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Old 21st June 2014, 12:44 PM   #1
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Default sziklai stability issues

Most of you know that i am an Sziklai lover !
In many of my posts you will find plenty of reference in various sziklai attempts that usually include some historical data .

The subject here is the Alchemist Forseti that is the most unique sziklai that ever designed since it features double sziklai pairs in the output .

Other than the output stage design the Alchemist Forseti is a well designed machine that has a reputation of sweet and round sound and plenty of kick since its a dual mono and features plenty of capacitance and good drive ability from the extra pair of outputs and drivers .

The only problem of the Alchemist APD 15 was the sonic performance that was limited by huge 680 pf miller caps on both the drivers and the outputs to keep stable with double an sziklai pair for an output stage .

-------------------------------------------------------------------------------------

so please ladies and gents allow me to quote these questions from my P3A thread in order to get some answers


---APD 15 senses the idle of the amplifier in the wrong place as we now know .

--- It senses over the outputs and not over the drivers , Plus that drivers all 4 of them share no heatsink at all ( may be not needed) but no heatsink at all means also no common behavior to the 4 drivers since these are behaving on their own when it comes to temperature

---Then APD 15 features huge miller caps of the size 680pf to keep stable in the output these may prevent oscillation but will do nothing to thermal stability .

Result :
Is there a chance that the designer tried to cure stability issues with the wrong way ? it is common practice that miller caps put a brake in oscillation issues but will only make the circuit worst if the source of the problem is thermal stability .

Why something is telling me that this amplifier may also work without the sonic killers miler caps on the outputs or have them replaced with something a lot lot smaller that will work as an oscillation stopper without killing the sonics ?

The only thing you have to do is redesign the thermals in the drivers and stability will come to visit you ....

Think please ...

kind regards
Sakis
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Old 21st June 2014, 05:25 PM   #2
djoffe is offline djoffe  United States
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Sakis,

Can you post a schematic?
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Old 21st June 2014, 08:42 PM   #3
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Default Schematic

Here is the schematic.

Sakis, I also like Sziklai OPS, however I don't like its implementation in ADP15. To be honest, I don't like many things in this design Examples of good designs with Sziklai - Audiolabs (8000A, 8000P, 8200M). They use Sankens without any additional compensation in OPS and the circuit is rock stable.

For this particular one I don't have any good idea so far, though I will think a bit more...

Cheers,
Valery
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File Type: jpg APD15-Forseti-schematic-1500x935.jpg (141.2 KB, 430 views)
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Old 21st June 2014, 08:48 PM   #4
Bigun is online now Bigun  Canada
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I don't like multiple output CFPs either. I don't see how they can be that well matched and it adds more opportunities for instability. Best to stick with only one pair CFP.
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Old 21st June 2014, 08:59 PM   #5
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OK - as an idea - I would do the following:

1) get rid of one of the drivers (source both OPs' bases from one driver);
2) add resistors (0.1 - 0.22 Ohm) serially with OPs' emitters;
3) reduce the resistor value at the drivers' collectors to some 100 Ohm (to allow more power to the drivers);
4) re-adjust the bias and see how it behaves without the caps at all.

Depending on the result:
- add base stoppers (2.2 Ohm) to the OPs;
- introduce the cap (47-100pF) between B-C of the driver.

Something like that.

Cheers,
Valery
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Old 21st June 2014, 09:16 PM   #6
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The Sziklai pair have high gain so any changes in bias current will be amplified many times.
I just use single end follower output setup and find it is very stable.
I even find quasi complimentary to be a little sensitive.
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Old 23rd June 2014, 08:24 AM   #7
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This will be long ( as usual )

At the first place i will have to thank all of you for your input ...still i will have to argue with many ...

@valery
Please remove from any of my threads the word Audiolab 8000 i see it as contamination... you are wrong 8000 was a horrible machine that obviously never failed since the power supply was too small fro such an output stage but the worst was the sonic performance due to the overall design .As about the thermals that was the worst part of the 8000 and the machine could maintain specs of bias only when the room temperature was about 20-22 degrees average ...Increase this to 28 and call me back ...

@Gareth
We have been there before in our private conversations ...sonic performance cannot be simulated ...Take uncle Charly AKA Destroyer X amplifier and simulate it ..Then built it with an average pcb and power supply ...Then built a version that will have for example 2X10.000 psu caps on board and very close to the output transistors I bet you that the one that features the psu caps on board will sound far better Can you simulate that ? Obviously not ...

