Stepped Input Attenuator Rin Value

[gg85]

Hey guys, i'm trying to get my head around building a stepped 100k input attenuator for my Gates STA build. I'm using a 24 pos switch, and want to build it with a fixed Rin and switched shunt. I've worked out my shunt values but just don't quite understand how to determine the Rin resistor. I'm using the Lundahl 1922 for the input (1:8). I don't know if i'm looking at the wrong specs but the technical docs mention a 47k ohm load...would that be the best value for my series resistor, or am I misunderstanding it all?

Thanks!!

 

[radardoug]

A stepped attenuator is just like a pot, but with fixed steps. So the total resistance of your divider should be 100K. It doesn't need to have an input resistor although it could if you want a fixed value of attenuation when it is full up.  So you could divide 100K by 24
(4.166K) and put a resistor of that size between every pair of contacts, and the last one to ground. That wont give you a log scale though.

 

[gg85]

Thanks for your reply! The only problem is that I am building a shunt type attenuator where only the resistance to ground will be changing (see attached). I understand there are different options, like building it with all resistors in series as you mentioned, or a ladder type which would require more decks on the switch...but in my case with the switches I have, a shunt type is the way I want to go...I just can't seem to find much info on figuring out how to determine the best value for the first (fixed) resistor. I'm think it would depend on the source load...possibly to keep the ideal frequency response of the transformer?

 

[gridcurrent]

appears you want a 100k stepped attenuator between transformer secondary and grid.
against my principles of an ideal mic pre;  but your concept should function fine.
you need to specify amount of attenuation per step.
here is a suitable calculator.  easier than a hand held TI.
http://www.mh-audio.nl/MySystem/SteppedAtt.asp

 

[gg85]

Yes this is how I'll be using it. I don't know if it's better that I'll be using it in a compressor rather than a mic pre, but it's nice to know it will suit for either purpose. I have checked that website and that is what I used to get my resistor values to ground at each step of attenuation, but I am still confused as to how I would find my series resistor which would be fixed. Being a 100k attenuator, would that just mean that my series resistor would be 100k? I apologise if this is a very basic thing, I'm just a little unsure and can't seem to find any information on it after reading and googling for the past few hours.

 

[emrr]

If you are building a Stalevel, why are you changing the attenuator design?

 

[gridcurrent]

Yes this is how I'll be using it. I don't know if it's better that I'll be using it in a compressor rather than a mic pre, but it's nice to know it will suit for either purpose.

in my opinion, if the input is configured to feed a compressor, you will want the 1922 hard  terminated into
47 kohms, not 100 k.  calculate the stepped attenuator based on 47 k.

 

[gridcurrent]

apologies,
did not realize a Gates STA was a compressor.

you should read what Doug says, he knows the Sta-Level better than anyone alive.

If it were me, would not use a step-up transformer with that unit.
folks have had great results with 1:1 coils feeding the compressor.

 

[gg85]

Thanks so much for your replies guys! Doug, the only reason I wanted to change from the 100k dual pot is for stereo matching (I am building 2) and fast recall. I know stereo use can be set up using a signal generator in my DAW, but I’ve been doing that with other outboard and if I had the choice of stepped or pot, the stepped option is what’s going to work best for me.

 

[radardoug]

I'm intrigued. You say you had worked out the shunt values, but didn't know what to use for the input resistors. Thats not possible.
Why not do it as others before you have done, with resistors around the switch as I suggested? That makes it easy to wire too.
If you are going to reinvent the wheel then dont go with the square design!

 

[PRR]

You can not design a variable-shunt pad without picking an input resistor.

Going from a transformer to a hi-Z grid, you do NOT want the variable input impedance of your proposed pad. You want POT. Scale the total pot for 47K. Pick attenuations. This is easy to work out on a slide-rule and paper, possible on the TI-30, but you've been shown an on-line special calc (ah, but it does not calculate the attenuations for you; there are others).

 

[PRR]

It is actually quicker to work it out than to scour the Web for a right calc, and *check* that it is right.

