Substitution for "1000pf 680v syrene" capacitor?

[SoaDMTGguy]

I am preparing to do the RFI modification for my Audio Research SP-10 preamp (see attached). The instructions call for 1000pf 1kV ceramic disc capacitors to be installed between all power supply output lines and ground, except for B+, for which they specify a 1000pf 680v syrene capacitor. This leads me to a couple of questions:

1. Does "Syrene" mean "polystyrene"?
2. Mouser only has 1000pf polystyrene capacitors up to 500v. Can I substitute a different cap type here?

 

[VivaVee]

Yes,"syrene" will mean polystyrene.
You could substitute a 1000pF 1kV NPO ceramic - KEMET C322C102KDG5TA is available at Mouser.
And you could, if you can bear the expense, use the same cap for all locations

 

[SoaDMTGguy]

Thank you for clarifying that. Is there a reason for NPO ceramic vs a different type?

 

[VivaVee]

Yes. NPO means that the dielectric has

1. a zero temperature coefficient (which is useful in a hot valve preamp)
2. does not change its capacitor value with applied voltage (which is very important in high voltage circuits - and that means, effectively, anything over 5V)
3. very low DA and DF, equal to any of the best plastic dielectrics with the possible exception of PTFE (which means that it does not contribute to distortion and noise)

In short, NPO ceramics are about as good as it gets. Just three problems:

1. 0.1uF is about as big as they get
2. cost
3. size
4. often confused with the lower grade ceramics dielectrics like X7R etc by people who can't/won't read...

You could read up on the distinction between ceramic dielectrics by going to one of the Tier 1 suppliers like Murata or TDK and checking out their website info

 

[Mark Tillotson]

Two things that make for a stable capactor value are: not absorbing moisture from the air, and having a low tempco.

Many types of plastic film readily absorb moisture from the air or expand a lot on temperature rise, PS and PTFE are both highly crystalline at room temperature and non-polar so perform well.

Ceramics don't absorb moisture, but only NP0/C0G types have a low tempco, many others have enormous tempcos as they are ferroelectric.

The typical application where PS caps would be used is an RF oscillator or a multipole filter circuit (active or passive). They are much less common now due to high-order filtering being done with DSP now and RF oscillators using PLLs.

 

[SoaDMTGguy]

Thank you all. You have perfectly answered my question!

1. often confused with the lower grade ceramics dielectrics like X7R etc by people who can't/won't read...

I very much did not want to be that guy!