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Old 1st June 2010, 03:09 AM   #61
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Hi Bas,

Do you aware that Cambridge A500 uses SAP15N/P? This is the manual http://x0546.orbbox.com/files/storag...56c2ba1a6b3a08

Panson
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Old 1st June 2010, 04:20 AM   #62
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Quote:
Originally Posted by Sebastiaan View Post
You can try base stoppers of a few ohm.
Hi Bas,

If base stopper resistors are used to avoid high-freq oscillation, did you try to use a ferrite bead or a little ferrite core in series with the base?

Regards,
Panson
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Old 1st June 2010, 05:20 AM   #63
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Default oscillation vs loading cap value

The book "Intutitive Analog Circuit Design" derives input impedance of a capacitive loaded emitter follower. When the loading cap value is small, the input impedance exhibits negative resistance. In contrast, large loading cap does not cause the input showing negative resistance. Base resistor is commonly used to offset the negative resistance. I tested the amp (49811+03N/P) with small 270 pF and large 2 uF load cap. The result is consistent with the book. I got oscillation with 270 pF load cap (10 R base resistors installed), but not for 2 uF. Increasing the base resistor to 240 R does not stop the oscillation. I then put 1.5 k which stops the oscillation for 270 pF capacitive load.
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Old 1st June 2010, 09:15 AM   #64
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and if it wasn't an integrated Darlington, you could try base stoppers on both the driver and output. How about 47r and 2r2 instead of 1k5
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Old 1st June 2010, 11:53 AM   #65
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Quote:
Originally Posted by AndrewT View Post
and if it wasn't an integrated Darlington, you could try base stoppers on both the driver and output. How about 47r and 2r2 instead of 1k5
I think that using a discrete Darlington is a good way to learn more.
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Old 1st June 2010, 01:17 PM   #66
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Quote:
Originally Posted by panson_hk View Post
Hi Bas,

The schematic is shown in another issue which is not available from the website. I can scan it and e-mail to you. Please let me know your e-mail address.

I have base stopper resistors already, but did not try putting cap across C and B pins. This is effectively adding a Miller cap in the output stage for which I don't feel "right". I might do that eventually for this Sanken part and/or other high-gain Darlington.

I am in line with you that the amp (simple topology) should not have so many caps around.

Cheers,
Panson
Dear Panson,

That is a very kind offer from you. Thank you in advance. Please send it to bas_music@yahoo.com

I think the sanken's really need the cap's from collector to base to stay stable. Without I never got the system really stable. But maybe with some practice you can do it. In my subjective listening sessions, The capacitors from collector to base made the sound better (warmer and less sterile) and the base stoppers make the sound dull and it lost it's "magic" So I would try to stay without the base stoppers but go with the cap's. The cap's between the source and sink really improve the performance. The bigger the electrolytic the better it sounded in my setup.

With kind regards,
Bas
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Old 1st June 2010, 01:38 PM   #67
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Dear Panson,

I want to add, that if you increase the (miller) compensation of the driver chip a bit the problems would be far less. And don;t forget the 3.3K and 75P to ground.

And yes you are right about the other things. If I loaded my first design with the STD03's with bigger value cap everything was fine, but small capacitance as result from mixed speaker wires caused oscillation.

With kind regards,
Bas
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Old 1st June 2010, 01:40 PM   #68
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Quote:
Originally Posted by panson_hk View Post
Hi Bas,

Do you aware that Cambridge A500 uses SAP15N/P? This is the manual http://x0546.orbbox.com/files/storag...56c2ba1a6b3a08

Panson
Thanks Panson, Yes I already had this schematic. In this case they put the 10uF capacitor between the diode string, but I believe it does the same trick to stabilize.

With kind regards,
Bas
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Old 1st June 2010, 01:59 PM   #69
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Dear Panson,

Sorry for the raining of post's... If it can be of any help, this is my final circuit that is tested with all kind of loads's and works super stable under all possible circumstances.

Note, the supply rails is relatively low, because we needed more current headroom in lower impedances. In this setup the STD3's can double output power into 4 ohm which was very important for this application. The base stoppers are zero ohm links and not used after all. Also not visible on this part of my schematic, each supply pin from as well the driver as the darlingtons are de-coupled with a 100nF ceramic SMD cap.

With kind regards,
Bas
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Last edited by Sebastiaan; 1st June 2010 at 02:01 PM.
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Old 2nd June 2010, 03:48 AM   #70
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Quote:
Originally Posted by Sebastiaan View Post
I think the sanken's really need the cap's from collector to base to stay stable. Without I never got the system really stable. But maybe with some practice you can do it. In my subjective listening sessions, The capacitors from collector to base made the sound better (warmer and less sterile) and the base stoppers make the sound dull and it lost it's "magic" So I would try to stay without the base stoppers but go with the cap's.
Hi Bas,

I just e-mail the schematic to you.

The 1.5k base stopper solves the oscillation problem with small load cap. But, the test unit then oscillates with 2 u load cap. EF output inductive reactance is proportional to the stopper. It is larger with a 1.5 k stopper. The test amp may become a oscillator at freq different from that of 220pF load cap. Need to check the freq today. I probably should try e.g. 1 k to see whether it is the optimum value.

I agree with you that C-B cap is an effective oscillation killer. On the other hand, I will use discrete Darlington to learn more the oscillation issue. Is it also happening in discrete form? My goal is to use as less cap as possible.

Cheers,

Panson

Last edited by panson_hk; 2nd June 2010 at 03:53 AM.
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