Subwoofer mating theory questions...

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bermtoog said:
They were TL's until recently I built them into standard, single ported reflex... tuned to around 90-100Hz. They are 8" Audio Nirvana single fullrange drivers, no crossover or notch filters of any kind.
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I'd suggest converting them to sealed with 6 dB highpass = 18 dB total slope. Excursion should reduce enough, especially at a higher crossover frequency, if the sub can handle that...er, I don't see that you mention WHAT sub.

bermtoog said:
George Anderson suggests a 50k pot for the Simple SE, I haven't purchased it yet... still trying to get all the parts on order...want absolutely as few components in the audio path as is possible...But, the impedance should go up below the cut-off frequency, right (on the FR side)? And the sub input impedance will be high anyway.

What do you think about going from the Rega to the volume control and then splitting: one stereo set to the subamp, one set to the filter caps on the tube amp. Would that work?
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I don't know.
- You can have a pot with no crossovers, it will just reduce the volume IF IF IF the input impedance of the amps were constant with frequency, which most are NOT (I think higher at high frequencies?). So just a pot in-line can be problematic, it will change the frequency response away from flat, and that effect can vary with volume position.
- You can have passive crossovers with no pot, designed to work with the input impedance of each amp. However the passive crossovers assume they are fed from a low source impedance...which a 50k pot is probably not at all volume settings.
- The Rega may be fine with all this, or it may freak out. Yes, the passive crossovers will go to a high impedance in their filtering range-but the phase will shift, which could make the Rega unhappy. And/or, the parallel combination of those rising impedances with shifting electrical impedance phase could end up making some weird impedance notch.
What you need for that is
a) A way to truly measure the input and output impedances vs. frequency of your Rega and amps [buy or make a spot frequency test CD, and then you need a HIGH QUALITY voltmeter that has guaranteed specs at least 20-20k Hz in a 2V measurement range, plus some test resistors]
b) Some help (another post, maybe in the amp forum) to electrically model all this in SPICE or something, or
c) A preamp...still leaving the crossover problem.
I'd encourage you to try A+B. It's not insoluble, just work, but if you can do it you'll be very satisfied. You may decide it is easier to wire in some op amps...even if you just change the SE input capacitor* you may feel better with a high quality output buffer to isolate the Rega. You can find help with that here as well, I'm sure.


bermtoog said:
So then it would affect the apparent gain to filter out as much of the power hogging lows as possible?
No, it will affect the apparent POWER. The filter cuts out the lows, so your mid/high gain in the amp would be the same. But cutting out the bass "frees up" the amp to put out just mids/highs.


*I'd post in the amp section to see what caps are recommended. First get your SE schematic to see what the current value is. Then you need to measure and/or calculate what the cutoff frequency is, and recalculate to raise it to 100 Hz or whatever you decide, depending on what sub(s) you use.
Plan on wiring several caps in parallel, and start with too small = too high of a crossover. Once it is in the audio band you can use your test CD/voltmeter to measure the actual cutoff, and keep wiring additional caps in parallel until you get down to the frequency you want. The paralleling also reduces series impedance.
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