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JensRasmussen 18th June 2004 09:12 PM

MOX builder’s thread
Please post pictures and other relevant material here


Variac 20th June 2004 12:31 AM

Here's more than you need about the MOX active crossover:

tiroth 21st June 2004 07:47 PM

Important opamp note
PLEASE do not forgot that the 3-pin transistor silkscreen on the opamp boards is correct for BC550/560, and REVERSE for MPSA42/92.

So depending on which you are using:

MPSA42/92: REVERSE of silkscreen
BC550/560: SAME as silkscreen

If you need more info, the relevant posts start here:

tiroth 24th June 2004 04:42 PM

The draft opamp assembly guide is complete!

Please offer criticism/suggestions.

Here are some example pics. There are large (warning: ~60k) versions of all pictures on my site.

JensRasmussen 25th June 2004 05:04 PM

MOX builder’s thread
MOX builders thread is about the construction of an adjustable cross over module, partially based on moamps original design – hence the name has been reused.

Thanks to moamps for his efforts in both the descrete opamp and the original filter design found here :

Also many thanks to Tyler who has made it all happen, for managing the group order and for his excellent page here:

The board is a module that can form the following signalpath:

My prototype board looks like this:

More info can be found here on my homepage

Active filter two - mox

Please post pictures and other relevant material here


BDP 2nd July 2004 04:24 AM


I have noticed two different resistor values listed for collector resistor R3 on the discrete opamp. One is 392 ohms and the other is 360 ohms. Which value would be more optimum? You had posted your findings on a very low DC output offset, with which resistor value? Was this one of the follower buffers, HP, LP filter or was it with gain?

Putting together my order of parts. It always adds up to more than you think. A couple hundred dollars just to populate six boards. Still deciding on a power supply. What is everyone else going to use?


Bas Horneman 2nd July 2004 11:34 AM


Still deciding on a power supply. What is everyone else going to use?
2*12V(33aH) batteries. 29$ each. (surplus) is what I am planning. Could be bettered I suppose..but not for the money and effort.


tiroth 2nd July 2004 01:01 PM

Batteries are great, but 33AH? Pretty stiff supply! ;)

BDP, I used 392. The offset is going to vary slightly depending on the supply voltage. You can trim it to zero by using a pot or a selected resistor in R4. I did not need to trim it for my use--the offset was so small that it did not affect the operating point in the MOX circuit.

I measured the offset both in-circuit with one sample and on the bench (Gain=1 buffer). The results were similar.

JensRasmussen 2nd July 2004 07:51 PM

Manual here Manual

Parts list


marsupialx 2nd July 2004 09:08 PM

Probably a stupid question...

Can two active xovers be hooked in parallel to one set of output jacks, ie one rolling off at say 200 hz and one going out to 2000?
Will it do what I want, reinforcing the lower bass, or will it screw something up?
A simple equalizer circuit built on the experiment hole portion of the board is probably a better idea. Thanks. I hope I can comprehend the answer. We'll see...

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