Modifying a discrete opamp

[robmil]

Hi

I've found a discrete opamp that I am going to build for a mic preamp project. the design is from Bryston but the input transistors I do not have but also are pretty noisy for my application.

I'm going to use the 2SC2547 and 2SA1085 which I have a few of. The biasing resistors on the schematic are pretty high value and I have no way of measuring. Is it a simple case of V=IxR? - yes I am a novice!!

Also if I replace the 15K for 1K and add a 1K to the other transistor collectors (that means 4 x 1K resistors) and also instead of 115K use a current source like J511 in each emitter pair, would that be better scheme.

I need to get the input stage noise as low as I can. The 2SC2547E/ 2SA1085E have a noise level well below 1nV which is good for my application.

Thanks for your help

R

[robmil]

Hi

I've now uploaded my modified input section. Will this work?

thanks

[Samuel Groner]

I need to get the input stage noise as low as I can.

What is the source impedance? Besides voltage noise you'll also need to consider current noise.

Samuel

[robmil]

Hi Samuel

Source imp anywhere between 200 ohms upwards to about 2K ish.
I've seen other designs using the 2547E but those opamps are pretty complex and I'd like to limit the parts count.

As an aside.... I saw the input configuration on your website and thought it might work in my application with some minor resistor changes.... hope you don't mind

Rob

[Samuel Groner]

For a mic preamp you usually want a balanced input and a wide range of gain. I'm not sure if you appreciate the challenges involved--at least your opamp plan doesn't sound like the best solution. Getting low distortion over a 0-60 dB gain range won't be easy without the use of current feedback topologies and/or multiple gain stages.

Samuel

[robmil]

The plan has been modified slightly with the use of an input transformer but I still would like to use a low noise discrete opamp afterwards. What constitues higher noise levels in the Bryston opamp compared to the Forsell or JE990 types?

[Samuel Groner]

Do you know what voltage noise, current noise and optimum source impedance is?

Samuel