On Tuesday, August 26, 2008 6:25 PM, Tom Fine wrote:

> You'll find that most phono preamps need a tweak here or there to 
> flatten the curve, much as using an MRL tape to align a tape 
> playback requires a tweak here or there of the EQ trimmers if 
> you've changed heads or components have drifted in the playback 
> amp.

Tom, I think you're on the right track.  Cartridges are generally
not all that flat, and there's a lot of variability between each
individual cartridge.  The weak link is often not the phono preamp, 
but it is the cartridge and the cartridge loading.  Even L-R balance 
can be quite variable with cartridges.

So the best thing anyone can do with RIAA (or any EQ) disc transfers 
is to carefully measure the frequency response of your complete 
transfer chain (from the cartridge to the DAW) using an RIAA 
calibration disc, and measure the deviations (assuming your 
cartridge is set up correctly).  Include this calibration/deviation 
information with the transfers so that anyone in the future can 
decide whether or not to compensate for those deviations.  At the 
very least, keep a transfer of the calibration disc so that damping/ 
loading issues and EQ deviations are all documented.  If you change 
anything in the playback/transfer chain (cartridge, alignment, 
cable, preamp, ADC, etc.), make a new transfer of the calibration 
disc for inclusion with all future transfers. It's good to re-run 
the calibration disc with some regularity to catch any drifting of 
system performance.

For fun, I collect calibration discs of all eras and EQs when
I can find them.  Some treasure music, I treasure EQ discs!

Interestingly, the ELP Laser Turntable had one of the flattest
frequency responses I'd ever measured.  Far flatter than any 
electromechanical cartridge, which is why I think the sound 
(harmonics, timbre) of the ELP Laser Turntable is quite accurate.  
However, there are other compromises with the Laser Turntable 
(like low signal-to-noise ratio) - nothing is perfect.

> Look at old stereo gear specs for the phono preamp, you'll see 
> that acceptable tolerance was +/-1-2dB in many cases.

The trained human ear can detect 0.1 to 0.2 dB differences.  A
1-2 dB error in EQ is then quite audible.  Which is why there 
are as many opinions as listeners, it seems, when it comes to
cartridge and phono preamp preferences.

Nonetheless, RIAA accuracy of the phono preamp is still 
valuable, as this will contribute less error to the overall
playback/transfer chain.

If you can find a phono preamp that follows the RIAA curve at
+/-0.2 dB you are doing really well.  At least that's one
element in the playback/transfer chain you don't need to 
worry about so much.  I think tone controls can get in the
way of audio transparency, so I'd rather make these fine
EQ adjustments for cartridge inaccuracies in the DAW.

> However, it's non-ideal in the first place to have only a 
> pressed RIAA LP of something to transfer. Much better to 
> expend the efforts and find and transfer the master 
> tape.

Sadly, master tapes sometimes go missing and a mint LP may
be the best that's available.

Eric Jacobs

The Audio Archive, Inc.
Tel: 408.221.2128
Fax: 408.549.9867
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