Coupling capacitors

[sb****33]

Hello everybody

I'm just curious about the values in this preamp : C4-C6 .22uF.
Why not 10uF like the output one C7 ?
Are they acting like low frequencies cut-off ?
There are such value in ARC SP6, for exemple.

 

[AndrewT]

the coupling capacitor acts as a High pass filter when combined with the following load.
The load is 1Meg in parallel to the grid input impedance.

I don't know how to calculate that input impedance.

If I assume it is 0.5M , then the total load after the coupling capacitor is:
500k*1000k/(500k+1000k) = 333k

The turn over frequency of this single pole filter is given by
F-3dB = 1 / (2 PiRC) = 1/(2*3.14*333k*220*10^-9) = 1/0.46 = 2.2Hz
The response will be down by ~1dB @ 4.4Hz

You need an estimate of the input impedance to allow you to check the F-3dB.

 

[AndrewT]

Your profile says you joined in July 2001, but under your nickname it states Jan 2009
What up?

 

[ErikdeBest]

Your profile says you joined in July 2001, but under your nickname it states Jan 2009
What up?

Weird indeed If one clicks on profile, the name Stan 33 something appears, with last activity in 2006.

 

[sb****33]

the coupling capacitor acts as a High pass filter when combined with the following load.
The load is 1Meg in parallel to the grid input impedance.

I don't know how to calculate that input impedance.

If I assume it is 0.5M , then the total load after the coupling capacitor is:
500k*1000k/(500k+1000k) = 333k

The turn over frequency of this single pole filter is given by
F-3dB = 1 / (2 PiRC) = 1/(2*3.14*333k*220*10^-9) = 1/0.46 = 2.2Hz
The response will be down by ~1dB @ 4.4Hz

You need an estimate of the input impedance to allow you to check the F-3dB.

OK, Andrew.
What about C6 and C7 ?

 

[sb****33]

Your profile says you joined in July 2001, but under your nickname it states Jan 2009
What up?

what nickname ? stan_33 ? I don't know who is it. I never entered that name.

 

[DF96]

I seem to recall other people finding that the site software can confuse people with what it considers to be similar IDs. stan_33 and sb****33 both start with 's' and finish with '33', so easy for a dumb computer to think they are the same person!

Raise a query in the web problems forum.

 

[AndrewT]

is "****" appearing because he typed ****, or because the Forum recognises that as a banned word and has overwritten his entry?

 

[AndrewT]

C7 is coupling capacitor, but the output looks wrong.
C6 is a bootstrap that creates one of those totem pole thingies that I don't understand.

 

[Merlinb]

R2 is incorrect. It should be much larger, say 100k to 1Meg.

 

[stajo]

V1b and V2a makes a mu follower. And yes R2 should be way larger

 

[Tubelab_com]

R2 is incorrect. It should be much larger, say 100k to 1Meg.

This is true, I am assuming 1 meg.

What about C6 and C7 ?

Those two capacitors are looking into a high impedance, about 1 meg ohm. If you put a 10 uF capacitor there it would take nearly a minute for the cap to fully charge, probably longer due to the changing characteristics of the tubes as they warm up.

C2 feeds into an unknown impedance since it goes to the outside world. 10 uF would be needed if the load was rather low. I would reduce it if the load is known too be a high impedance, say a 100K volume pot.

Large value coupling caps are not needed or wanted if they allow the low frequency response to extend below the audio range. Low frequency instability can occur if the power supply isn't properly decoupled at the frequency extremes. Warped (or worse not concentric) records will cause fits with your power amp trying to eat a roughly 1/2 Hz signal.

 

[sb****33]

R2 is incorrect. It should be much larger, say 100k to 1Meg.

You are right, Sir. On PCB, R2 is 470K. I picked up the schematic on the net.
By the way, what are your thoughts about my primary question ?

 

[Cal Weldon]

I seem to recall other people finding that the site software can confuse people with what it considers to be similar IDs. stan_33 and sb****33 both start with 's' and finish with '33', so easy for a dumb computer to think they are the same person!

Yes, it doesn't understand certain symbols and reverts to whatever is closest.

is "****" appearing because he typed ****, or because the Forum recognises that as a banned word and has overwritten his entry?

I suspect the former.

 

[Merlinb]

By the way, what are your thoughts about my primary question ?

C4 and C6 are driving high resistance loads, so they should be small capacitors. C7 may be driving a low resistance load (external), so it needs to be a big capacitor.

 

[piano3]

Large coupling caps are sometimes used to enable shunting of noise of a following high resistance by the preceding stage. This would only be useful in the early stages of a low level amplifier.