Capacitor across B+ and the input signal on the primary of the output transformer - diyAudio
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Old 3rd October 2015, 06:15 PM   #1
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Default Capacitor across B+ and the input signal on the primary of the output transformer

Hey guys. Sorry if I worded that title wrong. My question is what is the purpose of the capacitor across the primary side of the output transformer which is used on some amps, this one for example:

Look at that 3,3n capacitor on the output transformer. What does it do? What happens if it gets removed? Can it be a source of muffled sound in an amp?

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Old 3rd October 2015, 07:48 PM   #2
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Do a search for "conjunctive filter".
Safe to remove and listen for different sound - listen/scope for oscillation tho'.
Putting a small cap across the 82R resistor might help unmuffle your sound.
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Old 3rd October 2015, 09:29 PM   #3
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The cap removes transformer ringing. In HIFI it is supposed to be sized such that it removes high frequencies. If it is oversized, it could affect the audio range as well.
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Old 3rd October 2015, 10:16 PM   #4
tomchr is offline tomchr  Canada
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If you do remove the cap, do measure the output with an oscilloscope before connecting the amp to a speaker. The cap is often used to extend the bandwidth or tame the ringing of a cheap output transformer. If you just remove it, the amp may oscillate. Oscillation can happen at supersonic frequencies and destroy your tweeters.

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Old 4th October 2015, 06:26 AM   #5
teemuk is offline teemuk  Finland
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The capacitor "evens" rising inductance of the loudspeaker's voice coil at high frequencies. This in many ways helps to stabilize amp's operation in regards to overall oscillation as is or during "clipping". Preferably you'd fit a series resitor to the cap so that the amp doesn't have to drive "short circuits" at highest frequencies.
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