Over the past week, I repaired the ladder sections for the right channel. I had installed the ladder resistors in the wrong order. One of the ladder sections is shown in the picture below. The lowest ladder rung (least gain) is at the upper left and the the rungs increase in a counter-clockwise direction around the perimeter of the board. The resistors are the blue parts with the colored bands. I had mistakenly installed the resistors starting at the upper right and in a clockwise direction which resulted in a weird gain curve that looked nothing like the audio taper I was looking for. To correct this, I carefully desoldered the first set of resistors and soldered a new set of resistors onto the boards. I almost completed the right channel but in the process damaged the PCB traces on one of the ladder sections and need to make one new section. Fortunately I bough some spare PCBs.

With the corrected resistor arrangement, I did some initial testing of the attenuation curve and it is looking good. Final testing still to come.

While looking at a close-up of a ladder section, I want to point out a few things. For scale, the PCB is two inches (50 mm) square. Each ladder section has eight rungs (or settings or steps). There are eight tiny relays which are the ivory colored parts in the picture with the word “Japan” on them. Next to each relay are two parallel resistors which are the blue parts with colored bands. The resistors are precision 1% metal film resistors and two resistors are placed in parallel for improved accuracy. Often ladder attenuators will only have one resistor for each rung so doubling the number of resistors is a feature of my design.

The ladder sections are stacked to make a ladder attenuator with the desired number of steps. The picture below shows a 24-step attenuator using three ladder sections. At the front of the ladder stack is a connector board with the flex cable attached. The box like component next to the flex cable is a vacuum sensor that will allow the vacuum level in the chamber to be monitored. The fifth board, at the rear of the stack, has four relays used to select one of four input sources namely Phono, CD, Aux 1 and Aux 2.

The attenuator stack is suspended in the vacuum chamber by the metal rod and four small rubber bands. The mounting is meant to be resilient to prevent mechanical transmission of vibration from the relays to the enclosure. The vacuum stops the air-borne transmission of relay switching noise and the resilient mount prevents the mechanical transmission of switching noise.

Good idea but only problem is the flex cables are too long and stiff - I am working on correcting this and will talk about it in a future post (maybe tomorrow?).