Is it a sacrilege???....

[jeffdavison]

My passive subwoofs pump the subsonic dance from some L.P's

My system is mostly all tube... tube phono, Aikido gain stage, Passive TVC.
Crossover is a very modded Dahlquist LP1 that is an active low pass (opamps) and a passive High Pass ( one capacitor). A 75watt triode mode 6550 amp, and a solid state for the elephant farts.

I want to get rid of the sub pump with a rumble filter. I'm looking at an 18db/Octave @ 17HZ filter that passes through some good resistors and caps and has an opamp buffer output. I want to use a very high quality opamp (OPA627AM), but it has to go in front of the crossover so as to not mess up the high and low phase. I would have liked to put the filter on only the low pass out before the sub-amp, but have been advised is not the thing to do for sonics. This means that the entire audio signal will pass through the opamp. I've been told good things about the Buur Brown part... but it almost seems wrong to pass a tube signal through some sand, then back out to a tube... I guess that if I consider glass to be made from sand, then it won't be as hard to accept the concept ;^P

JD

[EWBrown]

The best approach, since you're already using an SS amp for the subs, is to split out the audio at the line level, then run that through an appropriate filter then into the sub amp, this way the main audio signal doesn't get twisted and tormented Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_04

The easiset way to make a simple audio splitter is to use a small shielded box and 6 RCA connectors, just feed each output with a 1K to 4.7K from the input, and that should offer enough isolation to keep things separate from the tube amp and the subs' amp. This approach is used by, and recommended by Bottlehead, the 6DN7 2WPC "SEX" amp feeds the Fostex or other full range hi-eff speakers, and let the powered subs do the heavy lifting in the "elephant farts" end of the spectrum. In their case, they recommended some relatively inexpensive Dayton 10 inch, 100W powered subs, one for each channel as they worked up to around 100Hs, rather than just the bottom octave. Due to the higher than usual LF response, the two subs provided better overall channel separation, though a single sub is perfectly fine in most instances.

If your preamp / amp has a tape monitor loop, then the sub signals can be convniently lifted there, in most cases.

Alternately, the sub amp could be fed with speaker-level output from the tube amp, this works, but results can sometimes be less than optimum.

HTH

/ed B in NH

[kheper]

It depends on what preamp you have. If your
preamp has more than one set of outputs
like mine: a bypass out and main out.
The bypass out is 'dry'. The main carries
a signal to whatever the tone and 'loudness'
controls are set at.

With this, you could use the bypass to feed
the tube amp and the main to the filter/SS amp.

On the other hand: Do you need a subwoofer
with a 75wpc amp? You will still get phasing
problems with any sort of cutoff filter. IOW,
less is more.

[EWBrown]

Most powered subs have a phasing switch, just to cover this. If your sub amp doesn't have this just try both "phases" and go with whichever sounds better, as there are many contributing variables that can affect this.

On the Bottlehead "foreplay" linestages, they have two sets of outputs, with the "sub" set being fed with 1K or 4.7K from the main outputs, this allows the volume controls to simultaneously control both main and sub volume levels, otherwise it's a matter of having to futz around setting main and sub volume controls every time.

/ed B in NH