3 way rotary switch for LCR pan - what to get?

[Humner]

I'm working on an LCR circuit for each channel of my summing mixer.

When the signal gets to the switch it will be unbalanced, however I'm having difficulties working out what rotary switch I need to get for my purpose.

Further to this, how do I calculator resistor values - as I understand it, when center is selected, there will be less signal going to the left and right buss. Is there a general ratio I need to consider, or does it change depending on the circuit?

 

[ruffrecords]

Opinions vary as to the best level reduction to the two channel when a signal is panned centre but they usually range from 3 to 4.5dB depending on manufacturer. For a 3dB drop in the centre you just need to make the two resistors 3dB larger than when L or R is selected. However, this can complicate the switching, and can mess a little with bus impedance if you are using  a passive mix bus rather than virtual earth mixing. Bottom line is that I have the feeling that a LCR switch is not intended to act like a pan pot - you make the selection then set the level in the mix, so there's no need to change resistor values for the centre position.

If you are happy with that kind of scheme the you can make an LCR switch with a centre off DPDT toggle . I have a schematic of this somewhere. If you are interested let me know and I will post it.

Cheers

Ian

 

[tuanuibi]

Hi Ian,
I'm doing the same myself and I was curious as to how you do the lcr switching with a centre off switch - three pole I assume...
If you could shed some light it would be much appreciated.
Cheers - Josh

 

[ruffrecords]

Hi Ian,
I'm doing the same myself and I was curious as to how you do the lcr switching with a centre off switch - three pole I assume...
If you could shed some light it would be much appreciated.
Cheers - Josh

Here is the basic schematic. You need a 2 pole 3 way switch. R1 an R2 are the bus feed resistors and I have assumed you want them connected to 0V when not used. You can do this easily with a rotary switch but there is a way of doing it with certain types of double pole double throw toggle switches with a centre off position; On-Off-On types. I think this is the sort of one you need:

http://uk.farnell.com/multicomp/2md6t1b5m2re/switch-dpdt-0-1a-20v-solder/dp/1550108

If you look at the MD6 type in this datasheet:

http://www.farnell.com/datasheets/1686095.pdf?_ga=1.192823854.564246154.1461574480

you will see that pins 2 and 5 are the two wipers. In the centre position  2 connects to 3 and 5 connects to 4 so pins 3 and 4 are the signal input. Pins 1 and 6 connect to 0V.

Cheers

Ian

 

[leigh]

Was just looking for an LCR circuit, and found this. Nice and clever!

But to address Josh's question about the type of switch needed – he was correct, that it can't be done with a center off switch. Contrary to your comments, Ian, what is shown in the schematic, as well as in the MD6 datasheet, is an On-On-On switch. It's still only double pole, but the center position mixes it up, with one pole going "high" while the other goes "low".

In other words, if its inputs were labeled A and B, the 3 positions of the switch would produce pairs of outputs  like this:
AA
AB
BB

At least that's my read on it. Just wanted to clear up any confusion for others using this circuit.

 

[ruffrecords]

I think the2MD6 type will work. Looking at the bottom line of the  attached section of the data sheet, in the middle position, pins 2 and 3 are connected together and pins 5 and 4 are connected together. This is the centre position so the bus resistors are connected to pins 2 and 5 and the input signal goes to both pins 3 and 4. When the toggle goes to the right, pin 2 is now connected to pin 1. . Pin 1 should be grounded and the resistor connected to pin 2 is therefore the left bus. Pin 5 is still connected to pin 4 so the signal still goes to the right bus via pin 5.

Similarly when the toggle is to the left pin 2 is still connected to the signal pin 3so the left bus gets signal but pin 5 is now connected to pin 5 which should be grounded.

heers

Ian

 

[leigh]

hi Ian - yes, we are in agreement! I just was pointing out the error in your comment above that the circuit needed a "centre off position; On-Off-On type" switch. That comment initially caused me confusion when trying to understand the circuit. After reviewing the switch datasheet and finding the MD6 row on there, I realized it was actually an On-On-On type needed.

 

[leigh]

Just curious, I was wondering if SP3T switches are doable for LCR setups?

I'm thinking the attached would work for a SP3T switch. It would have to connect to a virtual earth summing amp in order for there not to be leakage across the L and R channels. (I'm not an expert on this, but that is my understanding.)

Advantages: You'd have the option to give the pair of center position resistors a higher value than 10k, in order to follow a -3dB or whatever-you-like "pan law".

Disadvantages: Requires 4 resistors instead of the 2 used in Ian's LCR switching design above, plus a more expensive (?) switch.

 

[ruffrecords]

Unfortunately this scheme has crosstalk when switched left or right due the the two 10K resistors in series between the two buses.

Cheers

Ian

 

[leigh]

Yes, as drawn here – but I thought that by spec'ing a virtual earth summing node, that that would eliminate the crosstalk. Once the signal hits that black hole, it's not coming out again on another summing resistor. Right? That's the whole reason people use virtual ground summing to begin with?

Unfortunately this scheme has crosstalk when switched left or right due the the two 10K resistors in series between the two buses.

 

[ruffrecords]

You are right - with VE mixing, crosstalk would be virtually nonexistent.

Cheers

Ian

 

[leigh]

You are right - with VE mixing, crosstalk would be virtually nonexistent.

Cool - glad I'm finally starting to understand this stuff!

