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Old 9th January 2012, 05:30 PM   #11
AndrewT is offline AndrewT  Scotland
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I think C12 & C13 are in the protection circuit.
If they start passing significant audio signal then you are in some form of overload condition and the protection activation is now destroying the audio quality.
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Old 9th January 2012, 05:36 PM   #12
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Begin with C1 and C6 (input and NFB loop). C11, C12, C13 & C14 , that are part of the short circuit protection. Ceramic capacitor normally donīt need to be replaced unless they go open (very rarely) or short circuit (idem). There isnīt need for extra quality capacitor because they carry few audio voltages and currents. Use normal electrolytic capacitors, preferably rated to 105 deg Celcius. Also, I suggest you not to touch ceramic capacitors, they sometimes define high frequency rolloff of the amplifier or part of it (like C10 & C15). If you put a failed new unit or bad soldered, or any other mistake, the amplifier entirely will start to oscillate at a very high frequency, and the power output will be destroyed.
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Old 9th January 2012, 06:56 PM   #13
AmCan is offline AmCan  Canada
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Begin with C1 and C6 (input and NFB loop). C11, C12, C13 & C14 , that are part of the short circuit protection. Ceramic capacitor normally donīt need to be replaced unless they go open (very rarely) or short circuit (idem). There isnīt need for extra quality capacitor because they carry few audio voltages and currents. Use normal electrolytic capacitors, preferably rated to 105 deg Celcius. Also, I suggest you not to touch ceramic capacitors, they sometimes define high frequency rolloff of the amplifier or part of it (like C10 & C15). If you put a failed new unit or bad soldered, or any other mistake, the amplifier entirely will start to oscillate at a very high frequency, and the power output will be destroyed.
Wow! Thanks. It is interesting to see how the circuit breaks down into function. What about C16? What is the function of that position because there is a huge mylar there now, a 1uf 100V.
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Old 9th January 2012, 08:27 PM   #14
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Begin with C1 and C6 (input and NFB loop). C11, C12, C13 & C14 , that are part of the short circuit protection. Ceramic capacitor normally donīt need to be replaced unless they go open (very rarely) or short circuit (idem). There isnīt need for extra quality capacitor because they carry few audio voltages and currents. Use normal electrolytic capacitors, preferably rated to 105 deg Celcius. Also, I suggest you not to touch ceramic capacitors, they sometimes define high frequency rolloff of the amplifier or part of it (like C10 & C15). If you put a failed new unit or bad soldered, or any other mistake, the amplifier entirely will start to oscillate at a very high frequency, and the power output will be destroyed.
I've been told that old type ceramic capacitors can be microphonic. The newer multi layer ceramics are much better and have lower inductance at RF. The electrolytics at the input and the feedback loop can be replaced with "audio grade" ones.
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Old 9th January 2012, 08:31 PM   #15
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I am recapping my solid state power amplifier that uses transistors at it's output stage. The Circuit diagram shows a 10uf 250V electrolytic capacitor connected directly to the transistor. An electrolytic is currently connected there now that is 35 years old! I can fit a 10uf 250V MKP in the space but as a novice I am unsure whether or not it would be o.k. to use a better quality bypass cap or "tone capacitor" to accentuate the high frequencies, say a .01uf? In reading through the forums I have read that this may destabilize the bias current at the transistor and cause it to fail? The last thing I want is for the transistor to fail. Is this an appropriate place for a bypass cap?
I don't see why those are rated for 250V. I don't believe you need anything more than 100V rating. I'd replace it with a good grade audio electrolytic and I'd replace the 0.1uF ceramics with the newer multilayer types. Keep the leads as short as possible.
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Old 9th January 2012, 08:31 PM   #16
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Wow! Thanks. It is interesting to see how the circuit breaks down into function. What about C16? What is the function of that position because there is a huge mylar there now, a 1uf 100V.
I can't find C16 in your schematic.
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Old 9th January 2012, 09:04 PM   #17
AmCan is offline AmCan  Canada
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I can't find C16 in your schematic.
It's weird because it is right after VR1 and just before C13. I was going to use some RTE's for those .01, which are polystyrene film, real stable (2 % tolerance). They are not that expensive. Might be good especially if those positions determine high frequency roll off? Thanks for the suggestions. There are not many caps here so I would like to go all out a bit.
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Old 9th January 2012, 09:20 PM   #18
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It's weird because it is right after VR1 and just before C13. I was going to use some RTE's for those .01, which are polystyrene film, real stable (2 % tolerance). They are not that expensive. Might be good especially if those positions determine high frequency roll off? Thanks for the suggestions. There are not many caps here so I would like to go all out a bit.
VR1 appears to be your bias adjustment. C16 bypasses it apparently. I would leave it at the same value since I don't know what would happen if you change the value of it.

All of the 0.1 or so caps are probably for high frequency bypassing, so I would use multilayer ceramic. They have lower inductance at RF than polystyrene or any other film cap.
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Old 9th January 2012, 11:46 PM   #19
AmCan is offline AmCan  Canada
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VR1 appears to be your bias adjustment. C16 bypasses it apparently. I would leave it at the same value since I don't know what would happen if you change the value of it.

All of the 0.1 or so caps are probably for high frequency bypassing, so I would use multilayer ceramic. They have lower inductance at RF than polystyrene or any other film cap.
So as I understand it a bias bypass caps helps to deal with voltage fluctuation when the transistors over heat. Meaning that C16 is not that important to the quality of the audio signal as some other caps.

I am amazed by what I have learned here. It would appear then that the most important caps are the main power capacitors followed by the input capacitor (C1), and the negative feedback cap (C6). C8, C9, C10, C15 & the transistor caps for high frequency bypassing.

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 .
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Old 10th January 2012, 10:49 AM   #20
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The reason why sometimes any manufacturers do some straneous things is for me unknown. I had repaired SMPSīs where use 100uF 63V in a 5V rail (63 volt, not 6.3V!!). I believe there may be the wrong knowledge that big capacitors has lower ESR or resonant frequency, and in trust it is not. I canīt understand why to use 250V caps in 55V rail. 63V is quite reasonable. C16 bypasses the bias transistor so both drivers "see" the same HF levels at their bases.

Good luck!
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