Introduction
When building my own gainclone I decided to make a 12-step attenuator based on a rotary switch and some resistors just like the original gaincard amp.
Most people (including myself) think that a stepped attenuator gives better sound quality than a quality pot.
1.1 Why use an Attenuator
Why change your pot to a 12- or 24-step attenuator, or build your new amp with an attenuator instead of a potentiometer? Is it worth the trouble to change, and how much will you win?
Well, your mileage may vary, but in general the log pot used in many amplifiers could (over years) suffer from the following problems:
- Channel tracking might differ from 10% to up to 20%, with descrete resistor values it is possible (especially when measuring and matching before use) to get both channels within 1 a 2% over the full range.
- Cracking and popping due to wear of the carbon/cermet resistor elements.
- With dual-mono setup it is nearly impossible to get both channels in the same position (same gain).
I use an ALPS blue pot in my UL40-s2 amp, and I'm very satisfied. However, after the warranty period is over I might try an upgrade with just 12 resistors. Listening to the gainclones I've built with these switches its safe to say that this is a very good way to control your amp. Especially if both channels have their own volume control, it gives a reference for putting both channels in exactly the same position, something that's nearly impossible to do with a regular pot.There are standard 12-step switches available from your local electronics store, and these are most likely mass production pieces that will do perfectly. However, there are also 24-step pieces available but sold new these are far more expensive. A cheaper alternative is to look for used switches in army-junk shops.
The photo on the right shows the Lorlin switch of just € 1.50 or so with 12 resistors that makes up the 12-step switch for Geenkloon (and Cyclone). The (big) shielded microphone cable connects ground and signal in, the thin silver/PTFE wire outputs the attenuated signal to the amplifier stage.
In the remainder of this page, I will describe the construction for a 12-step switch but the principle if the same for 24-step switches as well, it just requires a little more resistors and soldering.
1.2 What's the problem?
Like most other builders I knew how a 12- or 24-step attenuator should be constructed, but I needed to calculate the right resistor values for each step based on the desired input impedance, the number of steps and the desired maximum attenuation. The input impedance is often specified by the project you're working on, in my case I was working on the Gainclone with an input impedance around 20 kOhms.
The number of steps results from available switches, the amount of money you want to throw at it and your ears. I discovered that I can easily live with a 12-step attenuator, but of-course a 24-step (another standard value) would do much nicer. However, it is very important to define the first steps really well as they determine the critical low volume levels (candle light music etc.) and should be such that the sound level does not disturb conversations etc.
The maximum attenuation in dB will be 60 or better. For a 12-step project make it at least 48dB, but for a 24-step switch you might want to use a value of 60 dB or better.
The most simple way to make a 12-step attenuator with logarithmic volume control is of-course to make sure that every step of the attenuator has the same value in dB (which results by definition in a logarithmic gain). However, I found especially that with 12-step controls it's much better to tune the steps to your own favorite sound levels. In my case I might want the first steps to be fine, a mid section with more coarse steps and the last three steps to have steps that are three times as steep.
2. Designing an Attenuator
In the following sections it is described how to design your own attenuator, and how to do the math for calculating the resistor values etc.
2.1 Architecture
Things to take into account when building your own volume control with a stepped attenuator are :
- What is the design of the next stage of the amp we're feeding the attenuator output into: Non-Inverted or Inverted?
- What is the output impedance of the CD-player, tuner or phono amp feeding into the attenuator?
- What is input impedance we want to create for the input of our amp?
- How many steps do you need?
- What are the steps in dB we want for Volume control: linear or a particular curve based on our preferred volume levels?
- What type of switch do we have: Make-before-break (=shorting) of break-before-make (non Shorting)?
- Is there a ground resistor from the input pin of the amp to ground (mostly used to give the amp a DC reference point), then we should take this resistor into account as it is a parallel resistor between wiper and ground and influences both impedance and gain (inverted amp).
Basic Principle
There are several ways to build a volume control for an amplifier, although all methods described below are based on a variation of the voltage divider. The basic circuit looks like this:
And the "gain" of a voltage divider is defined by the following formula.
And as we know the formulas for calculating the gain we can also work in the reverse direction and calculate resistor values for a given gain of the attenuator (which will mostly be negative e.g. -60 dB)
These two formulas are the basis of all attenuators described on this page.
