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Re: Mid Side M-S Matrix Uses No Precision Resistors
Posted: Fri Aug 20, 2010 11:55 am
by emrr
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.
Re: Mid Side M-S Matrix Uses No Precision Resistors
Posted: Fri Aug 20, 2010 12:30 pm
by mediatechnology
OK, got it. Absence of serial errors is the key IMHO.
I wished I had some of the old Kay Elemetrics attenuators...
Re: Mid Side M-S Matrix Uses No Precision Resistors
Posted: Fri Aug 20, 2010 12:34 pm
by Gold
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.
Re: Mid Side M-S Matrix Uses No Precision Resistors
Posted: Fri Aug 20, 2010 12:39 pm
by mediatechnology
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.
Re: Mid Side M-S Matrix Uses No Precision Resistors
Posted: Sat Aug 21, 2010 3:14 pm
by emrr
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.
Re: Mid Side M-S Matrix Uses No Precision Resistors
Posted: Sat Aug 21, 2010 3:36 pm
by Gold
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!
Re: Mid Side M-S Matrix Uses No Precision Resistors
Posted: Tue Jul 19, 2011 2:28 pm
by Gold
Re: Mid Side M-S Matrix Uses No Precision Resistors
Posted: Tue Jul 19, 2011 2:38 pm
by mediatechnology
Paul - Your build is awesome. You may hold the record for a rotary switch with the most poles.
Re: Mid Side M-S Matrix Uses No Precision Resistors
Posted: Tue Jul 19, 2011 7:09 pm
by Gold
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?
Re: Mid Side M-S Matrix Uses No Precision Resistors
Posted: Mon Jul 25, 2011 11:07 am
by emrr
Excellent work Paul. I hope it lives up to your standards.