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Project Specific Discussions => Mixers/Monitoring Systems => Topic started by: orangerec on December 15, 2020, 07:48:51 PM

Title: summing mixer with panner, short or not to short
Post by: orangerec on December 15, 2020, 07:48:51 PM
Hello,

I'm building a 24x2 mixer with the CAPI ACA cards. I've decided to incorporate a 3 way switch for the LCR panner.

https://www.mouser.com/ProductDetail/Alpha-Taiwan/SR2511F-0403-19R0B-E9-S-W-159?qs=8%252Br4Hz5Xir%2FmSW9A%252BgefJA%3D%3D (https://www.mouser.com/ProductDetail/Alpha-Taiwan/SR2511F-0403-19R0B-E9-S-W-159?qs=8%252Br4Hz5Xir%2FmSW9A%252BgefJA%3D%3D)

My question is should I use the Shorting (make before break) or Non-shorting (break before make) switches? Thank you in advance.
Title: Re: summing mixer with panner, short or not to short
Post by: boji on December 15, 2020, 09:02:29 PM
My common sense answer would be for routing mono channels, MBB.
For summing say, postfade aux buses to mono, or dropping out the sides of a stereo bus, BBM.

Title: Re: summing mixer with panner, short or not to short
Post by: orangerec on December 16, 2020, 10:16:06 AM
Thanks, boji. It's just input transformer (600ohm)>LCR switch>buss resistor>ACA. MBB it is. It's 24 channels. So, I'll be using 47k buss resistors as per CAPI's schematic. I'd like to incorporate a -4.25dB center(mono) pan law for Center position. 
Title: Re: summing mixer with panner, short or not to short
Post by: ruffrecords on December 16, 2020, 03:32:49 PM
It does not matter which type it is for an LCR switch. BBM is more common though.

If you want to be sexy you could do it wit a single pole centre off toggle switch and a couple of relays. Switching would then be so fast it would not matter.

Cheers

Ian
Title: Re: summing mixer with panner, short or not to short
Post by: boji on December 16, 2020, 04:21:18 PM
Quote
It does not matter which type it is for an LCR switch

In an extra-normal situation like switching on the fly, mid-mix, might MBB be less disruptive? 
Title: Re: summing mixer with panner, short or not to short
Post by: ruffrecords on December 16, 2020, 05:55:29 PM
In an extra-normal situation like switching on the fly, mid-mix, might MBB be less disruptive?
Debatable a) if it would be and b) whether you ought to operating an LCR switch on the fly in a mix.

In one case the level momentarily increases then goes to the desired level, in the other case it momentarily drops and then returns to the correct level. I find it hard to decide which one is worse.

As I mentioned. BBM switches are much more common. The only place I have seen these present a potential 'noise' problem is in EQ circuits when you switch caps with different dc charges on them. MBB does not solve this problem. The normal solution is just to ensure the dc voltage is distributed to all the caps by adding 4M7 resistor between adjacent switch contacts.

Cheers

Ian
Title: Re: summing mixer with panner, short or not to short
Post by: boji on December 16, 2020, 08:43:23 PM
Quote
Debatable...ought to operating an LCR switch on the fly in a mix.

Indeed, it was the only scenario I could imagine where connection type might matter.

Not to belabor the point, but if a MBB switch was "passively" routing towards resistors in front of VGSB rails, why wouldn't the switching noise be less or equal to BBM?

Edit: Guess we now have to worry about the point at which the bus resistors get lifted from ground rail as well.  On triple thought,  OP: K.I.S.S. and go with what is standard (what Ian said)   ;D
Title: Re: summing mixer with panner, short or not to short
Post by: abbey road d enfer on December 17, 2020, 07:30:07 AM
Not to belabor the point, but if a MBB switch was "passively" routing towards resistors in front of VGSB rails, why wouldn't the switching noise be less or equal to BBM?
Switching noise is constituted of at least 3 elements:
Title: Re: summing mixer with panner, short or not to short
Post by: boji on December 17, 2020, 03:09:25 PM
Thank you for the detailed explanation Abby!

Quote
BBM with create punches between positions , whereas MBB will create humps.

humps to punches-- does gender play a role?  ;D ;D
Title: Re: summing mixer with panner, short or not to short
Post by: orangerec on December 21, 2020, 12:24:35 PM
Thank for the info. I've ordered BBM switches. I love how much I learn on these forums/discussions. Yes, it's a basic LCR switch. I need that for the recall purposes. I'll be using 47kohm buss resistors for the L and R rails (recommended by CAPI for it's ACA) but what should I use for the center position to maintain a -3dB pan law? I'd actually prefer -4.25 or 4.5 pan laws. I believe that's what SSL and/or Neve used back in the day.
Title: Re: summing mixer with panner, short or not to short
Post by: boji on December 22, 2020, 05:17:15 PM
https://www.electronics-tutorials.ws/attenuators/l-pad-attenuator.html (https://www.electronics-tutorials.ws/attenuators/l-pad-attenuator.html)

Someone correct me, but I think you'll want a drop of 2.125dB per side for a total center reduction of 4.25.

