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.

 

[Somkereki]

Az X7R és mások erős feszültségfüggő kapacitással rendelkeznek, nézze meg az adatlapot. Az alacsony feszültségű sapkák problémásabbak. Ha elegendő előfeszítő feszültség van a kupakon, akkor megtalálja a végső kapacitását, így a tápegység leválasztása nem probléma.
Ezeknek a típusoknak az audiojel csatolására való használata nem jó választás.

 

[Somkereki]

I absolutely agree! I only use it for PS interference filtering, never in the sound path! If possible, I only use polypropylene or Mylar inductance control capacitors there. Many people do not believe how significant the effect of the condenser is on the sound.

 

[Somkereki]

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

I absolutely agree! In the RIA corrector circuit, the sound of the capacitors is very audible. I only use polypropylene and styroflex (polystyrene) or induction-free Mylar capacitors. The lower the signal level, the more critical the capacitor sounds in my experience. I remember back in the late '70s I tested the sound of the capacitors with a jazz orchestra recording recorded with a professional microphone at 19 tape speed from my Uher Report tape recorder. (In a gold-plated socket) Back then, I didn't have proper instruments and my ears were much better. Even the 19 kHz stereo plot signal was very disturbing, I had to build an extra hole filter in the stereo receiver. Isophone KK-10 dome speakers became known then. Another friend of mine brought it from beyond the "iron curtain". (West Berlin) Is this history now?