Series Attenuator
The series type consists of a chain of resistors connected between signal source and ground forming a constant input impedance. The wiper takes position between any adjacent pair of resistors and forms a classic voltage divider with all resistors from that position to the signal and all resistors from that position to ground. For the series attenuator you need a 12- or 24-step switch with only one deck.
The series attenuator has a big advantage: Only a simple switch with one deck of 12 positions is necessary, and only 12 resistors per channel are used. The simple construction makes it less prone to failure. The disadvantage is that at most positions of the switch there are more than one resistors in the path to the signal source and also more than 1 resistor to ground. At the lowest listening levels there are 10 or 11 resistors in the signal path.
Series attenuator with ground resistor
A common variation of the series attenuator is the attenuator with shunt resistor (signal to ground). The reason for such a shunt resistor is normally to provide a defined impedance to 
ground to the amp input. Common reasons for presence of such a resistor are:
ground to the amp input. Common reasons for presence of such a resistor are:- There is an audio capacitor between the line input (with it's attenuator) and the input of the Opamp to protect the amplifier from DC offsets. The resistor provides a DC ground reference for the Opamp input.
- The attenuator is of the break-before-make type and therefore between switch positions there is a moment where the ground reference is not present. A permanent resistor between the wiper and ground will provide such an impedance at all time.
- The amp is used as a power-amp only and in order to protect the speakers when disconnecting the pre-amp the resistor provides a reference to ground.
The figure at the right explains the principle: Rg is used to provide an impedance to ground and influences the attenuator (Ra and Rb for a certain position). When designing an attenuator for such environment it is good to remember that the effective input impedance of the amplifier at the signal input terminals will vary over the range of the attenuator.
As a result, care must be taken to select the right values for the resistors in the attenuator so that the impedance of the amplifier as seen by the source (Cdplayer etc) will not deviate too much from the desired values.
Ladder Attenuator
Figure 2 shows the setup of the ladder version. The rotary switch makes two connections at a time, connecting on one side to the signal in and to the other side to ground. The wipers are both connected to the signal out that is fed to the next amplifying stage. For the Ladder type you need a switch with two decks.
The ladder type of attenuator has a big advantage over the series version: Only one resistor is in the signal path and there is always one resistor to ground. The voltage divider always consists of two resistors Ra (to signal input) and Rb (to ground) and the wiper connects the two to the output of the attenuator.
The ladder attenuator therefore in fact contains 12- or 24- voltage regulators, which makes it the most elegant (and expensive) solution for volume control.
Resistor 12b may be omitted and replaced by a short to ground if the volume needs to be really 0 in the lowest position.
I updated the Excel spreadsheet, to include a ladder attenuator for both a 12- and 24-step switch.
Shunt Regulation
The shunt type is a mix of the ladder and the series version and offers in most of its range the advantages of both. The shunt type is not usable in all amplifier designs.
The advantages of the shunt regulator are in principle the same as for the ladder: Only two resistors (except in position 1) are in the signal path. The disadvantage of this design is that the input impedance is not flat but is a value of Ra + Rbx and thus changes with every value of x. For tube pre amps special care must be taken that the input impedance still is high enough for the tube pre amp.
In most cases resistor number 11 will be replaced with a hardware to ground so that there is no signal on the output in the lowest volume position. However, with a 12-step attenuator its a choice to use position 12 for very soft music and not completely shut off the amp.
2.2 Building a 12-Step Series Attenuator
Let's start with a 12-step attenuator for non-inverted designs such as my GeenKloon project. Such attenuator switch is based on the principle of voltage-divider: Eleven, Twelve (or twenty-four) resistors are connected in series, and the rotary switch positions between two resistors. The effective attenuation on pin "pos" of the voltage divider then can be computed as shown in formula 1a below. In this example I assume 11 resistors for a 12-step attenuator which means step 1 is full power and step 12 is 0 volume (wiper connected to ground). But it is equally possible to give step 12 a resistor value to ground which means that the volume then is low but never completely off.
Figure 1a is the schema for the attenuator I used for the GeenKloon (my non inverted version of the gainclone based on a LM3875 chip):
It is very advisable to observe the resistor values and solder them the right way (determine which is pin 1 and which is pin 12) because it's easy to build it the wrong way around which results in poor (hardly no) attenuation at all.
Well, the above formulas allow you to calculate the resulting dB attenuation on every position given that you know all resistor values already. And of-course this is where the problem is, since we would like to work the other way around: First determine the number of steps, the maximum attenuation in dB and the input impedance of the circuit and then calculate the corresponding resistor values.
