What characteristics should I look for in an amplifier to drive only the
bass speakers? The intended bass set up is a pair of Kef B139 drivers in
some sort of medium-sized transmission line enclosures. The top end will be
B110/T27, driven by a Millennium 4-20 Maplin kit valve amp.

I expect the size of the bass enclosures to be such that the roll-off starts
somewhat above the resonant frequency of the drivers. Given that the purpose
of the exercise is to attain increased lower bass (compared to the
B139-loaded large reflex cabs I have at present) , I'd like to get around
this if possible.

Would it be feasible to use an amp which has a non-linear frequency
response, such that the signal below the cabinet resonance is relatively
higher and thus counteract same? I was thinking that a boost might result in
failure like an overheated speaker coil, but I don't think we're talking
large powers, here. The 4-20 is nominally 20wpc, rated up to 27, but I
rarely have it over half volume, delivering full range program material. If
the power is kept within the limits, my feeling is that boosting the low end
might be a way to get a smaller cabinet size.

If this basic idea is worth considering, what type of amplifier would be
suitable in terms of making sure that the bass cones don't flap about
unneccesarily? Is this where stuff like damping factor comes in?

I should mention that I'd rather build an amp than buy one, and that I quite
like the idea of building a second 4-20, if only to give me a nice pair of
valve bi-amp monoblocks. I like my valve amp, but I'm not wedded to using
valves for the bass end - I'm more interested in what can help me to get a
tight, solid bass sound that stays clearly audible when it goes very low.


--
Wally
www.makearatherlonglinkthattakesyounowhere.com
Things are always clearer in the cold, post-upload light.

"Wally" wrote in message ...
What characteristics should I look for in an amplifier to drive only the
bass speakers? The intended bass set up is a pair of Kef B139 drivers in
some sort of medium-sized transmission line enclosures. The top end will be
B110/T27, driven by a Millennium 4-20 Maplin kit valve amp.

I expect the size of the bass enclosures to be such that the roll-off starts
somewhat above the resonant frequency of the drivers. Given that the purpose
of the exercise is to attain increased lower bass (compared to the
B139-loaded large reflex cabs I have at present) , I'd like to get around
this if possible.


What are your reflex box and port dimensions?
A TL won't work well at less than about 80l internal volume for a B139.

A TL should have a first order rolloff so a simple bass-boost shelf
filter will do.

You could try an air-suspension box - 30l or so, then a suitably aligned
second-order low boost filter followed by a steep high-pass filter.
That would stop it flapping about at low frequencies.

As for the amp, the only requirement is power.

Roger.

The Old Fogey wrote:

I expect the size of the bass enclosures to be such that the
roll-off starts somewhat above the resonant frequency of the
drivers. Given that the purpose of the exercise is to attain
increased lower bass (compared to the B139-loaded large reflex cabs
I have at present) , I'd like to get around this if possible.

What are your reflex box and port dimensions?

Internal dims in cm:
Box: 83h, 55w, 29d
Port: 9dia, 17deep.

About 130L. There are a series of slots cut into the back panels, by a
previous owner, 7mm wide, 300mm long, 11 slots on each cabinet. And there's
a bunch wadding stuff inside.


A TL won't work well at less than about 80l internal volume for a
B139.

Given that the current boxes look to be about 130L each, that sounds like
there might be scope for a good size reduction over the present ones.


A TL should have a first order rolloff so a simple bass-boost shelf
filter will do.

There's a thing - are phase effects in the crossover an issue at low
frequencies? If I wanted to make the filter a low-end band-pass, to kill
everything below a given frequency, would that be a problem?



You could try an air-suspension box - 30l or so, then a suitably
aligned
second-order low boost filter followed by a steep high-pass filter.
That would stop it flapping about at low frequencies.

Not keen on that - I really want to try out the transmisison line idea. I've
heard a lot of good things about them, but have never listened much and
certainly not had the opportunity to live with the sound.



As for the amp, the only requirement is power.

