2. amps-preamps
  3. 153287-59357

Tube amp output taps; use both?

Dodd Audio 120 MKIIIs have both 4 and 8 ohm taps.  B&W Matrix 801s have inputs for both mid/tweeter and woofer (which I know are electrically isolated); currently bi-wiring.  Two questions: 1) Would running one set of speaker cables from 4ohm tap to woofer and one set to mid/tweeter harm the amplifier?  2) If, answer to 1 is "no", would there be any advantage to doing so with respect to lower impedance/higher damping presented to woofer?  Have tried bi-wiring from each of the two taps, and preferred the 8 ohm; "swings" better, boxers versus briefs, but did note that the woofer seemed a bit more tightly controlled off of the 4 ohm tap.  Not sure if it's safe to try the "two tap" bi-wire configuration and also not sure if doing so would impart a tonal difference between the drivers that would be counterproductive to good sound, but obviously, the first question takes precedent over the second.  

Thanks for any insights.    
128x128xenolith
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xenolith's avatar
xenolith OP
130 posts
 
Thanks for the reply Ralph; you're one of the members that I know I can trust the validity of technical information from.  Your point about a coloration not found in real life is well taken; thank you. 

By tightly controlled, I meant that cone excursion seemed less on the 4 ohm tap compared to the 8 ohm tap at same volume setting on the preamp.  I admit that I could have imagined that though.

I'm very grateful and glad to know that this is a permissible experiment, but think I'll hold off on trying it...or the time being anyway.  This version of these amps are likely, if not absolutely, irreplaceable...in the near term anyway.  And as much as I do believe your information, and I do, I'm having a hard time getting over the thought of finding out that they don't work well as OTLs :)

Thanks again!


atmasphere's avatar
atmasphere
11,827 posts
 
What does 'tightly controlled' mean? Tighter bass? If so, you might want to think about the idea that while tight bass is often a lot of fun, it is a coloration not found in real life.


But to answer your question you can mix the taps without damage to the amp. This is because each of the biwire inputs of the loudspeaker have crossover elements for the drivers involved. So bass won't have a complete circuit on the 8 ohm tap while highs won't have a complete circuit on the 4 ohm tap- and your power tubes will thus see a linear load. Whether this sounds right ultimately is a different matter, but the amp should be OK with it.
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