Because this requires more than just a few calculations I wrote an Excel file that does just this and is easy to use. The following table contains example output from the spreadsheet for some typical impedance values.
| Attenuator | Attn dB | Resistor | 10k | 20k | 50k | 100k |
| Step 1 (lo) | -60 | R1 | 10 | 20 | 51 | 100 |
| Step 2 | -54 | R2 | 10 | 20 | 51 | 100 |
| Step 3 | -48 | R3 | 20 | 40 | 100 | 200 |
| Step 4 | -42 | R4 | 40 | 81 | 200 | 390 |
| Step 5 | -36 | R5 | 81 | 160 | 390 | 820 |
| Step 6 | -30 | R6 | 160 | 330 | 820 | 1,600 |
| Step 7 | -24 | R7 | 300 | 620 | 1,600 | 3,000 |
| Step 8 | -18 | R8 | 620 | 1,300 | 3,000 | 6,200 |
| Step 9 | -12 | R9 | 1,300 | 2,400 | 6,200 | 13,000 |
| Step 10 | -8 | R10 | 1,500 | 3,000 | 7,500 | 15,000 |
| Step 11 | -4 | R11 | 2,400 | 4,700 | 12,000 | 24,000 |
| Step 12 (hi) | 0 | R12 | 3,600 | 7,500 | 18,000 | 36,000 |
| TTL: | 10,041 | 20,171 | 49,912 | 100,410 |
2.3 A series attenuator with ground resistor
As described above in the architecture overview there are amplifier designs where apart from the attenuator there will be another resistor parallel to ground. This resistor is normally connected between the amp input and ground. the capacitor is there to assure that there will be no DC offset voltage at the speaker output. On the right the principle is found in a figure where I modelled the amplifier with a power opamp (as used in gainclones) and the attenuator with the Ra and Rb resistors in the ellipse. And for the sake of this exercise we assume that the value of the capacitor is such that the lowest frequencies in audio land are passed through this capacitor (for filter background the reader is referred to the RIAA/filter background pages). The ground resistor Rg can also be used to make a non-shorting switch more usable for an amp, in this case the capacitor may or may not be present. If the resistor is not present the resistor will probably be soldered directly on the attenuator between the wiper and ground.
These formulas are used in the spreadsheet (found on the download page).
2.4 An Inverted Amplifier Design
For inverted designs we must use different resistors values for the same dB steps as in a non-inverted setup. The reason is as follows:
The gain of an inverted design is determined by the feedback resistor and the effective impedance found on the inverted input of the OpAmp. And here lies the source of the problem; The attenuator is in series with this resistor on the inverted input and therefore part of the gain loop as well. Moreover, the output Z of the source device (CD-player, preamp, phono pre) is also in series and plays a role in the gain calculation, especially when using tubes on the input.
Important is to recognize that the attenuator itself still behaves exactly in either setup, and that the effective attenuation measured between input and ouput of the attenuator is independent from the amp architecture chosen. However, in an inverted setup the total attenuation/amplification of the amp is influenced by the attenuator itself and therefore we deal with it in this chapter.
The following figure illustrates the issue described above. Clearly is shown how the input resistor Ri is in series with the effective impedance of the attenuator and the output impedance of the connected source equipment (CD-player or other preamp). The attenuator is modeled with Ra and Rb: On any given position of the wiper, Ra defines the sum of all resistance to signal and Rb the sum of all resistance to ground (R_a + R_b = constant).
In the schematic, I omitted other components such as the input cap, as these were not necessary for an understanding of the issue. The gain of the OpAmp is therefore determined as follows:
Therefore the calculation method used for non-inverted designs does not work a 100% for non-inverted designs and that calculating the optimum gain requires even more compute power. Well, let's first see what role each component plays in the equations.
In order to make a rotary attenuator switch for inverted mode amplifiers, we need to find the resistor values for any given attenuation step between two switch positions. After all, it's nice to be able to calculate the attenuation for a given resistor setting, but rather we would like to work the other way around. For example: If I want a gain of -50dB on step 2, what resistor values for R_a and R_b do belong to that setting.
