COMPUTER EQUALIZATION OF 78s



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Having had several enquiries about using a computer to make up the equalization curves I should say that on the whole I think the preferable method is to use a dedicated preamplifier: however in view of the cost the computer system may be acceptable, particularly if you don't have many records to transfer. Equalizer by Brian Davies is free and will convert RIAA or flat (unequalized) to a number of other equalizations. If you have transferred a 78 at a lower speed and then speeded it up in an audio editor it will also adjust the equalization to suit. Prior to this being available I worked out some figures, and I have left them posted below in case anyone wants to experiment.

The following figures assume a magnetic cartridge properly terminated (usually 47k for moving magnet/moving iron: moving coil as manufacturer's specs) into a flat response before filtering. Ceramic cartridges won't give you a useful result.

When you make up a curve, remember that the turnover point isn't a sharp 'knee' as in the theoretical curve, but the 3dB point where the slope and the flat sections intersect, as in this drawing:

 

WESTREX is the easiest
Flat from 500 Hz upwards
+3dB at 250 Hz
+6 at 125
+12 at 62
+15 at 50
+18 at 30 and I should roll it off below that.

BLUMLEIN
Flat from 500 upwards
+3 at 250
+6 at 125
+12 at 50
+15 at 25 then roll it off below that

ffrr 78
+12 at 50
+3 at 150
0 at 300
0 at 1
-3 at 3.4kHz
-6 at 7 kHz
-9 at 10 kHz

DECCA 78
+11 at 50 Hz
+3 at 150 Hz
0 at 300 Hz
0 at 1.5kHz
-3 at 3.4 kHz
-6 at 7 kHz
-9 at 10 kHz

BSI 78
+17 at 25 Hz
+14 at 50Hz
+6 at 120
+3 at 350
0 at 700
0 at 1.5kHz
-3 at 3.2kHz
-6 at 6.5 kHz
-10 at 10 kHz.

 

CONVERSION FROM RIAA

If you are using a preamplifier which equalizes a phono input for RIAA (LPs) the following figures will convert (approximately) to the two main 78 curves:

TO CONVERT FROM RIAA TO WESTREX
-2 at 500 Hz and flat below that
+3 at 2 kHz
+6 at 4 kHz
+13 at 10 kHz

TO CONVERT FROM RIAA TO BLUMLEIN
-5 at 50 Hz and flat below that
-3 at 250 Hz
+3 at 2 kHz
+6 at 4 kHz
+13 at 10 kHz

TO CONVERT FROM RIAA TO BSI 78
-3 at 50 Hz and flat below that
-1 at 350 Hz and 500 Hz
+1 at 2 kHz and 3 kHz
+3 at 10 kHz

Note that in some cases you need as much as a 37dB gain swing, which is a lot!

The figures are pretty close, one or two may be a dB or so out, but in practice most records only approximate the theoretical curve. Try to get a smooth 6dB/octave slope between the 6db and the next points.

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