BA440 - Diode Question

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justinheronmusic

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Jan 13, 2015
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Newb question - Recently I swapped out D3, which was a 1N4148 Diode in my BA440 op amp and replaced it with the original AA144 diode. This is labeled D3 on the original BA440 schematic, and is in association with the BC461 transistor. Do I need to re bias the Op Amp after making this alteration?
 
justinheronmusic said:
Newb question - Recently I swapped out D3, which was a 1N4148 Diode in my BA440 op amp and replaced it with the original AA144 diode. This is labeled D3 on the original BA440 schematic, and is in association with the BC461 transistor. Do I need to re bias the Op Amp after making this alteration?
should have no effect on bias.
am a fan of the family of amplifiers:  BA338, BA340, BA438, BA440.
only the BA440 employs compensation capacitor  C6.
perhaps someone will chime in as to the purpose of D3.
 
ruffrecords said:
From the Neve docs:
"The diode D3 protects TR4 from overloading by removing its base drive just
before saturation."
wouldn't saturation imply the onset of clipping ? 
guaranteeing saturation is a characteristic of bootstrapping.
quoting from RCA's "Audio Amplifier Manual" APA-551, page 30:
"......provide the drive necessary to pull the upper Darlington pair of transistors into saturation."
of course, the BA440 is not an example of bootstrapping, just discussing driver transistor saturation.
 
Disclaimer: The op amps I have here are clones built for 500 series, found in the Chameleon Labs 581.

There was a 1N4148 in the BA338 as well, and changing to the AA144 made everything sound much tighter and more refined, but I was still left with booming bass. I also changed out the rest for BAX13 diodes - I may be crazy but to my ears this really evened out the sound. The sound before all the diodes swap was very accentuated in the low end, which is cool but I had to scoop out so much low end to about 250hz during mixing from anything that wasn't bass.  Though I have never heard a real 1081, I can't imagine that the low end is that pronounced. I have heard vintage 1073's, and the bass bump is there but it's not overbearing. Now it's still present but more subtle, closer to how I remember the bass of the 1073's I've used, but a little tighter and more defined.

I will buy another one in the future, and perform some more clinical audio examples, my hunch is that my fully modded unit sounds vastly different from stock. I replaced caps, transformers, and the diodes were the last thing; I finally feel like this thing sounds amazing.

 

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