[jtwrace]

Thanks Mike, any data is good.

Only and only if it's good data.  Otherwise I'd rather have none.   

[*Scotty*]

I should have said that I trust that any data Mike is willing to share will be worth having and represent a correct use of the test equipment he has available to him.
Scotty

[mgalusha]

I should have said that I trust that any data Mike is willing to share will be worth having and represent a correct use of the test equipment he has available to him.

Be sure that if I don't think the data is valid, it won't leave my basement. That said, I've been fooled before but hopefully I've learned over the years to not be fooled as easily.

[serengetiplains]

One could also, while the amp is playing, capacitively couple the + and – power rails to a junction that is used as the input to an amplifier or digital-capture device.  One could then hear and see what's happening on each rail, and even listen/capture the difference component between the rails.

[*Scotty*]

I think there's a song somewhere like that, I won't be fooled again.
Scotty

[Occam]

I think there's a song somewhere like that, I won't be fooled again.
Scotty

Who would sing it? 

Maybe as song about low lying fruit, not by any means strange.
When all you have is a hammer, we tend to see everything as the thumb...

[serengetiplains]

To attach large bypass capacitors to the output electrolytics, I have decided I will use 8 strands of small-gauge litz magnet wire.  Two matched capacitors will be used as a single bypass cap, with two litz wires per capacitor lead, wired as follows (substitute two strands of litz wire per strand of wire in the photo):

[Rclark]

Who would sing it? 

They did, actually. http://www.youtube.com/watch?v=SHhrZgojY1Q

[serengetiplains]

Larger bypasses will be wired as above.  Smaller bypasses (say, 0.1uF and 0.01uF) will be wired with their leads directly soldered to the electrolytic terminals.  Total number of bypass capacitors will therefore include a minimum of three capacitor values to spread any inductive resonances I'm creating.  I may have to ditch the aluminum bottom plate to best pull this off.  That's workable.  There's a small (what looks to be) heat sensor that fits snugly into the bottom plate, but I'm supposing that sensor doesn't communicate with the amp unless the sensor gets too hot.  The output FETs will be well heatsinked, so I'm unconcerned about the sensor.

[serengetiplains]

Here's a clearer representation of my not-quite-inductance-free capacitor wiring.  Cough.

[jtwrace]

Are you going to re-measure what was a superb measuring amp?

[serengetiplains]

I doubt I will, JT.  That's assuming the amp works when I'm done with it. 

My only interest is better sound, and if I attain that to my satisfaction, I'll, uhh, probably look for more ways to get more of that satisfaction.  I get really bored with a certain level of amplifier performance.  The Ncore is to me good, but like any amplifier with normal-grade parts and less-than-stellar power supply regulation, it lacks a certain mmm-wow.

[*Scotty*]

serengetiplains, what value, voltage and film type are the caps that you're are going to use.
Generally the bigger the cap the lower it's resonant frequency will be. If the resonant frequency of the cap falls below the frequency range you are trying to affect the bypassing effectiveness will be greatly diminished. As an example I called Hovland a few years ago when they were still in business and got Bob Hovland on the phone to ask him what the resonant frequency was of an 8mfd 100 v MusiCap. Bob replied that it was 140kHz. I went ahead and bypassed the 4 pole Jensen caps in the power supply of my Tripath amp anyway even though the switching frequency was over 700 kHz. It made a small worth while improvement but I don't believe it was due to noise reduction. I think it might have had more to do with having a small amount of very low impedance energy storage available for reproducing the leading edge of the waveform. There certainly wasn't an adequate amount of power stored at 54volts to run the amp for any significant period of time.
If you are actually trying have an effective bypass at the amps switching frequency the smaller caps will have to have leads that measure in length in the Millimeters or the lead inductance will defeat your purpose.
By the way have you thought about replacing the wiring that came stock with something better,that would surely be a quick and simple upgrade path.
Scotty

[serengetiplains]

Here's another tweak idea.  The output FETs probably operate more linearly when instantaneous heat is removed as quickly as possible from the devices.  The stock TO-220 insulators look to be ceramic (aluminum oxide) which, per Aavid, has a thermal conductivity of 15.06 W/mK at 75° (167° F) (8.71 BTU/hr-ft° F).  Compare to beryllium oxide at 221.94 Wm-1 °C-1 (128.2 Btu/hr.ft.°F).

