----- Original Message -----
From: "Jim Shorney" <[email protected]>
To: <[email protected]>
Sent: Sunday, April 17, 2011 12:59 PM
Subject: Re: [Drakelist] R-4B audio problem traced to low
beta Q7 (2N3394)
On Sun, 17 Apr 2011 15:13:23 -0400 (EDT), [email protected]
wrote:
Very good explanation...I understoof about 30% of it...but
then I am a
tech., not an engineer. Hi, Hi.
What made this tough is that Q7 was kind of working...so I
was
checking, replacing everything BUT Q7.
My experience as a tech in situations like this has taught
me to suspect
capacitors first, transistors next. I've seen transistors
fail in some strange
ways over the years. :)
I've recently learned that disc ceramic caps are not ideal
for audio coupling.
You guys have got me thinking about distortion and such.
I'm thinking about
putting the TR-7 on the bench and trying out that nifty
surplus distortion
analyzer that's languising out in the garage.
73
-Jim
The use of ceramic caps in audio circuits is somewhat
controversial. Ceramics of some types, mainly high-K types,
do tend to have a variation of capacitance with voltage and
some audio folks think this contributes some distortion.
However, in most coupling circuits there is a steady DC
voltage on the cap that exceeds the AC voltage. Also, I have
never seen any rigorous research on this showing that a real
difference exists. High-K ceramics do have high temperature
coeficient of capacitance but low-K ones can have zero
temperature variation (NPO types). There can also be a small
variation in effective capacitance due to interfacial
polarization (don't ask) but mica caps have the same
problem. This is of concern only were a cap is used for
precision measurement at low frequencies. Paper and plastic
film caps have virtually none of this effect. I doubt if it
has any significance for normal audio use.
Ceramic caps have the virtues of small sizes,
availability in high voltages, and great reliability and
long life.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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