Mid Side M-S Matrix Uses No Precision Resistors

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

Post by emrr » Fri Aug 20, 2010 11:55 am

mediatechnology wrote: I haven't thought about it but how does a bridged T increase the accuracy?
Resistor sets working in parallel, so you are trimming a preset standard ratio. No serial errors, or at least far less. I submit the existence of 1 and 0.1 dB step controls on the standard decade attenuator as evidence. That's my laymans assessment.
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Doug Williams
Electromagnetic Radiation Recorders

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

Post by mediatechnology » Fri Aug 20, 2010 12:30 pm

OK, got it. Absence of serial errors is the key IMHO.

I wished I had some of the old Kay Elemetrics attenuators...
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Re: Mid Side M-S Matrix Uses No Precision Resistors

Post by Gold » Fri Aug 20, 2010 12:34 pm

I don't know if you can make this topology work in your application but you might consider some form of gain trim that is not a potentiometric configuration. The resistor errors add along the string. IIRC In the mic preamp gain switches I built, I was able to keep the errors <<0.1 dB with 1% resistors by using a single resistor for each step. With 0.1% parts you should be able to do much better.
I haven't thought about it but how does a bridged T increase the accuracy?

The problem as I see it with just switching a single shunt resistor is that I want the attenutor to behave as a smooth volume control. When the adjacent shunt resistors are in parallel during the switch the attenuation will increase significantly. The other thing I don't like about that style is the wildly varying impedance.

With a bridged T the impedance is constant and is dominated by the series and fixed shunt resistors. You switch two resistors per position so accuracy is very good.

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

Post by mediatechnology » Fri Aug 20, 2010 12:39 pm

True about the drop-out beween positions. In the mic preamp the gain drops to unity for a few ms between steps but it wasn't too audible. Your mastering situation however would require seemless switching. I think that was the motivation for bruno2000 (live) to use pots in his preamp build.
Read Damore v. Google and ask yourself the following questions:
Are Googlers really the people you want in charge of your search results, browser, data or operating system?
Would you be comfortable riding in a Google self-driving car knowing that Googlers might want you dead just because they don't like you?
Would you trust a Googler with your children?

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

Post by emrr » Sat Aug 21, 2010 3:14 pm

mediatechnology wrote: I think that was the motivation for bruno2000 (live) to use pots in his preamp build.
Oh yeah, no jumps allowed if they ever go in the remote truck.
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Doug Williams
Electromagnetic Radiation Recorders

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

Post by Gold » Sat Aug 21, 2010 3:36 pm

If you want accuracy in the neighborhood of .1dB then I think a pot is only good for about +/- 3dB. I wanted +/- 12db but the largest switch Shallco makes is 46 position so +/- 11dB it is. It goes to 11!

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

Post by Gold » Tue Jul 19, 2011 2:28 pm

I finished the prototype input and output amplifiers for the console with the Shallcos.

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That's the M/S board in the monitor controller
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The 3 position small rotary switch switches between passive stereo through and the M/S board for mono and difference. The toggle switches are mute L and mute R. So I can have mono sum, L mono and R mono. The push button source selectors control a pair of Broadcast Tools 16x2 relay switchers. I have access to 32 stereo sources. The bottom toggle switch switches between the two 16x2 switchers. It's a Preview/Program switch since everything is four channels.

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

Post by mediatechnology » Tue Jul 19, 2011 2:38 pm

Paul - Your build is awesome. You may hold the record for a rotary switch with the most poles.
Read Damore v. Google and ask yourself the following questions:
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Would you be comfortable riding in a Google self-driving car knowing that Googlers might want you dead just because they don't like you?
Would you trust a Googler with your children?

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

Post by Gold » Tue Jul 19, 2011 7:09 pm

Thanks Wayne. It's a full H pad with a 10k input impedance and a 5k output impedance. I thought it was a good compromise to throw away 6dB and get a lowish source impedance but have a bridging load.

BTW I sent you an email yesterday for some more m/s boards. Did you get it?

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

Post by emrr » Mon Jul 25, 2011 11:07 am

Excellent work Paul. I hope it lives up to your standards.
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Doug Williams
Electromagnetic Radiation Recorders

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