Bryston 2B can be as good as any Pioneer or Sansui or any consumer amplifier on paper the basic differences between them are very few but still make the 2B hit the sky
**Small caps only 2X4700 per ch but located on board where these are supposed to be very next to the output transistors
**Dual mono /modular design
**Excellent choice of semis and proper use of them

That was enough to create a legend


@ valery ( latest post )
You are actually instructing me how to convert the APD 15 to an Audiolab 8000 ...Sorry but no thanks that wasn't my intention

@nigel
You might be correct ... Since it is impossible to run a background check to all of you I will have to place here mine so we know where are we standing from .
I repair amplifiers i do this all my life I am a freelancer so i have been repairing from the cheapest AKAI till Perreaux that will cost 5-6000 USD or UBA of 15000 USD
For every amplifier we repair we take feedback from the costumers measuring and we also make listening tests to every new amplifier that joins the list
Last year only we repaired in total 1000 audio devices
What i am trying to say here is that beyond living from this we record real life results from listeners ...

That will mean that for 5 years now we give P3A amplifier to listen to costumers while their amplifiers are under repair ...Each and every one of them was able to tell the difference .

So if you like Sziklai just make a P3A and all will be more than fine !!!
But if you look for more power with similar sonic performance the APD 15 was notorious for the kick coming from the output stage and the beefy power supply but the sonic performance was bad due to huge miller capacitors Now can we fix that ?

To farther support more my theory look at most Elektor /giesberts designs that suffered from the exact same issues like way too many stages current mirrors /sinks /sources in diamond input stages that oscillated probably for the same reasons and since the amp was hfe/beta/gain depending then the only solution was to add capacitors here and there to prevent oscillation .

yet again electronic circuits cannot be treated with medicine You will have to cure the problem where it starts ...

Thanks for your time
Kind regards
Sakis
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Old 23rd June 2014, 11:09 AM   #8
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There should be little difference with "off-board" caps if inductance and resistance are kept low by using twisted wires of reasonable gauge.

Anyway, I think this amp uses kind of a 2-pole compensation. VAS gain is shelved, then the caps at the CFPs take over.

It's a bit of an odd concept. I'd think it's somewhat lacking in current gain. Input LTP tail current and VAS current would not seem to be unusually high. Also, TIP32C in VAS? Yikes.
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Old 23rd June 2014, 06:42 PM   #9
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Sakis, no problem, let's keep the brand you don't like aside. But your question grabbed my interest

I did some modelling. As we can see now, the main issue of the original APD15 desidn - transistors. Both TIP31/32 and TIP35/36 are awful for the purpose. Slow, huge input capacitance transistors, provoking oscillation at rather low frequencies, that has to be suppressed with large lag compensation capacitors. Not a good concept by definition.
See bode, see the phase response. THD is incorrect just because of heavy oscillation.

I also attach the model with transistors changed to some more decent ones. You can see the difference. Also, TMC compensation would improve the distortion at the higher end of the frequency response.

P3A is a proven design, but it's possible to spoil its benefits, as we can see. Sure, the models are not perfect, live performance will be different, etc. But they give an indication, are they?

Press the soft-button at the bottom-left corner of the image to see it full size.

Cheers,
Valery
Attached Images
File Type: jpg 01-APD-15-Original.JPG (556.3 KB, 295 views)
File Type: jpg 02-APD-15-Corrected.JPG (550.2 KB, 276 views)
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Old 23rd June 2014, 07:28 PM   #10
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Quote:
Originally Posted by vzaichenko View Post
Sakis, no problem, let's keep the brand you don't like aside. But your question grabbed my interest

I did some modelling. As we can see now, the main issue of the original APD15 desidn - transistors. Both TIP31/32 and TIP35/36 are awful for the purpose. Slow, huge input capacitance transistors, provoking oscillation at rather low frequencies, that has to be suppressed with large lag compensation capacitors. Not a good concept by definition.
See bode, see the phase response. THD is incorrect just because of heavy oscillation.

I also attach the model with transistors changed to some more decent ones. You can see the difference. Also, TMC compensation would improve the distortion at the higher end of the frequency response.

P3A is a proven design, but it's possible to spoil its benefits, as we can see. Sure, the models are not perfect, live performance will be different, etc. But they give an indication, are they?

Press the soft-button at the bottom-left corner of the image to see it full size.

Cheers,
Valery
Lets suppose that with better transistors the all thing works better .

here is a question though ?
Does your software support a feature to add tolerance to any of the components ? even better can you mess up with the hfe of the drivers in the model to see how the results will come up ? I can imagine a 20%+or- just to see how its going to be

That will be an interesting thing to have answered .

Thanks for your time Valery
Regards
Sakis
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