Stereo stepped attenuator

I am using a Sabaj PHA1 headphone amplifier with my Sennheiser HD555. The sound quality is OK, but the amplifier gain is too high for the relatively sensitive and low impedance HD555. To be able to use the full volume range, I decided to build a stepped attenuator to reduce the audio signal received by the amplifier.

A signal attenuator can get relatively complex if you want to match the input/output impedance of the devices it is connected to. In my case, a simple voltage divider circuit was sufficient. The signal comes from a computer sound card output which should have a low output impedance and is connected to an op-amp based headphone amplifier that should have a high input impedance. Because of this, I assumed a simple voltage divider with a total resistance of around 10kOhm should to the job.

The goal is to find the signal attenuation gain that allows the use of the full volume range on the namplifier. To find this value, I used a 10kOhm audio potentiometer I had lying around and connected it between my sound card and the headphone amplifier. I put the headphone amplifier volume at 100% and the potentiometer volume at 100% attenuation. Then, while playing a song, I decreased the potentiometer attenuation until the sound got a bit too loud. Then I measured the resistance between the potentiometer input and its tap to be around 9kOhm. That means the attenuation was 90% (9kOhm/10kOhm). To get this attenuation value in decibels, we can use the equation

\(\text{gain} [\text{dB}] = 20 \log \left(\frac{V_{out}}{V_{in}}\right) \)

Since we are attenuating the voltage 90%, the output voltage is 0.1 times the input voltage, or -20dB. I also wanted to have a midway attenuation in case the input signal is less powerful for some reason. I settled on an attenuation of 50% of the voltage, or -6dB.

The desired attenuation levels didn’t need to be exact since it is used for audio listening, not signal analysis. I settled on a voltage divider composed of two 4.7kOhm resistors and one 1kOhm resistor as displayed in the following circuit.

stepped attenuator circuit
Single channel circuit.

The circuit shown is only for one audio channel. It needs to be duplicated for each audio channels. Using the selected resistor values, we get a stepped attenuator with gain values of 0dB (pass-through), -5dB and -20dB.

Why didn’t I simply used a dual potentiometer instead of a stepped resistor divider? With a potentiometer, I would have an infinitely variable attenuator. One problem with dual potentiometers is tracking between the two channels. The higher the attenuation, the worst the tracking tends to get. So, at high attenuation (low volume), the volume between the left and right channels tends to be audibly different. Using high precision resistors avoids this problem. To absolutely eliminate the tracking problem, I decided to go with 0.1% resistor. Probably overkill, but they are not much more expensive than 1% resistors.

A dual pole three positions (DP3T) rotary switch is used to control the attenuation level. A dual pole switch is required since we want to control both audio channels simultaneously. The three positions (triple throw) is required because we have 3 attenuation values (0db, -5dB and -20dB).

The resistors were soldered directly on the switch pins. Then, piggy back wires were soldered onto two 3.5mm jack to be used as input and output. Everything was soldered together within an ABS project box.

To make holes in the ABS plastic enclosure, I used wood drill bits. This works, but when drilling larger holes, the drill bit has a tendency to simply thread itself on the soft and malleable ABS instead of making a round hole. To prevent this, start with a small diameter bit, then slowly increase the drill bit size to enlarge the hole step by step. Also, don’t put too much pressure on the plastic to allow the bit to take out chunks of plastic instead of just piercing through. Next time, I might try to use a step drill bit, it should make the process of drilling the enclosure easier and faster. The following picture presents the finished circuit inside the project box.

Circuit inside the attenuator enclosure
Circuit inside the attenuator enclosure.

Finished to project by adding labels and a retro looking switch button.

rotating stepped attenuator
Finished attenuator.

The table below list the parts used in this project.

Description  Reference
Enclosure 1591XXLSFLBK
3.5mm jack 839-1411-ND
DP3T rotary switch CKN11067-ND
4.7k 0.1% resistor 4.7KADCT-ND
1k 0.1% resistor 1KADCT-ND