OK, based on this we can calculate the following formulas:
Therefore I made some calculations with a spreadsheet. Lets assume an inverted design with a feedback resistor of 220k and a resistor R_i on the negative input of 10k. Taking just the two into account we have a gain of 22 times (26 dB). Now take into account the rotary attenuator switch (R_tot = 22kOhms) and lets see how apart from the attenuation itself(!) the effective impedance changes the gain of the amp.
In the figure, we plotted the effective gain for a low-impedance source (e.g. CD-player) of 100 Ohms and a high-impedance source (tube preamp) of 2000 Ohms.
Finally, lets see how the ideal attenuation of just the switch changes in an inverted setup and what the resulting real steps in dB are when taking into account the variation in gain over the range of 12 positions. We do this by taking the gain (in dB) defined by the feedback loop above and add to it the dB attenuation by the 12-step switch.
The chart below shows how for any given attenuator with 12 steps gain in dB the required resistor values are highly dependent on the topology (inverted/non inverted) and the load on the input.
As shown above, if precision is your goal, the resistor values for inverted topologies differ significantly from the non inverted counterpart. The yellow line shows the behavior for non inverted designs, as a reference for how the same attenuator and the same resistor values in the feedback loop produce such different results in an inverted setup.
As to be expected, the gain of a non-inverted amp is dependent on the voltage divider made from R_a and R_b only and not dependent on the serial resistance of R_i and R_o. Therefore it's optimum values are far more linear than for the inverted setup as is shown by the yellow line in the chart above.
2.4b Alternative for Inverted amps
There is of-course several alternatives to the above solution. One would be to use a shunt design: One resistor permanently between input and ground. For the principle: Imagine in figure 2 above that the Rb side of the switch is not connected to ground.
Of course we need different values for the attenuator and therefore let's work out some formulas and maybe a new sheet entry in the spreadsheet.
<formula>
3. Building Attenuators
Now that we know how to calculate resistor values for a voltage divider, it's time to think about construction of the attenuator itself, the best switch to use, wiring, resistors etc. Below some examples are given for concrete applications.
What switch to use
Series Types
For my own gainclone designs I used cheap Lorlin switches so far. These switches are available from Conrad (Internet) and in shops. The website of Lorlin specifies several types of switches such as 6*2 or 12*1, in both shorting and non-shorting versions, with lugs or pins etc. Unfortunately, the non shorting (break before make) is the most widely used switch and is used for sources and output selection etc. For these applications a shorting version is hardly used and therefore shorting versions are hard to get. As the Lorlin switch is a closed design, it requires no maintenance and is self-cleaning (the inside only).
I found an open shorting switch in the conrad catalog (it is not advertised to be shorting but is definately is a shorting switch) and this one is easily modified to be useful for our application. Standard the switch does not have a stop and can be turned more than 360 degrees. With a very small drill and a bolt it is possible to modify the switch such that it becomes usable for an attenuator.
Anyway, should you be looking for 24-step switches or out-of-the-box shorting switches that are zombie proof etc. then alternative switches are available as well from antique, angela, reichelt etc.
Ladder Types
As shown on the right below there are some cheap alternatives for double deck switches, these will only cost a fraction of what a Dact switch will cost. These are of course 12-step switches and not the 24-step versions. However, for the average gainclone project this may just be good enough to work with.
As said, the Dact 24-step switches are amongst the best available. However, these switches are very expensive. Price differences between single of double deck switches is relatively low, so if you have room enough in your amp I would recommend to go for a double deck of 24 * 2 positions (sometimes incorrectly called stereo).
What Resistors to use
Metalfilm resistors do a better job here than carbon resistors is my experience. I have used Dale, Caddock and several types of metalfilm resistors. BC components (Philips) makes nice ones, but I do not hear much of a difference.
Then there is always the questionwhether carbon or metalfilm resistors provide the best sound in this application. As far as I'm concerned I would metalfilm resistors for an attenuator, but I like a clear and transparent sound. Carbon will make the sound slightly softer and if this is a design goal for you then by all means build your attenuator with carbon resistors. There is probably no good or bad only a matter of taste.
4. Examples
This section contains examples of attenators. The code in the subheaders refer to the corresponding worksheet in the spreadsheet found on the download page.
12-step series attenuator with shunt
Code | s12bn |
Descr | Series 12-step break-before-make non-inverted |
Project | Geenkloon |
First, lets look at the 12-step switch made for GeenKloon. The first version made for Geenkloon was based on a switch which was non-shorting. This means that between two switch positions the wiper does not make contact with either contact. As a result, for a short moment there is not path from the wiper to either ground or signal in, and therefore the amplifier stage following the switch will see a non-defined impedance. Loud pops will result from your speakers and possibly cause damage.
