The Lab and Tools


Our speaker lab is a table in the basement. An old Harmon-Kardon receiver serves as the signal switch, speaker A/B switch, preamp and amp. When I built up the prototype active crossover I discovered that it had no preamp output, so I drilled a couple of holes in the rear, mounted two RCA jacks, and tapped into the amplifier inputs with two 470 ohm resistors to protect the preamp and isolate the preamp outputs. Works swell.

We use an Analogic digital waveform generator which makes OK sine waves, but  is not low distortion. Good for frequency response testing,  as any function generator will do.  I also have an electronics lab with a scope, power supplies and several meters. Plus lots of parts. And a wood shop for the wood parts.  Table saw, router, sanders, drill press, hand power tools. etc.

A PC running Speaker Workshop measures driver impedances and calcuates Thiele-Small parameters. We haven't yet set it up with a microphone to measure frequency response.  This free program is pretty amazing. Using just a sound card, cables made from PC audio cables, a 10 ohm resistor and a test box with a speaker-size hole, it measures speaker parameters, crossovers, and just about everything else like a champ. It also does a decent job designing ported and closed boxes. It does take a while to set up the input and output volumes, and until I made the unshielded parts of the cables real short and well soldered, the 60/120 Hz power line pickup prevented the program from working.

I use the student Pspice 9.0 for simulation, which is available for free.  There are file size limitations on this version, mostly in the number of transistors in a design. But if you use the OPAMP primitive, it is an ideal op-amp and uses no transistors. You can always add bias currents, voltages and other  primitives to make them more 'real'.  But for audio they work swell.

For schematics and PC layouts, I use ExpressPCB. Their free tools are very easy to use, libraries are decent. Their Mini-board service is $59 for three 3.8 x 2.5, 2 sided boards, three day delivery. The limitation is that if you want to build boards, you have to use their service. Their PCB tool doesn't output Gerber files. Or even print files so you can do toner transfer. There are however tricks to get clean artwork layer prints (using screen capture and a paint program) . No limits on schematics though.










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Last Updated: May 20, 2005