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Tear me apart: Relay Step Attenuator
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duak:
I'm pretty sure that the open and close delays should be as identical as possible, especially for the relays with the greatest weighting ie. 32, 16 & 8 dB. If a relay takes longer to open, it may be an improvement when going from 31.5 to 32 dB, but when going from 32 to 31.5 it will be the opposite.  If nothing else, reducing the opening time shortens the time the output has to be muted during step changes as imacgreg suggests.

Just to think outside of the box, maybe there's another way to do this.  I built an audio attenuator with a bunch of precision resistors and a 24 position, 2 pole rotary switch.  The switch was make before break so it actually slid through a half step between positions so there was minimal disturbance when varying the attenuation. It would take 15 relays to replicate a 1 of 32 selector: consider a DPDT relay as a 4 to 2 selector.  The first bank needs 8 relays to reduce 32 lines to 16, the second bank needs 4 relays to reduce 16 lines to 8 and so on.  So 8 + 4 + 2 + 1 = 15.  The number of relays could be reduced by using 4PDT units, but the signal has to pass through 4 contacts.  When I think about what this configuration does, it's implementing a binary to 32 bit decoder and there's probably a theorem that says how many contacts are needed and after that it's a matter of selecting parts.  However, I've been wrong before.  Anyone have thoughts on this or other ideas?
Prehistoricman:

--- Quote from: duak on March 26, 2020, 12:35:23 am ---If a relay takes longer to open, it may be an improvement when going from 31.5 to 32 dB, but when going from 32 to 31.5 it will be the opposite. 

--- End quote ---

No?
Relay opening = less attenuation
If closing relays is faster, then (in any case), higher attenuation will occur before lower attenuation.

Consider your example:
32    = 01000000
to
31.5 = 00111111

The 6 1s in 31.5 will close first (if opening takes longer)
So briefly you will get this:
63.5 = 01111111

Then the 1 relay in 32 will open:
31.5 = 00111111
floobydust:
G6K relays are classified as "signal" type which seems to mean under 2A. I've used the Omron G6K-2P for low levels and they aren't so great, some have dirty contacts and need a beating to clean off the oxide. They also are problematic in reflow, the legs frequently haven't soldered well as the relay body seems to block heat. A few production runs ended up with dirty contacts after reflow which I guessed was due to off-gassing, as the relays are sealed. The relay is OK for switching power. Would not use for a step-attenuator.
You need a relay with AgPd contacts, and a minimum switching capacity specification that is uA, not A.
duak:
Prehistoricman,

It looks like I wasn't as clear as I should have been when I introduced the terms open vs closed for relays.  They really should refer to the state of a particular set of contacts.  I understand the relays as drawn on the schematic as being unenergized, with the magnetic gap and the N.O. contacts are open.  In this state the stage is attenuating.  When the relay is energized, the N.O. contacts close and the stage is bypassed.

If the release time is longer than the operate time (terms from the Omron data sheet) on the formerly energized relays, their N.O. contacts could still be closed when the formerly unenergized relays are operating and their N.O. contacts are closing. This means that on the -31.5 to -32 dB transition (and vice versa), all attenuators could be bypassed and pass a larger than expected signal to the output.  Do you agree with this analysis?

When I review what you highlighted in my reply, I have to say I can't remember what I was thinking of and I didn't make a note of it.  It was probably a fleeting thought that I can't explain.  Regardless, it's confusing and should be ignored.

One thing concerns me.  I don't have time to calculate the cummulative attenuation as the attenuator is advanced state by state.  It seems to me that each stage is loaded down by the cummulative input impedance of the following stages  This means that a stage's input impedance is determined by what state it is in and what states the following stages are in.  ie. the input impedance is not constant.  However, this may have been considered in the design but I 'm not sure.  Does anyone else see this?

Cheers,
BurnedResistor:

--- Quote from: duak on March 26, 2020, 05:45:36 am ---One thing concerns me.  I don't have time to calculate the cummulative attenuation as the attenuator is advanced state by state.  It seems to me that each stage is loaded down by the cummulative input impedance of the following stages  This means that a stage's input impedance is determined by what state it is in and what states the following stages are in.  ie. the input impedance is not constant.  However, this may have been considered in the design but I 'm not sure.  Does anyone else see this?

--- End quote ---

The dividers are all designed to provide the desired amount of attenuation under 10k of output load and to present an input attenuation of 10k. So no matter what the attenuation selected is, any divider engaged will always see a load of 10k.

Hence the output load adjustment: If the audio device this is connected does not have a 10k input impedance, I have included the possibility for a resistor and pot to load down the output. (Only works if the output load is lower ofc..., but the stuff I wanted to connect it too seems to have an input impedance of around 20-40k)
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