Dahlquist DQ-10 Passive Crossover

Sorry to bother you....

Hi Clyde -

I knew someone who did that exact thing. Jim Austin, who owned "Pro-Sonics" which made what was at the time one of the best sounding electronic crossovers you could buy. He was highly motivated because he wanted to design an electronic crossover for the DQ-10, and dump the passives. He failed in making them sound better, although simple bi-amping did improve them. His conclusion, IIRC was that the DQ-10 was designed from in front of the speaker, not behind. A series topology was selected,for the bass and lower midrange, then modified until it was right.
The the second series leg was designed and modified until it too was right. Finally both legs were combined in parallel, then tweaked some more.
Personally, I think both brilliance and luck were involved in the success of the final product.
I'd ask Jim for his notes, but he passed away long ago now.
In short though and pertinent to your question, being designed the way this was, you are not going to find a definitive explanation. I'm quite certain that if a top flight speaker designer today were given those exact parts to design a system around, you'd have something very different as a result. Would it be better? You'd have to conclude it would have to be with today's design tools.However, no designer would wish to start with those drivers, so the DQ-10 which we know and love would not even exist. That would certainly have been a shame since the darn things continue to pump out some great sounds all of these years later, flawed by today's standards as they are.
Good luck in your quest!

Yeah, thanks. I'm convinced that the concern for bass boost is why Jon separated the legs because the topology he used there would have messed up the rest of the series circuit. The resistors across the woofer, where a cap should be, bring the woofer Re below 3 ohms, and the midbass crosses over way under the published 400 hz. The cap in series with the midbass appears to be the only place he could put one in order to roll it off but he could have put it ahead of the bass resistors too. Maybe that didn't pan out. As an experiment I replaced the 80uf cap, which rolls the midbass off at approx 180hz (too low for the stock driver's capabilities IMO and the reason that replacing it improves the midbass), with a 36uf cap for a 400hz rolloff and the upper bass disappears. The lack of bass was a criticism of the prototypes, and according to J. Gordon Holt, Jon scrambled to correct it. A pretty imaginative change to the layout, I have to say. A desire to have the cosmetic appearance of a Quad and a reasonably sized bass enclosure really painted the DQ-10 into a corner.