Newbie here....treble and bass control for portable speakers

[Roger Johansson]

There is no such thing as a 'passive Baxandall'; the circuit *depends*
on negative feedback.

If he is using the tonestack.exe program to choose tonecontrol circuit
he may have read this passus in the help file:

"The James control was described in Wireless World during February
1949, although it is claimed that Michael Volkoff first used this type
of control in July 1939. Many would describe it as a Baxandall
control, although the true Baxandall control was an adaptation of the
James control using negative feedback."

The tonestack.exe is a cute program which is fun to play with.
I think it can be found on the web with the search word tonestack.exe,
or maybe just tonestack.
It is describing and simulating different tone control circuits which
have been used in electric guitars and elsewhere.

 

[John Woodgate]

I read in sci.electronics.design that Roger Johansson <[email protected]>
wrote (in <[email protected]>) about 'Newbie
here....treble and bass control for portable speakers', on Tue, 28 Oct
2003:

If he is using the tonestack.exe program to choose tonecontrol circuit
he may have read this passus in the help file:

"The James control was described in Wireless World during February
1949, although it is claimed that Michael Volkoff first used this type
of control in July 1939.

Quite right. I have the WW issues that document this.

Many would describe it as a Baxandall
control, although the true Baxandall control was an adaptation of the
James control using negative feedback."

'Adaptation' is a big word. It's not the same circuit as far as the
treble control is concerned, and gives different response shapes. It's
possible, and quite common, to use the James circuit in a feedback
configuration as well.

 

[Roger Johansson]

'Adaptation' is a big word. It's not the same circuit as far as the
treble control is concerned, and gives different response shapes. It's
possible, and quite common, to use the James circuit in a feedback
configuration as well.

I don't dispute the facts, I just wanted to suggest why he may have
got an idea about a "passive Baxandall".

By the way, I found your name in the help file too, as one of the
contributors or advisors or something.

"Tone Stack Calculator was written by Duncan Munro and is based on the
Tone Stack Simulator devised by Dave Cigna. Thanks go out to the
following people:"

"John Woodgate. For providing technical and historical information
about the James stack, Baxandall stack and sending me detailed
articles dating back to 1949."

The name of the program is "Tonestack Calculator" v1.2.
Should be easy to find on the web.
.....

To elfa: You have a limited voltage coming out of the radio so the
tone control may not dampen the signal very much, so you may have to
choose a tone control circuit accordingly.
It depends on how strong signal your amp needs on the input.

If you look at the curves in the tonestack program you see how much
different types dampen the signal, how low in the diagram the curve is
positioned. The James type ("passive Baxandall") is one of the worst
alternatives for you because it dampens the signal with 20dB in
neutral position. The "Bench" type can be adapted for low dampening.

Radio amateurs often use a filter to hear weak signals through noise.
A common filter is a series connection of 1Henry 100nF and 1kOhm.
It gives a strong presence filter, cutting away bass and treble and
enhancing the frequencies between 500-3000Hz.
This type of filter does not dampen the signal very much.

You probably have difficulties finding an inductor with 1Henry, but
you can try different transformers, try different connections on the
transformers.

You can see the results of such a series link in the tonestack program
if you choose the Bench type, lower the source impedance to 1k or
lower, because you radio has low impedance output.

Turn down the bass and treble pots to zero, and make them bigger, to
1M pots. That removes their influence.
Now you can use the mid link to try different values and settings, use
1Henry and 100nF for example, and set the mid pot to slightly less
than maximum.
See how the curve shows a high point at about 2kHz.
Bass and treble are cut away.
That is often a good filter for people who have problems with hearing,
and gives the clearest sound when listening to speech.

 

[YD]

Ooops....forgot another question. If I want to add a volume control between the
amp and speaker (the original speaker system didn't have volume control), what
value in ohms would you recommend in the way of a audio pot?

elfa

If you don't want to use the radio's control a 100k audio taper pot
between the tone stack and amp input would probably be fine. Have a
gander at http://www.discovercircuits.com and
http://www.epanorama.net/links/audiocircuits.html, there are some
preamp and tonestack circuits there. If you're going to place it
inside the box you might go all the way and use an active circuit.


- YD.

