Mid Side M-S Matrix Uses No Precision Resistors

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
Post Reply
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by mediatechnology »

Paul - Let me think about that.
Ruairi
Posts: 2
Joined: Mon Aug 18, 2008 5:12 pm

Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by Ruairi »

Hi all,

I can't believe I missed this thread! I've been working too much to hang out. This design is evolving fantastically. I use MS in mastering all of the time and now that I'm building an analogue chain based around my Pico I'll definitely be needing an analogue MS device, up to now I've done it exclusively in PTHD. I almost always use the MS wrapped around an eq, in my case I use the gains on the Massenburg eq to "Mute" the M or S channels, obviously a switch would be preferable. Gain wise I agree with Paul, in a mastering context +/- 3dB is enough, +/- 6dB would be a luxury. Personally I'd build the gain with a rotary switch in 0.5 dB steps but I wouldn't expect others to follow suit - in a tracking or mixing situation that kind of accuracy is unnecessary. When I use MS on mics in a tracking situation I push up the faders until it sounds right, no accuracy needed.

It's after midnight here and I've been working since 8am so I'm gonna reread the thread tomorrow and chime in with my 2 cents once I've absorbed everyone's thoughts, it may be too late to add anything but that's ok too because it seems to be going in the right direction.

I should echo Paul's thoughts that I will throw dollars into the pot for Roger's and Wayne's work either by buying a build from Roger or by paying a fair price for the PCBs. Obviously no one will be retiring on the spoils of the DIY M/S decoder market :mrgreen: so it's important that the work does not go unrewarded.

Cheers,
Ruairi

PS in other mastering news I just took delivery of a pair of PMC MB2 - http://www.pmc-speakers.com/product.php ... iew&pid=22
They are enormous, the front panel is 34"x15" and they are 21" deep! They sound and look very imposing indeed. I've moved into a larger space to accommodate them and develop my mastering business (with a Pico taking pride of place in my rack!).
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by mediatechnology »

Roger - There needs to be some means to bypass this stage to provide accurate gain in addition to the bypass we already have that goes competely around the insert. We need wire between the decoder's line receiver and the matrix. Without that, the 0 dB attenuation setting will not be exactly 0 dB (within 0.05 dB). If it's not bypass-able and built with say 1% resistors, the best separation we could expect in the decoder might only be 30- 35 dB. (Assuming that it operates in the M-S domain.) Even a trimmed stage for 0 dB will have significant drift which has to be far less than 50 ppm. Please add a relay to this.

I would keep this as a second board because some people might just want a simple encoder-decoder. Not everyone will be using this in mastering.
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by mediatechnology »

Yes happy Thanksgiving everyone!

Yesterday I had a chance to sketch a few things out with the 0 dB position being a wire to the previous stage. I'd like to find a simple way to provide this trim without the 0 dB gain error. I'm not there yet. Complete bypass of the stage is another option, one being "fixed" and the other variable.
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by mediatechnology »

Works for me. Then we can link them and when we come up with a trim method they can be inserted.
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by mediatechnology »

Very nice layout! An early Christmas present.
Mastertone
Posts: 11
Joined: Sat Jan 05, 2008 6:18 pm

Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by Mastertone »

Wow! Beuatiful!

Looks like im gonna be able to rebuild my console sooner than i expected- :D
Jonas Ekstrom
Mastertone
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by mediatechnology »

Oh yeah, do we need termination Rs on both drivers?
With THAT1646s they are not needed. If someone wanted to use SSM2142s or DRV134s they need to be pre-loaded in 600 ohms to be gain-accurate with active balanced (bridging) inputs. If there's not room for them don't worry about adding them.
User avatar
mediatechnology
Posts: 5442
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by mediatechnology »

Roger - Nice tight simple layout. I like it!

FWIW: The type of resistors that would be needed for external gain trimming that equal the matching accuracy of the balanced line receiver's internal ones would be something like these Vishays:

http://www.vishay.com/docs/63001/s.pdf

The 1240 line receiver's resistors can vary in absolute value by as much as +/-20%. And their tempco quite likely isn't as good as 1-2 ppm. But in terms of matching, they're laser-trimmed to 0.005% and their on-chip tempcos track very well. When you start to use external resistors and require a precise 0 dB gain, then you have to get something almost as good as the Vishays or get very creative in the switching at the 0 dB position. At any other selection, for example +2 dB, precision isn't as important. But at 0 dB it has to be as good or better as the 1240 or just be wire.

Leaving gain trim off-board just seems like a great idea for now. Leaving the mute relays off and using switches instead sure keeps the size and build cost down.
emrr
Posts: 570
Joined: Sun Mar 02, 2008 10:27 pm
Location: NC, USA
Contact:

Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by emrr »

I'm not attached to the relays.
Best,

Doug Williams
Electromagnetic Radiation Recorders
Post Reply