 

[leigh]

Excellent, that's how I planned to set it up if I go for LCR panning.  Just need to find an appropriate switch - decent quality at a good price.

Let me know what you find. Depending on how many you need, and what kind of control scheme you want, it's also worth considering using sealed relays.

Advantages: You'd have the option to give the pair of center position resistors a higher value than 10k, in order to follow a -3dB or whatever-you-like "pan law".

After further thought, I'm not entirely sure  if the circuit I drew would have a flat amplitude (0dB pan law) when in the center position. Be sure to check that on the bench before ordering your resistors!

 

[leigh]

Actually, I was thinking only three resistors would be needed with a SP3T.  Right, or wrong?

If you had only 1 resistor for the center position, then the L and R virtual earth summing nodes would be just a single node, right?

 

[orangerec]

Hi. leigh,

I'm building a summing mixer and I'm incorporating a three position rotary for LCR as well. I've read through this thread and noticed your original switch diagram. You guys mentioned the crosstalk problem and using a virtual earth? Is there any way you could post or send me you wiring diagram for no crosstalk? I'm going in: line input xfrm>3-postion switch>L/R buss>Capi ACA (loaded with John Hardys and Jensen output xfrms).  Thank you!

 

[Mike D]

Excellent, that's how I planned to set it up if I go for LCR panning.  Just need to find an appropriate switch - decent quality at a good price.

Cheers
Adam

When I was researching LCR panning I came across the 3-way tele switches like the following:
https://www.tubesandmore.com/products/switch-3-way-tele
It looks like Tree Audio are using them extensively.

Cheers
Mike

 

[joaquins]

Hi. leigh,

I'm building a summing mixer and I'm incorporating a three position rotary for LCR as well. I've read through this thread and noticed your original switch diagram. You guys mentioned the crosstalk problem and using a virtual earth? Is there any way you could post or send me you wiring diagram for no crosstalk? I'm going in: line input xfrm>3-postion switch>L/R buss>Capi ACA (loaded with John Hardys and Jensen output xfrms).  Thank you!

  With VE summing X-talk is as low as it gets. If you don't use VE summing you need to avoid the X-talk at each channel node, so a pan control after the fader makes it much worse, unless you add a buffer at each output. The usual aproach is to add the buffer, the pan, and then use a stereo fader. If you want to make several post fader mixes you should add a buffer after the fader to avoid X-talk between them.

  Depending on the channel count and your X-talk requirements you might be able to live with all the bad practices and still get usable figures.

JS

 

[leigh]

Hi. leigh,

I'm building a summing mixer and I'm incorporating a three position rotary for LCR as well. I've read through this thread and noticed your original switch diagram. You guys mentioned the crosstalk problem and using a virtual earth? Is there any way you could post or send me you wiring diagram for no crosstalk? I'm going in: line input xfrm>3-postion switch>L/R buss>Capi ACA (loaded with John Hardys and Jensen output xfrms).  Thank you!

I haven't used an "ACA" scheme ("Active Combining Amplifier"?). So I'm not sure if that is API's take on virtual earth summing or not...? Do you have a circuit diagram you could post? If it's just "passive summing with gain boost", then it wouldn't have the low crosstalk advantage of a VE scheme.

The way it would look with a regular opamp doing the summing is: each bus (L/R) would feed the inverting input of a summing amp. The non-inverting input of that same amp would be connected to ground. That way, (almost) no voltage appears at the inverting input – and because of that, a signal panned only to the L bus does not find its way back up the summing resistors of the other channels, and into the R bus.

A feedback resistor is used on the summing opamp, of course. The summing stage is typically followed by a second inverting opamp stage to flip the polarity back around. Sometimes both these stages are both run at unity gain, and sometimes not. (On the Trident S65, for example, summing resistors are 12k and the summing amp's feedback R is 4k7. Then this is followed by another inverting stage, fed through a 4k7 R with 12k for the feedback. So the two stages combined produce unity gain, but the first stage is run at a lower gain, for some advantage that I can't recall right now...)

 

[orangerec]

It is passive summing just feeding the opamps. Here is a link of the Capi ACA & Booster card.  There is a schematic at the bottom of the page.

http://capi-gear.com/catalog/product_info.php?cPath=22_46&products_id=403

I'm just coming in line level into my input xfrm>L/R buss >into the ACA card.  My question is how to wire a 3 way rotary, post input xfrm,  to select LCR buss?

Thanks for helping!

 

[leigh]

It is passive summing just feeding the opamps. Here is a link of the Capi ACA & Booster card.  There is a schematic at the bottom of the page.

http://capi-gear.com/catalog/product_info.php?cPath=22_46&products_id=403

I'm just coming in line level into my input xfrm>L/R buss >into the ACA card.  My question is how to wire a 3 way rotary, post input xfrm,  to select LCR buss?

Thanks for helping!

I can't fully tell from the block diagram there, but I would infer that their summing scheme is a passive sum + boost. (So, a non-inverting opamp stage, and not a virtual earth summing scheme.) Can someone who knows more about API stuff chime in?

If that is the case, then Ian's LCR switching scheme earlier in the thread would be better than the one I posted, since his won't have crosstalk between the L and R busses. If a channel is switched to Left bus only in his scheme, then its Right bus summing resistor gets connected to ground.

In my scheme, a signal switched to the Left bus can hop across those 2 middle resistors right into the Right bus - unless it's attached to a virtual ground summing amp.