Para / Series Mic Input Transformer Wiring

I'm working on an API style mic pre but I added a relay to switch the separate windings of the EA2622 mic input transformer primary between parallel and series.

Is this something that is found in other mic pres and if yes what is it called?

I'm in the process of making the panel and I need to give the button a name.

Also, I assume the default position should be parallel for the full 1:7 boost but low Z and then when the button is activated it switches to series for higher Z / 1:3 boost. So I might call it "Z"?

It is found in others.

It's kinda 'Z'.
It's really ratio=gain more than any other thing. 
It also affects transformer response because of source Z relationship. 
It's complicated.....

emrr said:

It is found in others.

It's kinda 'Z'.
It's really ratio=gain more than any other thing. 
It also affects transformer response because of source Z relationship. 
It's complicated.....

Click to expand...

Ratio! That's a good name.

Yes, it messes with gain and Z which makes it complicated. But it seems like a nice feature. It shouldn't be left out just because it's complicated and hard to name.

It might get left out because

not needed
additional failure point / mechanical contact

but can likewise be useful

emrr said:

It might get left out because

not needed
additional failure point / mechanical contact

but can likewise be useful

Click to expand...

Just to confirm my understanding, the point of this feature would be to switch the input impedance between 150 and 600 ohms at the expense of 6dB of gain and that running the primary in series to get 600 ohms might give a different tone (flatter response?) especially from higher Z mics like say an SM57 which IIRC is ~300R. Does that sound right?

The setting you call 600 ohms will typically look like a load of 1200 ohms or so. it is intended to lightly load mics with an impedance of between 150 ohms (very common) and 600 ohms ( rare these days) It also works well with condenser mics which typically have an output impedance of around 50 ohms.

The setting you call 300 ohms will load like a load of typically 600 ohms. it is designed to lightly load old fashioned vintage mics with an impedance of 50 ohms. You will probably never need it.

Cheers

Ian

Neve 1073 preamps typically have this feature, With a switch for 1200Ω or 300Ω input Z.
It's generally on the front panel on newer versions, But I think the originals had it on the rear - The Hi/Lo switch, next to the edge connector.

I'm sure I've played with a few other preamps which also make use of it, Can't remember specifically which ones though...

I'd definitely include it, If it was me building the preamp.
For the price of a switch, The extra option can't a bad thing.

Lee_M said:

Neve 1073 preamps typically have this feature, With a switch for 1200Ω or 300Ω input Z.
It's generally on the front panel on newer versions, But I think the originals had it on the rear - The Hi/Lo switch, next to the edge connector.

I'm sure I've played with a few other preamps which also make use of it, Can't remember specifically which ones though...

I'd definitely include it, If it was me building the preamp.
For the price of a switch, The extra option can't a bad thing.

Click to expand...

Ahh yes, I see it. On AMS and GA 1073's this appears to be called "Lo Z" or some permutation thereof. So "Lo Z" it is.

squarewave said:

Just to confirm my understanding, the point of this feature would be to switch the input impedance between 150 and 600 ohms at the expense of 6dB of gain and that running the primary in series to get 600 ohms might give a different tone (flatter response?) especially from higher Z mics like say an SM57 which IIRC is ~300R. Does that sound right?

Click to expand...

It's not 150 or 600 ohm input.  That is the intended source Z for the transformer.  An SM57 will be loaded even more lightly, and have a peakier response.  In 600, you may get a more extended response with most mics, in fact you may get a bit of smiley face EQ boosting extreme lows and highs. 

You need to wire it up and listen to really know it's value. 

The Warm Audio WA12 is an API based design and has a button marked 'tone' which switches the input from 1:4 to 1:8. When switched in (150R) it boosts the gain from 65 to 71db, and out its 600R. There's a piece in the manual about it and the choice of the word 'tone', alluding to vintage consoles being wired akin to the 150R setting. It gives a noticably thicker tone with a bit more HF roll off.