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Old 10th October 2008, 12:03 AM   #131
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The harmonic spectrum results shouldnt surprise too much, I was expecting this, but what is surprising me is that THD should be lower than the case with bootstrap. The lower distortion at low frequencies with ccs usually compensate for the higher HF ones.

One question though, when you did the last simulations between the 2 did you change both the LTP current source and vas current source or only the vas.

The lower high frequency odd order distortion is one of the reasons many love bootstrap circuit, and compare it to valve sound. Hugh from aksa likes this circuit and I have never come accross anyone that had something bad to say about his amps sound.

Current sources degrade at higher frequecies, but this is baffling me, is the bootstrap circuit superior at high and low frequencies.
This must have something to do to do with the dynamic impedance.......
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Old 10th October 2008, 12:19 AM   #132
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Quote:
Originally posted by MJL21193



I'll do this Andy and post the results.
First, I want try a run at a lower frequency to see if this backs what you say about the feedback cap.

Fundimental = 5Hz

stop time - 2000 seconds (a looooong run)

It's showing - a bit. This is 10 times greater than at 20Hz.
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Old 10th October 2008, 01:37 AM   #133
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Quote:
Originally posted by andy_c

So, in short, it would be interesting to see:

1) 20 Hz vs 200 Hz distortion of bootstrapped version (using long simulation times).

2) 20 Hz vs 200 Hz distortion of CCS/CCL version (using long simulation times).

Here they are:

Click the image to open in full size.

Click the image to open in full size.

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Click the image to open in full size.


These ran for a long time. 500 seconds at 20Hz, 50 at 200Hz.

Lower distortion on both counts for the CCS as opposed to the bootstrap. Surprised that distortion at 20Hz was better than 200Hz for the bootstrap.

I will see if I have time later to run these again, for verification.
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Old 10th October 2008, 01:40 AM   #134
andy_c is offline andy_c  United States
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Ahhhhh, it all makes sense now .
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Old 10th October 2008, 01:46 AM   #135
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Quote:
Originally posted by andy_c
Ahhhhh, it all makes sense now .

Yes, looking better. I used my cap model in the bootstrap version.

I forgot that the low frequency stuff needs an incredibly long time to get accurate results. Each of these took more than 10 minutes.

Still, look at those figures. It's not as cut and dried as it was made out to be, with low frequency edge going to the CCS, and the high going to the bootstrap.
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Old 10th October 2008, 01:49 AM   #136
PB2 is offline PB2  United States
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Originally posted by MJL21193


Hi Pete,
I can tell that you are becoming frustrated with my obstinance again.
I didn't want this to degenerate into an argument over what topology should be used. A discussion on the merits of each is what I intended. As far as this design goes, yes my mind is made up. I will not change the current configuration.

This is my point exactly.
John, I read you at the start saying that you just want to understand the differences so I am aware that you have no plan to change. I'm just trying to add to the understanding, and throw in my main reason for the CCS preference which is to eliminate the electrolytic cap. I do also wonder why we're not seeing the expected rise in distortion at LF. Might want to try 1 Hz or .1 Hz.
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Old 10th October 2008, 02:03 AM   #137
PB2 is offline PB2  United States
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Quote:
Originally posted by andy_c


I think a series RLC model should be fine. The app note gets pretty fancy because they're looking for models suitable for switching regulators. The ripple performance of switching regulators is very sensitive to capacitor characteristics.

Their 1-2 nH per mm of spacing, together with about 0.2 Ohms series R should do the trick.

Andy, they mention screw terminals for those caps and this suggests that they're computer or PSU grade that are of different construction with multiple attachment points to the plates. Most common electrolytics have one attachment point. I would not normally worry much about non-linear capacitor effects, but with the very low numbers here it might be a consideration. My point was that we're not even sure that the SPICE model is valid at these very low distortion levels. The RLC model for the cap is still obviously a linear model.

Pete B.
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Old 10th October 2008, 02:09 AM   #138
PB2 is offline PB2  United States
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Some interesting cap references:
http://www.intusoft.com/nlhtm/nl65.htm

http://www.intusoft.com/nlhtm/nl76.h...odel_For_SPICE

http://www.intusoft.com/nlhtm/nl44.htm#modeling

http://www.kemet.com/kemet/web/homep...56BCD004EBC04/$file/TechTopics%20Vol4No5%20Sep94.PDF

http://www.kemet.com/kemet/web/homep...56BCD004EBC0C/$file/TechTopics%20Vol8No2%20Jul98.PDF

Not saying that they all apply to this situation.
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Old 10th October 2008, 02:10 AM   #139
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Originally posted by PB2
I would not normally worry much about non-linear capacitor effects, but with the very low numbers here it might be a consideration. My point was that we're not even sure that the SPICE model is valid at these very low distortion levels. The RLC model for the cap is still obviously a linear model.
Yeah, at these sub-ppm distortion levels at low frequency, the cap distortion could very well dominate. I don't know how to model capacitor distortion in SPICE. I bought the Cyril Bateman CD, so I'm familiar with the nice work he did. I know he found that non-polar electrolytics have lower distortion than their polar counterparts. Might be something to consider. Mouser has the Nichicon non-polars for good prices.

Edit: Here's Cyril Bateman's web site:

http://uk.geocities.com/cyrilb2@btinternet.com/
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Old 10th October 2008, 03:56 AM   #140
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Tonight, while waiting for these sims to run, I plucked some transistors from a dead Kenwood receiver that I got at Goodwill for $2.00. I got about 20 each of 2SA992 and 2SC1845 - about $27 worth right there from B&D. Nice devices too.
A bunch of lesser beings: 2SA733 and 2SC945. About 20 each of those.

4 each of the ubiquitous outputs 2SC3854 and 2SA1490. These go with the other 2 pairs that I found in another receiver.
A good haul, for 2 bucks.
Long leads, I just had to cut them out.
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