What sort of output should I be looking at to match my 20w valve jobbie? I
don't know where the bass/mid crossover will be set, but probably roughly in
line with the existing Kef DN12 crossovers (100Hz?).


--
Wally
www.makearatherlonglinkthattakesyounowhere.com
Things are always clearer in the cold, post-upload light.

Wally wrote:


What are your reflex box and port dimensions?


Internal dims in cm:
Box: 83h, 55w, 29d
Port: 9dia, 17deep.

About 130L. There are a series of slots cut into the back panels, by a
previous owner, 7mm wide, 300mm long, 11 slots on each cabinet. And there's
a bunch wadding stuff inside.


That should be an excellent size for a bass reflex B139.
Pity about the slots. They probably reduce the Q considerably so
that your enclosure sounds like a TL anyway. I suggest you fill them in
and make the bass reflex work as it should.


What sort of output should I be looking at to match my 20w valve jobbie? I
don't know where the bass/mid crossover will be set, but probably roughly in
line with the existing Kef DN12 crossovers (100Hz?).

20w should be plenty. Why not download a free copy of Goldwave and
play with some music samples? You can filter at varying frequencies
and see just what power is required for different crossover frequencies.

Goldwave doesn't do first order or boost filters unless you write them
yourself. Anyone know of some free software that does?
You could play with different filters on a pc before building an
analogue filter.

Roger.

Old Fart at Play wrote:

About 130L. There are a series of slots cut into the back panels, by
a previous owner, 7mm wide, 300mm long, 11 slots on each cabinet.
And there's a bunch wadding stuff inside.

That should be an excellent size for a bass reflex B139.

I did some numbers on them years ago, and they're supposed to go pretty
low - approaching the f0 of the drivers.

Pity about the slots.

When I got the speakers, there were 1/4" plywood panels pinned and glued to
the back panels - I pulled them off to find out why and discovered the
slots.


They probably reduce the Q considerably so
that your enclosure sounds like a TL anyway.

I remember reading something about the resonant point being extended a
little lower as a result of the slots.


I suggest you fill them
in and make the bass reflex work as it should.

It would be worth a try, I guess. Ultimately, I still want to change to
smaller boxes, though. I'm thinking of a combined TL unit in the middle with
a couple of small B110/T27 cabs.


What sort of output should I be looking at to match my 20w valve
jobbie? I don't know where the bass/mid crossover will be set, but
probably roughly in line with the existing Kef DN12 crossovers
(100Hz?).

20w should be plenty.

Righto.


Why not download a free copy of Goldwave and
play with some music samples? You can filter at varying frequencies
and see just what power is required for different crossover
frequencies.

I must have a hunt for that.


You could play with different filters on a pc before building an
analogue filter.

Definitely part of the plan. :-)


--
Wally
www.makearatherlonglinkthattakesyounowhere.com
Things are always clearer in the cold, post-upload light.

Wally wrote:



It would be worth a try, I guess. Ultimately, I still want to change to
smaller boxes, though. I'm thinking of a combined TL unit in the middle with
a couple of small B110/T27 cabs.


Something like this, perhaps:

http://www.geocities.com/thevenialcr...n_progress.JPG

or

http://www.geocities.com/thevenialcritic/wip.JPG

Roger.

Wally wrote:



It would be worth a try, I guess. Ultimately, I still want to change to
smaller boxes, though. I'm thinking of a combined TL unit in the middle with
a couple of small B110/T27 cabs.


Something like this, perhaps:

http://www.geocities.com/thevenialcr...n_progress.JPG

or

http://www.geocities.com/thevenialcritic/wip.JPG

Roger.

Old Fart at Play wrote:

About 130L. There are a series of slots cut into the back panels, by
a previous owner, 7mm wide, 300mm long, 11 slots on each cabinet.
And there's a bunch wadding stuff inside.

That should be an excellent size for a bass reflex B139.

I did some numbers on them years ago, and they're supposed to go pretty
low - approaching the f0 of the drivers.

