Active Baxandall EQ resistor value swap & tolerance question

snk · Post by **snk** » 08 May 2018, 08:15

Hello
I want to build an active Baxandall eq, but im' missing the 3.9K resistors from the BoM (i have every other component).

However, I do have both 3.3K and 4.7K resistors : i am considering using 3.3K resistors instead.

My question is the following :
- I'm assuming it is safe for the circuit to put a capacitor with a "20% average tolerance" value (i mean 3.3k instead of 3.9k), so that the circuit will still work without a big change in sound and without messing the circuit or the other components : could anyone confirm ?

- I see from the layout that the 3.9k (R8 & R9) are both linked to the treble knob. Therefore, I think that changing the capcaitor values might change either the frequency where the boost/cut occurs, or the boost/cut amount : am i right ? Could anyone tell me what exactly changing these values will change in the sound, and if i should better go for a 3.3k (value below the suggested one) or a 4.7k (value above the suggested one) ?

Ice-9 · Post by **Ice-9** » 08 May 2018, 11:36

First of all, it will work.

you could place the 4k7 and 3k3 resistors in parallel which will give you 4k, which is much closer to the 3k9 that the circuit is using.

You could download the Duncan toneStack calculator and input the values to see what difference to it will make. It is a great little utility to use for testing different tone control circuits.
http://www.duncanamps.com/tsc/download.htm

The Baxandall tone circuit is called 'James' in the Duncan calculator, here is a link to some more in depth info on how the Baxandall/james tone controls work.
https://www.ampbooks.com/mobile/amp-tec ... -analysis/

snk · Post by **snk** » 08 May 2018, 12:50

Thank you !

you could place the 4k7 and 3k3 resistors in parallel which will give you 4k, which is much closer to the 3k9 that the circuit is using.

Oh, indeed, i didn't think about it. Good advice

You could download the Duncan toneStack calculator and input the values to see what difference to it will make. It is a great little utility to use for testing different tone control circuits.

Awesome ! Thank you !
So, from what i understand, the 3.9K resistors values will affect the frequency, right ?

In the meantime, i just noticed that my 1µF capacitors are polarized (despite not being electrolytics), so i am thorn between being lazy (ordering the right resistor value, and some non polarized 1µF electrolytics), or impatient (trying to find the correct polarity, and soldering the polarized capacitors on the veroboard).

Manfred · Post by **Manfred** » 08 May 2018, 21:13

So, from what i understand, the 3.9K resistors values will affect the frequency, right ?

Yes it does, open the PDF-file for more details:
download/file.php?id=28956

snk · Post by **snk** » 10 Jul 2018, 15:06

Hello

I finished the build (with the correct resistor values), and it works fine.

Imho, the treble boost is much stronger than the bass boost, but i'm fine with that.

One little thing which doesn't seem perfect : i hear a "pop" when the treble knob is turned to the minimum.
I am using brand new, sealed Alpha pots, so i am wondering if it may be a faulty knob or something from the circuit ? I have read on the effect page that some users experienced similar behaviour when engaging the circuit. It is not my case, the "pop" comes only when the treble knob is turned to the minimum. Should i try what the circuit creator suggested ?

"Try adding a 1M resistor from the input to ground. You can also try adding a 100K resistor from the output to ground. That should help eliminate any popping sound when using this as a stand-alone box. "

If so, am i right assuming that i should wire a 1M resistor from the input jack socket to the 9V power plug ground ?

Ice-9 · Post by **Ice-9** » 10 Jul 2018, 21:18

Note, my answer about the 3k3 and 4k7 resistors in parallel giving 4k is INCORRECT. (it would be 1k94) Thanks Sinner for pointing out my silly mistake