Aphex
Thermionics
- Division of Aphex Systems Ltd.
Model 1100 MkII Mic Preamp - Owner’s Manual
clock with almost no loading from the 1100 MkII.
If using a 75
Ω
, 1Vp-p clock system, then you can
daisy-chain the W.C. input to additional 1100
MkII’s or other equipment. If the 1100 MkII is the
last unit in line, the best option for terminating
the line as shown in figure 5.
Clocking The 1100 MkII With Brute Force
Not knowing what the 1100 MkII’s word clock
input may be driven by, we made it automatically
accept the brute force voltage levels.
When accepting brute force clock, do not
install a termination adapter - it would be
of no benefit. When the driving source is
not matched to the line impedance, then
adding a load impedance will not improve
are driven by a low output impedance voltage
driver and loaded by a high impedance receiver.
A length of coax driven by a brute forced output
driver will cause edge ringing and wave distortion
of the clock pulses. If the length is too long, the
distortion can cause clock jitter. It is important to
minimize the cable length.
Brute Force Clocking
It is an unfortunate fact that transmission line
techniques for word clock have not been imple-
mented in much of the digital audio equipment
presently available. This leads to real clocking
compatibility problems in today’s studios. Obvi-
ous problems are usually seen as “no lock” but
less obvious is sound degrading jitter.
Though ugly from an engineering viewpoint, the
only solution is such cases is to apply unmatched
transmission lines as best as one can and then rely
on “brute force” clocking to hold it all together.
With brute force clocking, 4-5Vp-p clock pulses
are fed out from a low impedance line driver
with no intention of impedance matching the
coaxial cable. Daisy chaining is not usually pos-
sible because the magnitude of jitter rapidly rises.
Very short cable runs are necessary. Every piece of
gear gets a separately driven feed, and no termi-
nating load is used. Figure 4 above illustrates the
Brute Force clock principle. We have found that
many of the commercially available clock detan-
gler and regenerator/driver boxes actually source
out brute force clock as their hedge against
unknown studio equipment characteristics.
Clocking The 1100 MkII With 75
Ω
Matched
Lines
First, you need to be aware that when switched
to the external clock (W.C.) mode, the 1100 MkII
automatically connects the word clock input jack
to the word clock output jack through a metal
relay contact. This permits direct feed-through of
Input Jack
Loop-Through Jack
Input Jack
75
Ω
Coax Feed
75
Ω
Coax
75
Ω
Coax
Loop-Through Jack
Hi-Z Clock Receiver
Hi-Z Clock Receiver
Hi-Z Clock Receiver
First
Device
N
Devices
Last
Device
Input Jack
75
Ω
Load
Adapter
Loop-Through Jack
Figure 3
Transmission Daisy-Chain
5Vp-p Output Level
Open Circuit
Still 5Vp-p But With Ringing
(Worse With Longer Cable)
Coax
Line
Brute Force
Clock Transmitter
Brute Force
Clock Transmitter
Clock Receiver
<10
Ω
Source
Impedance
<10
Ω
Source
Impedance
Figure 4
Principle of Unmatched Lines
And Brute Force Clocking
Page 10-2
Appendix A: Word Clock Cabling