 

[YD]

(in <[email protected]>) about 'Newbie
here....treble and bass control for portable speakers', on Tue, 28 Oct
2003:

There is no such thing as a 'passive Baxandall'; the circuit *depends*
on negative feedback.

I know, the correct name is a James circuit. But everyone and the dog
call it passive Baxandall, so WTH.

- YD.

 

[YD]

If he is using the tonestack.exe program to choose tonecontrol circuit
he may have read this passus in the help file:

"The James control was described in Wireless World during February
1949, although it is claimed that Michael Volkoff first used this type
of control in July 1939. Many would describe it as a Baxandall
control, although the true Baxandall control was an adaptation of the
James control using negative feedback."

The tonestack.exe is a cute program which is fun to play with.
I think it can be found on the web with the search word tonestack.exe,
or maybe just tonestack.
It is describing and simulating different tone control circuits which
have been used in electric guitars and elsewhere.

http://www.duncanamps.com/software.html. Dunno if it'll make much
sense to the OP but it's fun to play around with.

- YD.

 

[John Woodgate]

(in <[email protected]>) about 'Newbie
here....treble and bass control for portable speakers', on Wed, 29 Oct
2003:

I know, the correct name is a James circuit. But everyone and the dog
call it passive Baxandall, so WTH.

OK. I didn't realise that the information about 'James' had now
penetrated so widely among the cognoscenti. It dropped out of general
knowledge among audio people for around 50 years.

 

[elfa]

Ok, that's fine then. You can use a passive circuit at the amp input,
either in a separate box between the radio and amp or if space allows
inside the speaker box. Just cut the wire between the jack and amp and
put the control there. A passive Baxandall seems like just the ticket.
Be warned that there's quite a bit of attenuation so you may not get
all the volume this way.

- YD.

I hate to admit it but so far, I'm failing. No response out of the speaker.
The circuit I used is at the link below....midway down the page is the Baxandall
circuit described as 'Passive Baxandall Tone Stack Schematic'. Apparently it is
designed for a Fender guitar.

http://amps.zugster.net/articles/tonestacks/

Maybe the pot, resistors, and caps are the wrong values for the voltage coming
out of an earphone jack. The ground is the earplug ground to the radio on the
input side which is connected to the ground of the output jack to the amp.

Before I give up entirely, anyone care to recommend a different set of values
for the pots etc.

BTW...I REALLY do appreciate all the help that has been given so far.

thanks

elfa

 

[John Woodgate]

I read in sci.electronics.design that elfa <[email protected]>

I hate to admit it but so far, I'm failing. No response out of the
speaker. The circuit I used is at the link below....midway down the page
is the Baxandall circuit described as 'Passive Baxandall Tone Stack
Schematic'.

This should work, if you build it correctly. But the sound may not be
loud enough.

Apparently it is designed for a Fender guitar.

No, that's the one above, with the highly-special 350 kohm pot tapped at
70 kohm. The Passive Baxandall Tone Stack is actually a variant of the
James circuit, and was originated for the British Mullard 5-20
preamplifier (not the 5-10 add-on - that had different values and was a
straight James circuit).

http://amps.zugster.net/articles/tonestacks/

Maybe the pot, resistors, and caps are the wrong values for the voltage
coming out of an earphone jack.

No. It should work, at lest to some extent.

The ground is the earplug ground to the
radio on the input side which is connected to the ground of the output
jack to the amp.

What about the 'hot' wires? How have you connected them?

Before I give up entirely, anyone care to recommend a different set of
values for the pots etc.

You don't need different values. We don't have enough data to determine
for sure what's wrong. You are probably getting around a volt out of the
earphone socket. The output of the tone control circuit is then about
0.1 V (100 millivolts, 100 mV). You amplifier may not produce full
output with an input of 100 mV, but it should produce something.

It's JUST possible that the radio has an old-type earphone output that
needs a lowish-value resistor across it, not 8, 16 or 32 ohms but 1
kohm. You could try that.

 

[Roger Johansson]

I hate to admit it but so far, I'm failing. No response out of the speaker.
The circuit I used is at the link below....midway down the page is the Baxandall
circuit described as 'Passive Baxandall Tone Stack Schematic'. Apparently it is
designed for a Fender guitar.

