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Old 20th February 2003, 08:05 PM   #1
jwb is offline jwb  United States
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Default How many steps in custom ladder attenuator

Greetings prophets. I am building a ladder attenuator for my preamp, using a rotary encoder, some logic, and relays. I wonder how many steps I should include. I have always used 24-position switched, which has never been enough, but I don't know how many might be "enough". The attenuator is two channels, and adding a step costs a few marginal dollars. Groups of eight are convenient, because of the 8-bit shift registers I am using, so I am considering 32, 40, or 48 steps.

Schematics, artwork, and source code will of course be added here when I am done.

P.S. Hifi, someday I will finish those drawings and send them to you. Have patience!
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Old 20th February 2003, 08:14 PM   #2
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In my experience, 32 is plenty, but you want it to go down to about -60 dB at the next to bottom versus the max. This results in 2 dB per step. We do this, but we have to use a nonlinear set of values and a look up table to keep the resolution per step.
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Old 20th February 2003, 08:17 PM   #3
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Default Re: How many steps in custom ladder attenuator

The attenuator is two channels, and adding a step costs a few marginal dollars. Groups of eight are convenient, because of the 8-bit shift registers I am using, so I am considering 32, 40, or 48 steps.

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Go for the 48 steps; even with 31 steps, I am always fiddling with the volume control. I often wonder how people can use 24.
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Old 20th February 2003, 08:26 PM   #4
Jeff R is offline Jeff R  United States
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My preamp has, I think, 24 steps which, for me, is fine. What helps is that the preamp also as an attenuation switch. By flicking that switch, the volume drops down - one can use this to answer the telephone and when done talking, flip the switch again and not have to redial in the volume setting you once had.

What also works is that you can flip the attenuation switch on, and increase the volume setting, and get a finer resolution in volume settings. This only works for the lower volume levels, but I doubt this would be a problem, as this is where you would want greater resolution anyway. In effect, this technique gives a 24 step volume control extra steps.
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Old 21st February 2003, 06:56 AM   #5
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What also works is that you can flip the attenuation switch on, and increase the volume setting, and get a finer resolution in volume settings. This only works for the lower volume levels, but I doubt this would be a problem, as this is where you would want greater resolution anyway. In effect, this technique gives a 24 step volume control extra steps. [/B][/QUOTE]

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A shunt attnauator should be fine, but anything else in the signal path is not desirable ie. switch contacts.
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Old 21st February 2003, 09:19 AM   #6
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I once used a 24 step with per step 1dB + a 3 position switch with 3 x 24dB attenuation.
For me the 1 dB steps were really too small and after a while I always used three or more "clicks".

Now Ive got a 24 step ladder with 3 dB steps wich get a bit coarser below -60dB and am very satisfied. For me smaller steps are not neccessary.


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Old 21st February 2003, 12:15 PM   #7
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I'm also using 24 steps and it's enough for me.
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Old 21st February 2003, 06:08 PM   #8
BrianL is offline BrianL  United States
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One could build the attenuator that looks more like
a "relay DAC" that has far more steps than the number
of relays because certain attenuations are the result
of closing more than one relay simultaneously. I think
that this is what Nelson was trying to say.

In fact, I designed a shunt attenuator using
16 relays that would do 0.125dB steps over a
range of 0->60 db. This could be done with
readily available Vishay resistors. Even 0.1%
relays could be used if you had a way to calibrate
the attenuator and build the ROM lookup table.
Maximum error at any setting can be less than 0.05 dB

Siegfried Linkwitz maintains that the better the
accuracy of your audio system, the more you
need to set volume to the "correct" level. And he's
one smart cookie! This would argue for better
resolution than the standard 24-position attenuator.
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Old 21st February 2003, 07:03 PM   #9
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In fact, I designed a shunt attenuator using
16 relays that would do 0.125dB steps over a
range of 0->60 db. This could be done with
readily available Vishay resistors. Even 0.1%
relays could be used if you had a way to calibrate
the attenuator and build the ROM lookup table.
Maximum error at any setting can be less than 0.05 dB

Siegfried Linkwitz maintains that the better the
accuracy of your audio system, the more you
need to set volume to the "correct" level. And he's
one smart cookie! This would argue for better
resolution than the standard 24-position attenuator. [/B][/QUOTE]
-----------------------------------------------
I agree; is your design published anywhere?

With my system, I like to set to at least 1dB and would prefer 0.5. This gets the soundstage just right .
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Old 22nd February 2003, 12:13 AM   #10
BrianL is offline BrianL  United States
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Nope, not built nor published.

To be able to do w/o VERY expensive resistors
(0.005% Vishays at unique values) would
require laboratory standard AC calibrator.
Lost access to that before I was able to
assemble and test the project.

Someday...
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