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Old 10th January 2012, 03:31 PM   #21
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Originally Posted by AmCan View Post
I have not considered multilayered ceramic caps but I will look into them. Certainly it would be worth switching them in and out to see how they compare to the more (sexy) appealing film caps.

Thanks to everyone who helped me to understand this schematic better. I feel that I have a better understanding of signal amplification now .
Film caps are not appropriate for power supply decoupling (bypassing) because they have relatively high inductance. You are risking RFI problems in your amp if you replace the ceramics with film.
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Old 10th January 2012, 05:44 PM   #22
AmCan is offline AmCan  Canada
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Originally Posted by dirkwright View Post
Film caps are not appropriate for power supply decoupling (bypassing) because they have relatively high inductance. You are risking RFI problems in your amp if you replace the ceramics with film.
Not to adversarial here because I do not have a firm grasp of all of this concepts. I am just trying to go on the numerous things that I read and then try to make an educated decision. I believe that MKP type capacitors have been widely tested and reviewed in power supplies, like the Solen FastCap & Mundorf MKP (10uf value). Mundorf states that “The practically induction-free type of construction and the low loss factor of the MCap® results in a very "quick“ capacitor." Also the RELCAP RTE Polystyrene caps (.01uf value) have an extended foil construction and were constructed for this purpose. What am I misunderstanding here?
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Old 10th January 2012, 06:45 PM   #23
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I was referring to typical film caps, not super fancy ones that have been specially designed to have low inductance. However, it is not clear to me that even these caps have low enough inductance to be effective at RF frequencies. Bypassing DC power supplies is for one thing to suppress RFI in the circuit. I don't see any advantage in using film caps in place of good ceramic ones in the power supply decoupling.

Last edited by dirkwright; 10th January 2012 at 06:57 PM.
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Old 10th January 2012, 06:53 PM   #24
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Most film caps will have significantly less inductance than an equal value of electrolytic cap.
Some will have an inductance that is so much lower than equivalent or very special electrolytics that you can almost ignore it.

Bypassing at the PSU is only needed if the PSU will perform better with it.
What is needed is decoupling at the receiver end of the power rails. The big current consumers are the output devices. They can turn on demand in an instant and turn off almost as fast. They need decoupling. That's where to spend your time designing.
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Old 10th January 2012, 07:05 PM   #25
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Originally Posted by AndrewT View Post
Most film caps will have significantly less inductance than an equal value of electrolytic cap.
Some will have an inductance that is so much lower than equivalent or very special electrolytics that you can almost ignore it.

Bypassing at the PSU is only needed if the PSU will perform better with it.
What is needed is decoupling at the receiver end of the power rails. The big current consumers are the output devices. They can turn on demand in an instant and turn off almost as fast. They need decoupling. That's where to spend your time designing.
Yeah, I agree it's a good idea to add capacitance at the output transistors and bypassing those with a good low inductance capacitor. I did that in my Crown amps.
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Old 10th January 2012, 07:10 PM   #26
AmCan is offline AmCan  Canada
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Originally Posted by dirkwright View Post
I was referring to typical film caps, not super fancy ones that have been specially designed to have low inductance. However, it is not clear to me that even these caps have low enough inductance to be effective at RF frequencies. Bypassing DC power supplies is for one thing to suppress RFI in the circuit. I don't see any advantage in using film caps in place of good ceramic ones in the power supply decoupling.
I think some of this internal resonance adds to the so called sonic signature of the film cap giving the perception of a more tube like (distortion) sound or feeling of open space in the music.
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Old 10th January 2012, 07:20 PM   #27
AmCan is offline AmCan  Canada
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Quote:
Originally Posted by AndrewT View Post
Most film caps will have significantly less inductance than an equal value of electrolytic cap.
Some will have an inductance that is so much lower than equivalent or very special electrolytics that you can almost ignore it.

Bypassing at the PSU is only needed if the PSU will perform better with it.
What is needed is decoupling at the receiver end of the power rails. The big current consumers are the output devices. They can turn on demand in an instant and turn off almost as fast. They need decoupling. That's where to spend your time designing.
So getting back to my original inquiry then. If I use a 10uf cap at the output transistors would it be beneficial to also have a lower value cap like a .01 to address higher frequencies specifically or is it redundant and unnecessary?
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Old 10th January 2012, 07:26 PM   #28
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Decoupling has to handle a variety of frequencies.
The highest harmonics due to switching on a BJT from Zero current to output current go way above the audio frequency range, not just 10 times our accepted 20kHz hearing limit.
Expect the harmonics to demand current right up to the MHz range for fast transients.

These can only be met by ensuring the trace lengths are kept very short.
The round circuit route from the output device to the decoupling cap to the PCB Power Ground and back to the PSU (the other half of the decoupling pair) must be in the mm range. A 10uF cannot be placed in a route that short.
A 0.1" pitch ceramic (x5r) is very small and the leads can be trimmed quite short, 1mm each.
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Old 11th January 2012, 01:48 AM   #29
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http://www.designers-guide.org/Design/bypassing.pdf

Maybe this helps. I haven't read it all yet.
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Old 11th January 2012, 04:56 AM   #30
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Originally Posted by dirkwright View Post
http://www.designers-guide.org/Design/bypassing.pdf

Maybe this helps. I haven't read it all yet.
Thanks. This was helpful. It may be beneficial to use multiple bypass caps in parallel but I am going to insert them one at a time so that I can listen for any differences, good or bad. Starting by returning the amp to its original circuit design. I was trying to save myself a step by inserting multiple bypass caps at once but its not worth not knowing what kind of perceivable change I made if any.
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