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Old 1st December 2008, 08:40 PM   #21
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Sorry Dave for not answering the second question.

As the current was adjusted to be identical (withing 5 uA) for both versions the value for C13 should be the same for both as FT is a function of Ic.

So I guess values between 15 - 22 pF is about right.

Regards

Nico
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Old 1st December 2008, 08:46 PM   #22
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Quote:
Originally posted by Nico Ras
And that of the bootstrap. Depending how you look at it you can argue that the bootstrap is faster.

Hi Nico,

Good investigation.

Maybe I'm missing something, but neither one of these square waves looks right. They are both asymmetrical. Is the amp clipping or slew rate limiting on these square waves? The initial overshoot on the first one is terrible.

As a starting point, we should look at small-signal square wave response comparisons, and these should be symmetrical.

Might the amps have been occasionally clipping in the listening tests? It happens more than we think on well-recorded music with good dynamic range. It is easy to believe that CCS vs bootstrap amplifier circuits would have different clipping characteristics and sound different.

Cheers,
Bob
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Old 1st December 2008, 09:12 PM   #23
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Quote:
By AkSA - I can see it's slower, but that absence of overshoot is a boon.....
I came to this conclusion after building too many CCS amps.
One can see/hear this by simming then building both types.
I found that one can moderate the "CCS effect" (overshoot)
by compensating the VAS side of the standard CCS.
A good analogy would be to visualize the CCS VAS as a spring,
as compared to the Bootstrap as a shock absorber.
I have found both to have their strengths and weaknesses.
The only topology I think can bring about the best of both
"camps" would be a balanced VAS or full comp.Diff.(leach)
(aksa says they sound bad but I guess I'll have to build one to find out).
These force the VAS linear (more like a tug of war) and give
a neutral sound (that is what we want,...for our amp to sound exactly like our soundcard/cd/preamp)I guess I'll find out in a few days .
OS
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Old 1st December 2008, 09:28 PM   #24
AKSA is offline AKSA  Australia
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Hi Nico,

Your careful measurements revealed slightly lower levels of 5th, 7th and 9th harmonic. These are significantly bad sounding harmonics, are their origins in the overshoot you noted, which I've found also leads to offensive sibilance?

Nico, I wonder if, like me, you found that with the bootstrap you could relax the lag compensation slightly?

I'm learning LTSpice at present, finding it interesting, but am constantly reminded that correlations between reported distortion and the listening experience are thin at best......

I take my hat off to you for doing this important empirical research and sharing the results.

Cheers,

Hugh

Pete:
When you report me as saying full complementary sounds 'bad' you invite a volley of abuse from those that know better...... Frankly, I have listened to them at length, and they are clean and undistorted, but I prefer what I regard as the 'fuller' sound of a single LTP!
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Old 1st December 2008, 09:45 PM   #25
cbdb is offline cbdb  Canada
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how different are the freq responses of the two amps? (dosnt the overshoot mean a bump in f response in HF sounds (sibilance))?
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Old 1st December 2008, 11:16 PM   #26
GK is offline GK  Australia
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Quote:
Originally posted by ostripper

I came to this conclusion after building too many CCS amps.
One can see/hear this by simming then building both types.
I found that one can moderate the "CCS effect" (overshoot)
by compensating the VAS side of the standard CCS.
A good analogy would be to visualize the CCS VAS as a spring,
as compared to the Bootstrap as a shock absorber.
I have found both to have their strengths and weaknesses.
The only topology I think can bring about the best of both
"camps" would be a balanced VAS or full comp.Diff.(leach)
(aksa says they sound bad but I guess I'll have to build one to find out).
These force the VAS linear (more like a tug of war) and give
a neutral sound (that is what we want,...for our amp to sound exactly like our soundcard/cd/preamp)I guess I'll find out in a few days .
OS


That analogy doesn't make a lot of sense. To be blunt, the waveform response of Nico's amp looks mediocre is either case.
This says more about the design as a whole, rather than anything absolute about the virtues and vices of a bootstrapped VAS or a CCS VAS.

With regards to your alledged "CCS effect" (overshoot), I've never seen this as an issue in any of the amps I've built and I certainly have never had to resort to "compensating" the CCS side of the VAS. If this is an issue then I suggest that there is something more complex going on.

Square wave and clipping CRO shots of one such basic (CCS VAS) design starting here:

http://www.diyaudio.com/forums/showt...31#post1488531


Cheers,
Glen
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Old 1st December 2008, 11:29 PM   #27
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Quote:
Originally posted by G.Kleinschmidt

Square wave and clipping CRO shots of one such basic (CCS VAS) design starting here:

http://www.diyaudio.com/forums/showt...31#post1488531


Cheers,
Glen
Hi Glen,
To be fair, that was only a 10 watt amp. Problems increase with voltage, right?
Remember this?

Click the image to open in full size.

From here?
Bootstrapped high power amp, running at full voltage driving an actual speaker.
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Old 1st December 2008, 11:39 PM   #28
AKSA is offline AKSA  Australia
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John,

I must say I laughed when I saw R38 on your amp!!

Hugh
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Old 1st December 2008, 11:47 PM   #29
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Quote:
Originally posted by AKSA
John,

I must say I laughed when I saw R38 on your amp!!

Hugh

On the old one? The value, perhaps?

I use it on the new one - 200k.
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Old 2nd December 2008, 12:00 AM   #30
GK is offline GK  Australia
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Quote:
Originally posted by MJL21193


Hi Glen,
To be fair, that was only a 10 watt amp. Problems increase with voltage, right?
Remember this?

Click the image to open in full size.

From here?
Bootstrapped high power amp, running at full voltage driving an actual speaker.

I've only have scope shots of my 10W "CCS VAS" amp readilly at hand ATM, but this alledged CCS VAS "overshoot" doesn't exist in any properly implemented amplifier of such topology, 10 W or 1000W. Maybe if you do something silly like running a high Cob transistor for the CCS, biased from a high impedance source.

WRT the bootstrap VAS, it has issues of its own with respect to driver saturation and rail sticking on clipping that the CCS VAS doesn't have.

Cheers,
Glen
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