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Old 24th August 2004, 01:29 AM   #1
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Default Cheap Stepped Attenuator Project

Here’s a project for anyone who wants to build an audio taper stepped attenuator volume control on the cheap. Before anyone gets too excited this project is a prime example of “give-and-take” in design. To make it cheap, compromises had to be made. The good news is that it doesn’t compromise on sound quality, so you can try it and see if you like the results before you invest in a much more expensive unit without the compromises. I like to use this design when I’m building a prototype, as I can usually build it for less money than a decent quality stereo potentiometer, and it allows me to optimize the impedance to the circuit I am using it with. I will be slowly posting this over the next few days, so bear with me!

So, you ask, what are the compromises? The primary compromise is made in either the size of the volume change between steps, or the total range from loudest position to quietest. The best stepped attenuators offer between 24 and 30 discreet volume levels. This unit only has 12. The compromise I have settled on is 4.3dB steps between –48dB and –1dB. Others might prefer smaller steps distributed over a narrower range, but for myself I find 4dB steps aren’t too bad. The other compromise is that this is a series attenuator where all the resistors are in the circuit all the time, unlike ladder attenuators or shunt attenuators, which only use two resistors at a time for each channel.

So what do we need? First, a switch! For a stereo unit you need a 2 deck, single pole switch. I like the Grayhill 81073, which as I mentioned earlier, offers 12 switch positions. Lastly, you need resistors in the right order of resistances. For this project, I wanted a 22kohm impedance. The resistor series is as follows, where the position numbers indicate the switch position as seen on the back of the switch.

Position : Value (ohms)
Gnd – 1 : 86.6
1 – 2 : 54.9
2 – 3 : 90.9
3 – 4 : 150
4 – 5 : 243
5 – 6 : 402
6 – 7 : 665
7 – 8 : 1100
8 – 9 : 1780
9 – 10 : 2940
10 – 11 : 4870
11 – 12 : 7870
12 – Input : 1740
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Old 24th August 2004, 01:33 AM   #2
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A pic of the switch from front:
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Old 24th August 2004, 01:34 AM   #3
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A pic of the switch from the back:
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Old 24th August 2004, 01:38 AM   #4
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Over the next few days I will post photos and directions on how to put it all together. Also, I will post a spreadsheet that calculates the resistors values for any given range of total attenuation and attenuation step size for 12, 18 and 24 position switches.

Cheers
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Old 26th August 2004, 09:11 PM   #5
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Interesting.. But How Cheep?
'Bottlehead' sells a Dual Mono Kit switches (12 position,11 useable steps) and an assortment of Resistors .. with instructions for.. linear or log steps. for $40.
Granted ithe switches are plastic and not overly robust .. but they work surprisingly well in actuall use.
Would like more substantial switches..
Any linfo on the where and how much for the Greyhill switches.. for a vancouverite ??
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Old 26th August 2004, 09:16 PM   #6
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Digikey has Electroswitch with 24 position (23 usable, 1 stop). However, it is non-shorting, and requires more torque than comparable switches (not an issue for me as I motorized it for remote control).
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Old 27th August 2004, 10:41 PM   #7
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I don't have the exact prices of the top of my head, but I ordered everything from digikey, enough for two complete stereo attenuators, and I think it was around $50CDN including taxes and shipping.

The Greyhill switches I'm using are plastic and metal, but they are quite robust, and have a very positive feel when being switched. You can turn the shaft reasonably easy just with your fingers on the shaft too.
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Old 27th August 2004, 11:07 PM   #8
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Are those non-shorting switches?
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“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
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Old 28th August 2004, 02:03 AM   #9
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I have no idea if they are non-shorting or not, but so far I haven't found anything objectionable about their performance whatsoever. No clicks or pops on switching, smooth switching action with a very positive stop at each position. I have a Goldpoint series attenuator and with the exception of the greater number of steps I don't find any noticeable operational or audio difference. A nice note is that the same switch is also available in a 4-deck configuration which would be ideal for a stereo attenuator for a balanced circuit.
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Old 28th August 2004, 02:11 AM   #10
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Quote:
A nice note is that the same switch is also available in a 4-deck configuration which would be ideal for a stereo attenuator for a balanced circuit.
You only need two decks for balanced stereo. I think Peter Daniel was the first to post the circuit here (attenuator as shunt between inverted and non-inverted lines, after series resistors). Obviously all series/parallel resistances around this circuit need to be taken into account. I'm using that configuration to great success with Ro(e)derstein MK3 resistors, although resistances needed to get a proper dB attenuation sequence have to be recalculated, as the usual log curves do not give a good result if you do that. The only problem with this setup is that if a non-shorting switch is used (as in my case), the breaks between positions will result in maximum volume. I use an IR sensor that acivates the muting relays on in-between positions (and makes sure the motor doesn't stop the switch on an in-between position), so I get a momentary silence between switches, but it's not a problem for me. Shorting switches are harder to find at low prices, so this is an option.
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