Needless to say that these switches are not the first choice for building attenuators. But at the time I built Geenkloon I did not have an alternative and therefore I had to find a work-around.
The solution was simple: Connect a fixed resistor from the wiper to ground which will provide a defined impedance for the amp even between switch positions. And it's easy to live with the few drawbacks introduced by this design, the sound quality is very good.
| Step | from pin | to pin | Resistors (1/4W,1%) | dB attn | dB step | Impedance |
| 1 (low) | GND | 1 | wire | infinite | -- | 19,910 |
| 2 | 1 | 2 | 100 | -45.98 | -- | 19,910 |
| 3 | 2 | 3 | 270 | -34.62 | 11.26 | 19,904 |
| 4 | 3 | 4 | 390 | -28.37 | 6.11 | 19,885 |
| 5 | 4 | 5 | 750 | -22.40 | 5.71 | 19,813 |
| 6 | 5 | 6 | 1,300 | -17.01 | 5.02 | 19,592 |
| 7 | 6 | 7 | 1,100 | -14.14 | 2.61 | 19,320 |
| 8 | 7 | 8 | 1,500 | -11.32 | 2.55 | 18,842 |
| 9 | 8 | 9 | 2,000 | -8.59 | 2.50 | 18,043 |
| 10 | 9 | 10 | 3,000 | -5.63 | 2.85 | 16,566 |
| 11 | 0 | 11 | 3,900 | -2.87 | 3.07 | 14,270 |
| 12 (hi) | 11 | 12 | 5,600 | 0.00 | 4.33 | 10,451 |
| 19,910 | 45.98 |
Unfortunately I did not take the time to do calculations too carefully at the time, although in practise the attenuator works real well. That's the reason why I'll upgrade the geenKloon with an attenuator that is capable of playing less loud at lowest volume settings. Biggest problem being that due to the low value of the parallel resistor the impedance for high-volume setting drops significantly below the 20k. This makes the Geenkloon with this attenuator not really usable for tube sources.
Fortunately this can easily be corrected using another attenuator as described below, trying to get hold of the make-before-break (shorting) version of the Lorlin switch used for Geenkloon.
12-step series attenuator
Code | s12mn |
Descr | Series 12-step make-before-break non-inverted |
Impedance | 22k7 (over full range) |
Project | Geenkloon upgrade |
This attenator will be used as a modification to the first version of the Geenkloon switch. This is really a very cheap 12 position switch that needs a little modification as the switch does not have a stop. It is however a make-before-break (shorting) switch and thus it's worth a little work in order to get a really cheap fnished product.
I described the modifaction process in some detail below for the double deck switch of the same manufacturer. It is a simple process, drill a small hole through one of the little dimples in the top plate and use a little bolt and fix it in the hole. The bolt needs to stick at least 1.5 mm through the hole on the innner side in order to provide a "stop" for the rotary switch mechanism inside.
Now you can solder the little resistors (1% types of 0.25W, matched per pair) on the small terminals. By using a layout at the outside of the swicth it's possible to make a quite elegant attenuator that can stand some mechanical use without intentional shorting once in a while.
| Step | from pin | to pin | Resistors | dB attn | dB step | Impedance |
| 1 (low) | GND | 1 | 47 | -53.69 | -- | 22,729 |
| 2 | 1 | 2 | 82 | -44.92 | 8.77 | 22,729 |
| 3 | 2 | 3 | 150 | -38.22 | 6.70 | 22,729 |
| 4 | 3 | 4 | 330 | -31.44 | 6.78 | 22,729 |
| 5 | 4 | 5 | 470 | -26.47 | 4.97 | 22,729 |
| 6 | 5 | 6 | 820 | -21.56 | 4.91 | 22,729 |
| 7 | 6 | 7 | 1,430 | -16.69 | 4.88 | 22,729 |
| 8 | 7 | 8 | 2,400 | -11.97 | 4.72 | 22,729 |
| 9 | 8 | 9 | 2,200 | -9.15 | 2.82 | 22,729 |
| 10 | 9 | 10 | 3,300 | -6.12 | 3.02 | 22,729 |
| 11 | 0 | 11 | 4,700 | -3.09 | 3.04 | 22,729 |
| 12 (hi) | 11 | 12 | 6,800 | 0.00 | 3.09 | 22,729 |
22,729 | 54.00 |
And after surgery of Geenkloon, the finished attenuator section looks like this:
The only drawback I can think of of this switch is it's open construction. But on the other hand with a little cleaning or eve better, regular use of the Geenkloon I do not anticipate any cracking or so. After all, I use small boxes for the clone without openings so dust and dirt will stay out under normal circumstances.