[*Scotty*]

What also has to be considered is the operating temperature curve of the output MOSFETs. There will be a temperature zone where the FET is at its most linear and that is not room temperature. In the case of my old Tripath amp the heatsink that came with the EVAL board was a little bit of too much of a good thing. Even though the amplifier was left on 24/7 the output MOSFETS were below their ideal operating temperature. The amp sounded better after it had been playing music for about 10 min. than it did when you first pressed play on CD player.
Scotty

[serengetiplains]

serengetiplains, what value, voltage and film type are the caps that you're are going to use.
Generally the bigger the cap the lower it's resonant frequency will be. If the resonant frequency of the cap falls below the frequency range you are trying to affect the bypassing effectiveness will be greatly diminished. As an example I called Hovland a few years ago when they were still in business and got Bob Hovland on the phone to ask him what the resonant frequency was of an 8mfd 100 v MusiCap. Bob replied that it was 140kHz. I went ahead and bypassed the 4 pole Jensen caps in the power supply of my Tripath amp anyway even though the switching frequency was over 700 kHz. It made a small worth while improvement but I don't believe it was due to noise reduction. I think it might have had more to do with having a small amount of very low impedance energy storage available for reproducing the leading edge of the waveform. There certainly wasn't an adequate amount of power stored at 54volts to run the amp for any significant period of time.
If you are actually trying have an effective bypass at the amps switching frequency the smaller caps will have to have leads that measure in length in the Millimeters or the lead inductance will defeat your purpose.
By the way have you thought about replacing the wiring that came stock with something better,that would surely be a quick and simple upgrade path.
Scotty

Scotty, I understand that any bypass capacitors will operate in the lower KHz region depending on lead inductance, meaning they won't affect switching-related artifacts.  Per your experiment, and experiments I have performed with class D amps (Tact, Tripath), bypassing to my ear creates obvious audible effects, the overall sum of which I find quite pleasing.

I suspect bypass capacitors help shunt DA recovery voltages to ground.  DA is generally modelled as follows:





You can see from the model that capacitors release spurious voltages related to the original input (charging) voltage at frequencies determined by the parallel RC combinations, the R indicating, I think, a lower frequency given the lagged release.  Here is a graph that purports to show recovery voltage/time after release of the original charging voltage.  Nasty stuff.





How this data reflects a working circumstance is beyond my guessing abilities, except perhaps to say this: DA recovery voltages will carry a frequency related to the overall peak charge/time of a capacitor.  I suspect that the pulse-current on the output of the Ncore (or any class D amplifier) can be decomposed into lower (including audio) frequency and higher (including switching) frequency components.  This decomposition is performed quite handily by the LCR filter on the output.  I don't see why a similar decomposition cannot be performed prior to the output, with L provided by the capacitor leads.  This would suggest the capacitor can operate at audio frequencies despite the high switching frequency, thus removing audio-frequency noise from the output rails.  This is actually what I hear.

Fwiw, here are a few charts I created to test DA recovery voltages.  I used a good DMM with a 10Mohm input, which is too low for consistent data for capacitors of differing values.  But the charts at least give some correlative evidence of DA effects, particularly for same-size capacitors.














I doubt I've answered your question to anyone's (including my) satisfaction, but what we need is some explanation, however tentative or cursory, to give some insight into what's happening.

I will use 1000V 2.2uF Solen metallized teflons for the large bypasses (smallish!), and 100V 0.1uF and 0.01uF film/foil teflons for the on-board bypasses.

[serengetiplains]

What also has to be considered is the operating temperature curve of the output MOSFETs. There will be a temperature zone where the FET is at its most linear and that is not room temperature. In the case of my old Tripath amp the heatsink that came with the EVAL board was a little bit of too much of a good thing. Even though the amplifier was left on 24/7 the output MOSFETS were below their ideal operating temperature. The amp sounded better after it had been playing music for about 10 min. than it did when you first pressed play on CD player.
Scotty

Good point, and thank you.  I guess I'll test this little tweak separately by listening.  I suspect the average heat will not change; it's the instantaneous heat spikes I'm going after.

[serengetiplains]

A better model of DA is the Cole-Cole parameter method (graphics below from here: http://www.cktsim.org/modeling/da.pdf ).  This model characterizes DA as follows:







Z[A] is graphed as follows:







... leading to a capacitor model that includes DA, which looks like this:

[*Scotty*]

Per Parts Connexion price list it looks like you've got about $800 in 4 caps for "large" 2.2 uF bypasses. Are these caps superior sounding to foil and film caps such as V-caps CUTF and TFTF  series. As a less expensive alternative I might be tempted to try the Robert Hovland SuperCaps,2.2 uF 200v polypropylene film and foil at $29 a piece. Obviously I am biased towards the Hovland caps due to past experiences with them.
Scotty

[jtwrace]

Per Parts Connexion price list it looks like you've got about $800 in 4 caps for "large" 2.2 uF bypasses.