The problem with this circuit is that it dampens the signal very much,
and you cannot afford that when your signal output from the radio is
so low as it is.
But you may also have made a mistake in the wiring.

http://amps.zugster.net/articles/tonestacks/

Most of the circuits on this web page are designed for high impedance
sources and will not work well for you.
(maybe your amp has a low inpedance input)

A little further down on this web page you find a link to "Duncan
Amps' Tone Stack Calculator". Use that and download the Tonestack
Calculator program, it is this link:
http://duncanamps.com/tsc/index.html
In that program you can compare different tone controls and see what
output curves you get.

You can see that the circuit you are trying to use now, called James,
gives very low output level.

You have a limited voltage coming out of the radio so the
tone control may not dampen the signal very much, so you may have to
choose a tone control circuit accordingly.
It depends on how strong signal your amp needs on the input.

If you look at the curves in the tonestack program you see how much
different types dampen the signal, see how low in the diagram the
curve is positioned. The James type ("passive Baxandall") is one of
the worst alternatives for you because it dampens the signal with 20dB
in neutral position.
The "Bench" type can be adapted for low dampening.

Radio amateurs often use a filter to hear weak signals through noise.
A common filter is a series connection of 1Henry 100nF and 1kOhm.
It gives a strong presence filter, cutting away bass and treble and
enhancing the frequencies between 500-3000Hz.
This type of filter does not dampen the signal very much.

You probably have difficulties finding an inductor with 1Henry, but
you can try different transformers, try different connections on the
transformers. Leave all other connections open.

You can see the results of such a series link in the tonestack program
if you choose the Bench type, lower the source impedance to 1k or
lower, because you radio has low impedance output.

Click on the signal source and other components to change values.

Turn down the bass and treble pots to zero, and make them bigger, to
1M pots. That minimizes their influence.
Now you can use the mid link to try different values and settings, use
1Henry and 100nF for example, and set the mid pot to slightly less
than maximum.
See how the curve shows a high point at about 2kHz.
Bass and treble are cut away.
That is often a good filter for people who have problems with hearing,
and gives the clearest sound when listening to speech.

The reason why we need a little resistance in series with the
capacitor and the inductor is that such a circuit can self-oscillate
if there is no resistance, it will cause ringing and create a bad
sound. With a little resistance in the circuit this problem is
removed. You can see this problem yourself in the tonestack calculator
program. If you set the mid control pot in the maximum position in the
example above you will see how the curve shoots up above the zero
line, it amplifies the sound, but that will sound bad. The circuit is
ringing. Put the pot just a little lower and you will get a nice
curve.

You can try other alternatives in the tonestack program, but remember
that you need a curve which is as high as possible, close to the zero
damping line, zero decibel.

If you dampen the signal from your radio with 20 dB as you are doing
now, you will not hear much output.
Look at the James circuit with the controls in neutral position, the
straight line curve is down at minus 20 dB, that is what you are doing
now, that is probably why you hear nothing.
You also use very high resistance components, which dampens the sound
even more, I guess.

It would help if you could tell us what tone curve you like when you
listen to speech on a radio or a stero with good tone controls.
Do you lower the bass and treble and raise the volume to hear better?
That is what people in general need to hear speech better.
You may have some special problem with your hearing and it is
difficult to give advice without knowing more about what frequencies
you need to amplify.

 

[Roy J. Tellason]

(in <[email protected]>) about 'Newbie
here....treble and bass control for portable speakers', on Tue, 28 Oct
2003:

There is no such thing as a 'passive Baxandall'; the circuit *depends*
on negative feedback.

See the current issue of Nuts & Volts, P. 62, fig. 12...

 

[YD]

I read in sci.electronics.design that elfa <[email protected]>


This should work, if you build it correctly. But the sound may not be
loud enough.


No, that's the one above, with the highly-special 350 kohm pot tapped at
70 kohm. The Passive Baxandall Tone Stack is actually a variant of the
James circuit, and was originated for the British Mullard 5-20
preamplifier (not the 5-10 add-on - that had different values and was a
straight James circuit).

No. It should work, at lest to some extent.


What about the 'hot' wires? How have you connected them?

You don't need different values. We don't have enough data to determine
for sure what's wrong. You are probably getting around a volt out of the
earphone socket. The output of the tone control circuit is then about
0.1 V (100 millivolts, 100 mV). You amplifier may not produce full
output with an input of 100 mV, but it should produce something.

