Passive MS Decoder

[chris p]

I'm working on a passive MS decoder/encoder, and I'm getting myself in a tangle over phantom power and transformers. Will this work?



As in, will it pass on 48V?

[otto ruiter]

No, you won't get phantom through this circuit. It's designed for line level.

[chris p]

Thanks Otto.

Am I right in thinking that because the 48V is (in effect) applied equally to all of the secondaries, there is no current flow and so no coupling across the transformer?

I've since found a diagram on the Sowter site that does pass phantom, so I'll go with that. For anyone playing at home, it uses another 2 transformers, whose primaries sit in the diagram above where the outputs now are. The secondaries go to the outputs but are center tapped, and the centre tap goes around the outside of the network to join up with the centre tap on the input primaries, thus bringing the 48V across.

[rob]

that's a pretty low imp load on the mics, from 300R to 100R depending on the pot position. Have you thought about doing this at line level, post the mic pres. Then you won't have to worry about phantom and can use the gain of the pres to control the image width. The downside is you'd need line level transformers

R

[chris p]

Rob

Thanks for your comments

I did seriously think about simply building the Lundhal tranny decoder, which is line level, for use together with the inserts on an Apogee Ensemble pres 1&2. But then its the age old story about flexibility, and why I tend to go for external DI's rather than build them into my pres. You just never know when you might want to MS something into some other pres, or use those inputs for something other than MS. In the end, I've decided I would prefer a mic level decoder.

Luckily, Sowter have just the thing in their 8379, and it passes phantom.

[rob]

perhaps look at Joe's ( JLM ) passive mic attenuator as a better way of doing the level control



this approach may be easier on the load for the mic, the 150R could be scaled if you wanted a different range.

Also there may be no need to add those 300R termination resistors, indeed you could be better off without them

Rob

[chris p]

So the mic sees a more constant load, circa 2K, right?

I might try dropping the shunt to around 100 ohms to get that little bit more range in the volume control, so that would look something like this .....

[chris p]

Here 'tis

[chris p]

I had a friend around yesterday evening and thought I'd give the little box a whirl. Typical lounge room acoustics, no treatments.

Mics are a Rode NT1000 as the mid and an NT2A in fig8 as the side. Positioned about 1.5 meters away, above the guitar midline but angled down towards it, pretty much at the sound hole.

The sound file is 16/44.1 aiff, about -12dbFS peak, 15MB or so. Adjust as needed. And this is untreated except for a little fade in / fade out. No reverb or eq or anything. Not even any volume adjustments.

http://www.legalfinesse.com/webpics/karl.aif

Oh, and its a one take wonder.

[Howard Jones]

Hi Chris,

Where are the two case screws on diagonally opposite coners near the XLRs?

Howard

[chris p]

All screws are now present and accounted for. You don't really need a new photo do you?

I use plastic press in retainers (which you can see in the other diagonals) when building and testing. Saves a lot of time and panel scratches from slipped screwdriver heads.