Working on personal project, I found that by wiring up a matched pair of 6X4 rectifier tubes in a push/pull configuration I was able to increase the B+ voltage, while also doubling the mA output and creating a more regulated voltage. This revelation led me to bust open my prototype passive EQ and work out a more perfected version of this twin rectifier supply. I decided to change some power filter caps for different values and tweak some resistor values to get the most out of the twin rectifiers. Then I added on a pair of NE-2 neon regulators to get a perfectly rock solid 343v B+. The changes to the power supply resulted in an increased voltage of about 5v and more B+ amperage than the EQ would ever need. The changes to the B+ supply made me want to re-bias the EQ's amplifier tube... a change I will never regret! I mounted a pot on the front panel of the EQ so I could adjust the bias by ear, while simultaneously recording before/after changes into Pro Tools. I found the magic sweet spot, which was actually sweeter than ever before!!!
Previously with the EQ bypassed and audio running through the amp things sounded fairly transparent, with some coloration - but essentially no significant changes to the sound quality. The noise floor was measured at -76dB. After the power supply and bias changes the EQ's amplifier (eq bypassed) sounded AMAZING! It blew my mind, but it actually made the original audio sound more "real." I could hear subtle details in the source material that I didn't even realize was there. After listening to the original source material again I struggled to hear those subtle intricacies in the sound of the music... in fact it seemed like they weren't there. This amplifier was able to extract something from the music that was clearly there, but hidden from sight. That's the magic, folks! Well, as excited as I was about the sound, I decided to test the S/N ratio. The new noise floor was measured to be -85dB!!! Woah! Where did 9dB of noise go? Amazing.
To end this long rant of discovery, I will be building a new 1401 Stereo Mic Amp for LBA Studios in a couple weeks and all future builds will get the same. It will incorporate twin rectifiers, the new bias settings, and the front panel indicator lamp will be replaced by a more functional regulator tube glowing behind a clear jewel. What does this mean for the 1401 Preamp? More headroom for one, but also a richer, more detailed tone. It will also get the back panel branding, "Rev A."