Kenwood KA-3500 Volume Control

I really like the vintage Kenwood KA-3500 amplifier I use as my home stereo.  It sounds great and the brushed aluminum front has a sweet retro style.  However, whenever watching movies or listening to music, I'd always find myself loath to get up to adjust the volume.  The solution?  Replace the volume circuit of the original amplifier with a digital volume control circuit that could be controlled via a remote control or a rotary encoder.

The Arduino Micro is the computing brains of the operation.  It handles decoding/interrupts from a rotary encoder and an IR sensor.  It also interfaces with the audiophile grade Burr-Brown OP2311 digital volume control driver and the LED volume display board.  Writing the embedded C and worrying about Gray Codes, interrupts, code efficiency, and low-level bit handling was an interesting diversion from what I normally think about at work.

I also designed my own PCBs using CadSoft Eagle.  For someone with little experience in board design (this guy) this was a tedious but rewarding task.  I paid special attention to the board layout, followed data sheet suggestions, and made sure to keep the digital signals away from the line-level audio signals.  The effort paid off as the circuit is dead quiet.  Next up?  Wideband RF circuit designs (yeah right)!

Finished Product

Front of Amplifier With New LED Volume Indicator

Inside of Amplifier
Volume control circuit is mounted to the heatsink on the bottom left

Volume Control Circuit (1)

Volume Control Circuit (2)

Code

GitHub

Artwork

Volume Control Board

LED Volume Board

Top Board Masks

Board CAD (Eagle) Files

• Volume Control
• LED Volume Display

Bill Of Materials (BOM)

• Volume Control
• LED Volume Display

Links to References

• Kenwood manual
• IR decoder
• PGA2311 data sheet
• Bourns rotary encoder

The Awesome Radio Spectrum

I am always awestruck at the amount of invisible EM waves whipping around us at the speed of light.  Gigabits of information are bombarding us every second via these waves.  A lot of it is critical information like the ILS that guides planes when landing or the TV station that broadcasts the Bears game.  Some of it comes from the ground - FM radio, TV, and cellular.  Some of it comes from 13,000 miles away in space from things like GPS.

While flying to Chicago I decided to check out what the radio spectrum looks like 2,000ft above Chicagoland.  Using my trusty RTL-SDR and a rather lackluster antenna I was able to capture some spectrum plots amidst two skeptical row-mates who were baffled by the weirdo next to them.

It's always fascinating to be able to visualize these invisible sources of information and entertainment.  Below are three captures at different frequency bands.

A Lot of HDTV Stations

More HDTV Stations and the 700MHz Cell Band - 4G LTE anyone?

850MHz Cellular

For my own future reference and for anyone who cares to read this, below is a link to the US frequency allocations.  It really is crowded!

US Radio Frequency Allocations

Make Your Own PCB Boards

A few years ago I embarked on a project to add remote control functionality to a vintage amplifier of mine (a Kenwood KA-3500).  Sarah and I had gotten tired of having to get up during a movie to turn the volume up or down but my appreciation for vintage audio gear wouldn't let me replace the amplifier.

While working on this project I discovered an easy to use and inexpensive place to get PCB boards made.  It's known as OSH Park and is run by a fellow who lives out on the US west coast (Portland I believe).  He collects individuals' orders (CAD artwork), groups them onto larger panels, and then ships them out to a fab house.  The panels ship back to him, he breaks them down into individual boards, and then ships them back to you.  The boards look fantastic!

I've attached a picture of one of the boards I made using OSH Park's service.

Volume Control Board (2.5" x 2")

OSH Park
https://oshpark.com/