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A
Stereo Width Control Using the THAT1240

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A
Stereo Width Control Using the THAT1240

by
mediatechnology
» Fri Jan 04, 2008 4:45 pm

Hello everyone. Here's a circuit that's been rolling around
in my head for some time now that I thought needed to be out there. Every since
the 1240 M-S Matrix post everyone has been asking for a width
control.

This one uses two 1240s as a dual channel
potentiometer-controlled polarity inverter along with two 1240s in a matrix. It
was inspired by Rick
Chinn's design but updated using the 1240.

The
upper input side 1240 either subtracts or sums the left channel into the right.
Conversely, the lower input 1240 either subtracts or sums the right channel into
the left. The pot is a dual section linear control. At the center of rotation
the circuit provides "undisgronificated" stereo operation. At full CW rotation
mono, or correlated output is produced: At CCW rotation the L-R uncorrelated
component is output. The two 1240s on the output provide summation via pins 1
and 3. The overall circuit is non-inverting input to output.

The
resistors from the wiper of each pot to the ends serve three purposes. The first
is to bow the taper and expand the center of rotation. The second is to provide
bias current return for the 1240 input in the event the wiper momentarily lifts
from the track or becomes dirty. The third purpose is to provide an "adjust on
test" trim to calibrate the midpoint for a minimum of stereo crosstalk. If one
preferred to AC-couple the wiper to reduce rotational noise it could easily be
done.

A bypass option can also be added by breaking the inputs to the
second pair of 1240s' pin 3.

The inputs must be driven from a low
impedance source.

To use this circuit as a width control for an M-S
matrix requires it to follow the M-S decoder here:

viewtopic.php?f=6&t=112

http://www.waveulator.com/
http://www.ka-electronics.com/
http://www.linkedin.com/in/waynekirkwood
“Logic
will get you from A to B. Imagination will take you everywhere.”

mediatechnology

Posts: 1777

Joined: Sat Aug 11, 2007 2:34 pm

Location: Oak Cliff, Texas

Website

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Re:
A Stereo Width Control Using the THAT1240

by
Sine
Star Audio » Sat Feb 07, 2009 5:57 pm

Hi Wayne and any body else that can help,

I am
building this circuit and have a few questions that you might be able to help me
with,

1. the potential divider resistors across the pot,is there a value
suggestion here? or just trial and error? you mention a trimmer also, would this
replace and sit in any position of the divider?

2. you also mention AC
coupling the wiper, is that just a small cap across the wiper to ground? or
would it have to be a large value so as not to filter any audio to
ground?

3. Once i have achieved all that, the test for measuring
crosstalk so i can adjust it to a minimum, is that a case of, inserting tone in
the Left Channel then measure the Right for minimum and vice versa?

4. Of
course its unbalanced so if i understand correctly i could just add Balanced
Line Receivers and Drivers?

gee that's a lot of questions!

All
the best

Pete

Sine
Star Audio

Posts: 14

Joined: Mon Dec 29, 2008 12:17 pm

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Re:
A Stereo Width Control Using the THAT1240

by
mediatechnology
» Sun Feb 08, 2009 11:36 am

1. the potential divider resistors across the pot,is there a value
suggestion here? or just trial and error? you mention a trimmer also, would
this replace and sit in any position of the divider?

I
think I used 22K resistors and the reason for choosing that value was to give it
a better "feel." There isn't a lot of lee-way in the "feel" department
taper-wise and they're not particularly critical. You do want some resistance
there to provide a bias current path. If the values are low enough they divert
enough bias current that I really wouldn't worry about AC-coupling. The values
will be identical except for the difference caused by a padding resistor
required to make them equal due to the pot. Ideally, at center point, the
divider ratio should be exactly 1/2.

2. you also mention AC coupling the wiper, is that just a small cap
across the wiper to ground? or would it have to be a large value so as not to
filter any audio to ground?

The cap would be in series
with the wiper to the voltage divider formed by the resistors. I think you would
want a subsonic corner frequency and depending on the values of the resistors a
fairly large C. The reactance of the C becomes significant at low frequencies.
For 22K resistors I'd try 47 uF which places the corner frequency <<1
Hz.

3. Once i have achieved all that, the test for measuring crosstalk so i
can adjust it to a minimum, is that a case of, inserting tone in the Left
Channel then measure the Right for minimum and vice
versa?

Yes, that's one way to check it. To set the mid
point of the pot accurately (which is full stereo with minimum crosstalk) I'd
adjust the padding resistors (those in parallel with the existing divider) by
monitoring the outputs at pin 6 of the first 1240s. With the pots exactly at
electrical center, the voltage there should be essentially 0 - equal to the CM
rejection of the 1240. That number is typically -90 dB but I doubt you could
ever get that in practice due to mechanical uncertainty in the pot. Once you get
the midpoints the same for both sections of the pot - both 1240 outputs at 0V -
then measure the crosstalk.

4. Of course its unbalanced so if i
understand correctly i could just add Balanced Line Receivers and
Drivers?

Yes, I would use -6dB line receivers (THAT1246) and the 1646
which provides 6 dB gain. If you want an output transformer, let the 1646 drive
its primary.

