Please scroll down the page to view the various projects.
JFET BATTERY PHONO STAGE:
This is a Jfet phono-stage that I built for a friend in England specifically for creating CDs from LPs for regular use. The input stage is a SRPP type, followed by Lipshitz passive equalisation. The output stage consists of a resistive loaded Jfet DC coupled to an emitter follower to drive the computer soundcard. The Jfets were a nuisance to work with as they needed to be hand-matched for current and gain. The PCB is homemade using self-adhesive resist tracks on double sided board.
Click here to see schematic:
GUITAR PRACTICE AMPLIFIER:
EL84 PP guitar practice amp with Baxandall active tone controls. It also features switchable triode / pentode modes, variable saturation on one gain stage and an output level control. This enables the user to get the tone at apartment friendly levels.
Click here to see schematic:
is a late 40s Bush A.C.91. table radio having long, medium and short wavebands.
I had this as a kid and while back in England last Christmas, rediscovered it in
my dad's shed. I brought it back to the US and having replaced all the leaky
paper caps (they all were) it works. I cut the cases away from the original caps
and the replacements were a nice slide-fit inside so the under chassis
appearance is not ruined. Amazingly, the electrolytics were still usable. The
speaker (Rola permanent magnet) was in terrible shape, with a rusty pole-piece,
also a completely detached diaphragm. I had to carefully peel the cone
surround away from the frame then clean up the pole piece, also re-glue the
diaphragm. I centered the pole piece by wrapping it in thin card then tightening
the screws. I then pulled the card out, it was tight! I re-centered the
diaphragm (using the adjustable spider) by gently energising the speaker at
200Hz and fiddling until the tone was clear. It took a couple of tries before it
sounded right with the volume turned up though. For those "in the
know" on these old radios it does still have the back, in pretty decent
shape. Yes, the case needs finishing, I know.....
Tube line up is classic Mullard 5 valve: CCH35, triode-hexode frequency changer; EF39 pentode, IF; EBC33, ("Binode") double-diode-triode, detector, AGC rectifier and AF; CL33, output pentode and CY31, half wave rectifier. Special thanks are due to Russ Sadd for bringing me a spare set of valves when he visited.
Note the beautiful Mallard metalised valves!
Two-Way Speaker Project:
I was given a pair of Realistic speakers with rotted out drivers. However, the carcasses looked to be the basis for a project. I cut the original baffle out (open side in picture). This is now replaced by an MDF side panel. The outer baffle is solid red-oak, bolted to what was the side using a constrained damping layer. The other sides have been treated for damping using vinyl floor tiles (glued and stapled in place) and coated with Soundcoat which is a panel damping compound mixed with regular latex paint. The panel directly behind the woofer is covered with a Deflex sorbothane anti-acoustic reflection panel.
The woofer is a 8ohm carbon Audax HM210C0 8" unit and the tweeter is a 6ohm Vifa D27TG-35 1" soft dome type. The 1800Hz crossover is a 12dB/octave Bessel type. I chose a Bessel characteristic in a nominal attempt to obtain linear phase response. The tweeter is fed from a pad to 1/ obtain a nominal 8ohm impedance and 2/ compensate for HF baffle diffraction. The compensation was tuned by ear. I also find that the speakers sound best with the tweeters connected in-phase (this is not always the case with 2nd order designs). The woofer parallel capacitor is a Solen unit while the tweeter series capacitor is a combination of NOS paper in oil units. The 12ga inductors are of the copper foil, air cored type. Resistors are Caddock. The schematic shows the crossover in its final form after tweaking as the woofer broke in*.
The stand is basically a section of 6" PVC cast in concrete into an oak plinth. The pipe is located over a 1" thick circle of wood, glued and screwed to the base of the plinth. Three 4" coach bolts extend into the space above the circle and concrete was cast into the pipe over them thus forming an extremely solid connection to the base concrete. The rest of the space in the stand is filled with sand. A very tight fitting 1 1/2" thick circular plug closes the top of the pipe. The plug is secured held in the pipe by radial screws and is glued and screwed to an oak entablature to form a base for the speaker. The entablature is covered with a self-adhesive vinyl tile and in bolted using 1/4" tee nuts onto the speaker. The wooden knobs fitted to the top end of the floor spikes which extend through the plinth. This makes for easy adjustment! For finish, I intend to bring the concrete surface level with a rebate cut in the oak then cover using the linoleum pattern to match the 845 SE.
These speakers are still the basis of my system, however I have made a number of changes. *The Audax carbon drivers seemed to develop some sibilance and harshness in the upper treble. I concluded that as the cones break in, they also develop break-up modes in the upper treble. So, after a couple of crossover alignment experiments, I ended up reducing the crossover frequency to around 1.8kHz (from an original value of around 3.2kHz). Additional bracing was incorporated into the cabinets, also a 2nd deflex pad attached to the (underside of) the top of the cabinets. I am still very pleased with these speakers. Though cutting off the carbon drivers at 1.8kHz seem to be a bit of a waste of a 5k driver, I find that the benefit of the carbon cones really shows through in terms of speed. The sound has an immediacy reminiscent of mid range horns.
Phono Stage: Implementation of a jfet/6GM8 cascode front-end, two