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garbage 26th July 2004 02:42 AM

PMA's opamp/buffer pcb
Hi All

Wanted to share my experience in using an opamp/buffer on my passive preamp.

After reading about Corey Greenberg's article in the November 1991 issue of Stereophile here , and Burr-Brown's application note on combining an opamp with a buffer (pdf here ), and also carlosfm's thread.

I decided to mod my line drive passive preamp with an opamp and buffer. Also because I wanted to include a headphone jack to drive my grado sr80 as I forsee I'd be using them more often in future.

Eventually I got Pavel's PCB. And built a regulated supply using Pedja's schematics with some changes, such as using 12V secondaries transformers and using a bourns variable resistor (replacing 2.2k resistor) to tune the output voltage and larger caps after the rectifier, and i used 10uF cerafines for C1 and C2.

Some pictures are available herehere.

I used a R4 of 2400R and a R3 of 1000R giving a Gain of 3.4, according to the opa627 datasheet.

I forgot to buy resistors for R10(50R) and am running them jumpered for the moment.

For C6 and C7, I had a BG 4.7uF NX cap that I got recently(ordered that for DAC, but I already scrapped my DAC, so using them here). For C8 and C9, I had 22uF Cerafines.

Initial impressions are that the soundstage is much wider and the bass is much lower. I used to experience resonance with lower bass in my room, but with the preamp's bass,the sound is just great.

Future mods are to build another power supply and also to add a neutrik headphone jack and to box the power supplies up.

garbage 2nd August 2004 02:09 PM

just added a pair of interconnects that had 50R resistors soldered at one end to parallel signal to ground as suggested by PMA's instructions. the end with the 50R is to be connected to the amps. the outputs of the buffer has 50R in series.

it made more difference to the sound than i imagined. it is less edgy and harsh now. vocals sound much better. bass is still as tight, soundstage is still as wide.

i noticed that at the same volume control level, the sound is generally softer when using this pair of interconnects.

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