BYOC 5 Knob Compressor - BA6110 Input Bias

Started by wookie, March 24, 2014, 03:33:46 PM

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wookie

So I recently received a BYOC 5 Knob Compressor kit as a gift from my wife (yeah!).  I've been looking through the documentation on this kit, as well as the Ross compressor it's based on.  One of the changes from the stock Ross made for this kit is using a BA6110 as the OTA rather than the CA3080.  Reading the datasheet for the BA6110, I see that pin #3 is used for a bias current for the input diodes.  The BYOC kit leaves this unconnected, and the BA6110 has information on using the part in that manner, but per the datasheet "By connecting pin 3 to the Vcc by way of a resistor, the input is biased at the diode and distortion is reduced".  Reducing input distortion in an OTA sounds like an operational advantage.  Most of the curves on the datasheet note an input bias current of 200 uA, so maybe I'll give that a shot before I box it up and see how I like it. 

Two concerns with doing this that I was wondering if someone who's interested could comment on:

1)  Will giving 200 uA of bias current change the control current characteristic?  The datasheet plots different performance characteristics for biased and unbiased operation, so I'm wondering if I'm missing the fact that using that input bias will do more than just reduce input distortion.  I'm guessing if it even works, I'd need to recalculate the scaling between the envelope detector and the control input.

2)  Will reducing this input distortion remove some distinctive character from the circuit?  I've never used a Ross, so that's it's an unknown for me.  I'll still play by the "use it if it sounds good" paradigm in any situation, but I'm curious if anyone had opinion on how much of the Ross sound might be coming from the OTA input overloading?  Although I guess even if it is characteristic to the sound, there's a good chance overdriving a CA3080 sounds different from overdriving a BA6110...so it maybe changing something that sounds different from the original anyway.

Here's the links to anyone curious about the topic:

BYOC 5 knob compressor build instructions, schematic is on page 24
http://byocelectronics.com/compressorinstructions.pdf

BA6110 Datasheet, comment on the bias current taken form page 194 (actual page 4) 1st column about 80% of the way down.
http://urekarm.tripod.com/synth/BA6110.pdf

I'll post back with the impact of adding 200 uA of bias current to this thread once I build the pedal.  I've got two kids (1 & 5) who take precedent of pedal building though, so it may be a bit of time.  Thanks in advance to any input provided!