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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Do preamp input caps need to be polar (DC blocking?)?
Do preamp input caps need to be polar (DC blocking?)?
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Old 23rd September 2019, 05:44 PM   #11
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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On the type of 47n cap, ceramic cause microphonic issues when they age, (they turn into condenser microphones as the plates move) but there is not enough gain there to make it obvious.
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Old 27th September 2019, 01:22 PM   #12
Mark Tillotson is offline Mark Tillotson
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A condenser mike has moving diaphragm, not very similar, ferroelectric ceramics generate voltage when the crystal structure is distorted - its another word for piezoelectric. They generate voltage spikes when tapped with a stick, whether brand new or not.
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Old 27th September 2019, 03:06 PM   #13
DF96 is offline DF96  England
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Ferroelectric is not another name for piezoelectric, although some ceramics have both properties.

Some ceramic caps can act like a crystal microphone - quite different from capacitor microphone. Then there is the electret microphone, which is a type of capacitor microphone with a built-in bias.
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Old 27th September 2019, 03:36 PM   #14
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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The input impedance looking from the source is high, because of the base resistors are bootstrapped from the emitter, so this cap has reasonable value. In any case, I would prefer a higher one, like 0.33 or 0.47F.
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Old 27th September 2019, 10:47 PM   #15
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by DF96 View Post
Ferroelectric is not another name for piezoelectric, although some ceramics have both properties.

Whoops, yes, all ferroelectrics are strongly piezoelectric, but piezoelectric materials are not all ferroelectric (quartz being a good example). Many ferroelectrics are based on the barium titanate family.
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Old 29th September 2019, 04:27 PM   #16
raffajaffa is offline raffajaffa  United Kingdom
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Ok going slightly off original topic (preamp) I have a few questions re DC blocking on power amplifier. Specifically the Rotel series 980 etc .
The main cap is a 4.7uf. electrolytic with a 33.3K resistor accross.on the official spec sheet.

I noticed on one of the recommended mods to improve sonic performance that a PP of 3.3 to 4.0 would be a better bet. I think 3.9 is being used now. Historically I found reference to being able to use lower values for PP that electrolytics. Plus an explanation Ive forgotten.

Is their also an argument for altering the 33k resistor upwards to maintain the same results?....
Personally I have always removed this blocking caps on most of the many amplifiers for over 55 years to get improved sonics. .
Second question relates to Humble HIFI tests capacitors. This seems to be totally for crossover use? Do such opinions count for this DC blocking use?...
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Old 29th September 2019, 10:01 PM   #17
Mark Tillotson is offline Mark Tillotson
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As a blocking capacitor the key issue is the AC voltage across the capacitor (not the DC offset). Often the AC voltage across is tiny as the impedance of the cap is low compared to the loading impedance.


If this is the case the distortion the cap can produce is very small, even if electrolytic, because the bulk of the signal voltage is across the load, not the coupling cap.


Increasing the value of the capacitor reduces its impedance and thus further reduces any distortions. Often this is more economic than replacing a large value electrolytic with a film cap.
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Old 30th September 2019, 05:07 PM   #18
DF96 is offline DF96  England
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An electrolytic coupling cap will often be oversized in order to make distortion negligible. This is fine because electrolytics are easily available in large values yet small physically.

Swapping it for a film cap often means that you don't need such a large value. This is good because film caps can be physically large and so pick up hum and interference. You need to decide what LF rolloff is needed at this point in the circuit.

Coupling caps are there for a reason. Removing them is unlikely to improve the sound, but it may make the equipment less reliable.
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Old 1st October 2019, 12:22 AM   #19
Mark Tillotson is offline Mark Tillotson
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Another reason to make coupling caps large value electrolytics is because all their roll-offs combine - several -3dB at 6Hz rolloffs combined can easily make -3dB at 20Hz or worse.


This is also an argument for DC-coupling where feasible of course.
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Old 1st October 2019, 12:36 AM   #20
oon_the_kid is online now oon_the_kid  Malaysia
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For commercial units they need to have coupling caps at the input because they can never be sure of the source before it. If it is faulty with DC content etc. However if you are confident that the previous stage already has one. Then it would be okay.

Another reason is if your preamp is single supply, so there is a high positive bias. In which case the cap in the previous stage might be biased the wrong way and can lead to damage.

Oon
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