li_gangyi wrote:

Hiyee,
I know I know...this is a tube amp group...but since you all are so
good at it...I might as well just post here...I need a little low-pass
filter that will work for a Solid-State amp that I plan to convert to a
Subwoofer amp...say a few parts hooked together...like a 1st order
thingy...I'm not really familiar with this...I guess my cut-off point will
be like 87Hz or something?? can you guys recommend a schematic and some
component values?? Thanks in advance.
Regards,
li_gangyi

I have built a couple of subwoofer projects which
included a 12" speaker in a large ported box driven by an SS
amp to augment a small pair of floor standing speakers.
The L and R signal from the line stage part of the integrated tube power amp,
( used for most of the signal ) was taken out with two cables to two 22 k
resistors,
and at the join of these two R is a mono signal, and this is then fed to a high
input impedance
emitter follower transistor stage, with a cap from base to ground of 0.15 uF,
giving a first order pole at 96 Hz,
The emitter output from the transistor give a low impedance drive
( just like a cathode follower ) to another RC low pass filter, LPF,
4.7k and 0.33 uF, which gives a pole of 100 Hz.
Then another LPF with 22k and 0.082 uF, then another single transistor,
with its emitter output cap coupled to a 10k gain pot.
So all up, a 3rd order filter. The power amp has an RC input filter with a pole
at 150 Hz.
So ultimately, there is a 4th order filter above 150 Hz,
which is 24 dB/octave,
and you NEED this level of filtering so that all you get from the sub is
a deep rumble, and its very hard to tell if there is someone singing,
or playing a saxaphone.

The above filter, and 25v power supply is very cheap,
and the cost to DIY should be no more than USD $15.00

The filter I built is powered from the SS amp power supply.
It draws only 10 mA.
In my client's case, I had 4 sets of 3 different caps, to give cut off points at
27, 54 and 68 Hz,
to suit the speakers the guy already has, and allow him to experiment for the
best sound.
A 3 position x 4 pole switch allows this.
His speakers, Vienna Acoustics, with two small 5" woofer-mids, have a cut off at

55 Hz, and guess what, he prefers the setting I made for 54 Hz,
although his son prefers 68 Hz, and greater volume on rappy music.

With a bit of a learn, and a bit of a do, you can afford 10 bucks, eh?

Patrick Turner.