Stepped Attenuators and Signal-Source Selector Switches

I decided to create a new stereo stepped attenuator based entirely on the ladder structure. The ladder improves upon the series attenuator by setting up an array of many two-resistor voltage dividers and the means to switch to the desired pair. Now we are back to just three solder joints and just two resistors in the signal path—but at the cost of twice the switch contacts and twice the resistors used. This is the no-compromise approach to stepped attenuator design. The downside to this attenuator topology is that twice as many resistors and switch poles are needed. In other words, a comparable series attenuator will cost half as much as the equivalent ladder attenuator.

Attn-2 Stereo Stepped Attenuator Stepped attenuators deliver decisive clicks and precise attenuation. Sadly, they are never cheap—well, at least the high-quality ones are never cheap, as the switches with hard-gold contacts are expensive in the extreme. Thus, the need to be frugal with the number of switches used. The new Attn-2 uses two rotary switches, one 6-position switch (on the left) for fine decrements of 1dB and one 12-positon switch (on the right) for coarse 6dB decrements, which allows for 0dB to -65dB in 1dB steps for two channels.

The new Attn-1 stepped attenuator uses both series and ladder attenuators, with three rotary switches to yield a total of 99 possible decrements of attenuation in 0.6dB steps, which means we can step down to -66dB in -0.6dB decrements. The leftmost and rightmost switches hold eleven positions and are mono ladder attenuators for the left and right channels, with only two resistors switched in at a time, attenuating up to -6dB per channel, with a mute in the final position. The center switch cascades from the two flanking switches and it defines a stereo series attenuator with 12 resistors in series per channel, with -6dB decrements, establishing a maximum of -60dB of attenuation, with a hard mute in the final position.

The Attn-12 is a new stereo stepped series attenuator that holds 11 resistors per channel and offers twelve -6dB decrements, with a hard mute at the bottom twelfth position. The closest analogy to the series attenuator is a normal audio-taper potentiometer used as a volume control in 99.99% of audio gear. It consists of a rotary switch that holds many differently-valued resistors placed in series, creating a long chain of resistors, with each resistor connection attached to a switch contact. The rotary switch’s moving contact then selects between resistor connections, thus providing a selectable amount of fixed signal attenuation. The chain of many fixed resistors wired in series defines the total input resistance of the attenuator.

1.4 by 2 inch, PCBs that hold a single rotary switch and nothing else, other than termination pads for hookup lead. The Select-2 selector switch assembly accepts three stereo inputs, with both the hot and grounds of each signal source to be selected. So if a signal source, say a CD player, is not selected, neither its outputs or grounds make any connection to the line-stage amplifier.

The idea behind the Stereo-Freq switch is that you can choose between two low-frequency cutoff frequencies, say 10Hz and 100Hz or mute the signal. Why would anyone want to do this? Late-night or early-morning listening. Deep bass travels easily through your house or apartment. By selecting the higher low-frequency cutoff, we can listen without bothering others. But the real reason I made this new switch was that I needed a rumble filter with LP playback, as some LPs are horrifically warped.

The new Select-CM allows you to do this in mono. In other words, this is a special capacitor selector switch that is designed to work within dual-mono setups, either line-stage or power amplifiers. In addition, this switch differs from the stereo Select-C in that the Select-CM offers four positions: mute, C1, C2, or C1 & C2 in parallel.

A rotary switch for phono playback that presents three positions: mono, mute, and stereo. The signal leaves the phono stage and enters the Stereo-Mono switch's PCB. Since the rotary switch is a shorting type (a make before break type), there are no popping sounds between clicks. Kit includes the PCB, rotary switch, and several pairs resistor values to choose from, either carbon-film or metal-film types.

The new GlassWare Balance Trim is a simple ladder attenuator, with six positions, which can be used with a volume potentiometer (or stepped attenuator). The switch is a 2-pole, 6-position, shorting design that never places more than two resistors in the signal path. It allows making -1dB decrements in signal level. By using one per channel, fine balance trimming is possible.

The new Select LSA/HPA rotary switch and PCB allows you not only to select where the output signal goes, but which capacitors are used. The big coupling capacitors that attach to headphones, are usually only very good, whereas the smaller coupling capacitors that attach to our power amplifiers are usually much better although smaller in value, but not always in size. With the Select LSA/HPA switch, we can use both types of coupling capacitors, with the headphones getting both in parallel, and the line outs getting only the smaller valued capacitor.

The Trim-1 stepped attenuator offers 11 positions and can viewed as high-quality replacement for a potentiometer. In other words, it can be used in non-audio as well as audio applications. A Trim-1 can even be used as a balance control of sorts.

The A5 stereo stepped attenuator kit includes two shorting rotary switches (2-pol/6-pos) and one USA-made extra-thick (0.94"), high-quality PCB, with 2oz copper traces and two side solder mask and silkscreen.

The M1 stepped attenuator combines both series and ladder stepped attenuators into a single functional attenuator. The ladder attenuator's job is to provide six fine steps of attenuation (-1dB per step); the series attenuator, eleven coarse steps of attenuation (-6dB per step).

The TCJ Attenuator A3 is an interesting design: a hybrid attenuator that uses both series and ladder attenuators to yield the best compromise between flexibility, performance, and cost.

A3-Mini Stepped Attenuator

The standard A3 36-position stepped attenuator is 9 inches long, whereas this new A3 Mini is only 5.8 inches wide, but is otherwise identical. Where the resistors lay flat against the standard A3 PCB, the resistors stand perpendicular to the PCB on the new A3 Mini attenuator.

The three positions are: all transformers off, transformer 1 on and transformer 2 off, and both transformers on. With this setup, you can turn on the heater transformer first, so the tubes are given a chance to heat up, which will create an electron cloud over the cathodes, protecting them from the B+ voltage.

The Select-4 switch and PCB makes wiring up a line stage amplifier easier. Select up to six stereo inputs.

The capacitor selector makes wiring up an Aikido board an easy task, as the two coupling capacitor outputs from each channel attach to the small PCB and the two outputs leaving the switch allow choosing between coupling capacitors C1 or C2 or both C1 and C2 in parallel.

With a balanced system, we can easily flip the phase of the signal with the GlassWare Select-Phase switch and PCB. I get a lot of e-mail asking if I believe in phase. In other words, do I believe that we can hear the difference that phase reversal makes? I do. I have performed many experiments with phase and I have found that the difference is obvious, if one important stipulation is followed: you must pause at silence in between phase reversals. It doesn't take long, maybe a second or two, but this brief pause gives the ear and brain a chance to readjust to the change in phase. On the other hand, with no pause (and a two-position toggle switch), no difference can be heard with mad flipping back and forth. The Select-Phase switch has mute at the center in between phases.

FrontPanel Signal Selector & Attenuator The new FrontPanel makes wiring up a line stage (or integrated) amplifier much easier. Two channels of three inputs (both hot and grounds) attach to the PCB and two channels of 36 steps of attenuation are possible before leaving the PCB. When a line input is not selected, both its hot and ground are detached from the selector's output. The PCB is 1.4 by 6 inches and holds three rotary switches and pads for 24 resistors; the FrontPanel kit can be bought with (carbon-film or metal-film) resistors or without any resistors. The attenuator uses both a shunt and series stepped attenuation.

1.4 by 2 inch, PCBs that hold a single rotary switch and nothing else, other than termination pads for hookup lead. The Select-5 selector switch assembly accepts four stereo inputs, with both the eight signal hots and four grounds of the signal sources to be selected. So if a signal source, say a CD player, is not selected, neither its outputs nor its ground make any connection to the line-stage amplifier.