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Old 18th August 2012, 07:14 PM   #1
jm23hh is offline jm23hh  Germany
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Join Date: May 2007
Default Audio Research EC-3 modification

Hello all,

I have this crossover for many years now and used it in the past for my Magnepan speakers as a mid-/high crossover. Since I don`t own these for years anymore it`s now sleeping for long (I use old Quads today and find them much better...). The unit needs the usual repairs in the power supply, but that`s quite easy to do.

My point is: I want to modifiy the crossover for a steeper slope (it`s only 6db stock) for occasional use as a subwoofer crossover and I don`t understand the schematic fully concerning the filter section. I am able to repair my tube amps and even build some many years ago, but I never got really to crossover-building, even if I did this (also many years ago) for a friend (SS-based and it worked fine).

Because of my lack of knowledge, I bought the TJC Filter Designer software but it cannot simulate the ARC circuit to the full extent and I don`t get some details and differences.

The ARC schematic can be found here:
http://www.arcdb.ws/EC3/ARC_EC3_schematic.gif

My goal is to modify (not completely rebuild/slaughter the unit) in an easy way to get let`s say a 100Hz/18db crossover.

A (somewhat near) simulation of the stock circuit in TJC for 100hz/6db/LP looks like this:

(see arc-ec3_100hz_6db_lp.jpg)

A mod for 18db would probably look like this:

(see arc-ec3_100hz_18db_lp.jpg)

My first question is: Where has the 3,2nf (C3) cap have to go? In most tube crossover schematics this connects to ground. In the ARC stock setup this is not the case. It`s parallel to what I would call a feedback resistor that sets the gain of the stage and connects to the output of the stage, i.e. C22+R38 in the ARC schematic for 100hz.
And with a steeper slope, shold all caps connect to the out in a similar manner or the other way round - remove the connction to out for the cap(s) and simply tie them to ground (in the 18db case C1, C3)?

With the HP I am also somewhat puzzled:

(see arc-ec3_100hz_6db_hp.jpg)

To get a similar value for the cap (C12 in the ARC schematic), I get two! 300k resistors in the simulation, one as a feedback from anode to grid and another tied to ground. In the ARC schem. there`s only one apprx. 300k in the feedback loop ?!?

And finally a 18db HP sim:

(see arc-ec3_100hz_18db_hp.jpg)

How should I implement this with respect to the ARC schematic? Simply disabling the original caps and leave the existing feedback loop alone, then throw the new caps in and connect the resistors like in the sim?
-one anode>grid
-one tied to cathode
-the other two tied to ground

Any help appreciated. I know this is not the best crossover in the world, but I won`t do a complete rebuild and also I won`t throw it away...

Thanks in advance,

Joerg
Attached Images
File Type: jpg arc-ec3_100hz_6db_lp.jpg (137.7 KB, 74 views)
File Type: jpg arc-ec3_100hz_18db_lp.jpg (151.5 KB, 69 views)
File Type: jpg arc-ec3_100hz_6db_hp.jpg (141.0 KB, 68 views)
File Type: jpg arc-ec3_100hz_18db_hp.jpg (147.8 KB, 65 views)
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