Digitising LPs - a question for Graham

You'd need an extra 40dB of dynamic range/headroom because the output of a magnetic cartridge rises with frequency...

...turning this (the actual output from a RIAA record):





...into this:



For those who don't understand it, if you were to play a record with a cartridge having a "flat" response, such as a condenser/ceramic or crystal cartridge (good luck finding a good one these days) you'd have a near "flat" frequency response.

Like an car alternator without a regulator, the faster it turns, the higher the voltage. So the higher the frequency, a magnetic cartridge puts out proportionally more. The range 40dB equates to a ratio of 100:1.

The reason why the "flat" output in graph 1 (which is graph 2 rotated +45 degrees) is so bumpy is because the cutter head is driven by coils (it's a solenoid) so it is highly inductive, and without additional signal conditioning (a bit of low bass boost and treble boost in the cutter head amp) it would simply resemble a hill, like really ancient records do.

The RIAA curve came into being 1953 using "appropriate technology" of the day. They thought a +/-2dB accuracy was good enough - after all, they had tone controls in those days.

Still, 58 years later, nobody has been able to make anything better!

Edit: (Friday 30th) Nostalgia. If you want the sound of a lovely old warm sounding Dansette or you long for the sound of a good-ol ceramic cartridge the phono preamp doesn't need all those "tucks and curves" just a single pole 6dB per octave roll-off


Edited by Graham Slee - 30 Dec 2011 at 7:01am

Hmmmmmmmm, I have been pretty satisfied with the results of feeding the Reflex into an M-audio firewire interface (though not as good as just playing the record) - but being at a loose end for a day or two  I have taken it into my foolish mind to tinker some more.

Source of the question was the Terratec DMX 6 Fire and Phase 26 USB both of which I own (as also the newer - "I want" DMX 6 Fire USB {24/192 I/O, 48V Phantom power for Mic all singing all dancing}), appear, from the rather scant documentation, to utilise this method.

Specs quote the phono inputs as 47Kohm impedance and sensitivity to suit MM or HOMC cartridges, there is a button on the control panel (software) for "RIAA Filter" but the rather cryptic documentation hints that this is for monitoring only and the "howto" guides one to use the provided Wavelab Lite software to apply RIAA correction using a .dll provided as a post-processing filter.

Down the rabbit hole we go - using ASIO drivers is a "good thing", but being Steinberg proprietary, not all software can use them, notably the freeware Audacity (Steinbergs licensing conflicts with the Audacity license) BUT - it is possible to "roll your own" for personal use (I believe).

My usual preferred, minimalist recording software, offers auto level setting, auto start and auto stop - all quite handy in a stripped down interface uncluttered with all the paraphernalia of Audition or Sound Forge and the like.  Sadly, it does not support ASIO nor high bitrates, which I rather prefer for initial "takes", no harm in down sampling to 16/44 later.

Why now ? - some other darn fool wanted to do some archiving and this one opened its big yap and volunteered to help out - I predict much regret of this impetuousness in the coming days.

Trouble with this stuff and software and 'puters in general, it is the stuff they don't tell you that you REALLY DO need to know to get it all to work and the only way to find out that stuff is to try all the wrong ways and any other avenues of frustration that offer themselves in the process.

Probably not much different to designing phono stages and headphone amps and such eh? 

That's about enuff from Groucho as he goes orf to make 'is Marx -  - maybe this should have gone in Krabby Patty.

Lang may yer lum reek.

Oh BTB, had the lid off the Phase 26 (don't ask) and was impressed to see what appeared to be a decent looking digital pulse transformer on the coax out.

Let you all know how it went when I am done driving myself further round the bend. 

My method, for years, has been to record direct to recordable audio c.d. using an old Philips c.d. recorder/player connected to the tape in/out on my pre-amp. The results are very good.
The c.ds may, after finalising, then be played in car or converted to MP3 (highest quality) for adding to my MP3 player.