|9th March 2012, 02:43 PM||#381|
Join Date: Jan 2012
If there were big resonances AND big spikes in the music to trigger those
resonances, you'd see them.
Bass tones are poor at causing the "ringing" we're looking for, if the ringing
would be at high frequencies. And treble tones are poor at causing base ringing.
To search for high-freq ringing, you need
(1) a sin-sweep generator providing perhaps 1volt peak-peak
(2) a fast-edge generator, and a 555 "rectangle wave" IC at 1KHz period
would be fine; the edges of 555 are faster than 1microSecond, thus serving
to stimulate lots of candidate ringing; the fast edges are just like the slap
of a hand on the belly of a fat guy---the belly joshes around and "rings"
and soon stops because of dampening.
(3) music with fast edges, such as (your call) high trumpet notes? they don't
sound like pure tones to me, so probably are good waveforms to search for
The lowest resonance/ringing frequency would be two of the 2,200uF in a loop
with some of the wire. In series in the loop, they become 1,100uF. Assume
couple pieces of 4" (0.1meter) wire, which has inductance approximately of
MUo * length of wire in meters, or [ 4*pi*10^-7 ] * (0.1 meter) = 4*pi*10^-8
or 12.6 * 10*-8 ===~~~ 10^-7 Henry.
Since this is a nice round number, we'll assume the total loop inductance is 10^-7.
Resonate freq of 1,000uF (10^-3 Farad) and 10^-7 Henry is 0.159/sqrt(10^-3-7)
All other resonances will be higher.
I think you should try some loud high-freq unpure-sine material,
or build up a 555 oscillator running at 10,000Hertz. And use the scope
to check. The OPA627 has good power-supply rejection at 20,000 and lower
Examine the third row of plots in the section "Typical Performance Curves";
depending on how you've got the opamp connected (don't you hate those
weasel words? check out the schematic! and the compensation cap inside)
you should have at least 60dB (1000:1 reduction) and maybe 80db (10,000:1)
at 20,000Hertz. Thus 1milliVolt of rail-ringing would appear as 1 microvolt
between input to opam, to be amplified by whatever gain configuration you
are using (Unity?).
If we aim for damping factor of approx 1, which also approx Q=1,
then Rloss = Z(L at 16KHz) = Z(C at 16KHz). One is easy to remember. Use one.
But...what if Q=5, because Rloss varies with temperature or aging or brand_name
or Vdd? I like a Q=5, because its dampening, once the ringing has been triggered,
is 6dB per cycle. Thus if the initial half-sin wave is 10milliVolts, the following
opposite polarity half-sine will be 10mV * 0.70 ,
and the following same polarity half-sine will be 10mv * 0.7 * 0.7 = 5milliVolts.
I can easily predict how fast a Q=5 resonance will decay, and Q=5 does last
a while, so coloration is possible IF your PowerSupplyRejection is poor.
Back to Q=1. No ringing, thus no sin-response, no coloration, but you may see
the actual music or voice waveforms on the VDD, accurately appearing, if
the caps are small. [ hmmm I hope this does not become "folklore". ]
What Rdampen to use? Rloss = Z(L @ 16KHz) = Z(C @ 16KHz)
== 2*pi*16KHz * 100 nanoHenry = 10^+5 * 10^-8 ==> 10^-3
I expect the wiring resistance, the Caps ESR, or the solder!
to provide enough dampening.
|10th March 2012, 09:13 AM||#382|
Join Date: May 2004
Yes, I have previously gone that path trying to use LTSpice to model all possible RLC in the power supply caps and wires. The pictures were never pretty unless I padded resistance here and there. But in reality, I never like the sound of a power supply with padded resistance. Any resistance added creates ripples which are distortions.
I have built more than a few types of line level regulators. So far the best sounding one is Ikoflexer's shunt reg, but it is quite difficult to build. From learning from and tuning the Iko reg, I gained a bit of experience.
Now I want to sell one of the active speakers I previously built. I don't want to build a shunt reg in the active crossover, so this time I built a LM317/337 reg. With all the experience I gained from building the Iko reg, this time the LM317/337 regs seem to work fine, actually, very fine.
Finding no trace of ringing from my limited scope up to 10Mhz, and hearing the very clean sound from the speakers, I am pretty much sure that even if there are resonances above 10MHz, they would be so mild that don't really affect the sound.
I guess keeping the rail and ground paths short, using ground plane or thick wires, is the first step reducing possible resonances. I guess the 3 large 2,200uF has sufficient ESR to form the damping circuit which may reduce the possibility of ringing.
Last edited by HiFiNutNut; 10th March 2012 at 09:18 AM.
|23rd April 2013, 03:18 PM||#383|
Join Date: Jan 2002
Location: Sourhern MI
Question on power supply caps
Have been adding a 10 or so microfarad film cap slightly before the main supply caps. Reason I am thinking this would be a good thing is that any voltage spikes get taken by the film caps (more reliable and usually a higher voltage rating I've chosen for this reason).
Thinking this would make life easier and perhaps lengthen the life of the electrolytic caps.
Any idea if this idea has any merit or problems??? So far it has not caused any difference in sound quality that I can tell.
|23rd April 2013, 03:36 PM||#384|
Join Date: May 2007
No merit, unless you have an unusually high resistance/inductance path between the film cap and the main electrolytics.
Minor possible problem: any HF voltage spikes which do make it past the transformer and rectifier will now see a lower impedance path so the HF current could be greater. This means that is it even more important to ensure that the relevant circuit loop has a very small area to reduce inductive coupling into signal circuits - not always easy to do with large PSU components.
If you want a bypass, the right place for it is as close to the amplifier output circuit as possible i.e. as far away from the PSU as possible.
|23rd April 2013, 05:22 PM||#385|
Join Date: Sep 2006
If you can provide quantitative data about your configuration, I can show you the difference (just a pic of your set-up would already be sufficient to make a rough estimate).
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪
|23rd April 2013, 06:46 PM||#386|
Join Date: Aug 2003
Location: Santa Cruz, California
You're probably better off adding RC snubbers to the rectifier, which would reduce a major source of HF noise (ringing on diode cutoff) and also bypass a certain amount of external crud.
Here's a previous discussion:
Fast Recovery rectifier diodes
|23rd April 2013, 10:09 PM||#387|
Join Date: Jan 2002
Location: Sourhern MI
Thank you for the feedback. As it hasn't made any difference in the sound (that I can tell) and if it doesn't help lengthen the lifespan of the main caps, seems a waste.
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