Preamplifier - ADC impedance

My mic preamp goes to an external ADC and nowhere else.  The ADC sheet gives 10k input impedance.  What would be "better" if the preamp would have a 10k output transformer rather than 600?

Thanks!

no

Hi Alex,
Its not a simple yes or no answer I think ,lets say for the sake of simplicity your using a 1:1 transformer on the output ,with the matching impedence of 10 kohm you most likely will get a small bump in the high end frequency response,at the same time your transformer will be more or less unloaded ,which will reduce the amount of power the transformer has to pass,and  reduce the amount of transformer core distortion.Lets say you use a 1:4 transformer ,you will end up with about a 12db gain from the transformer itself , this will mean you might be wasting headroom and not driving the circuit fully .Depends alot on the input level your convertor needs to hit 0db fs digital.So if your preamp is capable of say +24 db at the output ,you add a 1:4 step up transformer ,now when your circuit is driven to full level the output from the transformer will be up around +36 db which is way too much level for almost anything.If you have a look at my previous post ,the transformer Im using has a step down ratio of 4:1,so Im loosing 12db of headroom.Although 600 ohm inputs are rare these days its generally a good thing to have a preamp capable of driving low z.What might be a good plan is to terminate your transformer with a variable resistance ,say a 20kohm pot, inject a squarewave of a few khz,monitor the output waveform ,and adjust the pot to get the best squarewave output without overshoot at the leading edge of the waveform,once youve found the sweet spot ,measure the pot value,and put this value of resistor across the outputs of your preamp .Heres a usefull piece of software that might help,
http://www.ymec.com/products/dssf3e/
It has a signal generator and a scope and works with almost any soundcard,
best of luck,

Thanks.  I will sleep on this, it is a bit over my level  

Second question. If i'm not mistaken, transformers are nice for isolating the circuits, and for driving long cables.  And they can add nice color or sound weight. 

But if we don't really want coloration, and the ADC is 50cm next to the preamp, what would be wrong with skipping the transformer and feeding the ADC unbalanced? 

In my case, the trafo is wired 2:1.  Output level (gain) seems to be the same in unbalanced. 

Agreed on all the points you made about transformers ,
I have a behringer Ada8000 convertor myself ,I didnt want or need the mic preamps ahead of the A/d stage ,so I literally wired  xlr's direct to the a/d chips themselves ,these chips want a balanced input of around +17 db to hit 0db digital .Yes you most likely could feed them unbalanced if you wanted ,but I think performance is slightly degraded if you do .
Depending on what kind of A/d convertor you use ,its most likely got a buffer stage at the input ,so grounding one of the input terminals and going in unbalanced should be ok .
I got around to testing my neve-ish clone earlier ,the expected gain loss that I anticipated ,due to the step down ratio of the transformer didnt materialise at all ,I'm guessing because the output of the transformer was expecting a much much lower  impedence than the 1 kohm I used ,I was able to get an output of about 27 volts rms out of it with no sign of clipping at all ,and bandwidth was ruler flat from 20hz to more than 50khz.
In most of my designs I like to keep the input transformer seperate from the preamp ,that way I can choose, depending on impedences involved which transformer suits the job best ,or use none at all if I want a less coloured sound .
Heres a pic of another of my creations ,EL84 based preamp ,drives a Sowter transformer to match the highish impedence of the valve to 600 ohms .
Simplicity pays big dividends when it comes to sound,thats my motto ,
All the best,

Tubetec said:

Heres a pic of another of my creations ,EL84 based preamp ,drives a Sowter transformer to match the highish impedence of the valve to 600 ohms .

Click to expand...

Got a schematic of this?