Phantom power blocker for synth outputs

[Newmarket]

In addition, for the sake of perfect sound, it is worth connecting a minimum 100 nF Mylar or polypropylene capacitor in parallel with the biolar electrolytic capacitors in order to eliminate the high-frequency phase shift. I recently repaired a friend's Quad 66 preamplifier, all coupling capacitors were 50uF / 25 V bipolar. I measured the phase shift at 20 kHz at 48 degrees. (With a Neutrik A2-D Audio Test & Service System instrument.) In addition, when testing with a square signal, I saw "endless ringing" on the oscilloscope, at the top of the signal. I installed 680 nF polypropylene capacitors everywhere and reduced it to 14 degrees. The ringing has stopped. Synthesizer sounds are full of harmonics, phase shifting can result in a bad sound.

If you like. Personally I'd probably just go with a higher value capacitor. Or add them in parallel

 

[Newmarket]

35 KV? You are brave! I worked in a plasma physics research laboratory with 10 KV capacitor banks, but I had to be so careful

Well in context I am in the "Low power" section so current capability is low. It will still hurt / stun you though. We actually go to 70k +. The "High Power" section has a lot more current on tap.

 

[Newmarket]

Cheap ceramic caps have a pretty severe

voltage coefficient,

Yes. This a known thing (ask me how I know 🙄)

and NPO caps have limited values, so if a coupling cap, better to use a film cap, or even electrolytic.

Yes

Most SMT resistors are thick film, metal film are many times more expensive and not very common.

Use "Thin Film" or MELF. Yes they cost more. Although I've put standard Thick Film into a product (sorry - some decades past now 😊) and no one complained - at Trade Shows. Nor did the test kit. Capacitors were a different matter though.
Big disadvantage with MELF is that they'll just roll away and drop onto the floor 🤣

Active devices, I don't think SMD suffers any disadvantage.

True. All else being equal you're simply minimising lead inductance.

 

[Cqwet Dbdfte]

If an unbiased X7R is fed with a high level white noise the larger amplitude HF signals would the likely be attenuated.
I don't think SPICE models include voltage coefficient.

 

[RoadrunnerOZ]

Big disadvantage with MELF is that they'll just roll away and drop onto the floor 🤣

MELF racing could become a thing 🐎

 

[Newmarket]

If an unbiased X7R is fed with a high level white noise the larger amplitude HF signals would the likely be attenuated.
I don't think SPICE models include voltage coefficient.

I think you're right about Spice models. Would be interested to know if others here know different. I usually (ie nearly always) just pull up the default cap and set its value.
Not certain what you mean by biasing X7R. Some proportionality wrt DC voltage across the cap ? I'm in the pub so not thinking deeply about it atm 😊
And to put in context - depending on your metric then difference can be small. Eg taking THD measurements where NP0 gave -90dB result, X7R returned same within a few dB. Long time ago now and @ 48kHz.

 

[JohnRoberts]

And to put in context - depending on your metric then difference can be small. Eg taking THD measurements where NP0 gave -90dB result, X7R returned same within a few dB. Long time ago now and @ 48kHz.

What was he bandwidth of the circuit being tested? THD products from 48k will be multiples of 48k, 96k, etc. If the higher harmonics are rolled off the measurements may look more linear than the reality.
===
I observed a bandwidth issue with THD measurements used to qualify RIAA playback paths. RIAA EQ is basically a low pass filter so HF THD is attenuated by the circuit being tested. I switched to using two-tone IMD (19-20kHz) for designing RIAA preamps and found it especially revealing of linearity issues. The IMD distortion components actually get boosted by the RIAA EQ instead of attenuated.

BTW I don't much care about the linearity x7R when used for PS decoupling.

JR

 

[Cqwet Dbdfte]

X7R and others have a strong voltage dependent capacitance, check the datasheet. Low voltage caps more problematic. With a sufficient bias voltage on the cap it would find its ultimate capacitance, so PSU decoupling not an issue .
Using these types for audio signal coupling is not a good choice.