Originally Posted by
edgewound
You can use a 175, which this system did, as long as it's not an early narrow gap model. 2410/2421/2460/2470 will work as well with a new diaphragm. I didn't do anything to this pair of 175s in order to preserve the wax seals per the customer and they swept great. The LE85/2420/2421 is a stronger motor which will yield increased upper end reach.
Here's the basic circuit. I ended up tuning it by ear by adding an additional 10uf cap in series with C2 which yields 8uf...and added a mechanical 50 watt L-Pad. Seems it brought the hi pass up to ~1800 or so. I can't really explain what happened electrically in detail, it just sounded right.