Newbie question about mic preamp output stage

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Bertu

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Greetings everyone,

My son is studying music recording and engineering, and in his unlimited faith in his dad he suggested we try to build a mic preamp for his 500-series rack together.

We are planning on using the THAT 1512 as input stage. He is interested in adding some 'color', but not necessarily replicating any particular unit, so we are thinking an output transformer would be the way to go. And that is where the confusion starts.

My understanding is that one probably needs to use a discrete 2520-style opamp to drive the transformer properly, but I haven't been able to find a circuit that does just that, here or anywhere really.

Does the 1512 input circuit amplify the signal all the way to line level? Does the discrete opamp in the output stage need any gain, either to get to the desired level or to 'overdrive' the transformer? Should we add gain to the opamp and then use a step-down transformer? Some people (CAPI) seem to use a variable t-pad at the output, I am assuming that goes at the transformer output?

Thank you for any tips or pointers, and thanks for reading.

- Bertu
 

Whoops

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dmp

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It sounds like you want to design a circuit yourself as part of the project? In that case, start with some research.
Read the datasheet for the THAT 1512 or see the basic page that describes how to use it
The 1512 unbalances the input signal AND provides adjustable gain (wire a pot to it). The only thing you need after that is to create a balanced out.
The simple compact option would be a balancing chip, like a DRV134 or equivalent. There is a complete simple preamp build.

A typical preamp is three basic functions: unbalance, gain, balance.

Your other goal "color" is more nebulous.

So look at a simple preamp design that has a good sonic reputation, like the API 312 schematic (you can find this with a websearch) and see how it achieves those functions.
The input transformer unbalances the signal and gives 13 dB of gain (1:5). The 2520 DOA give adjustable gain. and the output transformer creates a balanced out.

How do you want to pick and chose components to make up your preamp? 1512 + balancing? Then you wouldn't need the 2520, just put a 1:1 transformer after the 1512. That would be a simple way to start. In combining the components you just need to make sure they will be compatible. Can the 1512 drive the output transformer? Does it need a cap to block DC?
 

Bertu

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Thanks for the responses. We already built a compressor kit from diy.re and had alot of fun. Now it is time to try to come up with something on our own.

Yes, we did a lot of research and came up with the basic design of a THAT 1512 with adjustable gain for the input, and for now a THAT 1646 as 'modern' output. We are thinking of later inserting some basic EQ circuits between the two.

The color is supposed to come mainly from the vintage-style DOA and output transformer (which would replace the THAT 1646) , and that is where my questions are. My current understanding is that the DOA that is driving the output transformer has some small amount of gain to 'overdrive' the transformer, and if necessary a variable t-pad after the transformer would bring the level back down.

We are also not sure if the output transformer should provide any gain, or if it just balances the signal, but I suppose there are many ways to do this and we may just have to experiment.
 

JohnRoberts

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Color is an advanced concept and not objectively defined.

I prefer to learn how to make a clean canvas...

JR
 

Bertu

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Right JR, that's why we are starting it with a 'simple' neutral design that just uses THAT chips for the balanced in and out. Color may or may not come later, depending on our initial success.
 

ccaudle

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The color is supposed to come mainly from the vintage-style DOA and output transformer (which would replace the THAT 1646)

Check out the application notes from Jensen Transformers. A discrete op-amp should not add much "color" how ever you define it, be it frequency response variations, slight distortion, oscillations on peaks, etc., so I would not expect much difference between using a DOA and just putting an output transformer on the outputs of your output driver. You don't need the cross-coupling behavior with an output transformer, so you could just use a dual op-amp. Depending on how much output level you want you may even be able to drive the transformer single-ended rather than differentially.
 

Newmarket

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My understanding is that one probably needs to use a discrete 2520-style opamp to drive the transformer properly,

I find this statement a little problematic. The real question seems to me to be what transformer are you thinking to drive and into what impedance. Depending on that you might be fine with eg a '5532/4' solution, a discrete "small signal" transistor circuit or a Neve style heatsinked powerhouse etc.
And if the point is to add 'colour' then how much ? More colour means you may opt for a technically inferior transformer and drive it hard.
I suggest to take a look at the Sound Skulptor options / schematics / options to get an idea of where you are going.
 

Bertu

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Quick update: my son and I ordered one of those 500-series prototype boards from AML and started putting together the basics. THAT 1512 input stage with optional pad and phantom power, followed by a buffer and a THAT 1646 line driver.

Today we started testing and got a clean signal at the output, well at least clean according to the signal generator and display on our mid-range Hantek scope. Excited to see that we didn't screw things up completely.

Next steps are to add a variable high pass and 2-band parametric EQ. Eventually we plan to add an alternative 'classic' output stage using a DOA and Edcor transformer.

Thanks for all the help and suggestions so far. Also, looking at Sound Skulptor and DIY.RE circuit diagrams was definitely helpful. And thanks to everyone who is providing countless nuggets of wisdom on this board.
 
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