Fleximix bug fixes

Twenty five years have passed since Trident Recording Studios made the decision to build their own Multitrack Recording Console.

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Fleximix bug fixes

Postby Jim Williams » Sat May 02, 2009 7:04 pm

Thought I'd post some of the common design bugs on the Fleximix input modules and their fixes since there are so many modules out there.

There are a couple of resistors that should be removed. R24 is a gain shunt resistor no longer needed as IC3 is configured as a voltage follower. R40 is a 1k resistor that needs to be removed. It places a 1k load on the output of IC6, the insert return buffer opamp which will cause increased THD if using an original MC1741S or TL071 opamp. Place a 47 pf cap across R41, a 18k resistor. That stops the high frequency oscillations these modules have when the gain is high and hf EQ is boosted.

If you're inclined, adding some local .1 uf mono ceramic caps from pin's 4 and 7 to ground on some of the opamps helps. Add a 100 pf cap across R43 to limit the bandwidth of the fader opamp. Change the 33 ohm power input fusing resistors to 1/2 watt 10 ohms. That increases operating current. Remove the 16 volt Zener diodes as they create noise. For much lower noise and better THD, try National LME49710NA opamps.
Jim Williams
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Re: Fleximix bug fixes

Postby jproaudio » Tue May 21, 2013 1:48 pm

Jim,
Thanks for the great information! For the 33 ohm power input fusing resistors, are you referring to R19 and R20?

Thank you,
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Re: Fleximix bug fixes

Postby Jim Williams » Thu May 23, 2013 11:03 am

Those are easy to spot, they are mounted up off the pcb. I follow them with a pair of 1000 uf/25V Panasonic FR caps.
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