Wednesday, October 9, 2013
Yet another build of an OPAMP based piezo pickup preamp. The OPAMP again is an OPA2134 (socketed). The single cct. brd. holds both the differential PS, generated by a resistive network and capacitors and the preamp section itself. Control of treble is via a small pot and can be used to "tone-down" surface noise found on most 78 records. There is a battery test point at the front of the preamp to allow easy battery checks without opening the case.
A mustard cap is used on the input and a poly on the output. Internal wiring is solid core copper wire. The case is cast Al and again I have used the hammertone finish which I think is quite attractive. This completes my 78 setup with a new stylus in my Ronnette piezo cartridge. With some reasonable 78s I have managed to get some enjoyment out of the shellacks.
Tuesday, February 19, 2013
I built a single jFET preamp for my mate's double bass which is fitted with a piezo pickup under the bridge. I couldn't make it work properly and therefore this OPAMP based attempt. Mustard caps are all the rage with pedal makers and I just happened to have about 6 from my early electronic days mostly 0.1uf. Mustard cap on the input and polypropylene on the output.
The paint job is a poor attempt at hammertone. Right colour no hammer?? The OPAMP is my fav. OPA2134 and is running at 18V. Input impedance is a ridiculous 10M. There appears to be good gain and the pot seems to enhance treble as it is turned up. I work both sides of the proto-board and even cut a track for compactness.
Wednesday, January 2, 2013
Based on the high performance OPA2134 OPamp chip this headphone amp\driver is good sounding and portable. The schematic is based on the CMoy design with some changes in passive parts values. I elected to run it on two 9V batteries for better battery life and greater dynamics.
There is only 4" total of hookup wire in the signal path and all component-2-component connections on the proto cct. brd. Low ESR 200uf caps and a resistive network provide the differential voltages required to operate the chip without an output cap. The brd. is silver soldered with Mil.spec machined chip socket and Russian PIO caps on the input. Power only comes on when both phone jacks have plugs in them and the power switch (at the back) is on.The enclosure is impact resistant and lined with heavy Al foil for shielding and enclosure resonance control. The sound is clean and bright with good volume and dynamics. The amp is dead quiet with no signal.