Impedance Question on Subwoofer in Setup

[Twenty Watt]

I'm a little vexed over what impedance I should be targetting with my setup. Here's some background.

I'm running a Dynaco Stereo 70 which has 4, 8, and 16 ohm taps on each side. I'm using a Dynaco PAS 3 as a preamp. There is no subwoofer output or center channel output available to drive a subwoofer so I must utilize the "speaker wire input and output" patch bay on the back of the sub to get a signal to my sub and then out to my speakers.

I'm running a pair of Heresy II's which are 8 ohms each. I've got a JBL subwoofer driving a driver of unknown impedance. It's not anywhere on the literature.

I used my meter to check the impedance of the subwoofer + Heresy "chain" and it registered 198 ohms!. My meter is fine...I checked the single Heresy and it registered about 5.8 ohms which is where it should be for a published 8 ohm speaker.

Can anybody shed some light on this? Is this an acceptable way to drive a sub off of a Stereo 70 using the 4 ohm tap? Should I not do this? Anyhelp would be appreciated.

[Guest " "]

the 198 ohm impedance you measured could be as a result of ...

A passive subwoofer with an passive crossover intended to recieve output from your amp and them split the signal as lows to your subwoofer and highs to your normal speaker. In a 6db per octave passive crossover, The impedance value you measured included a high pass capacitor that runs from the subwoofer input to the output terminals that run to your Heresy chain. So your test circut actually consisted of a multi meter positive lead, capacitor, speaker, multi meter negative lead.

[Twenty Watt]

So what tap should I use off my Dynaco ST-70...the 4, 8, or 16?

[Guest " "]

Rule of thumb is the nearest tap setting above your speaker impedance. so for Heresy's with a sub woofer you could actually use the 8 or the 16 ohm tap.

You will get+3db more output on the 16 ohm tap than you would on the 8 ohm tap.

Your speaker impedance's change across the drivers. in the sub woofer range it's probally 3.9 ohms, the Heresy woofer 6 ohms, the mid driver 10 ohms, and the tweeter 6 ohms.