[j-curve]

So proud of my new toy...

Big knob for coarse control = 6dB/step.
Small knob for fine control = 2dB/step.

For those times when I need an in-line volume control, I now have something I can rely on, i.e. no crackling or channel imbalance, plus a nice low output impedance to drive my cans.

More details to come, including a circuit if anyone's interested.

 

[Garbz]

interested, although I probably understand what you did in any case.

 

[j-curve]

Hi Garbz,
Thanks for the expression of interest. Of course you're right, anyone with some basic electronics should be able to draw up the circuit, but I'll prepare a graphic anyway and post it for everyone else. For the moment, a few words about the parts, and the unexpected difficulties in sourcing them...


Why 6x6?
The good thing about 2P6T (double pole, six throw) switches is that you can actually find them in a retail store(!) They're cheap too, although if you count up the hours you'll spend drilling and soldering then the initial parts cost is probably less of an issue. Note that these must be make-before-break or "shorting" switches for clean audio during switching. If you don't mind having separate left and right controls, a simpler design using 1P12T is a viable alternative. Otherwise, good luck finding stereo shorting switches of 12T or above.
The matrix has other advantages, namely 36 possible gain settings, meaning a greater range of attenuation, better resolution (smaller steps), or both. My choice of 6dB & 2dB steps leads to considerable redundancy but is intended to keep things simple and minimize switching.

Precision switches
I found these Alps switches in Akihabara (Tokyo's electronics district). When I got them home and pulled out the multimeter to work out which pins were which, I thought I had been given ordinary non-shorting switches by mistake. The overlap between positions is so narrow that it's hard to find by hand.

Resistors in a haystack
Having got the switches sorted, how hard could it be to find a bunch of resistors? Guess again, because many of the required values are less than 10 ohms. Even if I was willing to pay Akihabara prices (40 yen per resistor for metal film

), those values simply aren't to be found. Some weeks later in Taipei I had better luck, so I bought ten of everything they had, including some wacky values like 4.99 and 5.76 ohms.

The box
It was there (Taiwan) that I spotted this amazing box. Believe me, the original intention was a plain yet practical metal enclosure, but once I saw this showboat box I began to question whether I really needed full shielding for a low impedance audio project and whether my soldering skills were exhibition grade.

Not just pretty, it's strong too. Naturally, while drilling the holes in the lid, the drill seized in one of those "whoops" moments that would have spelled catastrophe for any regular plastic. The ABS simply buckled - incredible stuff. That strength would also bring a disadvantage in that the walls were too thick to mount the 3.5mm headphone socket and the cable grommet directly. Both those holes had to be recessed from the inside with an engraving/cutting tool.

 

[tyre]

Good stuff, attenuators on a budget. I'm interested in seeing a schematic.

 

[j-curve]

OK, here's the circuit:-

Parts List
As mentioned earlier, S1 and S2 are 2P6T "shorting" (make-before-break) rotary switches. At the risk of stating the obvious, the circuit shows a mono attenuator so you need to duplicate for stereo using S1b and S2b.
For the resistors, the calculated "ideal" resistor values are listed, followed by the actual resistors I used in square brackets. All values in Ohms
Rin 4.7 [4.7]
R1 12 [12]
R2 3.6 [3.9]
R3 4.7 [4.7]
R4 5.8 [5.76]
R5 6.8 [6.8]
R6 7.3 [7.3, being 10 parallel 26.7]
R7 1.33 [1.32, being 2.2 parallel 3.3]
R8 1.45 [1.5]
R9 3.3 [3.3]
R10 8.3 [8.2]
R11 18.9 [18]
R12 23 [22]
Rout 150 [150]

Specifications
Minimum input impedance (with 32 Ohms headphones connected): 17 Ohms
Maximum output impedance (with a 16 Ohm source connected): 16 Ohms
Adjustment range: 40dB in 2dB steps
Insertion loss (with 32 Ohm headphones, 16 Ohm source): 5dB

Design
The design goals were:-
i) consistent 6dB and 2dB steps
ii) low output impedance
iii) acceptable input impedance
iv) low insertion loss
Being a passive circuit, some compromise is necessary, so these goals were combined to make an error function which was then minimised using the "solve" function in a spreadsheet.

Are Rin and Rout really necessary?
The impact of source impedance and headphone impedance on the step sizes was investigated. It turns out that the small steps get bigger if the attenuator is connected to a low impedance source and get smaller when connected to a high impedance source. Having the input resistor Rin reduces this effect. Another advantage of Rin is that it makes the input impedance of the attenuator less dependent on the volume setting of the switches. This is handy if you are sharing a headphone output with someone else, because you don't want your volume setting to affect their listening level.
To a lesser extent, the big steps are affected by different headphone impedances and Rout acts to reduce that dependency.
In summary, Rout could happily be omitted, whereas Rin provides two worthwhile benefits.

Step sizes
Steps should not be too fine and absolutely must not be too wide. If you can't set a comfortable listening level then the project is a failure. That said, I find the 1.5dB steps on my portable CD player annoyingly narrow. Steps of 3dB would look good on paper (nice ratios) but I suspect 3dB may be a bit coarse, hence the 2dB choice. After a few hours of listening with the attenuator, I'd say 2dB is almost too narrow, but overall I'm happy. A revised design would have 2.5dB and 7.5dB steps with Rout omitted.

No zero
There is no "off" position, i.e. the attenuator is always letting some signal through. If that's a problem, the fully anticlockwise tab of the "big step" switch S2 could be connected to the ground rail instead of the junction between R7 and R8. This gives a no-output setting but also reduces the attenuation range by 6dB. Personally, I'd add a mute switch to implement this feature.