ELMA stepped attenuators

Looking around for an output attenuator of my vari Mu compressor, I found this:

http://www.elma.de/eu/products/rotary_components/Audio%20Switches/25

Is there some information about these on the forum?
Are they any good? How much are they? Where to buy?

JH.

[quote author="Viitalahde"]Just buy a plain Elma 2x24 switch and solder the resistors in yourself. [/quote]

Yes I've just read on and saw their "ladder" type isn't what I'd expected
for a constant impedance ladder - so using a plain switch means that one?
http://www.elma.de/eu/products/rotary_components/Rotary%20Switches/6

What would I have to expect to pay, and who sells them?

Do a search on stepped attenuator calculations and you'll find a lot of stuff.

Also, visit Goldpoint's website:

www.goldpt.com and check out DIY reference for some values.

Click to expand...

The calculations / resistor values are no problem. Just looking for hardware, hoping I don't have to spend a fortune ...

JH.

"Resistors not included"

looks like your gonna have to do it anyways. :grin:

[quote author="asm"]"Resistors not included"

looks like your gonna have to do it anyways. :grin:[/quote]

That pcb which is part of the switch would be nice nevertheless,
if the lyout were right. But as far as I see it, it's not a real ladder configuration. They just swich between resistor pairs, top end and center.

JH.

> it's not a real ladder configuration.

In many ways, it is better than a ladder. Drawbacks: twice as many resistors, and a low and non-constant output impedance.

And the switch is the same, the problem may be the nifty resistor board.

> hoping I don't have to spend a fortune...

If you were closer to Chicago USA: Triode Electronics ran out of resistor boards so they are selling a 4-pole 23-position Shorting Alps switch for $20: http://store.yahoo.com/triodeel/4pol24posals.html You can NOT beat that price for 4P23T. They will ship almost anywhere, and say that euro post offices are often faster than inside the US.

This is probably a really stupid question, but, being "four pole" does that mean it controls four seperate inputs? If you have a mono signal, you just want single pole, and if you are controlling a stereo signal with one knob, you want two pole, right?

What would a four pole be for in an audio app?

Joel

Yes every pole is for a separate signal.
For example you need 4 poles if you want to make a single volume control for a stereo balanced signal (i just did this). Per signal you have + and - and beeing stereo means you have 4 signals.

Flo

Or if you want to make a constant Z balanced attenuator.

[quote author="PRR"]
If you were closer to Chicago USA: Triode Electronics ran out of resistor boards so they are selling a 4-pole 23-position Shorting Alps switch for $20: http://store.yahoo.com/triodeel/4pol24posals.html You can NOT beat that price for 4P23T. They will ship almost anywhere, and say that euro post offices are often faster than inside the US.[/quote]

Hi PRR - thanks for the link! That looks like a good price for a 4pole 23pos switch indeed! I consider buying one of these, even though I expect some hefty shipping costs to Germany.

I wonder what the price for a 1pole 23pos switch would be, even the regular price, when I could get in directly in Europe, maybe in Germany. For a ladder I'd only need a single pole. But then again rafafredd is right, I might go for balanced and really constant impedance with 4 poles!

But wait - if 29.95 is the regular price for the 4pole version, a 1pole version should be considerably cheaper, so does anybody know a retailer in Germany (or Europe) who sells them?

JH.

ou need 4 poles if you want to make a single volume control for a stereo balanced signal

Click to expand...

Gotcha. Thanks guys!

> if 29.95 is the regular price for the 4pole version

It isn't. I've paid $25 for a 2-pole 12-position switch, and 4-pole with more than 3 positions are not a standard item at any distributor.

All the standard switches today are "12 pin": 1-pole 12-throw, 2-pole 6-throw, 4-pole 3-throw, etc. RadioShack, Elma, everybody has those. There are about 3 low-price makers in the $2-$10 range, and high-price brands are stocked at industrial distributors like DigiKey and Newark.

It "should" be "easy" to stack several wafers on one shaft. But the $2 RadioShack is pressed together, can't be stacked. And nobody seems to want to carry long-shafts and stacking hardware. DigiKey has one line of stackable pieces: $23 per wafer and $35 for the shaft-bushing-detents. So to duplicate what TriodeEl is offering would cost over $130. If DigiKey even has the wafers in stock; they often don't have everything in the catalog.

What has happened is: today if you need more than Off-Warm-Hot 3-way switching, you are probably going to put a computer in the thing. That works very well for washers, toasters, crank-grinders, and most other modern toys. So the need for a simple passive switch with a lot of postions has vanished. We audio-heads don't want a computer mangling our audio, but our needs are far too small to keep a factory busy.

