Idea for a 255 step attenuator, is it good ? - diyAudio
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Old 26th June 2003, 10:38 PM   #1
Prof is offline Prof  United Kingdom
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Lightbulb Idea for a 255 step attenuator, is it good ?

I have just had a conversation with a friend about Gainclones, DIY audio etc. He told me that a friend of his had built a volume control that used relays and a binary counting IC. This got me thinking that it might be a good idea to use this as my Gainclone volume control. 8 resistors wired in series, each having a pair of relay contacts connected across it. The relays can be controlled by a binary counter IC, this being stepped by either a simple UP or DOWN switch arrangement or, as done in his design, a potentiometer feeding an A to D converter. Anyway what I am wondering is that is this arrangement a good idea as far as audio quality is concerned ? How would this "variable resistor" be best used, connected to the input with a fixed resistor to ground or with a fixed resistor from the input and have the resistor/relay chain going to ground ?

I hope I have explained this idea clearly enough for someone to give advice if this is worth building.
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Old 27th June 2003, 12:01 AM   #2
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I imagine that you are aware of the Remote Relay Volume control:

Remote relay volume control kit.

The general consensus is that relay switched resistors sound good.

If your very simple approach works I imagine a lot of people would like it as a simple and cheap option. Can you get over 8 levels with this idea? The pot on the gainclone cannot be eliminated I have heard, unless a resistor is substituted, so your resisitor values will have to be chosen to be similar to the pot. Of course these would be the most useful values anyway.

Also look at the Passive Preamp thread for info on shunt type controls, etc.
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Old 27th June 2003, 06:56 AM   #3
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With 8 resistors and 8 relays, 255 volume levels can be selected. I`m going to look into this idea and design a circuit, i`ll post the results soon. As long as having 8 resistors with relays (very small ones) connected across them doesn`t reduce sound quality, then this could be a really good volume contol for a Gainclone.
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Old 27th June 2003, 07:17 AM   #4
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Prof

The first issue will be to select a relay and actually listen to it in an audio chain. You may like to compare to the sound of a solder joint and high quality silver switch. I somehow think you'll go through quite a few relays before you find one not too obtrusive. Your choice of 8 relays assumes you can get resistor values which relate as powers of two; no idea how easy/difficult this may be. Then you'll need a controller which won't do simple up/down counting but convert to log, right? For any given volume you'll be listening to a few of the contacts and a few of the resistors - not a very minimalistic approach. I would probably prefer 30 relays and 30 sets of fixed resistor dividers as 30 is even more steps than i need.

cheers
peter
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Old 27th June 2003, 07:20 AM   #5
Variac is offline Variac  United States
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In the threads I mentioned about every possible issue with this idea has been discussed
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Old 27th June 2003, 08:46 AM   #6
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This kind of regulation (lets assume they have been borrowed from Alephs P designs from Mr. Pass) has advantages that with only 8 switches (high quality,silver,gold plated contacs are recommended) and 8 resistors one gets 256 combinations.Disadvantages of this design is that unlike in stepped atenuators it works partly as shunt regulator and partly as ladder one.It makes many series/parallel combinations,so the preamp or DAC (depending what sits before that regulation) doesn't see steady impedance.
In stepped attenuator this imput impedance is steady (not in shunt type).But when we consider stepped atenuator we have to know what kind of are we talking about.There are series ones ,ladder type and shunt type (I don't think that one is the way volume control should work so don't consider this one).
The series one for 24 positions needs:
2 deck ,24 position switch,and 48 resistors for unbalanced,
4 deck ,24 position switch and 96 resistors for balanced
But when we listen for example on second volume level signal goes through 22 resistors and at least the same solder joints.
In this case I would prefer the solution with relays and 256 positions becouse it probably doesn't sound worse.
The true ladder type where in every position there are two resistorsne in signal path and the other to ground is a truly outstanding solution,becouse in every position signal goes through one (always diffrent resistor).But now for ladder type we need (24 position):
4 deck,24 position switch , 96 resistors for unbalanced
8 deck,24 position switch , 192 resistors for balanced
Also it's almost imposible to find values of resistors calculated for that kind of attenuator.This one is trully minimalist design and offers outstanding performance (I think only transformer volume control might be better),but we probably would have to order custom designs resistors.
Assuming above I thing the solution with relays and controling them chips is worth considering.When people say stepped attenuator,they usually mean the simplest one-series attenuator where in worst case we are listening to 23 resistors and solder joints.In that case I thing 8 relay type is going to be better (more minimalist),becouse here in worst case we have 8 resistors/solder joints ,and not all of them are series but part is parallel.
Bartek
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Old 6th July 2003, 08:26 PM   #7
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I`ve built an 11 step shunt attenuator, it sounds good. I`m going to stick with this.
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Old 7th July 2003, 03:46 PM   #8
Variac is offline Variac  United States
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Hi Prof- did you use the IC's to control it?
It has a total of 11 settings?
Is it simple?
Mark
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Old 7th July 2003, 05:23 PM   #9
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Check this post...

http://www.diyaudio.com/forums/showt...7685#post97685

Didn't occur to me until recently that this scheme could work well with an inverting GC. The first op-amp could be omitted if your preamp or source has a nice low output impedance...
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File Type: gif attenuator.gif (4.4 KB, 954 views)
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Old 7th July 2003, 06:04 PM   #10
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Mhenessey,

I saw your post earlier, even commented on this. But now I think it cannot work, because it would be impossible to calculate the resistor valus for monotonic range. You need a ladder network. Did you try to calculate them?

Jan Didden

" Can adders multiply? Yes, on a log table"
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