Help with a schematic, please...

Hello,

I'm trying to get a good understanding of the Rane MP44 mixer, found at:

http://www.rane.com/pdf/mp44sch.pdf

My question involves the line input circuits on the first page of the schematic (or page four of the document, after the silk screens).
Look at IN B LEFT, for example.

There is a LPF and a voltage follower input buffer. That part I think I'm ok with. However, directly after the voltage follower, there is a HPF. My questions concern the HPF (C=22uF, R=20K).

The capacitor, for example C67, is marked "22/16". I assume this is 22uF and 16V? Which works out to a cut off point of 0.36 HZ, which is great. However, the capacitor is drawn as non-polar. What kind of non-polar caps are available at 22uF? Are they using a bipolar cap here? Or am I way off on this line of reasoning? Is this HPF really necessary? Would the Op Amp have such a high DC bias?

Any help would be greatly appreciated - Andrew

> a cut off point of 0.36 HZ

The stage it feeds can be as low as 5.1K, so it is more like 2Hz.

> the capacitor is drawn as non-polar. What kind of non-polar caps are available at 22uF? Are they using a bipolar cap here?

Surely an electrolytic, whatever you call it.

> Is this HPF really necessary? Would the Op Amp have such a high DC bias?

The following stage can have gain of 5, so a small offset becomes bigger. More important, the input stage has no DC blocking at its input, so any DC coming out of the source would be passed along. I'd put DC block at the input; Rane apparently felt that few sources have enough DC to bother the input stage but it should not get into the gain-trim stage or the pot.

Thanks for the response, PRR.

Could you use a polar electrolytic cap as the 22uF capacitor? I am just wondering if the cap would be properly +/- oriented as the signal passed through it? Would you not need to use some type of non-polar capacitor in the circuit?

Cheers - Andrew

http://www.panasonic.com/industrial/components/pdf/ABA0000CE97.pdf

http://www.panasonic.com/industrial/components/pdf/ABA0000PE26.pdf

Thanks for the links. I'll take that as a 'yes' to using a bipolar electrolytic here.

Cheers - Andrew

Sorry, another question...

Do you have any idea why Rane would have put the 22uF cap and 20K resistor after the voltage follower, rather than just putting a 22uF Cap in front of the 20K resistor at the input (as you suggested)? That seems to make more sense, but Rane has made a lot of mixers, so I wonder what's going on with that?

Thanks - Andrew

> idea why Rane

I would not do it that way. Rane has a lot more happy customers than I do. There are many ways to skin cats.

to some extent the Rane gear is overly bullet proof
hence they do add many of the safe bits that the DIY'er may choose to remove and get a more direct signal path

10uf, 22uf and 47uf are typical BiPolar electrolytic caps that you may find in this sort of gear. Neve 51 Yamaha PM3k and 4K and so on

There are some data suggesting that biploar 'lytics are lower distortion that polar even at zero bias (Jensen, Cyril Bateman), and even when compared to 'lytics with a polarizing voltage across. Bateman has written a number of articles about caps for Electronics World.

They are bulkier though, and still leaky and high dissipation factor/dielectric absorption compared to film. 22uF films get mighty big. Low voltage 22uF ceramics exist but use high-K ceramic material and are lossy and piezoelectric.

I bought a huge bag full of 20uf polypro caps for something like 2$ a peice.. they are about 3.5"x1.75".. :green: