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Old 11th January 2013, 08:11 PM   #21
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That wasn't the cause Effebi

Look deeper in the schematic Try to understand how really this amp work ..The secret lays elsewhere ...

Kind regards
Sakis
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Old 11th January 2013, 09:23 PM   #22
jaycee is offline jaycee  United Kingdom
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P3A does work well if built well, but that CFP output can be a pain. My suggestion of adding local power supply decoupling of 100uF in addition to the 100nF on the schematic often does the trick.

Likewise if you use fast output transistors, you should use fast drivers too - such as the MJE15032/3 devices. I am not a fan of using the BD139/140 there, as the modern ones are not as good as the old Philips devices were. MJE243/253 also work well.
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Old 11th January 2013, 09:57 PM   #23
effebi is offline effebi  Italy
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Quote:
Originally Posted by sakis View Post
That wasn't the cause Effebi

Look deeper in the schematic Try to understand how really this amp work ..The secret lays elsewhere ...

Kind regards
Sakis
Hi Sakis, nices to hear from you again...
but,
I do not see anything difficult to underderstand:
imput: diff. pair with LED current source
VAS: single BJT with bootstrap.
Output sigle CFP pair.
Compensation: Cdom on the VAS
Simple and beautiful.

And about the problem: seen on scope on square wave ,
changed CB capacitor, problem disappeared.
What am I missing?
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Old 11th January 2013, 10:39 PM   #24
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your case is a bit different since if i remember well you are using cans in the outputs am i wrong ?

Change a pair of drivers to any other model ( given as a fact that miller cap remains 100pf ) and observe the problem to see if its better or worst ....but change the drivers with anything available but different to see what will happen and get back to me ...


Kind regards
sakis ( good to hear from you too !! )
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 11th January 2013, 10:50 PM   #25
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@ jay cee small joke if i am allowed :

When i worked for the local Nak-Adcom dealer in my very young days that company also represented JRC RAtheon ship radars , so inside the bench was a tech named Jim Pappas repairing radars next to me and every time there was a super weird Japanese transistor burned beyond recognition in some weird also ( for me ) stage of the radar Jim come over to my bench virtually still a BC 547 from my parts a couple of resistors and a couple of IN4148 installed those on the fly and radar was working all over again .

Through the years it actually become an internal joke and in any case any of us 3 techs inside the bench was into any possible problem the solution of course was to """ADD A BC 547 AND A COUPLE OF RESISTORS "" and you will be just fine ..

Similar to the above we both follow close P3A threads and every now and then you talk about those two capacitors of 100uf ...fair ..and helpful but this circuit has a bit more juice than that ... you need to look a bit more carefully to stop thinking EFP when you look at the schematic ....

Add 100uf decoupling and you will be just fine !!!

Kind regards
Sakis
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Old 12th January 2013, 06:28 AM   #26
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P3A is obviously inspired by Motorola App. Note 485 circuit, but without complicated output protection, and bootstrap instead of transistor CS for VAS. Original amp designed by Motorola applications engineer Richard Ruehs used 50pF compensation in VAS and 50pF for lower driver. Obviously even with low values of these caps Sziklay outpuit stage can be stable, depending on drivers and output transistors used. If one decide to use ceramic caps for these positions depending on type of ceramic and temperature inside amp, 100pF value specified in Rod's P3A circuit will go down due to negative tempco of ceramics.

Has enyone tried 2SB649/2SD669 or 2SA1837/2SC4793 as drivers in P2A?
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Old 12th January 2013, 08:39 AM   #27
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Also, in CFP output stages 0,33R wirewound emmiter resistors may cause instability or oscilations in the range 5 to 50 MHz due to their inductance. I found 2W 0,33R MOX resisitors in the local shop and shall use them in P3A that I intend to build soon. Since CFP output stages are more prone to instability than other output topologies it is better to prevent any possibility of oscilations by using non-inductive metal oxide emmiter resistors.
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Old 12th January 2013, 09:31 AM   #28
dadod is online now dadod  Croatia
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Quote:
Originally Posted by ivanlukic View Post
P3A is obviously inspired by Motorola App. Note 485 circuit, but without complicated output protection, and bootstrap instead of transistor CS for VAS. Original amp designed by Motorola applications engineer Richard Ruehs used 50pF compensation in VAS and 50pF for lower driver. Obviously even with low values of these caps Sziklay outpuit stage can be stable, depending on drivers and output transistors used. If one decide to use ceramic caps for these positions depending on type of ceramic and temperature inside amp, 100pF value specified in Rod's P3A circuit will go down due to negative tempco of ceramics.

Has enyone tried 2SB649/2SD669 or 2SA1837/2SC4793 as drivers in P2A?
Ceramic capacitors used for compensation should be of COG/NPO type, and that one has temperature stable capacitance.
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Old 12th January 2013, 09:46 AM   #29
jaycee is offline jaycee  United Kingdom
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Originally Posted by dadod View Post
Ceramic capacitors used for compensation should be of COG/NPO type, and that one has temperature stable capacitance.
Indeed. I used this type of ceramic capacitor when I built a P3A (on Veroboard too!) and it was stable. Ordinary ceramics can go as much as 50% off value, usually lower, in use.
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Old 12th January 2013, 10:39 AM   #30
AndrewT is offline AndrewT  Scotland
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Originally Posted by ivanlukic View Post
............. wirewound emmiter resistors................. due to their inductance............
......... MOX resisitors ............. non-inductive metal oxide emmiter resistors.
Are you sure that wirewound have less inductance?
How much less?
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