Increasing Subwoofer Plate amp Impedance

So i've long been aggravated by the mediocre power of my powered subwoofers, even on full tilt.

I looked them up today to find input impedance is a mere 30k ohms, by comparison with my ST 70, at 270k ohms.

Am I right in assuming I'll get more out of them if I increased the input impedance to something in the order of 270k? And if so, how can I do it? Is it as easy as putting a 240k ohm resistor in the input signal path?

Tube Nube wrote:So i've long been aggravated by the mediocre power of my powered subwoofers, even on full tilt.

I looked them up today to find input impedance is a mere 30k ohms, by comparison with my ST 70, at 270k ohms.

Am I right in assuming I'll get more out of them if I increased the input impedance to something in the order of 270k? And if so, how can I do it? Is it as easy as putting a 240k ohm resistor in the input signal path?

No. You need an buffer that has i high input impedance and low output impedance.
A cathode follower ( for the tube-addicted) although you will have to build your own as i
have not seen a kit or a usable board smaller then "tolarge", but you could try :
http://boozhoundlabs.com/collections/all/products/bhl-audio-jfet-buffer-preamplifier-kit-2sk170

It's a buffer with j-fet as active elements. I will try this myself .

Maybe you can work around it by changing the output impedance. How many drivers, and how are they hooked up?

You could double the impedance if you have multiple drivers wired in parallel by re-wiring them in series ...

Not sure how you mean, sKiZ', but the way I'm set up, I'm driving my VTA ST 70 from one of the pre amp outputs, and driving my powered Rhythmik subs also from the pre amp from its second output.

One way I contemplated was to get Bob's attenuator, to attenuate the input to the power amp, then just drive both power amp and sub amps harder from the pre amp. I rarely get above the 9am setting on the pre amp volume control.

Thanks for this link, Pete. So thinking out loud, my pre amp would be seeing an input impedance very close to the ST 70's, while the buffer output impedance is much lower than my SP 14 pre amp output impedance, it probably doen't matter much--the volume control on the sub amp will do the work of getting the level right.  

Have I got that about right?

Noob, long time no hear. Glad yer alive.
I might suggest another route; get a pair of Dick Vandersteen's in-line hi-pass filters and put 'em twixt the preamp and line amp. They are impedance-specific so you've gotta have your numbers right on the amp input side. But they're not expensive, I think like $75. One coax lede goes to the main amp, the other to the woof. Just a thought.

Hi Dave,

Yes, alive and well indeed! Good to hear from you here. I guess the impedance selection performs the buffer function Pete referred to above?

I assume that, also being a high-pass filter, there's the benefit of relieving my Dynaco from trying to do the heavy lifting of low frequencies.

Initially I ran my pre-amp to the sub-woofers high pass filter, then back to the power amp, but the signal suffered a lot of muting. I concluded that the high pass filters in the Rhythmiks just aren't good enough for hi-fi.

Do you notice a degradation inflicted by the vandersteen hi pass?

Tube Nube wrote:Hi Dave,

Yes, alive and well indeed! Good to hear from you here. I guess the impedance selection performs the buffer function Pete referred to above?

I assume that, also being a high-pass filter, there's the benefit of relieving my Dynaco from trying to do the heavy lifting of low frequencies.

Initially I ran my pre-amp to the sub-woofers high pass filter, then back to the power amp, but the signal suffered a lot of muting. I concluded that the high pass filters in the Rhythmiks just aren't good enough for hi-fi.

Do you notice a degradation inflicted by the vandersteen hi pass?

None whatsoever. In fact they're required equipment on any Vandy subwoof, whether the 2Wxxxs or in the 5Axxxs. The 5As have about 10,000 dip-switches in their bypass filters to accommodate any combination of impedances, or if you're sticking with one amp, just get the necessary fixed input value for about 1/50th the price. Either way, they split off at about 200 Hz to your subwoofs, at which point I quit blowing rectifiers and fuses.

I loaded up Room EQ Wizard last week, and was interested to see how my mains really fall off fast starting around 160 Hz. When I added the Sub Woopers, they only gave me 3 or 5 dB at 70Hz over my mains, but by that point the mains were already attenuated by some 20 to 30 dB.

I've gotta get some boost out of those Subs, but also will want to high-pass my VTA 70 to spare the mains with the bother of coping with anything below . . . I dunno, maybe 160 - ish Hz.

I can't find that cross over / high pass filter you're referring to, Dave. Is it currently being manufactured?

Best sub for the money.

http://www.rythmikaudio.com/F12G.html

Hey Corndog,

Yes, I think that's the one. Or the two--I have a pair. It'll be nice to get them doing their job at long last!