It's JUST possible that the radio has an old-type earphone output that
needs a lowish-value resistor across it, not 8, 16 or 32 ohms but 1
kohm. You could try that.

Take a look at those high component values. As I understand it elfa's
using one of those computer speakers with built-in amp. These tend to
have very low input impedances and will load down the tonestack
something fierce. That or there's some miswiring.

If there's space inside the enclosure for a small circuit board the
best bet would be using an active circuit and tap power off from the
amp.

- YD.

 

[John Woodgate]

(in <[email protected]>) about 'Newbie
here....treble and bass control for portable speakers', on Fri, 31 Oct
2003:

Take a look at those high component values. As I understand it elfa's
using one of those computer speakers with built-in amp. These tend to
have very low input impedances and will load down the tonestack
something fierce.

Why would they have 'very low' input impedances? What is 'very low' in
this context?

That or there's some miswiring.

Probably.

 

[elfa]

(in <[email protected]>) about 'Newbie
here....treble and bass control for portable speakers', on Fri, 31 Oct
2003:


Why would they have 'very low' input impedances? What is 'very low' in
this context?


Probably.

I've checked and double checked and the proper components are in the right
place.

The input is the input of the earphone jack (center part of plug) with the outer
part (barrel) being ground. The output is connected to the center part of
another earphone cable connected to the speaker amp. Both barrels (ground) are
tied together.

elfa

 

[YD]

(in <[email protected]>) about 'Newbie
here....treble and bass control for portable speakers', on Fri, 31 Oct
2003:


Why would they have 'very low' input impedances? What is 'very low' in
this context?


Probably.

They're designed to be plugged into a low impedance source like a
radio or a sound card so they may want load it a bit. I've seen
reports of sound cards misbehaving if they're not adequately loaded
though I don't have any first hand experience of this.

Measuring the three speakers I have at hand one came out as 100 ohms,
one as 1.3k and the last as 45k. Even the 45k one is lowish compared
to the 500k pot in the tonestack.

- YD.

 

[YD]

I've checked and double checked and the proper components are in the right
place.

The input is the input of the earphone jack (center part of plug) with the outer
part (barrel) being ground. The output is connected to the center part of
another earphone cable connected to the speaker amp. Both barrels (ground) are
tied together.

elfa

Looks right. See my other post regarding impedances and loading. Your
tonestack circuit is for tube amps and expects about 1 megaohm on the
output. If you have a DVM try measuring the amp input (between tip and
barrel). Seems you might have to go for an active circuit after all.

- YD.

 

[elfa]

Looks right. See my other post regarding impedances and loading. Your
tonestack circuit is for tube amps and expects about 1 megaohm on the
output. If you have a DVM try measuring the amp input (between tip and
barrel). Seems you might have to go for an active circuit after all.

- YD.

Damn...afraid of that. Well, I gave it a decent try. I was looking for
something passive. At this point I think I'll just toss in the towel. I guess
my best bet is to find another speaker with bass/treble controls built in (not
too many around).

Thanks to all for your input.

elfa

 

[Roger Johansson]

Damn...afraid of that. Well, I gave it a decent try. I was looking for
something passive. At this point I think I'll just toss in the towel. I guess
my best bet is to find another speaker with bass/treble controls built in (not
too many around).

You can use this kind of tone control.

___
signal-|___||---o------o-------------o------amp
A | | |
| | | |
'------' | |
| |
| C|
--- C|inductor
--- C|
| |
| |
| |
| |
.-. .-.
.--->| | .-->| |
| | | | | |
| '-' | '-'
| | | |
ground ---------o--------------o------------ground to amp

The signal comes in at left and you put a pot in series in the signal
wire, 50k or more.

When that pot is in one end position its value is zero, so the signal
goes through without dampening. Set it a little bit lower than that so
the sound is a little lower than when that pot is fully on.

The other two pots dampen treble and bass.
The capacitor can have a value of 100nF to 1uF.
A higher value can cut away more treble.
The inductor is a part of a small transformer.

With this circuit you can also use the first pot as a volume control.
Try different settings and connections on the transformer until you
get good sound. This is a circuit you can experiment with.

The capacitor and pot to ground will dampen the treble and you can
regulate how much with the pot.
The transformer and pot dampens the bass.
The volume pot has to be a little lower than full volume to give the
tone controls possibility to work fully.