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

If you got to www.partsexpress.com they sell a little low pass (and
high pass) plug in modules that may be what your looking for. Don't
know how good they are but they are cheap.



On Sun, 13 Jul 2003 23:48:53 +0800, "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


Dan Santoni
DTS Audio
Hamilton, ON
Canada

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.

Hiyee,
Perhaps you can gimmi a quickly drawn out schematic...thanks in
advance.
Regards,
li_gangyi
"Patrick Turner" wrote in message
...


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.

In article , Patrick Turner
wrote:

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,

Don't these 22k resistors create a noticeable amount of cross talk, given
the relatively high impedances involved in tube amps? Why not include a
couple of high input impedance emitter follower transistor stages in front
of the resistors?


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

John Byrns wrote:

In article , Patrick Turner
wrote:

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,

Don't these 22k resistors create a noticeable amount of cross talk, given
the relatively high impedances involved in tube amps?

Not much cross talk because the junction of the R is shunted with a cap to ground,
and the bass signals are usually the same for both channels.
The take off point from the line stage is from a relatively low impedance point,
being the wiper of the 100k log volume controls and in normal use, its at a low
impedance setting; at 12 oclock, at -20dB, the output impedance
of the pot is 9k.

The stereo info is contained in F above 250 Hz, and the cap shunting the 22k x 2
prevents much cross talk above this F.
By 5 kHz, where you sure don't want any cross talk, my method is
quite OK.


Why not include a
couple of high input impedance emitter follower transistor stages in front
of the resistors?

This is the more purist way of doing it, but then its an extra stage.

Mr Gangyi has to learn about transistors and filters.
Before I ferret through a mountain of paper circuits I've used,
and scan them in and transmit them, I think he ought to just try fooling around
with a few 10c small signal transistors, and a few R and C elements.
It will be a revelation.

He should find out that in common emmitter mode, with a bypassed emitter,
the transistors producing 3 vrms are hopelessly distorted, over
several %, but at 4 mA, the gain will be huge.
Then when the emitter R isn't bypassed, the gain will plummet,
and the thd will then seem similar to a tube, but still not as low.
Then in emitter follower, the transistor is at its best, ie,
has the lowest thd, and competes well with a cathode follower.

The way I did it means there is very low thd, and very little circuit complexity.
This sort of project is just perfect to train the mind to basic analog electronics,

some understanding of filters, and cascading them, settinjg bias currents,
choosing load values, understanding load lines, and it'll put a rumble in a lounge.

There would be many readers wondering where to start;
I suggest they buy a few small NPN signal transistors, and some pre drilled
board for experimental circuits, some 0.6 mm dia solid hook up wire,
and first build a little +/- 12 volt supply, and a volt meter,
and a book about basic transistor circuits.

Opamps could easily be used instead, but in this case, there is no need.

Since my idea needs emitter followers, cathode followers made from
12AU7, 12AT7 would be quite excellent.
But there is a lot more trouble to be gone to for just a bit of low bass.

There is no need for any gain in the active devices.
The following SS power amp input sensitivity can be
adjusted for say 0.5 v for full power by adjusting the FB network
if need be. Some presettable gain pot for the sub amp is usually necessary.

Patrick Turner.






Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/

Hey hey~! I'm not a Mr. yet~! at least in a "lawful" manner coz I'm 14...and
yeah...I have messed with some filters...but I guess it's my component
values that need changing...coz they dun cut at the frequency that I
need...and I dun use transistors...only passive...meaning I've got no
boost...right?? Can you point me in the right direction to build a
simple...filter...
"Patrick Turner" wrote in message
...


John Byrns wrote:

In article , Patrick Turner
wrote:

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,

Don't these 22k resistors create a noticeable amount of cross talk,
given
the relatively high impedances involved in tube amps?

Not much cross talk because the junction of the R is shunted with a cap to
ground,
and the bass signals are usually the same for both channels.
The take off point from the line stage is from a relatively low impedance
point,
being the wiper of the 100k log volume controls and in normal use, its at
a low
impedance setting; at 12 oclock, at -20dB, the output impedance
of the pot is 9k.

The stereo info is contained in F above 250 Hz, and the cap shunting the
22k x 2
prevents much cross talk above this F.
By 5 kHz, where you sure don't want any cross talk, my method is
quite OK.


Why not include a
couple of high input impedance emitter follower transistor stages in
front
of the resistors?

This is the more purist way of doing it, but then its an extra stage.

Mr Gangyi has to learn about transistors and filters.
Before I ferret through a mountain of paper circuits I've used,
and scan them in and transmit them, I think he ought to just try fooling
around
with a few 10c small signal transistors, and a few R and C elements.
It will be a revelation.

He should find out that in common emmitter mode, with a bypassed emitter,
the transistors producing 3 vrms are hopelessly distorted, over
several %, but at 4 mA, the gain will be huge.
Then when the emitter R isn't bypassed, the gain will plummet,
and the thd will then seem similar to a tube, but still not as low.
Then in emitter follower, the transistor is at its best, ie,
has the lowest thd, and competes well with a cathode follower.

The way I did it means there is very low thd, and very little circuit
complexity.
This sort of project is just perfect to train the mind to basic analog
electronics,

some understanding of filters, and cascading them, settinjg bias currents,
choosing load values, understanding load lines, and it'll put a rumble in
a lounge.

There would be many readers wondering where to start;
I suggest they buy a few small NPN signal transistors, and some pre
drilled
board for experimental circuits, some 0.6 mm dia solid hook up wire,
and first build a little +/- 12 volt supply, and a volt meter,
and a book about basic transistor circuits.

Opamps could easily be used instead, but in this case, there is no need.

Since my idea needs emitter followers, cathode followers made from
12AU7, 12AT7 would be quite excellent.
But there is a lot more trouble to be gone to for just a bit of low bass.

There is no need for any gain in the active devices.
The following SS power amp input sensitivity can be
adjusted for say 0.5 v for full power by adjusting the FB network
if need be. Some presettable gain pot for the sub amp is usually
necessary.

Patrick Turner.






Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/

li_gangyi wrote:
Hey hey~! I'm not a Mr. yet~! at least in a "lawful" manner coz I'm 14...and
yeah...I have messed with some filters...but I guess it's my component
values that need changing...coz they dun cut at the frequency that I
need...and I dun use transistors...only passive...meaning I've got no
boost...right?? Can you point me in the right direction to build a
simple...filter...

Well, the simplest (first order) filter is one resistor and one
capacitor. The resistor is in series, the capacitor is shunted to
ground. Like this (you'll need to have a fixed-width font to see this
properly):

o--------/\/\/\/\-------+------o
R |
|
input ----- output
-----
C |
|
o-----------------------+------o

The value of R should be at least an order of magnitude lower than the
impedance at the output (in other words, in parallel with C). For most
tube amps this will be quite high, 100k to 1M or so, usually the volume
control pot. So, for example, if it's 500k, you'd want to choose R to be
around 50k. Let's choose 47k.

Then choose your corner frequency. Let's say you want the rolloff to
start at 50 Hz. Multiply the frequency F by the resistance R, and divide
that into 159,000 for the value of C in microfarads. In our example,
47,000 * 50 = 2,350,000. Divide that into 159,000 and we get about 0.068 uF.

The formula is derived from the formula for capacitive reactance. The 3
dB corner will be that frequency at which the reactance equals the
resistance R. It's given by the equation:

F=1/(2*Pi*R*C) with R in ohms, C in farads, F in Hertz, Pi ~ 3.1416.

Cheers,
Fred
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