Project files: Line Attenuators

What is it?
If you are using a preamp with gain you may have problems with only using a fraction of the available range on your volume control which is very annoying. The problem is usually caused by too much gain and/or an incorrect gain structure. If it is not possible to reduce the gain of one or more of the amplifiers in the chain, a solution can be to use inline attenuators from e.g. Rothwell Audio instead. These are quite expensive though, and they only come in predefined attenuation levels so for testing purposes a DIY-option such as I am presenting here might be better.

The attenuator is built on a small board with RCA sockets for input and output, as well as an option for fitting two parallel resistors on the output side. The gives two (or even three) selectable attenuation values. The selection can be either by jumpers or even via a switch to make the boards suitable for testing etc.

How big are the boards?
The board measures 1.75″ by 0.9″ (app. 44 x 23 mm) – plus of course the off-board part of the connectors.

What is the status of the boards?
The board is in v1.0, meaning it has been tested and confirmed working.

Does it use any special/expensive/hard-to-find parts?
The RCA sockets are clones of Vampire RCAs. They are normally the best board-mounted RCAs I know of and available on ebay. If you don’t want to use connectors or can’t find them, just connect the signal via a 0.1” header (or a JST XH/Molex KK connector) instead.

Anything else I need to know?

  • Important: The reason that Rothwells are built into the RCA-plug is to keep the signal path short and especially the load capacitance on the output side as low as possible. Use the shortest possible cables on the output of these to avoid the cables inducing an RF-rolloff.
  • The resistor values are quite important and should ideally be matched to the source and load impedance. I’ve used this thread (post #6) as a starting point but it’s worth reading up on the theory behind the operation as there are a few trade-offs involved.
  • The center-to-center spacing of the RCAs is 1.1″ (28mm)

Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.


Inching forward…

Another long(ish) break from posting – this time mostly courtesy of some extremely nice late-spring weather and a couple of house-related DIY-projects. Just about the only thing that has moved forward (at least enough to notice) are my ICEpower 700ASC-based mono blocks (which I discussed here). A couple of weeks ago I got the mounting plates I designed for the modules + supporting circuitry which meant I could drill the chassis and start putting some mechanicals together at last.

Some of you may have guessed that this is where my BalBUF design is supposed to end up, but there was a piece missing. A matching power supply to drop the 700ASC’s 15V aux power supply to something more manageable for the OPA1632 (which gets very hot in operation). Because I was running out of space in the enclosure I wanted to use, a key design criteria was that the PSU should be “stackable” with the BalBUF board.

I quickly found what looks like the perfect device for this use – the TPS7A39 from TI – which is a dual pos/neg low-noise regulator with the right specs. Unfortunately, it is also only available in a 3×3 mm leadless package and as my odds of hand-soldering that are pretty much = 0 I dropped that pretty quickly. Instead I went for a bog-standard LM3x7-based design, but managed to squeeze it down to size because of the modest heat sinking requirements.

In a nod to “reusability”, which is something I always aim for where possible, the PSU board includes SMD resistors on the bottom in front of the caps, which means it can also be used with the unregulated supplies on the other ASX-boards such as the 50ASX and 125ASX. This means that you can use the BalBUF with any ASX-module without a separate offboard supply for the low-voltage circuitry, and because the BalBUF and the PSU stack on top of each other it should be very compact. Assuming everything works as expected with the 700ASC when I test it, I’m pretty sure that means I’ve just figured out what to do with my last remaining pair of 50ASX’es 😀

The sketch for the rear panels is also pretty much done, but given that Schaeffer/FPX panel work is getting more and more expensive I have decided not to order the rear panels “blind”, i.e. before I have tested that the monos work electrically. If this weather continues, that might be a while though 😀