Pity about the slots.

When I got the speakers, there were 1/4" plywood panels pinned and glued to
the back panels - I pulled them off to find out why and discovered the
slots.


They probably reduce the Q considerably so
that your enclosure sounds like a TL anyway.

I remember reading something about the resonant point being extended a
little lower as a result of the slots.


I suggest you fill them
in and make the bass reflex work as it should.

It would be worth a try, I guess. Ultimately, I still want to change to
smaller boxes, though. I'm thinking of a combined TL unit in the middle with
a couple of small B110/T27 cabs.


What sort of output should I be looking at to match my 20w valve
jobbie? I don't know where the bass/mid crossover will be set, but
probably roughly in line with the existing Kef DN12 crossovers
(100Hz?).

20w should be plenty.

Righto.


Why not download a free copy of Goldwave and
play with some music samples? You can filter at varying frequencies
and see just what power is required for different crossover
frequencies.

I must have a hunt for that.


You could play with different filters on a pc before building an
analogue filter.

Definitely part of the plan. :-)


--
Wally
www.makearatherlonglinkthattakesyounowhere.com
Things are always clearer in the cold, post-upload light.

Wally wrote:


What are your reflex box and port dimensions?


Internal dims in cm:
Box: 83h, 55w, 29d
Port: 9dia, 17deep.

About 130L. There are a series of slots cut into the back panels, by a
previous owner, 7mm wide, 300mm long, 11 slots on each cabinet. And there's
a bunch wadding stuff inside.


That should be an excellent size for a bass reflex B139.
Pity about the slots. They probably reduce the Q considerably so
that your enclosure sounds like a TL anyway. I suggest you fill them in
and make the bass reflex work as it should.


What sort of output should I be looking at to match my 20w valve jobbie? I
don't know where the bass/mid crossover will be set, but probably roughly in
line with the existing Kef DN12 crossovers (100Hz?).

20w should be plenty. Why not download a free copy of Goldwave and
play with some music samples? You can filter at varying frequencies
and see just what power is required for different crossover frequencies.

Goldwave doesn't do first order or boost filters unless you write them
yourself. Anyone know of some free software that does?
You could play with different filters on a pc before building an
analogue filter.

Roger.

The Old Fogey wrote:

I expect the size of the bass enclosures to be such that the
roll-off starts somewhat above the resonant frequency of the
drivers. Given that the purpose of the exercise is to attain
increased lower bass (compared to the B139-loaded large reflex cabs
I have at present) , I'd like to get around this if possible.

What are your reflex box and port dimensions?

Internal dims in cm:
Box: 83h, 55w, 29d
Port: 9dia, 17deep.

About 130L. There are a series of slots cut into the back panels, by a
previous owner, 7mm wide, 300mm long, 11 slots on each cabinet. And there's
a bunch wadding stuff inside.


A TL won't work well at less than about 80l internal volume for a
B139.

Given that the current boxes look to be about 130L each, that sounds like
there might be scope for a good size reduction over the present ones.


A TL should have a first order rolloff so a simple bass-boost shelf
filter will do.

There's a thing - are phase effects in the crossover an issue at low
frequencies? If I wanted to make the filter a low-end band-pass, to kill
everything below a given frequency, would that be a problem?



You could try an air-suspension box - 30l or so, then a suitably
aligned
second-order low boost filter followed by a steep high-pass filter.
That would stop it flapping about at low frequencies.

Not keen on that - I really want to try out the transmisison line idea. I've
heard a lot of good things about them, but have never listened much and
certainly not had the opportunity to live with the sound.



As for the amp, the only requirement is power.

What sort of output should I be looking at to match my 20w valve jobbie? I
don't know where the bass/mid crossover will be set, but probably roughly in
line with the existing Kef DN12 crossovers (100Hz?).


--
Wally
www.makearatherlonglinkthattakesyounowhere.com
Things are always clearer in the cold, post-upload light.