This informal web page contains ideas and potentially upcoming web site additions. The schematics on this page may contain errors. Last updated: July 27, 2010
Mouse over the images on the left to view a full size version.
Rubber feet for chassis are expensive. Felt self-adhesive pads sold at craft shops provide a reasonable alternative. I bought the sheet of felt pads shown in the photo for ~ $2.00 last year. They are high enough to allow clearance of #6 bolts. I don't bother with #4 bolts — they are too wimpy for me. While felt feet lack the absolute traction of rubber feet, the cost savings is excellent. Click for another photo .
The Wheatstone bridge is essentially a pair of voltage dividers. In classic form, each leg is a resistance with a detector connected across ports A and B. If ports A and B have equal voltages and R1 = R2, then R3 and R4 must also be equal; the bridge lies balanced or in a null state. If you remove R4 and measure an unknown resistance, the bridge will return to balance after adjusting pot R2 to equal the unknown resistance. Bridges can be arranged to measure unknown C, L and other values by calibrating the 10 turn pot and using equations.
Another AF return loss bridge I designed this Spring. The bridge is lightly coupled to high impedance buffers. This unfortunately amplifies both the bridge and common-mode signals, however, performance is good. The differential amp and low pass filter provide substantial gain because any AF amp under test must be driven with a low amplitude signal (< 600 mV open-circuit). A 49.9 ohm R gave a return loss of 56.73 dB! It's great to be rewarded after so much toil and parts wastage. I almost gave up working on this circuit - design is hard work.
Breadboard of the above audio frequency R-L-B. Click for another. Built using Classic Ugly Construction, this circuit was the most stable, accurate and sensitive of the 5 designs tested this Winter-Spring. 1% metal-film resistors were used on the bridge and instrumentation + low pass filter amps. Matching R1-R4 as shown is critical, so 1% Rs are required. I will describe these experiments, some new audio stages and additional info concerning instrumentation amps on a new receiver audio page this Fall.
Work to improve the website photography continues. I am fortunate enough to receive email advice from professional photographers who want to help improve the site by giving me tips and constructive criticism. Lighting remains my greatest concern, although some improvement might be noticeable.
Sometimes you advance to the basics.....Understanding, not copying promotes lasting knowledge. This Winter-Spring, I studied BJT operating parameters such as current gain, voltage gain, power gain + input and output impedance. These experiments will be embedded in future content.
The chassis for an experimental distortion analyzer. Hardware and containers can get expensive. The 2K ten turn pot + calibrated knob were harvested from a dead piece of test equipment 10 years ago. Light weight aluminum chassis provide a cheaper alternative to die-cast when RF tight enclosures aren't required. Now the ugly boards must be built and tested - that's the hard part.
The evolving oscillator for a distortion analyzer — now finalized and chassis mounted. To center the output into a 995 Hz fixed-tune notch filter, precise fine tuning is needed - hence the 10 turn pot. The fine-tuning circuit and low-pass filter were designed using the ideas of Ken Kuhn. His web site is currently my main inspiration and info source. The sine wave is as good as it gets on a DSO. Most people can't perceive under 3% distortion on as a scope.
The Wien bridge oscillator and 4th order low-pass filter in the chassis. I spent a little money on parts to get the lowest distortion possible — 1% metal film resistors, Burr Brown op-amps and 1% capacitors for the oscillator. Click for a wider shot. This instrument uses a split power supply. The low-pass FL is a Chebychev with 3 dB of ripple. Built proudly using Classic Ugly construction. Now it's onto the high-pass and notch filters...
A 12 watt or so Ampeg Rocket guitar amp that sounded terrible. I rebuilt the entire pre-amp and installed a Fender tone stack (Bass, Treble, Fixed Mid). The owner wanted a 12AX7A for the preamp stage - I milled a copper PC board piece (see photos). It sounds great and has very low noise. Such a sweet sounding amp. Click Click. As you age and your hands become unsteady, it's less fun to work on high-voltage circuits.
Prototype design of the heart of an upcoming distortion analyzer- the high pass and notch filters. Hum and low frequency noise from the amplifier under test is attenuated (~ 57 dB at 60 Hertz) before the signal is notched. S1 is thrown and opened to obtain the notched and unnotched AC voltages. (V Notched / V Unnotched) x 100 = the percent harmonic distortion. Onto the breadboard...
From my home province — Steve, VE7SL melts solder + builds and describes cool stuff for his Radio Notebook. Steve's work is steeped in history and tradition. Thanks for sharing!
Click for a Russian language mp3 audio file. Voloyda, a builder in Ukraine, wanted an A - B comparison of Class AB versus Class B (cross-over distortion) in an audio power amp. In the audio file, I tweak a potentiometer biasing a pair of power followers to give contrast between the 2 amplifier classes. Under biased AF amps sound terrible in any language. The audio source was a cassette player. I ran it into my latest receiver AF power stage. The speaker output was recorded, digitized and stuffed into the mp3 file. 73 Володя, Тодд.
Remember to write the construction date + circuit purpose on the back of your copper boards. This board was exciting for me; featuring an excellent 1 KHz square wave oscillator that is op-amp low-pass filtered before going into a prototype Wheatstone bridge. Oscillator output noise is filtered by the RC low-pass tank circuit (19.4K + 0.1 uF) + the dead-band input region of the 4093 Schmitt trigger — performance rivals the 555. Click for the output tracing. This experiment is incomplete. The oscillator prototypes were built using 4011 NAND gates; I will have to update the writing on the board since the 4093 was ultimately chosen.
The subtitle is now Amateur Radio Electronic Design reflecting the web site purpose. Some builders ask me if I am anti-kit building? Not at all; kits have their place. This is a scratch homebrew site for people who like the adrenaline rush from creating and tweaking their own designs. Scratch homebrew is about learning, curiosity, a desire to push boundaries and passion for creativity.
My call sign in Morse code - I hope to talk to you on 40 meters