12-step series for Inverted Amps
Code | s12bi |
Descr | Series 12-step break-before-make inverted |
Project | Cyclone |
The Cyclone project is an inverted amplifier. And therefore, the switch is part of the feedback loop of the inverted amp and the impedance will influence the gain (as does the voltage divider also).
How to use a non-shorting (break before make) switch in your amp? What I did is add a parallel resistor from the wiper which connects to the chip input, to ground. That parallel resistor will make sure that between the positions of the attenuator the input of the amplifier is in a defined state. For a 20K input impedance, a value of 47k or 100k would be nice, for a 50k attenuator I would use a 100k resistor. In general, use a resistor value at least 2 times the desired input impedance.
Important with the table below is to know that i used a parallel resistor of 47k and assumed an output resistance of the source device on the signal input of 100 Ohms, an input resistor of 10k and a feedback resistor of 220k for the inverted amp.
| Step | from pin | to pin | Resistors | dB attn | dB step | Impedance |
| 1 (low) | GND | 1 | 82 | -49.35 | 5.94 | 23,724 |
| 2 | 1 | 2 | 82 | -43.41 | 5.49 | 23,723 |
| 3 | 2 | 3 | 150 | -37.92 | 5.92 | 23,722 |
| 4 | 3 | 4 | 330 | -32.00 | 5.67 | 23,715 |
| 5 | 4 | 5 | 680 | -26.33 | 4.73 | 23.688 |
| 6 | 5 | 6 | 1,200 | -21.60 | 3.98 | 23.595 |
| 7 | 6 | 7 | 2,000 | -17.62 | 2.83 | 23,327 |
| 8 | 7 | 8 | 2,400 | -14.79 | 2.57 | 22.835 |
| 9 | 8 | 9 | 3,000 | -12.23 | 2.57 | 21,994 |
| 10 | 9 | 10 | 3,900 | -9.36 | 2.87 | 20,582 |
| 11 | 0 | 11 | 4,700 | -5.71 | 3.65 | 18,487 |
| 12 (hi) | 11 | 12 | 5,200 | -0.14 | 5.57 | 15,766 |
| 49.35 |
As you can see, the resulting impedance as seen by the source device (CDplayer) is not constant but is between 15k8 and 23k7. But it stays above the 22k for most of it's range and in practise this is not a show-stopper.
Building a 12-step ladder Switch
Code | l12bn |
Descr | Ladder 12-step break-before-make non-inverted, or a make-before-break type with grounding resistor on the opamp input. |
Impedance | 50k Ohms |
Project | tbd, would fit Gainclone and Cyclone |
For a ladder switch one needs a rotary switch with 2 decks. The wipers of both decks are connected with each other and with the input of the amp. For each of the 2 decks, the resistors are connected on 1 side with the switch contacts and are connected with each other and either ground or signal input on the other side.
The switch above and the one the right is a 12*2 switch. Although it's an open construction, which means cleaning once in a while in order to avoid corrosion or dirty contacts, it's build quality is excellent. And if you, like me, use the switch in an amp enclosure which is more or less air tight, corrosion will not play a large role anyway.
 |
see screw in top plate |
Only problem: Out of the box the switch does not have a stop (it's possible to turn it 360 degrees). But there is a work-around, and it's possible to fix the stop. Drill a small hole through one of the 12 marks/dimples in the top plate. Then us a little m1 bolt and screw it into place. Done. For just € 3.00 you made yourself a 2*12 switch that can be used to make a ladder attenuator.
Of course the switch can also be used to build a stereo version of a series attenuator, bit apart from the easy of use the technical construction is not different from the mono series attenuator described on this page and therefore not discussed further in this section.
However, in order to be protected even when the switch would degrade over time and would not be shorting in every position, I used a parallel resistor just in case. And unlike the series attenuator, for a ladder attenuator the parallel resistor does not cause the input impedance to vary over its range as long as you take its' value into account for every resistor pair on the ladder.
