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The AMNESIS amp: a good amplifier, like a gentleman, has no memory.
The AMNESIS amp: a good amplifier, like a gentleman, has no memory.
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Old 16th August 2018, 11:16 AM   #211
Symon is offline Symon  United Kingdom
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Hi Max,
I realise I forgot something while running the simualtions so while they seem stable dynamically I need to run tests to check phase behaviour ...

I've been checking further and the two pole compensation increases the gain at 10Khz, and the faster roll off give a 25 - 30 degree phase margin. and we have about 10Db gain margin.

Capacitor C15 generally appears to improve distortion, but as noted above in practice this could lead to instability. Bob Cordel recommends avoiding these. But in this case it appears to work with a reasonably large value that isn't very critical, usually these capacitors are just a few pF, so someting to pllay with.


I also had a throught about current limiting, the MOSFET cascodes probably provide current limiting which is selectable using different zener voltages. So under fault conditions just need fuses to kill the power.


Regards,
Symon
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Old 17th August 2018, 10:22 AM   #212
maxlorenz is offline maxlorenz  Chile
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Quote:
Originally Posted by Symon View Post
Hi Max,
I realise I forgot something while running the simualtions so while they seem stable dynamically I need to run tests to check phase behaviour ...

I've been checking further and the two pole compensation increases the gain at 10Khz, and the faster roll off give a 25 - 30 degree phase margin. and we have about 10Db gain margin.

Capacitor C15 generally appears to improve distortion, but as noted above in practice this could lead to instability. Bob Cordel recommends avoiding these. But in this case it appears to work with a reasonably large value that isn't very critical, usually these capacitors are just a few pF, so someting to pllay with.


I also had a throught about current limiting, the MOSFET cascodes probably provide current limiting which is selectable using different zener voltages. So under fault conditions just need fuses to kill the power.


Regards,
Symon
Hi Symon,

Capacitor parallel R feedback was tried (5 to 15pF) at an earlier stage and proved to increase oscillation tendency. I don't quite remember which version of the amp it was. Oscillation, I suspect, is due to either layout problem, or to some sort of local feedback (Colpits) or other

Two pole compensation is a technique with which I don't have any experience and I would not feel comfortable trying it, if the amp starts doing crazy things

Today I will try to experiment with "triple bootstrap". Wish me luck.

Oh! I forgot to mention that always the trick that halts negative peaks oscillation has been bypass cap (this time 150n) from drivers' cascode base to ground, not to output. I did not try RC, which was recommended by the creator of Ovation amp (who is a member but whose nickname I don't know) for VHF oscillation on his amp. Worth a try IMHO.

Cheers,
M.
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Old 18th August 2018, 10:32 PM   #213
maxlorenz is offline maxlorenz  Chile
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Yesterday, only listening session and a barbeque party took place
Creedence's "The Midnight pecho" and things like that.
No upgrades.
Today, duty.
I will anticipate that the board without the bootstrapped output cascode has less focus and less depth of images. Probably tomorrow I will be in position of fully state the former...and probably also will declare maxlorenz' rules to musical reproduction.

Cheers,
M.
Attached Images
File Type: png AMNESIS TRIPLE BOOTSTRAP.png (92.2 KB, 49 views)
Attached Files
File Type: asc AMNESIS TRIPLE BOOTSTRAP.asc (28.5 KB, 4 views)
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Old Yesterday, 07:23 AM   #214
JOSI1 is online now JOSI1  Germany
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Hi Max,

the power resistors for bootstrapping require more board space for layout.
Which power resistors (brand, seize) do you recommend.
Can these resistors be mounted vertically or horizontally one upon the other (with 2-3mm distance to the PCB).
Can a few resistors 0.25W / diodes be mounted on the solder side to save board space or trace length.
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Old Yesterday, 10:56 AM   #215
maxlorenz is offline maxlorenz  Chile
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Quote:
Originally Posted by JOSI1 View Post
Hi Max,

the power resistors for bootstrapping require more board space for layout.
Which power resistors (brand, seize) do you recommend.
Can these resistors be mounted vertically or horizontally one upon the other (with 2-3mm distance to the PCB).
Can a few resistors 0.25W / diodes be mounted on the solder side to save board space or trace length.
Hi dear JOSI1,

