Dear Mr. Moriarity,

Well, I finally used to scope to prove to myself
that I wired the phase splitter correctly.

I placed 100 mV p-p 1000 hz sine signal into C15. I get
proper 100 mv p-p 1000 hz sine signals that are
180 out of phase into pins 2 and 3 of U1.

R12 seems to be misspecified as well. On the
pdf schematic, R12 shows as "10W". I assumed
at the time that the W was simply a misconverted
omega symbol, so I had soldered in a 10 Ohm
resistor there.

U1 does not amplify at all!!! So, I figured
that perhaps it was a mis-typed "K". So I tried
a 10 K resistor.

With a 10K resistor as R12, I got a VERY nice
volume that filled the entire room. The only bad
news is that when I remove the input signal, I get
about 400 hz motorboating.

I wired a pot in place of R12, and found that the
threshold resistance was somewhat below 800 Ohms.
Much less than that, and U1 did not amplify. But
any value needed for U1 to amplify caused at least
a few hz of motorboating.

Please keep in mind that I am using ground plane
construction and that both leads and traces are
reasonably short.

Also keep in mind that I constructed only the
phase splitter and audio amp stages. Basically,
everything between C15 and the speaker, along with
the necessary D1/LED1/R4 voltage regulator.

My remaining hypothesis is this: Is it that I have
too much gain in U1? Should
I change the value of the capacitor, C18,
between pins 1 and 8 of U1 to adjust the maximum
gain in the hopes this will kill the motorboating?

I don't know how else to solve the motorboating
problem, especially in a circuit that NA5N billed
as inherently resistant to motorboating according
to the explanation sheet.

Again,

The Eternal Squire


John Moriarity wrote:
Or I should report the emitter voltage more
exactly. I'll verify this. Tell me something,
how terrible an idea is it to use disk ceramic
for C15, C16, and C17? I have heard of
disk ceramics being capable of conducting DC
if made badly.


The disc ceramics should be ok, as long as they're
good.

I've never built this circuit, I'm just looking at it
as a paper exercise. DC measurements will usually
locate the problem. Spice may be overkill ;-)

GL & 73,

John - K6QQ