The Black Hole......

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#5,201
Hans Polak said:
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Starting from an amplitude earlier as expected, the output of the amp started to produce a distorted square wave.
When replacing the ceramic NP0 cap by a silver mica cap, the problem was solved.
I mentioned at the same time that it was not at all my intention to disqualify NP0 caps, but it was a strange non expected experience.
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Could be very well a parasitics effect. I recall snipping pin 1 and 5 of an opamp in SOIC, to lower the inverting input and output parasitic caps by 0.1pf and stop the oscillations. Was a video amplifier with 250MHz bandwidth and required linear phase.
 
#5,202
scott wurcer said:
Where? Most diaphragms are mylar.
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Especially for you the second time. http://www.gyraf.dk/gy_pd/g7/gic_s.gif
As for the membranes, the first ones were made of PVC. Mylar was chosen as the membrane material because it is thinner, lighter and more resistant to tearing. Moreover, the membrane itself is not a capacitor, it is covered with gold and is a flexible composite electrode made of gold reinforced with mylar, which is one of the capacitor plates in the microphone capsule, see the diagram.
 
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#5,205
Bonsai said:
I wonder if the COG cap was cracked or damaged Hans.
Have to say that is a weird problem.
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Both channels reacted the same, so I don't think a damaged cap was the case.
Going back 14 years in my archive, I saw that the square wave offered to the anti-Riaa had 1usec rise time, quite an unrealistic risetime to be fair.
Anyhow, I could hardly get 100mV undistorted from the output, translating for this case into a 2mV square wave from the output of the AD797, which had the complete Riaa network in its feedback.
At the input, spikes of almost 50mV 1usec wide had to be coped with.
Probably like Syn08 suggested, some parasitic effect like the Mica cap having a higher ESR made the bandwidth enhancement a bit less aggressive or whatever.

See scope pictures with undistorted 100mV output and with the distorted squarewave at 150mV out.
After having replaced the NP0 with Mica silver, 150mV was flawless.
But again, for a Riaa amp nothing to worry about because far outside a realistic range, but intriguing when comparing caps.

Hans
 

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#5,208
C0G and NP0 are not ‘ceramic’ dielectrics AFAIK.

I use them in a power amplifier for the main comp caps and get 7 ppm at 200 W into 8 Ohms. I suspect the real distortion levels are close to 1 ppm since most of the distortion is from the generator as confirmed by the generator supplier specs.

Let’s not fall into a C0G/NP0 are sub par rut . . . They are not.
 
#5,211
GUNFU said:
Especially for you the second time. http://www.gyraf.dk/gy_pd/g7/gic_s.gif
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Gyraf is a DIY friendly business with the usual claims of dramatic improvement with capacitor upgrade (they aren't the only one). Most OEM mics from even the best manufactures use ceramic caps otherwise why would all the upgraders exist?

This mic is a DIY project not a product.
 
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#5,216
How do I switch the radiation pattern in this scheme? There are many similar patterns on pipes too. However, the radiation pattern cannot be switched without a capacitor. The CK12 capsule allows three types of radiation patterns in one microphone. At the same time, Gyraf, AKG and similar microphones sound great with the right choice of condenser type. http://www.gyraf.dk/gy_pd/g7/gic_s.gif
 
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#5,218
No, you are not in the subject, but especially about tube microphones. I showed you this example because it is the most sensitive point in the signal path. And even their gold-plated polyester diaphragms in capsules are not polyester capacitors at all. Moreover, in the best models of tube microphones, capacitors are used, with a polystyrene dielectric.
 
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