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Old 4th July 2010, 06:53 AM   #11
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You could bypass R47 and R48 with small (below 100pF) caps to speed up the output stage, if you're adventurous.

I revamped the simulation commands and modified the circuit a little to decrease simulation noise floor. The high noise floor was exaggerating THD figures. The spectrum also changed, I don't know what the cause of this is, however I have noticed that high simulation noise floors in LTSpice can create a phantom dominant 2nd harmonic.

I replaced the signal caps with voltage sources, since caps in the signal path create a slanting FFT noise floor due to imperfect operating point solution.

- keantoken
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Old 4th July 2010, 02:14 PM   #12
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Thank you , kean. I now know more. what a nice low noise floor !

Wow , this amp has ridiculous low 5th and 7th ( the evil harmonics ) Too bad everyone is too stuckup to build this amp. They would rather build schoolboy 30 year old designs and discuss blackgate caps and other esoteric B_S.

A shame , it is still simple , 25 components and a cheap $9 OPS (njw0281/0302) which can overwhelm any speaker out there.

Edit : 20k is impressive , too ... for a simple amp.

OS
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Last edited by ostripper; 4th July 2010 at 02:32 PM.
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Old 4th July 2010, 04:44 PM   #13
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Quote:
Originally Posted by ostripper View Post
it is still simple , 25 components
It could be said that simplicity is in the eye of the beholder. Personally I try not to shy away from increased part count. In one way DIY is not a good way to "push the edge" of audio electronics; not enough money and not enough time.

I notice your amp has 63db of open loop gain with a corner frequency of 600Hz; ouch. If you play with the compensation a bit, I believe you will see improvement in the FFT.

If you're adventurous, I'll suggest what I would do. The supremely enlightened Michael Bittner granted us his favor and revealed unto us Symasym some time ago. I wonder if his unconventional use of compensation didn't go unnoticed. Most people just add a cap and then are done with it. The high-value caps I use in my headphone amp would send it into convulsions if I didn't decrease the Q factor with series resistance; but when I do, look at the result. I think there is much to be gained by experimenting with compensation.

I believe this type of compensation is also more stable into capacitive loads; it reacts to current draw instead of voltage output. The drawback of the conventional methods is that the compensation doesn't react if the output voltage doesn't change - hence indomitable instability into capacitive loads. Voltage-type compensation is counter-intuitive for voltage drive amps isn't it? Voltage drive amps are immune to inductive loading, little need to protect against it.

Notice the dip in phase required to produce the steep rolloff. Savor the economy.

The spectrum benefits much as well. 11th harmonic has been reduced by 22db! Before and after OLG and FFT plots.

- keantoken
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File Type: png Mongrel_FFT.PNG (30.3 KB, 1962 views)
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File Type: png Mongrel_FFT_Altercomp.PNG (30.5 KB, 276 views)
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Old 4th July 2010, 05:01 PM   #14
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Here is the simulation file.

If you look at the placement of C4 in post 9, this cap is injecting switching trash from the output stage directly into the VAS input, which the LTP has to shunt away. This is another reason to avoid that type of compensation.

- keantoken
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Old 4th July 2010, 05:09 PM   #15
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I just noticed, in your posts the higher order harmonics are much lower than mine. When I first opened your amp in LTSpice it was anything but stable. I had to increase compensation drastically to get phase margin within bounds. I'm not sure if this is normal, but it explains your better FFT results.

Also, don't forget the 5pF cap in the feedback network! The transistor parasitics here can be seen as a capacitor from Q4's base to ground. This increases VHF response and instability, causing a blip in the VHF if you look at the OLG response (set your AC analysis farther than 10MHz!). The 5p cap swamps the parasitics out and increases stability.

- keantoken

Last edited by keantoken; 4th July 2010 at 05:14 PM.
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Old 4th July 2010, 05:21 PM   #16
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Quote:
I just noticed, in your posts the higher order harmonics are much lower than mine.
I destabilized this amp on purpose with the sub 22pF Cdoms. You mean a 5Pf in parallel with the main GFB resistor ? They call that lead compensation , I am not using that in the working model and have no oscillations. Still , to be on the safe side , why not ?

Quote:
I had to increase compensation drastically to get phase margin within bounds.
You mean the 47Pf Cdom ? 56pF takes the overshoot out of the squarewave plots and set unity gain at 800khz.
Also , I got -102 degree margin at unity. would low "90 ish" be better ?


OS

Last edited by ostripper; 4th July 2010 at 05:26 PM.
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Old 4th July 2010, 05:26 PM   #17
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If you're using small Cdoms, I'm sure the 5p cap couldn't hurt. I just cringe when I see that blip. It gets in the way when I'm trying to adjust compensation and stuff, just easier to have it out of the way.

There seem to be a lot of amps that don't adhere to the -100 degree phase margin standard, so it can't be too bad.

- keantoken

Last edited by keantoken; 4th July 2010 at 05:40 PM.
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Old 4th July 2010, 05:32 PM   #18
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Keen , submit your "version" (another ksim) of the mongrel with the changes and I will check it out.
This time , don't forget the output inductor .

On your second plot above "the dip" was a cause for concern to Andy C , he called it a "2-pole response" and said it might
cause problems ?????


ThanX , OS

Last edited by ostripper; 4th July 2010 at 05:37 PM.
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Old 4th July 2010, 05:51 PM   #19
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I think Andy is right about it being two-pole response, but it's not done in the same way it's done in this article:

http://www.analogzone.com/col_0628.pdf

I haven't had problems with my headphone amp, though admittedly power amps are different.

The output inductor is present in the simulation, I just use a different symbol than you do. I take the output FFT from the before the inductor, not after.

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Old 4th July 2010, 05:55 PM   #20
Dr_EM is offline Dr_EM  United Kingdom
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Quote:
Originally Posted by ostripper View Post
Wow , this amp has ridiculous low 5th and 7th ( the evil harmonics ) Too bad everyone is too stuckup to build this amp. They would rather build schoolboy 30 year old designs and discuss blackgate caps and other esoteric B_S.
This is actually "my sort of amp"! Having built the Symasym (twice) I've been very impressed with it's neutral, clean performance. This amp seems like a similar design, excellent technical specs but more juice! Only thing is, I don't need one right now

I asked about compatibility with Symasym as I'm running a 3-way tri-amplified setup and in the back of my mind had sort of planned to, one day, integrate the 2 Symasym and 1 P101 into a 6-channel unit, for tidyness. However, it seems as though this design might be more compatible with the Syms while providing that extra current for the bass section, OTOH the P101 also seems quite excellent in this regard so I don't really know! I measured it as having the same phase as the Syms at least.

I think it's great work anyhow, I certainly don't mind a more complex design if the parts go toward creating better performance
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