When you go to use this you'll notice that when panned to
full mono there's a significant gain build-up (up to 6 dB if the source is mono)
and when panned to full difference a significant fall-off in gain (full
reduction if the input is mono). The exact level shift depends on how wide the
source is. I don't know if any designs compensate for this, but the level
changes are what is naturally-occurring in the source material.

EDIT:
Pete - Here's another matrix trick you might want to take a look at:
viewtopic.php?f=6&t=110

http://www.waveulator.com/
http://www.ka-electronics.com/
http://www.linkedin.com/in/waynekirkwood
“Logic
will get you from A to B. Imagination will take you everywhere.”

mediatechnology

Posts: 1777

Joined: Sat Aug 11, 2007 2:34 pm

Location: Oak Cliff, Texas

Website

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Re:
A Stereo Width Control Using the THAT1240

by
Sine
Star Audio » Sun Feb 08, 2009 4:20 pm

HI Wayne,

Thank you very much.

The values will be identical except for the difference caused by a
padding resistor required to make them equal due to the pot. Ideally, at
center point, the divider ratio should be exactly
1/2.

I'm OK but one thing, I'm confused by the padding
resistor you mention? we are talking about the 2x 22k resistors?

The
other circuit you mention looks very cool, would make it easier to cut for vinyl
with the mono-ing of the bass.

best

Pete
(ps i have more
questions but i will start a new post for that...)

Sine
Star Audio

Posts: 14

Joined: Mon Dec 29, 2008 12:17 pm

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Re:
A Stereo Width Control Using the THAT1240

by
mediatechnology
» Sun Feb 08, 2009 4:32 pm

I'm OK but one thing, I'm confused by the padding resistor you mention?
we are talking about the 2x 22k resistors?

Yes, those
are the padding resistors for an ideal pot. With an inexact pot (all those
without ground taps in the center), one of the 22Ks will have a third resistor
in parallel to make a 10K pot exactly 5K in the center. As a pratical matter you
could have a 10K pot in parallel with a 50K trimmer and do the same
thing.

The other circuit you mention looks very cool, would make it easier to
cut for vinyl with the mono-ing of the
bass.

Exactly.

http://www.waveulator.com/
http://www.ka-electronics.com/
http://www.linkedin.com/in/waynekirkwood
“Logic
will get you from A to B. Imagination will take you everywhere.”

mediatechnology

Posts: 1777

Joined: Sat Aug 11, 2007 2:34 pm

Location: Oak Cliff, Texas

Website

Top

Re:
A Stereo Width Control Using the THAT1240

by
Sine
Star Audio » Sun Feb 08, 2009 4:36 pm

Yes, those are the padding resistors for an ideal pot. With an inexact
pot (all those without ground taps in the center), one of the 22Ks will have a
third resistor in parallel to make a 10K pot exactly 5K in the center. As a
pratical matter you could have a 10K pot in parallel with a 50K trimmer and do
the same thing.

gotcha! so really i'm looking for a
pot with Center Detent that has exactly 5K either side, if not adjust trimmer to
fix?

Sine
Star Audio

Posts: 14

Joined: Mon Dec 29, 2008 12:17 pm

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Re:
A Stereo Width Control Using the THAT1240

by
mediatechnology
» Sun Feb 08, 2009 4:45 pm

Yes, exactly. Though a center detent may not be necessary.
The center is fairly wide. Most of the action is near the ends. I'd say about a
third of the rotation in the center is subtle. The outer thirds are where stuff
starts to happen.

An ideal solution is a dual stepped switch with a bias
resistor at the 1240 input and the attenuation values calculated in
consideration of the Rbias load. The center position can then be grounded. The
calibration of those steps would be an interesting experiment.

http://www.waveulator.com/
http://www.ka-electronics.com/
http://www.linkedin.com/in/waynekirkwood
“Logic
will get you from A to B. Imagination will take you everywhere.”

mediatechnology

Posts: 1777

Joined: Sat Aug 11, 2007 2:34 pm

Location: Oak Cliff, Texas

Website

Top

Re:
A Stereo Width Control Using the THAT1240

by
raf
» Sun Feb 08, 2009 4:50 pm

Sounds like a job for my nifty NKK MRA switch footprint!
Well, if 5 step either side of center is enough.
I love these switches!

raf

Posts: 670

Joined: Sat Aug 11, 2007 4:04 pm

Location: California

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Re:
A Stereo Width Control Using the THAT1240

by
Sine
Star Audio » Sun Feb 08, 2009 4:53 pm

wow, very nice idea! might be a little above me but I'll
certainly give it some serious thought.

Sine
Star Audio

Posts: 14

Joined: Mon Dec 29, 2008 12:17 pm

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Re:
A Stereo Width Control Using the THAT1240

by
soundtech
» Wed Jan 29, 2014 7:00 pm

Hi, new here, just came across this cool circuit. I would
like to give it a try, using a 2-pole 23-position rotary switch in place of the
dual 10k pots. However, I'm unsure as to how it would be connected in the
circuit. I get the grounding of the center-point of the switch, but I don't know
what to do with the end of the switch that corresponds to the grounded ends of
the pots. Any direction here appreciated.

Paul

soundtech

Posts: 2

Joined: Sat Jan 25, 2014 4:37 pm

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