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Muddy audio in AF power amp
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wn4isx
The new transistors are oscillating at about 1GHz, the frequency is frequency modulated by the incoming audio!
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The cure was a ferrite bead on the base and collector (now that's a first for me) and a 10pF SMD capacitor between base and emitter.
I grabbed my Tiny SA on a wild hunch, found a spike and borrowed a Signal Hound USB spectrum analyzer to confirm the oscillation.
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The semiconductor industry constantly "improves" their products.
Original 1N4004 switch slow enough to be a minimal EMI source, modern 1N4004 switch fast enough to be EMI sources from hell.? The silicon is purer, the junction is thinner and the turn off time is a lot faster.
Someday really soon I hope to post a simple test device to see how much commutation noise a diode produces.
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On Mon, Dec 9, 2024 at 02:46 PM, wn4isx wrote:
I am trying to figure out if this is the start of a new conversation, or if it is meant to be a reply to another one.? Without any context, I assume it is the former.
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(It is quite challenging for me, trying to keep up with so many conversation in every interesting group.)
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With regards to RFI, oscillations, and changing,silicon, we do live in interesting times.? :-(
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Andy
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¿ªÔÆÌåÓýExactly as I suspected. That means THERE IS harmonic distortion on a simple assembly. Thanks for confirming! Can I ask if you noted any of the harmonics at the audio part of the spectrum? ? Nuno T. From: [email protected] [mailto:[email protected]] On Behalf Of wn4isx via groups.io ? The new transistors are oscillating at about 1GHz, the frequency is frequency modulated by the incoming audio! ? The cure was a ferrite bead on the base and collector (now that's a first for me) and a 10pF SMD capacitor between base and emitter. I grabbed my Tiny SA on a wild hunch, found a spike and borrowed a Signal Hound USB spectrum analyzer to confirm the oscillation. ? The semiconductor industry constantly "improves" their products. Original 1N4004 switch slow enough to be a minimal EMI source, modern 1N4004 switch fast enough to be EMI sources from hell.? The silicon is purer, the junction is thinner and the turn off time is a lot faster. Someday really soon I hope to post a simple test device to see how much commutation noise a diode produces. ? ? ? ? ? -- Nuno T. |
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wn4isx
There weren't any harmonics but the audio sounded, and I hate this discriptor, muddy, smeared.
There might have been a very weak comb effect.?
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I fed band width limited white noise, sharp cut off at 20,000kHz, 9dB/octave, ran the amps at 10W and the mid range, ~1000 to ~4000Hz was "depressed."?
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In one of his good amps, the peaks of the white (random) noise form a nearly straight line, in the 'bad' amps, the midrange is down about 6dB.?
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I've seen noise spectra of many amps and never anything like this. I'd have expected the upper frequencies to be messed up.
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And the need for a ferrite bead in the collector lead as well as the base and a 10pF cap between base adn emitter....
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This was a dozzy!
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We modified enough transistors for 5 amps, they all worked fine and then we went out for a nice steak, his treat.
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¿ªÔÆÌåÓýThank for the feedback. ? Hope you enjoyed the meal! J ? Nuno T. ? From: [email protected] [mailto:[email protected]] On Behalf Of wn4isx via groups.io ? There weren't any harmonics but the audio sounded, and I hate this discriptor, muddy, smeared. There might have been a very weak comb effect.? ? I fed band width limited white noise, sharp cut off at 20,000kHz, 9dB/octave, ran the amps at 10W and the mid range, ~1000 to ~4000Hz was "depressed."? ? In one of his good amps, the peaks of the white (random) noise form a nearly straight line, in the 'bad' amps, the midrange is down about 6dB.? ? I've seen noise spectra of many amps and never anything like this. I'd have expected the upper frequencies to be messed up. ? And the need for a ferrite bead in the collector lead as well as the base and a 10pF cap between base adn emitter.... ? This was a dozzy! ? We modified enough transistors for 5 amps, they all worked fine and then we went out for a nice steak, his treat. ? -- Nuno T. |