Bypass Caps & Transistors

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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?
 

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This capacitor forms a filter to high frequencies, so they can quickly go to earth, without going to the bulk capacitor in the power supply. I suggest to use "low ESR capacitor" here, placed as close to the transistor as you can, and if possible, rated to 105C. Normally, there must be some capacitors of different values to decouple several ranges of frequencies. By example, a 1micro 50V electrolytic resonates about 100KHz. To any frequency above 100KHz, this cap is not a cap, is inductive. So, there must be a .1 (By example) to decouple the range in 100KHz, to, say, 500KHz. You can tell me 500KHz is not present in normal audio program, but if the supply is not well decoupled, the harmonics generated INSIDE the can reach low signal stages via the power lines/buses and start oscillate in a frequency where the amp can blown easily.

Best regards from Barrio Garay, Buenos Aires, Argentina.
 
This capacitor forms a filter to high frequencies, so they can quickly go to earth, without going to the bulk capacitor in the power supply. I suggest to use "low ESR capacitor" here, placed as close to the transistor as you can, and if possible, rated to 105C. Normally, there must be some capacitors of different values to decouple several ranges of frequencies. By example, a 1micro 50V electrolytic resonates about 100KHz. To any frequency above 100KHz, this cap is not a cap, is inductive. So, there must be a .1 (By example) to decouple the range in 100KHz, to, say, 500KHz. You can tell me 500KHz is not present in normal audio program, but if the supply is not well decoupled, the harmonics generated INSIDE the can reach low signal stages via the power lines/buses and start oscillate in a frequency where the amp can blown easily.

Best regards from Barrio Garay, Buenos Aires, Argentina.

Thanks Barrio. There are 2 .01uf 100V (green circle) and 2 1uf 50V caps (red circle) and 2 .1uf caps on the Driver Board immediately preceding the transistor. The transistor is directly decoupled with the 10uf 250V capacator. (The Driver Board is supplied by the main power supply bank of 4 15k Cans.) I bought a 10uf 250V Mundorf Mcap MKP and a .018uf 300V Sprague Vitamin Q to connect directly to the transistor. I just wanted to make sure that I wasn't doing something wrong by bypassing the MKP circuit so close to the transistor. My interpretation of your response is that this will not harm the transistor if I parallel these 2 caps onto the transistor. Actually; The purpose of the Driving Board makes sense to me now.
 

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This capacitor doesn´t affect the transistor itself, it is for bypassing the power buses. It deviates high frequency signals to ground instead of going to lower signals stages via power lines.

My name is Osvaldo, Barrio Garay is my location in the earth:

Location of LW1DSE ? Google Maps APRS

Ja ja ja...

Sorry about that Osvaldo :sorry: Don't I look stupid. I should have paid better attention.

Thanks for the explanation about the signals bypassing to ground. That makes sense.
 
Don't bother with the caps in red or blue, they are not in the normal audio path (that is the short circuit protector).

Thanks for that information. I am just learning how to read circuit diagrams. Can you tell me how do I make a determination about what caps are in the audio path and deserve better parts? I appreciate the advice.
 

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OK, don´t care the "renaming". The circuit is so little I can´t look correctly. The capacitors under doubt are all that are involved in the signal path: input coupling, negative feedback and bootstrapping. Remember that you can use capacitors some microfarads more than original (if you have a 15uF, and don´t have one, you can put 22uF), but never less than original (10uF is not a good idea). And, if you have to replace with different voltage ratings, use the smaller near to the original (If orig is 40V, use 50, but no 100V), they are bulky, has different self resonating frequency, and some of them can´t be formed properly at a voltage so much less than rated.
 
OK, don´t care the "renaming". The circuit is so little I can´t look correctly. The capacitors under doubt are all that are involved in the signal path: input coupling, negative feedback and bootstrapping. Remember that you can use capacitors some microfarads more than original (if you have a 15uF, and don´t have one, you can put 22uF), but never less than original (10uF is not a good idea). And, if you have to replace with different voltage ratings, use the smaller near to the original (If orig is 40V, use 50, but no 100V), they are bulky, has different self resonating frequency, and some of them can´t be formed properly at a voltage so much less than rated.

If when you scroll over the thumbnail you will see a little expansion button in the bottom left corner of the image that will blow it up to it's original size. I understand the basics of replacement values but would like to know where I might benefit from upgrading to a better quality film cap?
 
Thanks for that information. I am just learning how to read circuit diagrams. Can you tell me how do I make a determination about what caps are in the audio path and deserve better parts? I appreciate the advice.

My best summation from studying the schematic is that the audio signal starts with C1, C2, (3,4,5 Main Power Bank) C6, C16, C13, C12. Is that the audio signal path? Although I realize that amplification is only as good as your weakest link even if it is a negative feedback cap I would still like to know how to follow the signal.
 
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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 don´t need to be replaced unless they go open (very rarely) or short circuit (idem). There isn´t 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.
 
Begin with C1 and C6 (input and NFB loop). C11, C12, C13 & C14 , that are part of the short circuit protection. Ceramic capacitor normally don´t need to be replaced unless they go open (very rarely) or short circuit (idem). There isn´t 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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Begin with C1 and C6 (input and NFB loop). C11, C12, C13 & C14 , that are part of the short circuit protection. Ceramic capacitor normally don´t need to be replaced unless they go open (very rarely) or short circuit (idem). There isn´t 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.
 
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.
 
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.
 
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.
 
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 :cheerful:.
 
The reason why sometimes any manufacturers do some straneous things is for me unknown. I had repaired SMPS´s 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 can´t 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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