Look in the spreadsheet for examples of 50K attenuators (or any impedance value you choose yourself), the spreadsheet takes the optional parallel/shunt resistor into account and computes a ladder attenuator with 12 positions.
In this case I used a 100k parallel resistor which is effectively always in parallel with the resistor 2 in the table. Therefore, the voltage divider consists of the following resistors: Resistor 1 to signal in and the two Resistors 2 and the parallel resistor to ground.
| Step | from pin | to pin | Resistor 1 | Resistor 2 | dB attn | dB step | Impedance |
| 1 (low) | GND | 1 | 49,900 | 47 | -60.53 | -- | 49,947 |
| 2 | 1 | 2 | 49,900 | 82 | -55.70 | 4.83 | 49,982 |
| 3 | 2 | 3 | 49,900 | 150 | -50.47 | 5.23 | 50,050 |
| 4 | 3 | 4 | 49,900 | 240 | -46.40 | 4.07 | 50,139 |
| 5 | 4 | 5 | 49,900 | 390 | -42.21 | 4.19 | 50,288 |
| 6 | 5 | 6 | 48,700 | 787 | -35.97 | 6.24 | 49,481 |
| 7 | 6 | 7 | 47,000 | 1,580 | -29.76 | 6.21 | 48,555 |
| 8 | 7 | 8 | 47,000 | 3,160 | -24.01 | 5.74 | 50,063 |
| 9 | 8 | 9 | 43,000 | 6,800 | -17.29 | 6.72 | 49,367 |
| 10 | 9 | 10 | 37,400 | 16,000 | -10.47 | 6.83 | 51,193 |
| 11 | 0 | 11 | 30,100 | 33,000 | -5.63 | 4.84 | 54,912 |
| 12 (hi) | 11 | 12 | 0 | 100,000 | 0.00 | 5.63 | 50,000 |
| 60.53 |
As you can see, even with the parallel resistor in place, the effective impedance is dependent on the resulting parallel resistance of R2 and the parallel resistor (therefore the value is 100K for the highest gain setting, as two resistors of 100k in parallel yield an effective resistance of 50k Ohms).
24-step series attenuator from old switches
Code | s24mn |
Descr | Series 24-step make-before-break non-inverted |
Project | tbd |
Look at the geenkloon and Cyclone projects for serial attenators based on "cheap" Lorlin 12-position switches. As said above, especially with inverted Gainclones, care must be taken when selecting the resistor values.
It is possible to combine two single decks into one double deck and then make either a stereo series version or a mono ladder version.
5. More About the Spreadsheet
5.1 Why a spreadsheet program
Over the last months I received several requests for the spreadsheet or for additional information regarding the sheet. And based on the input I received I have made changes (hopefully improvements too) to the spreadsheet program.
5.2 This Excel program helps
I made this small Excel program that will help you in calculating the appropriate values for your own switch. It's easy to use and self-explanatory and allows the user to tweak every value by hand and see the results. The published value is based on non-inverted design and has versions for the following types of switches:
- 24 step make-before-break (AKA shorting) switch for non inverted amps
- 12 step make-before-break (AKA shorting) switch for non inverted amps
- 12 step break-before-make (non shorting) switch for non inverted amps
- 12 step make-before-break (non shorting) switch for inverted amps
Note: It does contains a setup for inverted topologies based on make-before-break switches. I'm still working on a version that helps better with break-before-make-inverted designs.
Download your version here: <Excel program Link>
The break-before-make variant in the sheet is a very cost effective way to build your 12-step attenuator with a cheap Lorlin switch of € 1.50 a piece and an additional shunt resistor from wiper to ground (to avoid damaging "plops").
Unless you're familiar with Excel, do not change the sheets because you might screw up the chart or other Excel references.
5.3 How to use the program
In order to work with the Excel program, a few decisions should be taken in order to choose the right sheet for your application.
- First of all, determine the type of switch you have, for my programs either a 12- or a 24- position switch. Also make sure you know whether it is shorting (or make before break) or non-shorting (break before make).
- If you have a lot of standard resistor values at home, or you want the program to only use a selected set of resistor values instead of the complete E96 range: Fill out the last sheet "e96 std values" and in particular the column D.
- Determine what steps you like for the various switch positions and fill them in in the appropriate column (on the screenshot it would be column D rows 9 - 19).
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