Please Note: The peak to peak figures used in these posts should not be taken as absolute, real world values.
The values are intended to show the relative differences that result from configuration changes under simulated conditions.
One of the reasons I've been very keen on flat impedance and flat peak-to-peak voltage variation across frequency is that I have a sneaking suspicion that regions of higher peak-to-peak variation result in "enhancement" of the frequency range it occurs in.
The peak-to-peak plots I've been posting are inidicative of the amount of variation from a centre level. Ideally the voltage should have zero variation around 4.000000V, but that does not occur in any of the Vref mods. The original buffer is possibly closest in terms of varying around 4V, but there are difficulties correctly compensating for load resistance which makes it less than ideal. Lowering output impedance reduces the shift significantly, but having a signfincant roll-off in outpt impedance means the amount of shift under load drops as impedance decrease resulting in a frequency dependant variation in Vref.
So all of these variables point to an optimal Vref having very low output impedance, with minimal roll-off due to RC filtering.
Over at DIYAudio TNT has observed that his modified board with 3300uF sounds woolly. From the photo TNT has posted of his board, it appears he's jumpered the series resistor, and compensation cap. As observed in my last Vref diatribe the feedback loop compensates for the trace resistance up to the series resistor, so effectively this configuration has no series resistance before the buffer output cap. While this config has 2mOhm output impedance at low frequency there is a 24mOhm peak around 10KHz.
While some contributors have suggested peaking is good providing impedance is low elsewhere I think there is evidence to suggest that is not the case.
Using my test loads from previous post I've simulated the way TNT's jumpered series resistor mod + 3300uF will behave.
100Hz - 50uV peak - peak. Ringing and overshoot on load step edges are apparent. Once the ringing settles the behaviour is good.
1KHz load - 51uV peak-peak. The instability in response to load steps at this frequency is obvious. The step response even looks woolly.
At 7.5KHz, 112uV peak-peak, but a clean response to load stepping.
If the Vref configuration was altered by replacing the series resistor jumper with 0.01R - to alter the config to a "LowRes" mod rather than "NoRes" mod.
100Hz - 49uV peak-peak. The obvious thing here is the lack of ringing, and very small overshoot but the settled step level is close to peak value.
1KHz 49uV peak -peak. Clean step response with no ringing and minimal overshoot.
7.5KHz 41uV peak-peak. Lack of output impedance peaking results in significant reduction in peak-peak variance.
The main here difference is TNT's Vref config is significantly under-damped whereas the LowRes Mod is slightly under-damped.
Bambadoo's experience is also enlightening...
His inital mod was 0R/0.1R/449R - Soren's original suggestion for the Vref. With 4000uF capcitance it behaves like this:
100Hz 408uV peak to peak. Nice and clean, but significant peak to peak variation.
1Khz 218uV peak - peak. Nice and clean, perhaps over-damped, but still high peak - peak variation. Note that the variation is highest at low frequency and Bambadoo reported boosted bass. Coincidence?
7.5Khz, 31uV peak-peak. Clean, low level variation. But does the fact that the load variation at 100Hz is 380uV higher than at 7.5KHz enough to create a sense of boosted bass?
Applying the LowRes mod to Bambadoo's 4000uF config (0.01R replaces 0.1R)
100Hz 48uV p-p. Significant improvement in peak to peak variation...
1KHz 48uV p-p. Peak-peak variaiton is virtually the same as at 100Hz.
7.5KHz 35uV p-p, slight roll-off in impedance and reduction peak-peak variation starting to appear.
Taking the idea of tuning the capacitance and Vref a step further I've suggested a further tweak - but it is heavily capacitance dependant.
From the sims I've found there is an ideal capactiance//output impedance balance. The general trend seems to be that the lower the output impedance, the more capcitance that is required to eliminate output impedance peaking.
As an example, Bambadoo's 4000uF per VRef is slightly over damping the "LowRes" Mod. It's not practical to source 0.008R resistors to tweak output impedance but there are otherways to solve.
As mentioned in a previous post the balance of the "compensation cap" resistor and "feedback resistor" determines the portion of voltage from each sense point, and as a result the amount of compensation for the output impedance of the Vref.
By increasing the portion taken from the remote sense point it's possible to adjust the overall output impedance of the Vref buffer to suit the capacitance.
The way I've been modelling this is to set the series resistor to 0.1R, the compensation cap resistor to 499R, and then adjusting the feedback resistor. With Bambadoo's 4000uF I've simmed a 40R value for the feedback resistor is close to optimal. The 499:40 ratio means that 92% of the feedback comes from the remote sense point, and effectively reduces the output impedance to 8% of the value of the series resistor, giving the equivalent of a 0.008R series resistor.
At 100Hz for example this has the effect of dropping the peak to peak variation to 41uV from 48uV.
1Khz 41uV p-p variation
So the basic effect of this tweak is to reduce the P-P variation at lower frequencies, but doesn't lower p-p variation at say 7.5Khz which remains at 35uV in the sims. The advantage of this is that the variation between 100Hz and 7.5Khz is reduced from 13uV with the LowRes Mod, to 6uV with tweaked configuration. A "First world problem" at this level, but a worthwhile goal I think.
* A play on "Ignoring the elephant in the room"