MM Turntable into 1Meg Ohm DI, it works-ish!

[L5730]

Hi all,

I thought it might be useful to post this for anyone else wanting to 'have a go' and not finding any useful information online, or rather lots of "don't do it" messages.

I hooked up the old NAD 5120 with Ortofom OMB (?) MM cart to the RME Babyface Pro fs analogue 3/4 inputs. These inputs are for unbalanced line level sources like CD players, cassette decks for example, but are also capable of handling a passive (normal) electric guitar pickup output so one can play guitar or bass in directly and use amp sims or re-amp the recording later. The cartridge/stylus, from what I gather, is expecting to see a 47k Ohm impedance. Mic preamps are not suitable as they are in the 1-10k Ohms range and will massively screw up the frequency response. The DI inputs on this interface are both 1Meg Ohm so may also add some frequency response problems but won't be anywhere near as bad and won't 'load down' the cartridge.
Next is RIAA de-emphasis. Vinyl is created from an audio source that is EQ'd to suck the bass out and boost the treble, we're talking +/-20 dB at the extremes here. This is so so that more play time can be cut into the vinyl disc and for other technical reasons that are well published. When using a device without this RIAA EQ de-emphasis everything is going to sound very bad, no bottom end and ultra fizzy treble. It's pretty easy to simulate this curve in software and there are quite decent and accurate implementations around.

So RCA Left and Right out f the turntable, into the two 1/4" unbalanced connectors on the side of the BFPfs, set to -10dBv and gain at maximum +9dB.
Comes into the DAW and I apply an approximation of the RIAA EQ curve. Apply +24dB of gain and we are now listening to a vinyl record on the computer, and it sounds decent enough.

I am not saying that this method is anywhere near the best method. The signal is simply far too small for those inputs on this device to do much with. It would be preferable to purchase or build a small amplifier circuit with adjustable impedance to handle different cartridges that may arise. This could then feed mic pre amps balanced and just be cleaner and more accurate overall. Or one could buy an off-the-shelf phono pre with built in RIAA EQ and just use it as is, very easy and mostly decent enough results. Heck, one could always utilise the phono pre in an integrated amplifier if it has one and has tape record outputs.

Unfortunately, I've discovered that the left signal and right ground wires broke free from their solder point. Tiny ultra thin wires that lead up to the 4 pole socket for the tone arm, nothing to really solder there. Other issues need attending to with this deck, but it was nice to pull it out of storage and mess about with it. Also nice to confirm that I could use the BFPfs for this and get a working result, even if not perfect.
I forgot how much of a faff dealing with such large and fragile, thin and bendy discs of plastic was. Why would anyone want to go back to vinyl when digital solves so many issues, and is crazy convenient? Each to their own, not for me though.

 

[DVDdoug]

I'm pretty sure high impedance will mess-up the frequency response, but I don't know how badly or in what way. There's a reason cartridges have a specified load.

The capacitance load is also important (some of it comes from the cables).

The inductance & capacitance creates a resonance just-above the audible range, but it has some effect down in the audio range and it creates a boost (which is needed for flat response). If the capacitance is too high, the resonance comes-down further boosting the highs. If the capacitance is too low you get high-frequency roll-off.

...If you had a test record you could modify your software RIAA EQ to compensate.

 

[somebodyelse]

https://linearaudio.net/record-replay-riaa-correction-digital-domain includes designs for buffers to stick between mm or mc cartridges and a mic input, as well as the EQ part that's the main content of the article.

 

[AnalogSteph]

The DI inputs on this interface are both 1Meg Ohm so may also add some frequency response problems but won't be anywhere near as bad and won't 'load down' the cartridge.

As mentioned, that's not how this works. You need the parallel input resistance to provide adequate damping for the LC circuit resonance formed by the cartridge L and input C. Lower R means you can accept more C but also means that frequency response won't extend as high. People commonly do that (e.g. 36-39k) for fussy AT cartridges designed for 100-200 pF loads, which is hard to achieve unless your preamp has lower-than-average input capacitance to begin with (standard 220 pF need not apply). With some other cartridges like some Shures you may see benefits going over 50k. 100k seems realistic with a low-C prepre more or less directly at the tonearm.

The good news is that adding resistance in parallel tends to be relatively easy.

 

[L5730]

Oh believe me everyone, I know this is not the correct way to do it (eg. adding 20+ dB of digital gain is raising the noise of the ADC by that same amount too) but from what I have always read was that it's not possible and will sound terrible and the sky will fall in. None of that is true in my case. It sounded OK. I wouldn't use my janky setup to archive vinyl though, just a proof that it worked.
I couldn't be bothered to hook up cables to the phono in and tape out on the integrated amp in the other room. If I had done, it would no doubt sound better, albeit with RIAA baked in. Was a convenient way to find out the wiring needs soldering though.