Yesterday I worked a little on the output. First one side "shared bootstrap", that is only one bootstrap (2 resistors per polarity) and the other side "triple bootstrap" (four resistors per polarity) and some listening tests were performed. I had to bypass again the bases of the drivers' cascode. See below. I had the feeling that the "shared" was a bit loose on the bass and less focused as I went to upgrade it for the "triple", which was easier to stabilize, meaning getting rid of the oscillation on peaks. This was done with 27R in series with 100-150nF at the bases of the cascode T. But now I found the article by the creator of the Ovation amp (I believe he is Bonsai; sorry if I'm wrong) where he deals with the same problem but at input. See attached. I will upgrade as he advices for there is still a low grade VHF oscillation/noise that stops when I pause the source.
The problem I did not preview is that, the zener's leads being very long, I solder them in different lengths to the bases, meaning different parasitic inductances.

With 32V PS, I use 3X 0.6W resistors for bootstrap without problems, Though higher wattage can lead to less noise. I think 2W will be more than enough. As for brands, I don't have an opinion yet. I suppose any good quality metal film would be OK...
Resistors can be located underside. No problem. Due to different (unpredictable) inductances, it is better to plan for the "triple" and then taste what configuration gives better results.
They can be piggy-backed but horizontally to avoid they acting as antennas.
The trick is to place the zener (or damper R-bypass capacitor; see attachment) as near as possible from cascode's base.

Cheers,
M.
Attached Files
File Type: pdf BONSAI Cascode-Oscillation-in-Audio-Amplifiers1.pdf (774.6 KB, 9 views)
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Old Today, 10:53 AM   #216
maxlorenz is offline maxlorenz  Chile
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Guys, I made another strategic error: I stopped reading Dr. Kolinummi's book just before it gets interesting (for this project, I mean) exhausted with the first part...go and get it if you haven't yet.

It reinforced me in my philosophical worry about the following, and correct me please if I'm wrong:

Our VAS is cascoded, right. A BJT in common-emitter gets help from a BJT in common-base, meaning the cascode element receives two signals: THE signal at its emitter and a DC signal, referenced to ground, which establishes the working parameters (Vbe) of the former. This transform the couple into a quimeric transistor having multiple advantages, amongst which are: it limits Early effect on the common-emitter; it allows to pick a high quality specialized transistor for this position (I am open to suggestion on the best here) due to the low working Vce it faces.
All good until now, rigth?

Well, in the driver and output sections things are a little different because the cascodes (which are cascading common-collector units, BTW) not only receive the two signals commented above but receive also a third one: a copy of the output (being EF, one can say a copy of the input) so a form of positive voltage feedback. In that way it is more like a bootstrap than like a cascode. This is not just semantic. Or we must invent another term. It is not a classic bootstrap in the sense that it does not pull-up the working voltages, like the bootstrap for the CCS part of the voltage reference for the DC bias part of the same cascodes, which works like a positive current feedback to keep the CCS properly working near ideal situation, or that is what I understood. So, what do you think about the "bootstrap" positive AC feedback connection we have here? Maybe it is helping the sense of dynamics, maybe it helps (or deteriorates) linearity...I have to read further this excellent Kolinummi book.

BTW, go and check the Ovation amp site. It's a joy to read and full of useful theoretical and practical information.

Ovation e-Amp: A 180 Watt Class AB VFA Featuring Ultra Low Distortion

Attached, two simulations with the modified (as recommended by the above site) bypassing for the cascodes, with maximum values they seem to tolerate. In case of oscillation (highly probable) lowest values that cancel all oscillation must be used.
This could be modeled and simulated beforehand but I wouldn't know hot to do it exactly.

Cheers,
M.
Attached Files
File Type: asc AMNESIS TRIPLE BOOTSTRAP.asc (28.8 KB, 2 views)
File Type: asc AMNESIS SHARED DOUBLE BOOTSTRAP.asc (27.8 KB, 2 views)
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