A Stereo Width Control Using the THAT1240

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mediatechnology
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A Stereo Width Control Using the THAT1240

Post by mediatechnology »

Update 12/8/2014: See https://www.proaudiodesignforum.com/for ... ?f=6&t=681

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.

Image

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:

https://www.proaudiodesignforum.com/for ... ?f=6&t=112
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Sine Star Audio
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Re: A Stereo Width Control Using the THAT1240

Post by Sine Star Audio »

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
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mediatechnology
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Re: A Stereo Width Control Using the THAT1240

Post by mediatechnology »

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: http://www.picocompressorforum.com/foru ... ?f=6&t=110
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Sine Star Audio
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Re: A Stereo Width Control Using the THAT1240

Post by Sine Star Audio »

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...)
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mediatechnology
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Re: A Stereo Width Control Using the THAT1240

Post by mediatechnology »

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.
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Sine Star Audio
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Re: A Stereo Width Control Using the THAT1240

Post by Sine Star Audio »

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?
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mediatechnology
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Re: A Stereo Width Control Using the THAT1240

Post by mediatechnology »

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.
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Sine Star Audio
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Re: A Stereo Width Control Using the THAT1240

Post by Sine Star Audio »

wow, very nice idea! might be a little above me but I'll certainly give it some serious thought.
soundtech
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Re: A Stereo Width Control Using the THAT1240

Post by soundtech »

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
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mediatechnology
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Re: A Stereo Width Control Using the THAT1240

Post by mediatechnology »

Paul thank you for joining us! Sorry I just now saw your post.

The ends of the switches would be grounded exactly like the pots are.
The resistors from the "wiper" of the switch to the top and at bottom are there to provide a bias current path.
Only one resistor is required for bias, but the second one, of equal value pads the "pot" equally.
The wiring if it were a switch or pot would be the same.
With a switch you have the option to vary the taper from either side of center.
I got pretty good results with a pot.
I'd build the circuit wit a dual pot and experiment before committing to a switch.
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