API line input with input transformer.

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JW

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Hi folks

I've been tinkering around re-building my Yamaha console's input strip into a 536 API style channel strip. I've already done two channels and they work great. The console is already taking many API concepts, design wise (seemingly) so it was fairly easy to use all the existing busing/aux/direct out etc architecture, and build in the API line amp, that is (536) with a DOA de-balancing, and a second DOA for driving the bus, (not concerned with eq at the moment) I still have a question or two about the pan pot, but the upshot is it works nicely as a direct copy of the API circuit (albeit with different opamps)

So anyway, I wanted to go a little further and out of interest, see what it sounded like with the Yamaha's 1:3 input transformer and the multi-deck pad switch that they've implemented and see if I can take the unbalanced secondary of this input transformer and go directly in the API circuit. The consideration here (for me) is not to try to squeeze mic preamp functionality into this as well, (transformer isn't appropriate anyway for a 312 copy) but to treat the circuit strictly as a line amp for receiving from the recorder. Yet, to use the Yamaha's input transformer in that functionality. And have two or three steps on this switch, which exist already, where I can actually juice up the input a bit should I want to.

Essentially, on the Yammy schematic, you can see that the first 5 positions of the switch (line amp mode) pad the input, and bypass their mic preamp (IC1), and take the signal into IC2, where the negative input is grounded through a cap and resistor.

So, let's say I take the signal from the Yamaha schematic, right before the 4.7uF cap that goes into the + input of IC2 and instead patch that into the 33uF cap at the + input of the first DOA of the API 536 circuit.
In this case, (different from API where the DOA is debalancing the signal,) it would already be debalanced by the input transformer. Should I then do the same thing Yamaha does with the negative input? Ground the negative input's cap? (250uF in API's case, 100uF in Yamaha's case)?
 

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JW

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Part of the API schemo. Thanks Boji for the redraw.
 

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JW

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BTW, what are the 12K and 22K resistors from the + input to ground doing? (Yamaha schemo IC2)

And p.s. here's a picture of what one of the input strips looks like right now. Notice little green CAPI 2S-LA card I squeezed where the input switch would have gone. I've even built the passive hi pass filters right onto the switches. The interstage transformer is zip tied right now but I think will eventually go in the back of the console, rather than the channel strips. The existing Yamaha pcb is unpowered and unused except for the eq which I can switch in or out.
 

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CJ

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you might get more responses if you repost this in The Drawing Room section of the forum, maybe a Mod can move it for you.

Which Yamaha, the PM 1000 or what ever it is called?
 

JW

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No. Pm2000. Also, my discombobulation in asking questions probably exhausts the reader. A simple picture is better of course. Question is, will it work? I drew the combined schematics out (including the pitchforks from Yamaha.) See the ? on the 6.8K series resistor. I'm not sure if that's needed. Feel free to move, moderators. Sorry about that.

edit: the value of the resistor from the transformer + secondary to 470pF cap is 27K (I forgot to mark it. This is the Yamaha value)
 

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