0.1R series resistor, and 1K feedback resistor, gives very different results with the same 20 cycle 6mA loading as above.
100Hz (216.1uV p-p ripple):
1KHz 1.202mV p-p ripple):
and 10KHz (197uV p-p ripple):
There is a difference in the level of peak to peak ripple, with the stable implementation having higher levels of ripple.
The question is do you opt for very low ripple with ringing and "verging on instability" or low ripple with stability...
470uF "stable" buffer
This basically the setup I have on my DAM1021 at the moment. The mod really needs a 220nF cap to be fully stable, but I've had to use stacked 100nF caps until I put in another order.
Modeled config: 470uF/16V Panasonic Polymer, 0.1R series resistor, 499R feedback resistor, 200nF compensation resistor.
100Hz (86.6uV p-p ripple):
1KHz (765.6uV p-p ripple):
10KHz (469uV p-p ripple):
Looking at Søren's factory mod, it seems the overall level of ripple is the only real draw back. Otherwise it is a thing of beauty, that is mirrored across the entire audio band.
10KHz (1.142mV p-p ripple):
Missing the obvious...
The "obvious" that has remained unobserved is that the plots in "Part 2" effectively show peak deviation from 4.0V in dB.
So the plots indicate a) the level of ripple, and b) the amount of variation in the level of ripple vs frequency.
For example the "Factory Mod" has very little variation in ripple between 100Hz and 20KHz. In other words there is little frequency dependent variation to the overall level of Vref.
The "big cap" mods have spectacularly low levels of ripple at some points but the peaking introduces a level of frequency dependant variation in vref, which is less attractive.
In searching for the "ideal" vref mod it may be that we need to strike a balance between overall ripple level, and the stability of ripple across the audio band, to minimise frequency dependant variations.
1000uF cap variants
Just to show different behaviour of various buffer configs...
These plots are all at the same voltage and time scale.
Stock + 1000uF, 20KHz sine load:
Factory Mod + 1000uF, 20KHz sine load:
Factory Mod + 1000uF + 0.01R , 20KHz sine load:
While not of the above configs this should give an idea of how a factory mod/940uF/0.01R combination stacks up against a stock+1000uF combination.
The +940/0.01R basically matches a +470uF/0.01R config up to 5KHz at which point the extra capacitance gives additional ripple reduction.
The main point of interest is the behaviour of the 0.01R resistor replacement of the 0.1R. As zfe points out this is getting close to trace resistance, so it may be possible to simply jumper across the existing resistor.
position. I've got some 0.01R on order so I'll give them a try to start with.
The 0.01R + 470/940uF drops the 100Hz noise by 20dB compared with the standard "factory mod". At 20KHz the roll off of the RC filter is the main determinant to ripple level, but you loose a little LF attenuation as you push the caps value up. It's not apparent with 1000uF, but very obvious effect with 2000uF in the simulations.