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Old 29th April 2013, 01:22 AM   #11
dheming is offline dheming  United States
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Quote:
Originally Posted by soldersplash View Post
FYI - You don't need 36 relays for a 36 position attenuator. It is possible to make 127 step using 7 relays. Arranged as 1/2dB per step gives a practical 0dB to -63dB attenuator. Think binary...

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Are you referring to the R-2R resistor ladder topology? They do indeed save on relays, but they also have widely varying impedance. There's also quite a few components in the signal path at times. Plus they need a binary counter to operate the relays which means a micro-controller of some sort.

None of the above traits are desirable to me. My goal with this project is to build a true ladder attenuator with passive operation. If there are any tricks to doing that using less switches I'm open to input.
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Old 29th April 2013, 02:43 AM   #12
Bonsai is offline Bonsai  Taiwan
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Non that I know of that will not end up with more than 1 set of contacts in he signal path.
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Old 29th April 2013, 11:27 AM   #13
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Originally Posted by dheming View Post
Are you referring to the R-2R resistor ladder topology? They do indeed save on relays, but they also have widely varying impedance.
Not necessarily R-2R but that kind of thing. Input resistance can be fixed vs. level setting. Output resistance varies still. But that will with you project as is, I think?

Quote:
There's also quite a few components in the signal path at times. Plus they need a binary counter to operate the relays which means a micro-controller of some sort.
Yes, more components in path, it is true. For me that just meant finding sealed signal relays and high quality res. Binary counter, however, does not necessitate using a microcontroller.

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None of the above traits are desirable to me. My goal with this project is to build a true ladder attenuator with passive operation. If there are any tricks to doing that using less switches I'm open to input.
I have no tricks for less switches and maintaining constant input resistance. Soz.

All the best with your project. I'm not on a mission to dissuade you. Hopefully my input will be useful/interesting/thought provoking for others visiting this thread.

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Old 29th April 2013, 12:58 PM   #14
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Quote:
Originally Posted by cliffforrest View Post
But they will be along soon to tell you how magnetics screw up
Not so much the magnetics, but the required lumped mass, and the cost.

Contact force of Reed switches is rather low, and they're prone to vibration.
So the attenuator has to be a bit bulky, which comes as a gratuity by using a stepper motor, the natural choice for accurate switching.
An Astrosyn type-14 mini stepper motor, as used by Weiss, easily adds $60 to the total cost (which is cheap, compared to what e.g. an Airpax stepper motor used to cost)

A Hamlin MDSR-4 reed switch now does a few dimes each, but before 2k they cost two times as much, >100 wholesale, without counting inflation.
(made a stepped attenuator similar to Weiss' in the late '80s, had to resort to the cheapest ones available, MDRR-4, still ~$60 just for the Reeds)

Very nice ones to look for is the Hamlin MDC series, mercury wetted.
An MDC-3 e.g. was over $5 each at 100 minimum in the early '90s, NOS ones now sometimes $50 for hundred.
(downside is the diameter, 0.1", requires an H33 magnet)
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Old 29th April 2013, 01:16 PM   #15
DF96 is offline DF96  England
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My concern about reed switches is their small contact force, and lack of wiping action. Being sealed in an inert atmosphere will help, of course, but if contacts can be a problem for audio then I would have thought that reed switches would be particularly bad. I guess mercury wetting will help, though.
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Old 29th April 2013, 01:24 PM   #16
AndrewT is offline AndrewT  Scotland
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I would expect that reed switches do have a wiping action.
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Old 29th April 2013, 01:44 PM   #17
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Originally Posted by DF96 View Post
their small contact force, and lack of wiping action.
Makes one wonder how long it will take for someone to think up a 48-60 piano key rotary attenuator with 25-cent mercury tilt switches.
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Old 30th April 2013, 03:03 AM   #18
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What should be the ideal step size 1dB, 2dB ????
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Old 30th April 2013, 03:06 AM   #19
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Also optimising magnet size to angle increment so one relay closes as the next one opens will be difficult.
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Old 30th April 2013, 10:21 AM   #20
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Originally Posted by greenKnight View Post
What should be the ideal step size 1dB, 2dB ????
When I built a stepped attenuator I rationalised that I wanted a 'smooth' changing of level. The just noticeable difference in level is about half a dB so that's what I went for.
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