drivers with 604

[selmerdave]

I'm considering adding a pair of woofers to go (in parallel) with my 604E's for the principle reason of lowering the impedance the amp is seeing, it would seem to make more sense than to add an impedance matching device. I'm looking for something that I could run full range (the woofers on the 604's are running full range) that wouldn't muddy up the sound. 515's? I thought of 416's as they slightly improved low bass might balance the 604's but maybe the mid's would muddy up what I'm getting. Suggestions?

Dave

[joyspring]

The big question is why do you want to lower the impedance?</p>

If it's an impedance peak that you're attempting to flatten, a zobel LCR filter is very effective. If it's more output you're seeking, you need a larger amplifier.</p>

JBL and UREI have done what you propose - that is, adding an additional LF driver - to increase power handling capacity and LF output in their Model 4435 and 813/815 studio monitors, respectively.</p>

HOWEVER..., they both low-pass the second/additional LF driver(s), usually about 200Hz.</p>

Why? To minimize anomalies in radiation pattern because of the physical distance between the two drivers. Lobing and cancellation in the polar response of multidriver systems become exceptionally difficult to manage as wavelengths get shorter.</p>

To run the second driver full-range, even if it has the exactly the same frequency response and directivity as the 604E LF driver, will most likely have a detrimental effect on audio quality and will definitely result in a haphazard polar response that will degrade the excellent stereophonic imaging and localization that point-source coaxial 604 systems exhibit.</p>

Lastly, because the likelihood of sourcing a second driver with the same T-S parameters as the 604 LF, designing a proper enclosure for the drivers becomes problematic, unless of course you partition the drivers. Then matching the LF rolloff becomes yet another issue...</p>

BobR</p>

[GM]

OK, 16ohm 604E's are based on the 16ohm 515B, so at a glance the woofer choice seems to be a no-brainer. A big/low DCR inductor to roll it off to get the appropriate baffle step compensation will increase sens 6dB and keep comb filtering from becoming an issue, especially if done as a bipole. Of course the tradeoff is a cab twice as big......

If you're happy with the current efficiency, another option is to mount the LF in the 604's cab and put the 604 on an open baffle to keep the woodworking/space issues to a minimum, though you'll still need the inductor on the LF. As long as the c-t-c distance between the two is <1WL of the XO point [WL/pi preferred], off axis comb filtering isn't normally an issue in a typical HIFI room, especially as low as we're talking here.

GM

[selmerdave]

Thanks for the replys GM. The answer to why is that my amp (which I'm very fond of) is quite unusual in that it has only a single winding from the OPT, and the output impedance is I believe around 6 ohms. It works fine all be it with half the power (around 1.2 w/side for me) at the 16 ohm load but I believe overall sound quality would be likely improved with a better impedance match. It is stable with the terminals shorted and many people use 2 ohm speakers with it, but somewhere around 6 ohms sounds like it is ideal but I would probably have to settle for 8 ohms. Many people using this amp (decware) use impedance matching devices (zero autoformers) but at $400 a pair I think it would be better to spend the money on drivers and possibly gain a bit in the efficiency department (and perhaps capitalize on the high effeciency of the 604 hf), although I realize there are many issues as you pointed out. I was thinking of a large br cabinet like the 820 (?) that used 2 15" drivers although obviously I would have no need for a spot for a horn, but maybe your idea of open baffle combination would be better and bring less complications. Just in the mulling it over stage. Thanks for the input.

Dave