Constant impedance relay-resistor logarithmic attenuator

[x-pro]

To moderators - I am not completely sure where is the best place for this one. If it is wrong place here, please feel free to tell me off and move it.

Hi all!

I post here the circuit I've designed about 2 years ago for Creek Audio Limited and (with permission from the company) did publish it in an article on different kinds of remotely controlled volume attenuators in Russian-language magazine "Radiohobby" No 2, 2002. Obviously, this idea and this circuit is copyright of Creek Audio Limited, however it is already in a public domain for more than a year and half.

Here it is - as a scanned extract of my article in RadioHobby with a (hopefully) clear enough diagram:

This circuit has several important advantages over existing variants:

1) It provides completely constant input impedance independant of the attenuation position.

2) It gives absolutely precise logarithmic curve - the only limiting factor here is the tolerance of resistors. For 0.1% resistors the maximum error would be less that 0.02 dB over most of the attenuation range.

3) It uses only one switchover relay contact for each of the binary bits, so a full stereo single-ended (or mono balanced) attenuator with 64 dB range could be build with only 6 relays (you may need one more for a complete mute) . You can have a 0.5 dB steps with one more relay and so on. Multichannel attenuators could be easily made this way.

4) It has reasonably low output impedance (not more than a usual potentiometer type volume control)

5) It controlled by a simple parallel binary code, so a simplest control circuit could be designed from a couple of CMOS chips - as I did for a very first prototype.

The idea in this circuit is very simple:

Each part of the attenuator (when engaged) always presents the same load to the previous segments and also needs to be loaded onto a same load - on the diagram it is 10 kOhm, however it is easy to change this to any reasonable value. The only requirement is that the eventual load, i.e. amplifier input, should present the same load as well. This way the attenuation of each segment does not depend on the position of any of them. Something similar is used in RF attenuators for constant input-ouput impedance attenuation. However the usual "RF" approach requires at least 3 resistors and two switchover contacts per segment. My version is much cheaper and simpler.

I hope you'll enjoy it.

Alex Nikitin (x-pro)

[Nelson Pass]

I'm certainly happy to have it posted here

[x-pro]

Thanks!

Alex

[JoeBob]

Wow, after looking over it for a while it seems pretty nice indeed. I've been seriously thinking of a relay based volume control, just wasn't sure which route to take. Thanks alot because I really think I'll go this route.

I just don't get how you can do stereo single-ended with only 6 relays? I see you doing 128 steps with 6 relays per channel, yes, but not stereo...

[mlloyd1]

If you don't want balance adjustments, use DPDT relays ?

And I agree; it is indeed a very nice design!

It's getting harder to hold out for Wayne's!

mlloyd1

Originally posted by JoeBob Wow, after looking over it for a while it seems pretty nice indeed.....
I just don't get how you can do stereo single-ended with only 6 relays? ....

[x-pro]

Originally posted by mlloyd1
If you don't want balance adjustments, use DPDT relays ?

And I agree; it is indeed a very nice design!

mlloyd1

Yes - DPDT relays (like Omron G6K I've used) will do stereo w/o balance. Channel crosstalk was below -100 dB up to 20 kHz with these relays and a careful layout.

Thanks!

x-pro

[Eccu]

Douh.... this was thread what I really need... thanks!!!!

[OliverD]

Funny as it is, I have implemented almost the same circuit in my preamp, without knowing about your development. Didn't realize it was different from what was posted on the net already. I'm using Takamisawa relais (DPDT) to make it balanced. Two seperate PCBs per channel.

[janneman]

Originally posted by x-pro
Thanks!

Alex

No, thank YOU!

Alex, how do you proceed to calculate this? You select a load impedance first, then work backwards towards the input? I would be intrested in this with different values etc.

Jan Didden

[peranders]

X-pro, you could also use T-link or Pi-link to get 1 dB, 2 dB, 4 dB etc or whatever step you choose.

Have you tested T-link?
Did your attenuator "click"(I mean electrically ) when it was operated? Did you have to trim the turn-off times for the relays(using special coil arrangements)?

Did you have a microcontroller or did you use a simple counter with a demultiplexor for the control?

How about the frequency response when you used 10 kohms impedance?

x-pro, do you have a nice formula for calculating a link?

Fred, you don't have worry. I'm only curious.

Check this out!
T-link attenuator with constant impedance


As I understand T-link is good for little attenuation and and PI-link is better for much attenuation? Is that right?