Good day guys,
I've had this idea in my head for longer than I care to remember. A digital Phono Pre. Why? I don't know, just because. Well, maybe because in the digital domain, filters, depending on type, are relatively easy to implement and always deliver the same results (as in, 2+2 = 4, always). A quick glance at the internet and even a random comment here in these forums mentioned they exist, but my interest was really sparked with an IEEE paper called "Digital Realisation of Phono (RIAA) Equalisers" by Juha Backman in 1991. Technology has come a long way since then, so much so that a microcontroller can sample and process the data at 96Khz without breaking a sweat.
I took it upon myself to build a proof of concept (it doesn't really qualify as a prototype) using parts I have available, and trying to spend very little. That means, no new routed boards, simple power supply from an AC/AC wallwart, a few wires flying around, and a mock-case. But my specification was: at least RCA phono out, plus headphone out. I like testing on headphones, gives a very quick insight on how things are working and they reveal a lot of detail.
The other part was the programming of the digital filter. I ran a few simulations on Python to compare digital vs. analog RIAA filtering, and the errors are smaller than 0.1 dB up to around 10 KHz, and around 1 dB at 20 KHz. There's some tweaking that can be done to minimise the digital filter error, but I'm not that far ahead yet. Good things though, once the microcontroller is up and running, the filter programming took about 2 hours or so (some mistakes from my part).
The trickiest part is the analog preamplifier. Too much gain before the ADC and the input is overflown, not good. Too little gain and we might get digital artifacts with digital amplification. I settled for 20dB gain on the pre, 20dB gain on the digital filter. I'm sure than can be improved.
In summary, we have:
- AC/AC wall wart, 9V
- Diode bridge rectifier + linear voltage regulators: -9V and +9V
- One preamp, 20dB gain, using one NE5534 per channel, full range
- One microcontroller board with 96Khz/24bit ADC and DAC (PCM1802 and PCM1781), digital RIAA filter
- One headphone amp plus volume knob (based on the O2 / Cmoy design)
And this is what it looks like:
On the second image, from left to right, microcontroller board incl. ADC and DAC, headphone amp board (middle), power supply (top right) and analog pre (bottom right).
Issues to address:
- Power supply. The box should be made bigger, so it can fit a toroidal transformer and the device can be switched on and off permanently. Perhaps try better now noise linear regulators? Provide one regulated output per component.
- Preamp. Increase gain a bit, see if it works without overdriving the ADC, try a simple discrete stage perhaps?
- Digital filter. Separate controller board from ADC and from DAC, and isolate them with a digital isolator (TI has one, forgot the number now). Separate regulators for each. Try different ADC's / DACs? Adjust filter values to improve RIAA curve accuracy.
- Headphone amp. This headphone amp ROCKS dude! It needs nothing, it's so f***ing perfect!
- Grounding. There seems to be an issue with ground loops, I'll have to have a look at that. I know I have issues on the cable from the TT to the amp, because no matter what, each time I change the connection, it produces nasty hum unless I wiggle it a bit about. That happens with each amp, not just this one.
And yes, the sound. First I had to deal with some hum, nasty stuff, at which all this sounded quite dull. Once that was removed (there is still a minimal hum component, but on very close inspection only) the sound bloomed man! Seriously, it doesn't sound bad at all. I tried some Commitments and Michael Jackson's Thriller (I find it's a very hot mastered LP), and the detail as well as the highs are all there, at least all I need.
Is it detailed? Hell yes! You can hear any tiny pop or click or imperfection, if there is, as well as music detail. Not saying that the phono pre in my integrated does a bad job, but this baby, as it is, put a big smile on my face (maybe because it actually works?).
Still, I reckon it's a bit soft (as in volume), without A/B comparison to other phono pre's I still need to up the gain a bit, say 15 - 20 dB, and that's the trick. So for the moment, back to the workshop (I still want to listen to records from time to time), do some tweaking, perhaps take the TT to the workshop and check the cabling, oh!, and hook it up to the suitcase class D amp, now, that would be a cracker!
Alright fellas, time to stop procastinating and do some honest work now.
Cheers,
Alberto