In my last post I described an improvement to the DÆ v3.0 attenuator called “wiper motion” that is intended to eliminate the noise when switching between attenuator levels.

This improvement has worked very well indeed.

There are now only a few tweaks (famous last words) that I want to make to the design to reduce the number of hours required to assemble an attenuator. Reducing the assembly time will of course also reduce the cost.

It turns out that the dual magnet yoke described in a previous blog post has eliminated switching transients between steps but the angular alignment of the yoke must be very precise to ensure that only one reed switch is closed when the yoke comes to rest at a ladder step. If the yoke angle is only a little bit off, two reed switches close at the same time resulting in an erroneous amount of attenuation instead of the ultra precise attenuation that is expected from a stepped attenuator.

So far I have been able to fine tune the yoke angle in the handful of attenuators I have assembled for testing but this fine tuning procedure is tedious. Ideally I would like more tolerance in the required yoke angle so that the attenuator functions properly even when the manufacturing tolerances of reed switch sensitivity and magnet strength stack-up in different ways. By reed switch sensitivity I mean how close the magnet must be brought to close the reed switch.

The picture below shows a close-up of a reed switch. There are two metal contacts in a small glass chamber. When a magnet is brought close to the reed switch, the tiny contacts close turning the switch on.

This is a picture of a completed v3.0 attenuator level. Note the reed switches flanked by two resistors around the perimeter.

I found the hard way that the sensitivity of the reed switch depends on it’s installed orientation in the attenuator level. The sensitivity was different when the switch was installed according to close-up 1 compared to close-up 2 below. At times the reed switch wouldn’t even close when the magnet was directly adjacent to the reed switch when installed in the orientation shown in close-up 1. This finding is contrary to the literature about reed switches that indicates a reed switch is (should be?) insensitive to installed orientation.

Ideally, I would find a combination of reed switch and magnets that is insensitive to the installed reed switch orientation and I would use a slightly more magnetically sensitive reed switch so that I could return to a single magnet yoke.

I devised the “Reed Switch Tester” apparatus shown below to study the magnetic sensitivity of the reed switches at various simulated yoke angles for a number of configurations including single and dual magnet yokes.

This short video shows the “Reed Switch Tester” in action.

And the results….

Like any good test, I wound-up confirming a few of my suspicions but also learned a few things I didn’t expect. Here are the main findings:

In the test jig, the reed switch remained closed even with a single magnet yoke at great distances. This was not at all expected as the completed attenuator level requires a dual magnet yoke and the reed switch closes only when the magnetic is within 6.5 mm. It turns out that the leads to the reed switch are magnetic. When a magnetic material is placed near a magnet, the magnetic field lines are concentrated by the magnetic material. Hence the long metal leads of a stock reed switch increase it’s sensitivity. When installed in an attenuator level, the reed switch leads are trimmed and are much shorter than stock. When I trimmed the stock reed switch leads and replaced the leads with a non-magnetic metal (copper), the results became much more reasonable but the reed switch sensitivity in the test jig was still greater than in a completed attenuator level. Also the reed switch's sensitivity was independent of its installed orientation in the test jig whereas in the attenuator level, the orientation of the reed switch matters;
In the attenuator level the reed switch is flanked by two metal film resistors (see picture above) which are also made of magnetic materials and concentrate the magnetic field. I re-ran the tests with the reed switch flanked by two metal film resistors with their leads trimmed to a length approximating the lead length in the attenuator level. Finally with this arrangement, the reed switch sensitivity in the test jig agreed with the observations in the attenuator;
I was able to confirm that the installed orientation of the reed switch mattered so unfortunately careful alignment of the reed switch is required during assembly;
I tested a more sensitive version of the reed switch but alas a dual magnet yoke is still required and the installed orientation of the reed switch still mattered.

But hope springs eternal - I am working on an update to the v3.0 attenuator that uses a flex printed circuit board for the ladder sections. This v3.1 design significantly reduces the assemble time because the components on the flex PCB can be installed by machine. The flex PCB uses MELF surface mount resistors that are magnetic but significantly smaller than the through hole metal film resistors of v3.0. As a result, the reed switch becomes the largest piece of magnetic material near the magnetic which has fortunately reduced the sensitivity of the assemble to installed reed switch orientation. For machine assembly it is very important that the installed orientation of the reed switch doesn’t matter because the automated assemble equipment doesn’t adjust for the installed orientation like is possible with hand assembly. A dual magnet yoke is still required. Only downside of this design is the flex PCB is more expensive and takes longer to fabricate than more conventional rigid PCBs.