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Old 19th October 2014, 09:58 PM   #1
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Default Rotary stepped attenuator (series) instructions?

First, I'm cheap.

Looking at gold point stepped attenuators I balked at the pricing.

I decided to see what people were saying about the cheap Chinese stepped attenuators on eBay.

I couldn't find much but after reading a lot on the gold point web site, I knew that the basis of a good stepped attenuator is a rotary switch with shorting contacts so I started looking for one and found a Centralab PA4002.

When I searched for that part number, I found an article from 1979 using that very switch as a stereo stepped attenuator.

The article was written by W. Marshall Leach Jr. and can be found toward the bottom of this web page:

Dr. Leach's Amplifier and Speaker Projects

It seems pretty straight forward but most things 35 years old need a little refresh.

If anyone has positive or negative opinions on this type of stepped attenuator or the switch I'm using, I would love to hear it.

Any way, I'm still cheap and I'm thinking I should buy a quantity of one resistor value to make a linear taper and use a law faking resistor to give it audio taper (idea borrowed from ESP).

That idea satisfies my cheap side, its less math and the parts order will be way easier.

That all seems like a good idea to me but once again I would love to hear some more informed opinions.

I've also got ideas for a cheap balance control but let's stick to the stepped attenuator for now.

Thanks for all the opinions and help.
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Old 27th October 2014, 07:09 AM   #2
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Gajanan Phadte
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Old 28th October 2014, 02:58 PM   #3
oshifis is offline oshifis  Hungary
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23-positions is simply not enough. You need 1.5dB max 2dB steps, and at least 60dB range. High precision resistors are not necessary. I would use 5% SMD resistors, and true log, not law-faking. Calculation is easy on an Excel table, but I am sure you can find it readily on the net.
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Old 29th October 2014, 07:18 AM   #4
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You don't need fine steps over the entire range only around where you usually listen.
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Old 29th October 2014, 07:31 AM   #5
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Quote:
Originally Posted by oshifis View Post
23-positions is simply not enough. You need 1.5dB max 2dB steps, and at least 60dB range. High precision resistors are not necessary. I would use 5% SMD resistors, and true log, not law-faking. Calculation is easy on an Excel table, but I am sure you can find it readily on the net.
23 positions not enough? I have 6 positions on my shunt attentuator, and only use 3 of them (the 3rd is only used for DVDs which always have a lower volume than CD). In reality I only need position - loud - but my wife always tells me to turn it down when she is the room.
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Old 29th October 2014, 07:44 AM   #6
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Hi Lead Ear,

Im in same position as you, planning to build my first stepped attenuator also.
I bought pair of these, now waiting them to arrive:

PP36 21 Russian Military Rotary Switch 35 Steps 2 Plates 2pcs | eBay

First thing may be checking contact surfaces and then using some of those internet calculators for correct resistors values.

-Skoo
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Old 30th October 2014, 09:35 AM   #7
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Quote:
Originally Posted by hazard500 View Post
23 positions not enough? I have 6 positions on my shunt attentuator, and only use 3 of them (the 3rd is only used for DVDs which always have a lower volume than CD). In reality I only need position - loud - but my wife always tells me to turn it down when she is the room.
Hmm.. This sounds good.
Attenuator i mentioned in earlier post above has 35 steps.
So, i can divide those by two and maybe try Takman Carbon and Takman metal film in same attenuator and compare sound. Only have to decide whih design to do: shunt, ladder or series.
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