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"Can a suitable value always be found to give zero DC?" ...

Posted by andyr (M) on March 30, 2014 at 20:55:13

In Reply to: RE: An interesting JFET-based active XO design from TNT Audio ... posted by neolith on March 30, 2014 at 20:00:29:

I don't think that should be a problem, neo. People like Holco make resistors in all sorts of strange values and, if you create a "parallel resistors calc" spreadsheet, you can play with using several values to achieve the desired end-value.

Alternatively, if you need a value which is not quite the same what you end up with, using several available parallel resistors ... then you can make up a variable resistor combination which should be able to give you the final, fine level of adjustment you need.

Simply parallel:
* a fixed resistor which is around 10% greater than the value you need to end up with, and
* another fixed resistor plus a pot.

EG. if the value of the pot across the 22.1 ohm Source resistor in the schematic turned out to be 2,690 ohm, to get zero DC offset, you would use:
* a 3,160 ohm Holco, in parallel with
* a 10K resistor plus a 10K pot (set to 8,086 ohms).

I use this method for 7 crucial resistors in my 'Muse' phono stage - yes, you still have a pot in the mix but it has a resistor paralleling it which takes 70 or 80% of the current ... so the degradation associated with a pot is, IMO, substantially reduced (compared to just using a pot). And I use Vishay 1280G pots for this purpose, anyway.

(Again, using a "parallel resistors calc" spreadsheet is essential, here.)


Regards,

Andy

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