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Stepped Attenuator - Page 2

post #16 of 31
Thread Starter 

Getting the exact logarithmic values shown here is NOT necessary. Do few linear approximations and curve fit the function.

This is actual calculations that I have made.

Requirements: 50K 12 steps -> 11 resistors

50 = C * (Exp(r * 11) - 1)

Minus 1 is required since Exp (r * 0) = 1 and we want Zero here.

Ln((50 / C) + 1) = r * 11

r is some arbitary constant associated with my requirements. C is some constant that control the steepness of the graph.

r = Ln((50 / C) +1) / 11

The stepped attenuator's resistance must change according to this rule.

A(x) = C*(Exp(r * x) - 1)


r = Ln((50 / C) +1) / 11

Note this resistance is the total resistances of all resistors between signal-out and ground. So calculate the values of resistors appropriately.

I only had 2.2K and 7.5K in the number I needed. So I had to set C to 4 ~ 5 after doing some additions and subtrations.

50k = R1 * Y + R2 * Z

where Y + Z = 11

For ease of calculation, I decided to have Y = 5 or 6. After staring at graphs for a while, I decided if Y = 6, the graph look more logarithmic.

If you curve fit this experimental results, you get C ~ 4.8. Clearly C must be experimentally determined. However, I am very certain that C cannot be small. Small C will give you very low values except close to 11 and that ain't good. My curve fit is attached.

I guess I got super lucky and hit the sweet spot. The attenuator sound very good and natural. (Natural I mean is that decay over steps are natural)

post #17 of 31
Thread Starter 

Here is something I found on internet a while ago. This is for 24 step attenuators. (SERIES!!)

post #18 of 31
How many poles would be needed for a dual channel step attenuator? 5-6?
post #19 of 31
Thread Starter 

2-pole means there are two contacts. What we want is 2-pole 2-deck 24 (or more) position rotary switches. (although you may end up with 12 position due to cost. .... )

2-pole 1-deck means there are 2 contacts but there is only 1 set of rotary switches.

You get the point?


post #20 of 31
How do the poles relate to the decks and to the way you should wire the attenuator up to the circuit? Would it be like using a shunted potentiometer (basically acting as a variable resistor)? Was it 2 poles per deck, so each pair of poles is for a channel?
post #21 of 31
Thread Starter 
Possum -

No. of decks has nothing to do with No. of poles.

A deck just means a group of switches.
A pole means a contact.

You can't have 2 deck 1 pole. But you can have 1 deck 2 pole. You see?

There are 2 deck 2 pole. (which means you have 1 contact for each deck.


P.S. You still don't get it, you should go see it for yaself in stock room ... You can come and see the attenuator ... I am still at school.
post #22 of 31
i'm tempted to pick up some Goldpoint attenuators. Some of their old-modl series-type attenuators are priced quite reasonable now (40-60USD, don't remember exactly.).

my question is, is this "better" than a similarly priced pots, noble, alps, whatever? i plan to get the kit, and as much as i like soldering, soldering 20-odd resistors doesn't sound THAT fun.
post #23 of 31
You must use good resistors. If you don't, then there's no point in spending US$60 on the switch. You would still get the benefit of good channel matching with "plain" resistors but the sound quality wouldn't be quite there.

But you're buing a kit so that's (should be) a moot point. They use Holco resistors, right?
post #24 of 31
I am really wanting to replace the Clarostat 580 pot in one of my first generation Phone amps so are these switches SMALL must be no larger in Dia than a 580 is square. Can you post the url to the Kit your are using. Thanks
post #25 of 31
Thread Starter 

I don't think they are that small. Rotary Switches are large. GoldPoint makes several different models but they tend to be pretty large also.

Only solution is to use smaller resistors. The smallest rotary attenuator I have seen is from DACT.

My 12 step attenuator is about the size of Nobel pot. 24 stepper will be 1.5~2 times larger and won't fit my Szkeres amp chasis. (I will have to redo Szkeres amp if I want to install 24 steppers.)


P.S. I want to choose most transparent resistors as I can get. So carbons should not be used. Halcos may be good but I hear good things about RN55 and RN60 for fractions of price. Note the reliability that goes with these resistors. Halcos will break if the electrodes are stressed. CMFs are commercial equivalent of RNs. Upgrade from there to Halco may not make much difference. I would suggest using expensive Vishays. I would personally like to try out some non-inductive ww resistors for giggles.
post #26 of 31

no way!

ppl, nothing I've ever seen is as small as a 580... as dual pots go, that's one tiny thing.

Check out the Elma ones,

The 04 frame is what the audio ones are based on, and the Audio Series N one is probably the smallest 24 position you'll find. And, since they're bolt-on boards, you can etch your own if ya want. Headroom is using a version of these.
post #27 of 31
Ya to big If any of you runinto one about 1/2" dia. please post.
post #28 of 31

MDACS for volume control in headphone amps?

Originally posted by ppl
I have toyed with the idea of using a Digital Pot with the 1802 chip as sugested by Apaheared. I have been resistent to using this technology because of the added Noise and Distortion. I like the Idea of Close Channel tracking, This is most importent with headphones. ...
I agree; I've wondered about something like this also. I think the signal levels involved (since we're talking headphones) is a great match for MDAC based volume controls. I am considering a design using a Cirrus CS3310 driving a buffer (haven't decided which one yet - I also like the LM6321) directly. The CS3310 appears to be one of the better designed MDACs for volume control on the market (I have also seen that it appears in a number of high end audio products). I'm thinking this would make a great headphone amp and give me the opportunity to make something "ML380 preamp like"

National makes the LM1972 MDAC which looks promising. This device and the CS3310 use serial bit streams, so you'd best use a microcontroller of some sort (PIC, AVR, 8051, etc.). No one really uses the AD7112 and I wonder why?? It seems that it would be a good part to use also.
post #29 of 31

Burr-Brown digital volume control

Here's info on a new device from Burr-Brown. It looks pretty good but you need a micro to run it.
post #30 of 31
I have started a new thread concerning controlling digital pots without a microprocessor.
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