John Byrns wrote:
>
> In article <47E4A1B3.B670989F@[EMAIL PROTECTED]
>,
> Patrick Turner <info@[EMAIL PROTECTED]
> wrote:
>
> > There are NO other brands other than Quad *I know* of who now use 10%
> > CFB,
> > and none at all in the 1950s and 60s, except Lincoln-Walsh, who may
have
> > made amps with CFB BEFORE Quad made their famous type II power amps.
> > I invite someone to name other makers who they think did use CFB.
>
> It probably wasn't 10%, but Bogen used CFB in many of their amplifiers,
> see this link for an example:
>
> http://www.schematicheaven.com/hifiamps/bogen_db20df.pdf
>
> Regards,
>
> John Byrns
You are quite right John.
Most amplifiers manufactured in the US were not im****ted to Oz
in the 1959s and 60s.
Hell, they all needed 120V mains and we only had 240V.
And we had quaint tariff laws, and US proces for any darn thing
were high, and when folks dreamed of nice amps,
they thought of Quads, Leaks, Radfords, and other Bitishalia.
The Bogen is quite sophisticated in its FB arrangements and elsewhere.
There sure is local NFB in the output stage from OPT to OP tube
cathodes.
I dunno how much. 6L6 like KT66 will need at least 10% to do much good.
It has a substantial conventional global NFB to one 12AT7 cathode.
Then there is variable current FB from speaker return current path of
0.27 ohms and 0.47 ohms.
The pot makes the current FB either positive or negative, and
drastically changes
the amp Rout between very low and quite high respectively.
The 4uF below the 68 ohms at V1 reduce the current FB at HF,
lest the current FB spoil stability.
This is the classic way positive current FB could be applied to
better bass performance.
They also use the 12AT7 filament as a resistance to get -12.6Vdc grid
"back" bias
for the output 6L6.
This looks quite silly to me, but for them it worked.
Patrick Turner.
|