Edit: Also https://www.rfcafe.com/references/electronics-world/resistor-pads-electronics-world-july-1959.htm

Title: Re: summing mixer with panner, short or not to short
Post by: abbey road d enfer on December 22, 2020, 05:42:11 PM
Thank for the info. I've ordered BBM switches. I love how much I learn on these forums/discussions. Yes, it's a basic LCR switch. I need that for the recall purposes. I'll be using 47kohm buss resistors for the L and R rails (recommended by CAPI for it's ACA) but what should I use for the center position to maintain a -3dB pan law? I'd actually prefer -4.25 or 4.5 pan laws. I believe that's what SSL and/or Neve used back in the day.
It depends very much on the topology used for panning. There are 3 most common topologies (see attached).
Which one are you going to use? If you ordered DP3T switches, I would suggest using #3.


Title: Re: summing mixer with panner, short or not to short
Post by: orangerec on December 24, 2020, 10:30:46 AM
https://www.electronics-tutorials.ws/attenuators/l-pad-attenuator.html (https://www.electronics-tutorials.ws/attenuators/l-pad-attenuator.html)

Someone correct me, but I think you'll want a drop of 2.125dB per side for a total center reduction of 4.25.

Edit: Also https://www.rfcafe.com/references/electronics-world/resistor-pads-electronics-world-july-1959.htm

Thanks. Reading up on Lpads now.
Title: Re: summing mixer with panner, short or not to short
Post by: orangerec on December 24, 2020, 10:40:34 AM
It depends very much on the topology used for panning. There are 3 most common topologies (see attached).
Which one are you going to use? If you ordered DP3T switches, I would suggest using #3.

Yes. #3 makes the most sense. I got unbalanced signal post input transformer feeding the LCR switch then to the 47kohm buss resistors. I just have to figure out the values the Center position to maintain a 4.25dB panning law.
Title: whe when centred needs to be 4: summing mixer with panner, short or not to short
Post by: ruffrecords on December 24, 2020, 05:03:37 PM
Yes. #3 makes the most sense. I got unbalanced signal post input transformer feeding the LCR switch then to the 47kohm buss resistors. I just have to figure out the values the Center position to maintain a 4.25dB panning law.

The sum of the two resistors  needs to be 4.25dB higher than 47K.

4.25dB is 1.63 times.

1.63 * 47K = 76K7.

76K7 - 47K =29K7

So I guess the additional resistors need to be 30K.
 
Cheers

Ian
Title: Re: whe when centred needs to be 4: summing mixer with panner, short or not to short
Post by: orangerec on December 26, 2020, 01:09:29 AM
The sum of the two resistors  needs to be 4.25dB higher than 47K.

4.25dB is 1.63 times.

1.63 * 47K = 76K7.

76K7 - 47K =29K7

So I guess the additional resistors need to be 30K.
 
Cheers

Ian

Oh! I meant the panning law is -4.25 in the center position. Right? we want the center to be down because the two sides sum together.
Title: Re: whe when centred needs to be 4: summing mixer with panner, short or not to short
Post by: abbey road d enfer on December 26, 2020, 10:28:08 AM
Oh! I meant the panning law is -4.25 in the center position. Right? we want the center to be down because the two sides sum together.
Yes, and that's exactly what Ian proposes.
Title: Re: whe when centred needs to be 4: summing mixer with panner, short or not to short
Post by: orangerec on December 26, 2020, 12:31:45 PM
Yes, and that's exactly what Ian proposes.
Makes more sense now. I'm having some trouble figuring out how Ian came about 1.63 times. I just don't want to follow directions, I'm also trying to learn and understand. Thank you!
Title: Re: whe when centred needs to be 4: summing mixer with panner, short or not to short
Post by: abbey road d enfer on December 26, 2020, 04:53:59 PM
Makes more sense now. I'm having some trouble figuring out how Ian came about 1.63 times. I just don't want to follow directions, I'm also trying to learn and understand. Thank you!
4.25dB computes to a ratio of 1:1.63
Standard logarithmic calculus.
Title: Re: whe when centred needs to be 4: summing mixer with panner, short or not to short
Post by: ruffrecords on December 26, 2020, 07:12:18 PM
4.25dB computes to a ratio of 1:1.63
Standard logarithmic calculus.

If you wanted to know what 1.63 times is in dB then on your calculator you would do log(1.63) x 20

and the answer would be 4.25. Do do it the other way round you reverse the process so:

First you do 4.25 divided by 20 which gives 0.425. The on your calculator you do INV log( Ans) and get 1.63

On some calculators the INV button is labelled SHIFT.

Cheers

Ian