inline attenuators diy question

[Mr.D]

Hello

I need to build an attenuator for the Y component in a analogue component chain.

Ideally I'd like to build one with variable attenuation for experimentation and then assuming I find the right level of attenuation build a fixed one.

So my question is...

is soldering a resistor (variable or not) in series into a phono cable all I need to do?

Or would I have to something more complicated to get attenuation.

reason for doing this is to fix a problem which is caused I suspect by a slightly over gained Y component on some fussy kit.

cheers

 

[eviljohn2]

Resistor inline should be just fine mate. Not sure what it will do for the overall impedance though.

 

[Mr.D]

cheers

would building a more complex circuit allow me to attenuate without altering impedance?

My electronics knowledge is somewhat limited but I can solder alright.

There are some step attenuators available online but I wanted something quick and easy.

I also have a 10db coax inline attenuator rated for 75ohm . I can convert that to phono easy enough but I suspect I'll need to vary the attenuation eventually?

I've even tried hooking in two BNC to phono adaptors and a couple of phono couplers just to see if that would produce any attenuation ...hardly scientific and didn't have any appreciable effect.

 

[eviljohn2]

Have you got the frequency response for the attenuator you tried? It may only be suitable for audio frequencies.

I can't think of an obvious way of being able to vary the cables resistance without affecting the overall impedance as at video frequencies you're up into waveguide and transmission line territory where the maths gets very complicated. You could try adding in a variable resistor and see how you get on, obviously if that doesn't work too well then you'll want to look at other things. Maybe Joe or Gordon will be able to help a bit with the equipment side of things too.

 

[Alan Mac]

cheers

would building a more complex circuit allow me to attenuate without altering impedance?

To keep the impedance constant you need three resistors arranged in a “Pi” or “T” configuration.


Taking a 75 ohm “Pi” attenuator as an example:

Attenuation Series resistor Parallel resistors

1 dB 8.6 ohms 1305 ohms

2 dB 17.4 ohms 654 ohms

3 dB 26.4 ohms 439 ohms

4 dB 35.8 ohms 331 ohms

5 dB 45.6 ohms 268 ohms

6 dB 56.0 ohms 226 ohms


Alan

 

[Mr.D]

Cheers again

I've not tried the attenuator I have yet. It was originally added in the downlink from my aerial by the installer. I subsequently removed it but it should be okay for video : just a question oof whether the 10db of attenuation is sufficient or too much.

There are a couple of options on ebay including a variable attenuator : again coax based designed for aerial downlinks I imagine but from what I can gather its rated for 75ohm applications so shouldn't do anything to terrible to a Y component.