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Caps topology

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Hi all

look at the following auto-bias
output stage.
It is better the pic #1 (classic)
or pic #2.
PSRR is better in cir #1 but current
flows through 2 caps while
in cir #2 mainly through only
one cap.

any comment?

Federico
 

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fscarpa58 said:
Hi Tube_Dude



Absolutely no!

Very little AC current flows through the PSU cap of
cir #2. It is clear since that path has
more resistance ( the bias res in series) than the
other.

You can simulate it

Federico

We are at the age of simulation and people don't to put a hand and the nose in the real stuff..;)

Retourning to pic 2...the upper part of the power suplly capacitor(B+) is concerning AC signals a "ground"...in the pic 1 the cathod resistor is decoupled to ground...in the pic 2 the cathode resistor is also decoupled to B+ (AC ground)...

I hope i have been clear!
 
Hi Tube_dude

the upper part of the power suplly capacitor(B+) is concerning AC signals a "ground"...in the pic 1 the cathod resistor is decoupled to ground...in the pic 2 the cathode resistor is also decoupled to B+ (AC ground).

I completely agree, but it does not matter.

In pic #2, from the catode to B+, we can see
two paths (a and b) in parallel, as you can see in the following
pics. These paths have not the same resistance.

a) Continuous line (small res > big current)
b) dashed line ( big res > small current)

bye

Federico
 

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In your second exemple you show the AC path from the cathode to B+ but you forget to trace the arrow through the power suplly capacitor to ground...because that AC current must be decoupled to ground...;)
If that path doesn't existe the cathode will be not decoupled and the circuit will have cathode feedback...:)
 
you forget to trace the arrow through the power suplly capacitor to ground

Sincerely, Jorge

I do not understand what arrow.
Can you trace that arrow for me?
Are we both speaking of the current
that travel trough the tube ? true?

Frank, maybe the circuit #2
cancels out too much noise.
Yes, the noise which reachs
the catode tends to cancels out
the other but it is amplified (1+mu times)
and becomes dominant.

bye

Federico
 
diyAudio Senior Member
Joined 2002
Hi,

Yes, the noise which reachs
the catode tends to cancels out
the other but it is amplified (1+mu times)
and becomes dominant.

Sure, but you need to calculate what amount of PS noise you inject into the cathode in order for it to cancel out at the anode of the tube.

Assuming the valve has a u of 4 you inject 1/4 of the PS noise into the cathode for this to happen.

Ratio = C1C2/ (C+C2)

Cheers,;)
 
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