[sdiy] ASM Capacitors/Bench Power Supplies

[Czech Martin]

IMHO the things that matter are leakage and soakage.

Leakage will give problems in low frequency tracking (frequency
is lower then expected).
Soakage (hysteresis) is also only visible at low frequencies.
I'm not so sure about the effect.

I think the specific materials and components must be 
looked at. Brand names or chemical compound names give only
a rough information.

A voltage follower with very low input current is a must for
this, a CA3140 is not suitable, better this low current Burr Brown
or a CMOS (National has a good one).
Or a BS170 DMOS.

Sometimes I have the impression that things are mixed up.
For RF stuff you want high Q oscillators. Low losses to
get less phase noise.

Since our music VCO do not have inductance (L), we can not really
talk about Q. As long as no big frequency error (scale or drift) is introduced,
any cap is ok for music VCOs. It can be that this leads to the
same stuff that the RF guys like, but I think for some different reason
(in most cases).


m.c.

-----Original Message-----
From: Ian Fritz [mailto:ijfritz at earthlink.net]
Sent: Donnerstag, 31. Juli 2003 18:50
To: Neil Johnson; Steven Downhill
Cc: Synth DIY (E-mail)
Subject: Re: [sdiy] ASM Capacitors/Bench Power Supplies


At 03:32 AM 7/31/2003, Neil Johnson wrote:
>Steven,
>
>1. Polystyrene capacitors are needed for the oscillators because of their
>excellent low leakage and stability.  As is the way, they are therefore
>more expensive than polyester or ceramics, which are fine in other places
>(I use polyester for audio path, and ceramic for decoupling).

Does anyone know of a reason not to use NP0 ceramics for the integrating 
cap? I briefly tried both NP0 and polystyrene in my last VCO and didn't see 
any difference in waveshape or tracking.  The NP0 has a smaller 
tempco.  Also it seems to me that the high-grade mica units, although 
expensive, should work better than polystyrene.

Also, PEN (polyethylene naphthalate) and PPS (polyphenylene sulfide) look 
interesting.  Mouser has some CD PPSs in chips, the only ones I have seen 
readily available.

   Ian