Good Phono preamp circuit?

[tvi]

The JFETs are avilable for this:

[amplifierguru]

Hi Nuvistor,

In practice the Ccomp could probably be reduced by 5 times giving a 10V/us slew rate FOR THE FIRST STAGE. This would be comparable with that of the chip.

Zinsula,

I'm sure the 2SK170 would be fine at 2mA (your comparison compares a 2SK369 at 3mA with the 2SK170 at 1mA) once Fletcher Munson and resistor noise is added. Still an order of magnitude below surface noise.

Cheers,
Greg

[nuvistor]

Hi ampguru -

I've done some LTspice simulation of your discrete + opamp circuit with 2sk369 and NE5534 models, acknowledgement to Uwe Beis for the NE5534. I tried flat gains of 5 and 50, Rf=1k, corresponding to RIAA MM and MC gain beyond 50kHz.

With your compensation values I had no success with stable feedback at gains of 5 or 50.

With Miller-integrator compensation of 20pF around the op amp with 5534 Cc=22pF I had success at 50 but not at 5. Avol is 130dB at 20Hz, 77dB at 20kHz, 33dB at 2.4MHz with phase margin of 45°. Perhaps some combination of input lag and Miller compensation could work at 5.

The 2sk369 drain capacitance affects stability, I cascoded with 2N4392, without cascoding 45° pm is at 50dB, 300kHz.

All of this reinforces my opinion that amplifiers with 3 gain stages are difficult to compensate simply at low closed-loop gains.

[PeterMoreton]

FYI, I just had a reply to an inquiry about 2SK146's:

"thank you for your enquiry via www.mark5.com.

We are pleased to quote:-

6 min x 2SK146 at £11.37 each
Delivery from Australia by Air Parcel Post at £15.44 10 days approx"

Expensive, but maybe ok if a group of folk get together to purchase?

[audiodesign]

PeterMoreton,
Have you created the pcb for the Walter G. Jung phono pre ?

[Bonsai]

if you follow the link to my web page below, here is an article on RIAA preamps and a spread sheet to calculate the equalization network using the Lipshitz equations.

It is fun to build using discrete components, but value for money and performance probably favors an op-amp approach.

Note that in the Baxandall circuit I discuss, you should use a buffer after the 2dary post filter - something AndrewT raised and that I will attend to when I write part 2. Right now though I am up to my eyeballs in my next power amp. ;-)



RIAA Phono Equalizer, phono amplifier | hifisonix.com