Coupling vs De-Coupling

Can anyone explain to me if there are differences in coupling and de-coupling caps.
If there is, what preferences for both.

 

No-one biting?

Coupling caps are used to connect signals from one circuit to another, whilst blocking the DC component that may be there.

Decoupling caps are used to short AC signals, often to a PSU rail, in order to keep the DC component, whilst losing the AC part.

One is in effect a high-pass filter, one a low-pass.

There are definitely preferences, but little universal advice to offer, you need to analyse each application individually.

For example 'decoupling' caps are often used in amplifier feedback networks, and since this is very much a signal path application, it may have different requirements from a PSU decoupling capacitor.

At the end of the day, no cap is perfect, and all have differing imperfections. You have to decide what the effect of the imperfections are in each application.

Andy.

 

Thanks Andy

SO, if i have some caps on the anode supply of a valve circuit would these be classed as power supply caps?

 

Cap on the anode would be decoupling caps for the PSU, to short any AC to earth and therefore smooth the DC. Are you building something? Not a valve buffered Gainclone by chance

 

Just changeing a few bits on my Cornet, and trying to get a better understanding on the way.

Lee, have you ever replaced smoothing caps wit PP?
I heard it can be beneficial sometimes

 

hmmm strictly speaking, if you look at a valve amp schematic, both the coupling cap. which isolates the following grid from say the preceeding anodes dc puts in a lo freq roll off or hi pass filter around say 5 hz, and the cathode bypass, which allows dc on the cathode, whilst letting the ac signal give the most gain on the valve, will also be a lo roll off or hi pass.

All caps cause phase shifts, and I suspect this is why they are considered undesirable, as well as an additional bit in the signal path.

Coupling is to do with both isolating (blocking dc) and developing a lo freq.( I think the cornet is valve? , so you don't want say 200 volts from the prior anode which gets the gain, to be on the following grid, or the difference between the 200v that would be on the grid and the 2 v on the cathode could be too much and knacker the valve. For that reason the cap blocks this dc)

de coupl is to do with eliminating ac feedback, whilst keeping dc feedback.( as feedback reduces gain, and your signal is ac, eliminating this local feedback allows you to raise the gain of the stage, as one is sort of the opposite to the other!)

regulators are sometime decoupled for smoothing, and power supply lines similarly, the big caps for both filtering and current store, and the smaller for bypassing the bigger ones, and getting rid of the inductance of the supply lines.

usually for coupling its in the signal path, and not too high values, so caps are considered the smaller the value, the better quality the dielectric material can be, so you can have things like... polyester, polypropylene, polycarbonate, polytetrafluroethane (ptfe, teflon), or paper in oil.

decoup, bigger values and thus electrolytic, things like black gates, oscons, stargets, silmics.

 

cheers LCD

I stuck an rusian mil PIO on the signal coupling position a few days ago - gave a nice result, TBH better than the damn boutique stuff i forked out for before, but it is inline with a few of those so maybe ive hit the balance.
Could do with a tad more bass maybe, but that may also be my TT.

I currently have 47UF 450V 'lytics on the B+ rail, im considering changing to 50UF PP in oil and possibly a choke inline.
If i can find a choke at a good price i think it will be worth the experiment
Ive heard that replacing the 'lytics with PIO or PP in oil add's some more body to the sonics, so hopefuly worth playing with

 

I haven't actually put PP in place of lytics, but know people who have and say it is worth it. On the other hand others just bypass the lytics with good quality PP and then silver mica, going down 1/100th at a time.

I've never seen large value PIO. About the biggest I've got is 8uF and that is the size of a twiz bar. Mind you my ICW 47uF are the size of coke cans.

 

yep, the ones im looking at are around beer can size