A twisted tale about a logarithmic relay attenuator

[Russ White]

Originally posted by AndrewT
Hi,
I tried your resistor calculator to produce a 200r output impedance with a 60source and 200out for the first stage and then 200source 200out for the remaining 4 relays. -38.75db in 1.25db steps.

Unfortunately the input impedance drops very low whenever a relay pulls in to start attenuating.

Is there a trick connection that maintains a higher input impedance and still maintain the low output impedance?

Use a buffer in front of the attenuator.

Alternatively using a higher output impedance will bring it up to a much more managable level. A value of 800R to 4K output impedance resluts in generally acceptable values, and sounds great. The values you used are quite low.

[Russ White]

Originally posted by Nordic
Allthough I'd probably wait for the integrated Rev C (Rev D?).

When I say integrated REV C, what I mean is power amp and attenuator in one box. no preamp. Same REV C circuit.

I think we could arrange for you to get a PIC when we get things into production. For now, if you are wanting to try this soon, you might see if you can etch the programmer, (actually you could build it on protoboard) and order some for yourself.

Cheers!
Russ

[Nordic]

I looked at those programmer links, where does the chip go to be programmed, or is the chips on the boards the ones being programmed?... I'm thinking it was the processor chip on there...

[Russ White]

Originally posted by Nordic
I looked at those programmer links, where does the chip go to be programmed, or is the chips on the boards the ones being programmed?... I'm thinking it was the processor chip on there...

The spot on the PCB for the socket is where you put the PIC to be programmed. There is no other chip on the PCB, just passive components. Here is how mine looks:

[Nordic]

I see, thanks, feel more confident now... have one or 2 projects that are 99% done, which I will complete first, but pics seems to be a usefull skill... even saw a flyer a few weeks ago by a private guy that teaches people how to program pics.... been years since I worked with any programming language... think Visual Basic 4 was the last programing package I worked on.

Think I'll go scratch my old hardware pile for a video card or something to rip a D-sub connector from in the mean time.

[Duck-Twacy]

Originally posted by Russ White

Here is the plan. I will be posting PDFs for a single side version(actually a few versions) and Brian and I will likely produce kits for a more optimized double side version(s).

Cheers!
Russ

Good plan! I'll take the kit

Hope you will also offer an input selection (partly for balanced, partly for single ended)

Great work.

[Russ White]

I think I am on the verge of ordering PCBs so I want to run the design past you folks so that I can be sure you are getting what you need (and most of what you want)

First the nitty gritty:

1) The PCB is designed for through hole G6K relays, and either through hole or SMD(1206) resistors.

2) Controller is designed to accept input from a single 5-20K potentiometer which is read by the uC's ADC to set the volume. It of course, is not in the signal path.

3) I left .1" ceneterd pads on the controller board so that those who wish can easily create their own firmware(or use mine) to drive auxiliary features. The possibilities here include: LCD display, IR reciever, rotary encoder, mute, startup delays (soft start) for relays, etc...

4) The PCBs are compact at only 1.5" x 4" and stackable (use 5/8" or so M/F standoffs).

We will make alternate firmware hex files available to download on our site.

Here is the attenuator PCB itself:

[Russ White]

Here is the controller:

[promitheus]

Looks great!
Will the control board be able to contorl input selection or will that be another project?

[Russ White]

Originally posted by promitheus
Looks great!
Will the control board be able to contorl input selection or will that be another project?

The source selector we are creating (Darwin) is designed to integrate with this or the kooka, or even this + XBOSOZ. and it has its own controller.