48 steps, starting from 0dB, goes to -47dB.

-47dB is 0.004467.

0.004467 times 47,000 is 210. This is your bottom resistor.

Each 1dB step is a 1dB or 1.12202 bigger resistor.

5111  --- top resistor
4555.4
4060
.....
404
362
322.6
287.5
235.6
210  ------ bottom resistor

While some of these numbers look frighteningly exact, that's just math. When stacked in an array, 1% precision will give inaudible (nearly unmeasurable) error.

 

[gg85]

Thanks PRR! I think it might be easier for me to go with the series resistor design. I think I just like the idea of only having 2 resistors in the circuit as opposed to several, and a 4 gang switch for a ladder design was a little out of my price range for several channels of STA...I still do not entirely understand why I would build for 47k. The schematic has a dual 100k pot to the grid, and it seems whether I use the lundalh 1922 or sowter 9062 along with a dual pot the value would still be 100k so should i not build my stepped attenuator for the same value, only in series so it functions like the pot would?

 

[gg85]

Is this because the original transformer was 600:10k (10k x10 = 100k) whereas I'm working with 600:4.8k (4.8k x10 = 48k) with 47k being a closer more available value...is my thinking correct with this? Sorry if these questions seem trivial, just trying to fill in the gaps in my understanding!

 

[abbey road d enfer]

Lundahl 1922 is supposed to be loaded with 47k, Sowter 9062 with 50k.
Anyway, in the STA Level, the secondary load impedance is defined by the combination of the pots (2x100k) and the parallel resistor R43 (200k). You can very well use 100k stepped attenuators (200k combined) and a 68k parallels, that would result in 50k if that's what your xfmr secondary want to see.

 

[emrr]

Original xfrmr is 600:50K loaded 200K by attenuator, loaded primarily by the source Z. 

 

[abbey road d enfer]

Original xfrmr is 600:50K loaded 200K by attenuator, loaded primarily by the source Z.

My schemo shows an additional 200k across  the attenuator, for a resulting 100k.
It may or may not matter much.
In order to know it if actually matters, takes measuring the xfmr with various loads.
It's very likely the designer of the STA Level underdamped the secondary in order to enhance the frequency response at higher frequencies, probably to compensate subsequent losses.

 

[emrr]

My schemo shows an additional 200k across  the attenuator, for a resulting 100k.
It may or may not matter much.
In order to know it if actually matters, takes measuring the xfmr with various loads.
It's very likely the designer of the STA Level underdamped the secondary in order to enhance the frequency response at higher frequencies, probably to compensate subsequent losses.

Right....I was going from memory. 

The same transformer is used in lots of other Gates products, many with no secondary load whatsoever. 


Lest we forget, a Stalevel arrived from the factory with an input pad in place on the primary, and most people actually want that left in...except the new trend weirdos who insist on input overloading everything. 

 

[PRR]

The transformer maker specified a *specific* loading impedance for best result.

The shunt attenuator's input impedance varies ALL OVER the place.

The shunt attenuator puts "high" current through the switch contact. The potentiometer with a naked-grid load puts "no" current through the switch contact. (However it appears StaLevel has other loading.)

If the shunt switch is not make-before-break it goes to MAX level between positions. The potentiometer connection may use the more common break before make; the open-grid does invite stray crap in the transition but proper shielding usually does not cause woofer-blow.

I say good resistors in series act as a single resistor, but I am sure other preachers give other dogma.

 

[radardoug]

The transformer maker specified a *specific* loading impedance for best result.

The shunt attenuator's input impedance varies ALL OVER the place.

The shunt attenuator puts "high" current through the switch contact. The potentiometer with a naked-grid load puts "no" current through the switch contact. (However it appears StaLevel has other loading.)

If the shunt switch is not make-before-break it goes to MAX level between positions. The potentiometer connection may use the more common break before make; the open-grid does invite stray crap in the transition but proper shielding usually does not cause woofer-blow.

I say good resistors in series act as a single resistor, but I am sure other preachers give other dogma.

I'm with you, Mr Preacher!