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Old 13th August 2007, 03:22 AM   #1
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Default Sziklai output

I made few amps based on this schematics (call it clone Wink ) because I really liked the sound of the original. Every time i had nasty oscillations what were solved with playing with caps values. This time I have no luck. I think that this is Sziklai output (correct me if I'm wrong) and It's one of disadvantages of that design. I would like to hear about your idea... What can i do to get rid of problems?
BTW, I can't find Sanken's models so on the sim schemo are much slower Onsemi substitute Confused

BTW, I already posted this thread on some other forum but still have no real suggestion. Only thing I know is that I have no enough knowledge to modify this circuit. One solution is to skip Sziklai output and solve oscillations problem, but it will be great if I can keep the sonic
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Old 13th August 2007, 03:51 AM   #2
anatech is offline anatech  Canada
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Hi Moby,
Try using a zobel network on your output. All leads and traces need to be short at your output transistors as well.

You may need some C-B caps on your outputs a well and the first transistors are too big and slow. Try using MJE1503x types there for less capacitance and more speed.

-Chris
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Old 13th August 2007, 05:41 AM   #3
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Put a 10 ohm resistor in parallel with L1.
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Old 13th August 2007, 06:09 AM   #4
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Thanks for reply. Ok about zobel, but this topology at full swing runs first transistors for about 25w. Regarding speed ,they are 35mhz . I forget to mention that I used 2sa1295/2sc3264 .

I don't understand what will do C-B caps on output tr
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Old 13th August 2007, 11:08 AM   #5
VHF man is offline VHF man  Australia
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FWIW I've managed to get one of these working. The devices used are 2SA5171 / 2SC1930 (drivers) and Hitachi lateral mosfets for outputs. For best results the drivers need to be as fast as possible and you need to add gate resistors to control the gain roll off of the mosfets. 100 ohms worked well in my design and I'm using 2 devices in parallel per side. Without the gate resistors there was a very nice 300MHz oscillation but with the gate resistors added I can't see anything on the CRO within a 500MHz bandwidth driving resistive or speaker loads.

You may ask does it sound good? The answer is yes, very.
I built an amplifier with this output stage for my next door neighbour and he is over the moon. sounds smooth as silk like a class A but punches like a big AB and runs cold with just 100mA quiescent.
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Old 13th August 2007, 01:02 PM   #6
jaycee is offline jaycee  United Kingdom
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The biggest problem with the Sziklai is that making the design work well on paper/simulator is only half of the challenge. The Sziklai output stage is very sensitive to PCB layout... so you may have a design that works great in theory, but oscillates like crazy when you make it for real.

Personally, I prefer the triple darlington
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Old 13th August 2007, 01:15 PM   #7
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Thanks a lot, I will try as you posted.
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Old 13th August 2007, 01:15 PM   #8
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Only thing I don't know is how to treat BJT's since you described Mosfet situation. Yes, this topology sounds kind of A class. Only way I can do is to put cap over output to inv input of op-amp but I have an impression that I will ruined the sound that way. Some local compensation is needed but I don't know what. Oh I forget to mention, this is not just drawing this is real AMP I made that completely but have that issues stated before
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Old 13th August 2007, 10:12 PM   #9
VHF man is offline VHF man  Australia
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Quote:
Originally posted by Moby
Only thing I don't know is how to treat BJT's since you described Mosfet situation.
At first glance I thought you were using mosfets but the situation is same for BJT. Try first a small amount of resistance in series with each base of the output devices (ie 10 ohms). Alternatively you could use a single ferrite bead. And yes layout is very important. and it's important regardless of topology to keep supply impedance low. I tend to use an independant RF groundplane for this purpose to avoid stability problems.
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Old 13th August 2007, 10:44 PM   #10
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Thanks. i understand. VHF, is that project you did available for public?
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