Triode El's $30 price was for the switch plus resistor boards. They ran out of boards so they are dumping the switch at $20. However if you tried to buy just one of that switch, anywhere else, you couldn't! It must have been a special order: either Triode El put up the $10,000 set-up costs for Alps to produce that switch, or Triode El found surplus from some failed product which had paid the special-order costs. So if you might ever use ANY kind of 23-way switch, I think you should buy it now.

[quote author="PRR"]> it's not a real ladder configuration.

In many ways, it is better than a ladder. Drawbacks: twice as many resistors, and a low and non-constant output impedance.
[/quote]

I was thinking a little more about this.

First thought was it's just like a potentiometer, only stepped, so it doesn't have any constant impedance features at all.

Second thought thought was: Ahh, but you don't have to make the sum of upper and lower resistance the same for each step! Then indeed one could choose the resistors such that input impedance (for a given load) is constant (might be important for the driving amplifier), while output impedance is lower than that. (Generally a good thing.)

Third thought: But what about switching? In order to avoid glitches, make-before-break action is to be chosen. But this means that during switching, the amplifier sees half the impedance, doesn't it?

JH.

In the "second thought", what you're describing does indeed exist, and it's called an L-pad. It has a constant input impedance and a variable output impedance. It's often used as an attenuator before speakers.

[quote author="NewYorkDave"]In the "second thought", what you're describing does indeed exist, and it's called an L-pad. It has a constant input impedance and a variable output impedance. It's often used as an attenuator before speakers.[/quote]

I see - but doesn't #3 ruin it all?

JH.

It depends on the kind of attenuator. Instead of f*cking around with Elma or Goldpoint non-constant impedance attenuators--which are really made for audiophile use with "modern" high-impedance equipment--I would suggest buying a good rotary switch and building a bridged-T attenuator, or a bridged-H if you can find a switch with enough poles.

...and here's a bridged-T attenuator calculator if you don't want to crunch the numbers by hand.

http://www.microwaves101.com/encyclopedia/calcattenuator.cfm

[quote author="NewYorkDave"]It depends on the kind of attenuator. Instead of f*cking around with Elma or Goldpoint non-constant impedance attenuators--which are really made for audiophile use with "modern" high-impedance equipment--I would suggest buying a good rotary switch and building a bridged-T attenuator, or a bridged-H if you can find a switch with enough poles.[/quote]

That's what I intend to do: Buy this 4-pole switch without pcbs and find the best solution for the resistor configuration.

At the moment, I see the following options:

Going Ladder, like the original UA175. Throwing away 6dB, but pretty nice apart from that. Might even build a balanced version of the ladder, only needs 2 poles.

Going bridged T. If I got that right, most of these old 600R attenuators are built that way? (With 4 poles, I can even build a balanced version!) But I wonder what becomes of the constant impedance during switching. I drew up a 4dB attenuator and 6dB attenuator according to that calculator from your link. 600R for both - fine. But when two sets of contacts are closed at the same time (make-before-break), even this will show a reduced impedance of 424R, according to a quick simulation. Is this ok? Is that how the old Davens work?

Going L-Pad (I just learned the word from you). The variable/low output impedance is ok for me, but I must make some calculations how much the input impedance falls during switching of these. Probably more than the bridged T - I expect almost down to 300R at high attenuation rates.
I have no idea if 300R (1/2 Zo) would be tolerable in that application. That's what makes me hesitate.

JH.

For what it's worth, the old 600-ohm Daven ladders presented an input impedance of 300 ohms in the "off" position. I really don't think a momentary, slight drop in impedance when switching positions is anything to worry about.

If you really are worried about it, build your bridged-T or bridged-H with the series and shunt arms made up of several resistors in series (like the attenuator designs I posted), rather than using a separate resistor for shunt and series in each position. That should minimize impedance variations when in-between positions.

[quote author="NewYorkDave"]
If you really are worried about it, build your bridged-T or bridged-H with the series and shunt arms made up of several resistors in series (like the attenuator designs I posted), rather than using a separate resistor for shunt and series in each position. That should minimize impedance variations when in-between positions.[/quote]

Bingo! That is it. Dind't think about that.

JH.

Does anyone have any opinions on using SMT resistors
in a stepped attenuator. I've read some of the HiFi guys
claim they don't sound good. (what ever that means) I'm
laying out a 24 position stepped pot and I'm using metal film
Vishay 1206 packages. Is there any preference on a different brand?


RonL