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Studio Testing

I've finished all my tweaks and felt confident enough that the preamp is stable to be taken to my studio, and connected to my main mixer. So I bolted my prototype PCB to a plywood, brought it and my bench power supply to the studio for some real-world testing.

It passed with flying colors. It is so quiet... the mic sounds really nice... big and warm, and with lots of high frequency response and fidelity! (Remember, it can go to 40Khz clean).

I also tried it with my Triton keyboard (for line input testing)... excellent sound reproduction too.

I am happy and pleased with the results.

WE'RE GOING FORWARD!!!

Next phase:
Design the +48V regulated power supply using a 2 stage regulator for close to zero hum! Basically, a combination of IC regulator and discrete voltage regulation power supply circuits.

Oscillation... Getting rid of it!

A few weeks ago, I was complaining about my Sowter 8751 transformer oscillating at max output. This prompted me to try different kinds of output transformers. I thought the problem was with the Sowters.

So I tried some Edcor transformers and they worked fine. But it was also oscillating at max output. Quality was alright. They're cheaper too. I may offer this an option to the kit. Specify if you want Edcor or Sowter output transformer (or no transformer at all).

Tonight, I did more tweaks to the circuit... to increase gain and stability. I added a resistive load after the 1st stage, changed the feedback resistor of the 2nd stage for increase gain.

So I was playing with the Edcor transformers, and after many experiments, I got rid of oscillation at max output.

.... Hmmm....

This got me thinking... what if I try again the Sowters? So I hook up the Sowters, and experimented again.

SUCCESS!!!

The preamp is now stable with Sowter transformers even at max output. In fact, maximum voltage output increased to 14.04 Vrms (19.85Vpeak-peak) or +25.16 dBu!!!!

Gain of preamp was computed to be +47dB.

I'll have to redesign the PCB again to incorporate the tweaks from tonight.

New PCB Layout

Brought the preamp to the studio again to play and hear it. It's working great. I love it. It's clear, deep bass, crisp highs, and LOUD!!!

Incorporated my latest fixes to the revised PCB design.