On Fri, 6 Jun 2008 11:19:44 +0300, "Iain Churches"
<IainNG@[EMAIL PROTECTED]
> wrote:
>
>
>"Paul" <paulguy@[EMAIL PROTECTED]
> wrote in message
>news:o70h44pprf4j2agmd3mutmmpfd00ev1010@[EMAIL PROTECTED]
>> On Tue, 20 May 2008 21:57:48 -0500, flipper <flipper@[EMAIL PROTECTED]
> wrote:
>>
>> ....snip!.....
>> Here I am replying to an old thread.
>> I tried using a positive internal feedback loop in a SET amplifier
>> that has a global negative feedback loop. By coupling two unbybassed
>> cathode resistors to each other through another resistor, I was able
>> to reduce the 2nd and 3rd harmonics by about 10db, with a small change
>> in overall gain. I'm impressed with this "trick"! Really, it is a very
>> small modification, and distortion is reduced by a factor of 3!
>
>Please clarify. Do you mean joining the bottom ends of the two
>resistors with a common (third) resistor to ground? What were
>the values?
>
It's similiar to a classic circuit from RDH4, p 354, figures 7.51A,
B. In my case there's a 5751 with 470 ohms cathode resistor
(unbypassed), and a 6SN7GT with 220 ohms cathode resistor
(unbypassed). The positive feedback resistor is 3300 ohms, it goes
from cathode to cathode. (R4 in fig 7.51B). The cathode resistors are
small compared to the plate resistors (220K, 50K) so there isn't much
loss in gain.
The positive feedback is derived from the 6SN7 plate current, so I
believe that it will actually lower the effective plate resistance.
Negative feedback derived from the cathode resistor tries to make the
device a constant current output (Hi-Z), so positive feedback should
do just the opposite! RDH4 confirms this behaviour.
Unfortunately RDH4 says that the positive and negative feedback
loops should not be returned to the same node (it says "inadvisable"),
and that's what I did. However, the impedances of the various
components effectively make that node into a summing point without
interaction between the two feedback networks. What's more im****tant,
the measurements bear out that the positive feedback loop did work as
predicted.
2nd harmonic was -60db at 15W output, 3rd harmonic was -70db. With
the added positive feedback network, both harmonics dropped by
approximately 10db.
I should have some decent schematics in a week or so.... I'm
fighting with Microsoft Visio (grrrrrr......). I'm also setting up a
web site, so you can get pictures, schematics, measurements, etc. The
server is an old cantankerous Dell machine, but it has been fighting
me over my selection of RAM and SCSI drives....
What is interesting, is that the even harmonics can also be reduced
by appropriate biasing of the driver for the output tube(s) for a SET
amp. You can pre-distort the signal to cancel some of the output tube
distortion. I don't use this (there's something cheesy about this kind
of thing) , but I have set it up, and it does work! Since even
harmonics are the result of signal asymmetry about the zero axis, the
previous stage can cancel that (it's opposite phase) by its own
compensatory asymmetry.
-Paul
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