Audio interface as phono preamp - impedance and capacitance loading

[billybuck]

I would like to digitize phono output flat and apply RIAA (or other de-emphasis curves) in the digital domain. I realize there are issues with this, but I'm persisting because a) I'd like to see if there are advantages to declicking/decrackling LP output before de-emphasis is applied and b) I would eventually like to digitize 78 rpm discs, which use a variety of emphasis curves.

The mic pre's in my Scarlett may be quiet enough and have sufficient gain for this, but my phono cartridge (Shure V15vxMR) is known to be sensitive to capacitance and impedance loads. Is there a relatively inexpensive, DIY solution to stepping the Scarlett's XLR mic inputs to the 47k/100pf load my cartridge would like to see? I'm ready to solder.

 

[DVDdoug]

The input impedance on a mic preamp is typically around 1-2K. There's no way to increase it without modifying it.

A parallel resistor on the input will decrease the impedance.

A series resistor will increase the impedance but it makes a voltage divider with the voltage proportional to the resistance-impedance so 90-95% of the signal would be lost across the resistor.

There is probably a resistor at the input solely for lowering the impedance and if you can locate that resistor you could possibly remove or change it.

Most audio editors/DAWs work in floating-point (super-high resolution) so you could remove the existing EQ or apply your own EQ curve for processing, and then un-do those changes.

 

[levimax]

I do exactly what you want to do using a Steinberg ADC which is the same thing as your Scarlett. I built this "balanced transmitter" but changed the input resistors to 47 K (designer said no problem and it is no problem) https://sound-au.com/project87.htm . It is easy to build if you get the PCB. With this I run very short cables from TT to transmitter to keep capacitance low and then you can run balanced to your ADC as long as you like with no noise or other issues. The ADC will have plenty of gain for a MM cart, for MC I run a SUT in front of the balanced transmitter which again provides the correct load for the SUT. RIAA in digital is no problem (I use FIR filters) and noise is much lower than before and I don't have to worry about placement of long runs of RCA cables. For your use case I think it would work perfectly. Good luck and have fun.

 

[billybuck]

Thanks for the assistance. I ended up going with a $120 “flat” phono preamp from Tracer Technologies, who make hardware/software designed for vinyl and 78 restoration. I suspect it’s a $20 garden-variety phono pre that’s been modded to remove the RIAA curve, and I’m perfectly OK with that because it’s quiet and adds the necessary gain, while keeping the standard phono loading.

I’m using it in a real-time de-clicking setup where the output is digitized at high res, de-clicked on the fly with ClickRepairRT, then Equalizer APO adds the RIAA, low-bass summing, and room correction via convolution before sending the audio out to my receiver’s SPDIF input. Works very well - even hacked-up thrift store LP’s sound great. Except for a bit of latency, it’s just like playing LP’s normally, minus the noise.

The whole thing runs on a mini-pc sitting demurely behind my audio gear. When I want to save a vinyl rip, I just launch Audacity via Remote Desktop and use its loopback function to capture the processed audio being sent out.

Sort of a poor man’s Sugar Cube, but with more flexibility for tinkering and DRC. Plus, I think the performance of ClickRepair is hard to beat. I’ve put it up against Izotope RX’s Declicker, and stuck with CR because it’s less CPU intensive and kinder to transients.

Applying the RIAA in the digital domain does seem to have benefits. One is that the declicker comes first, so the boosted treble seems to help it identify noise a little better. Second, the accuracy of applying it digitally does seem to subjectively improve on the sound of my previous Gram Amp 2 pre.