Final modification for max +53dB boost and implementation of LPF

A project log for Audio - Phono preamplifier boost

Clean hack of the otherwise very solid ART DJ-Pre II phono preamp with the aim of improving the existing design and enhancing its features

jovanJovan 06/04/2023 at 18:520 Comments

I see that I haven't updated the status of my modification work, so let me give you the last update.

On the ART-DJ-PRE-II I modified the following:

- The old 5k Log Potentiometer was retained but I separated pins 1 and 4 from the existing line and bridged that connection with a 1M resistor with the addition of a 100pF NP0 capacitor between pins 1-2 and 4-5. In this way, I released the potential to get +54dB of gain, and I managed to suppress (LPF) the higher frequencies outside the audible range with the reservation that at least the 2nd harmonic of the sound at 16kHz can appear without loss (illustration below) by doubling LPF cutoff frequency.

- I then added a 3.5mm stereo banana connector (metal shielded), which is tied to the output using a shielded separate Tasker C121 2x0.25 cable.

I thought about adding the logic for the XLR connector and installing it, but unfortunately, I'm running out of space in the case itself, and I don't need it at the moment, so I don't want to complicate it. I also gave up on the idea of installing an ADC with the addition of a BT controller, which would enable wireless streaming, since as far as I know such a solution already exists in the DJ-Pre-II version.

I am currently working on my own solution for a +60dB Phono Pre. Amp. which in its current design achieves a FOM to RIAA as high as 65105!, which means that the deviation from the RIAA defined curve at any point is less than 0.01dB. Of course, this implies the use of a passive RIAA network, which is at the optimum limit of -22dB with manually matched components. For now, even if it's only in the protoboard phase, the results obtained are fantastic, but more on that in another place and some other time.

Best regards,

dr Jovan I.

p.s. If you need expert help taking your devices to the next level, I'd be happy to help, so feel free to call.

An after final assembling I tested it with my Pioneer PLX-500 Direct drive turntable and additional Pro-Ject Pick it 25A Phono cartridge. And and enjoyed the richness of the sound.

For some future MC cartridges, the current +54dB gain will be insufficient, so I started working on the ultimate audio preamplifier for +60db with minimal noise and based on passive RIAA, ultimate OPA 1612 and the use of PP MKP and FP capacitors.

All Op.Amps used in test:

I even discovered along the way that some of the earlier purchases arrived as fakes such as this LM4562 on the right of next figure. Can you spot surface finish?

Best regards
Dr. Jovan Ivković