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Old 6th January 2012, 11:12 AM   #1
AmCan is offline AmCan  Canada
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Default Bypass Caps & Transistors

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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Old 6th January 2012, 11:41 AM   #2
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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.

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Old 6th January 2012, 12:48 PM   #3
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Originally Posted by Osvaldo de Banfield View Post
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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Last edited by AmCan; 6th January 2012 at 01:17 PM. Reason: Mislabeled Diagram
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Old 6th January 2012, 04:04 PM   #4
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This capacitor doesnīt 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.

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Old 6th January 2012, 09:36 PM   #5
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Originally Posted by Osvaldo de Banfield View Post
This capacitor doesnīt 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 Don't I look stupid. I should have paid better attention.

Thanks for the explanation about the signals bypassing to ground. That makes sense.
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Old 8th January 2012, 08:30 AM   #6
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Don't bother with the caps in red or blue, they are not in the normal audio path (that is the short circuit protector).
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Old 9th January 2012, 02:27 PM   #7
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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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Old 9th January 2012, 03:20 PM   #8
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OK, donīt care the "renaming". The circuit is so little I canīt 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 donīt 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 canīt be formed properly at a voltage so much less than rated.
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Old 9th January 2012, 03:35 PM   #9
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Originally Posted by Osvaldo de Banfield View Post
OK, donīt care the "renaming". The circuit is so little I canīt 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 donīt 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 canīt 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?
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Old 9th January 2012, 05:19 PM   #10
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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.

Last edited by AmCan; 9th January 2012 at 05:25 PM.
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