Hi,

After a lot of measures and simulations I came to the following conclusions :

- I've used the "Jensen" procedure to determine the optimal load, and finally the best load is 9k1 + 470pF

- That trafo (LL1528) can't with one load show no ringing for low impedance sources (20R typical condenser mic) AND show a full bandwidth for high impedance ones : 300R seems to be the limit for an acceptable bandwidth, 200R is fine.

- The mic/line switch, that adds 4k7 on Jonte's schematic, cannot be adapted for the LL1528 : to have 20dB attenuation, you have to add too much series resistance, or to add another resistance in parallel to the primary to lower the source impedance and get back low/highs. I think I'll remove that switch and use the preamp with the PAD instead, or even better : through the unbalanced DI input. Input impedance is around 770R with the PAD engaged for all audio frequencies, considering the reactance of the Zobel network.

- Even without any load, the LL1528 at the input stage shows some high frequency lossed (1-2dB notch at 5kHz, starting at around 1kHz). When the gain is at the maximum (no NFB at all in the first two stages) this can be seen at the output of the preamp, but for other values of the gain the NFB flattens the response.

Final conclusion : you can use a LL1528 for this project, but it's not the ideal choice.

BTW I measures the leakage inductance by shorting the secondary and measuring the primary's inductance with an LCR. I also measured the inter-winding capacitance, and found the following values :

- L(leak) = 1mH

- Cw = 120pF

I calculated the primary inductance from the datasheet low frequency response and found 12H, so the secondary is 300H. That model seems reliable after some simulation/measures comparisons.

Best regards.

Eric