Paris 2016 devinyliser

1. AES PARIS 2016

Engineering Brief EB3-5
Non-intrusive Rumble Filtering by VLF
Crossfeed with High Filter Slopes
Douglas Self
1

2. The Problem

● Vinyl disc replay gives rise to large amplitudes of
subsonic noise.
● This tends to peak significantly around 8 – 12 Hz
due to cartridge/arm resonance.
● This is only an octave away from 20 Hz, the
bottom of the RIAA specification. A quite
sophisticated conventional filter is required.
● The subsonic noise results mostly from vertical
stylus movement so the electrical signals are in antiphase
2

3. Stereo crossfeed

● Stereo crossfeed at low frequencies cancels anti-phase
signals, but in-phase signals unaffected.
● Usually low frequency sounds are panned to the centre,
for trackability and to make use of both stereo amps.
● Therefore the bass response is unaffected by stereo
crossfeed.
3

4. Passive crossfeed

Fig 1: Anti-phase rolloff is -3 dB at 32 Hz, falling at 6dB/octave
4

5.

a) Not usable
b) Path-2 phase corrected
With (b) the 6dB/octave rolloff is on the anti-phase signal,
but there is +6 dB LF boost on the in-phase signal .
Fig 2: Notional crossfeed circuits using 2nd-order lowpass
filters
5

6.

Fig 3: Langvad/Macaulay crossfeed arrangement with Path
3 added to cancel in-phase +6dB LF boost
6

7. Fig 4: Low-impedance implementation of crossfeed with Path 3

Fig 4: Low-impedance implementation of crossfeed with Path7 3

8. Fig 5: Freq response for anti-phase signals of Fig 3 circuit

The anti-phase response has a +2dB peak,
and a rolloff slope of only 6dB/octave
Fig 5: Freq response for anti-phase signals of Fig 3 circuit
8

9. Fig 6: allpass filter added in Path 1

9

10. Fig 7: Frequency response changes as allpass delay is increased, moving to steep slope of 18 dB/octave

10

11.

Fig 8: As allpass delay further increased, steep slope
develops a useful notch at 8.5 Hz. After that notch moves
up in frequency and section below it has slope of only
6dB/octave
11

12.

• Note that the steepest slope is 18
dB/octave, not the 12 dB/octave that
would be expected from the 2nd-order
lowpass filter.
• The steepest slope is also 18 dB/octave
when 3rd and 4th-order lowpass filters are
used. However, these filters give deeper
notches at 8.5 Hz.
12

13. Fig 9: THD+N for in-phase signals at 2 Vrms (L and R channels) 'Gen' is testgear THD

13

14. Fig 10: THD+N for anti-phase signals at 2 Vrms (L and R channels) 'Gen' is testgear THD+N

14

15. Awkward resistor values

Three possible solutions:
• One E96 resistor. (1xE96) Stock problems
• Two E24 resistors in parallel. (2xE24)
On average 3 times more accurate in setting
nominal value than 1xE96.
• Three E24 resistors in parallel. (3xE24)
On average 30 times more accurate in setting
nominal value than 1xE96.
More resistors mean lower effective tolerance
.
15

16. AES PARIS 2016

Engineering Brief EB3-5
Non-intrusive Rumble Filtering by
VLF Crossfeed with High Filter
Slopes
[email protected]
douglas-self.com
16