| Quad Eight EQ312 modules |
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Over the years i've racked many pairs of Quad Eight MM312
Modules. These units feature a 3 band inductor based eq, with
each band switchable over four frequencies, a line input transformer
with a passive 5 position level control. The electronics are
formed around two identical class AB discrete op-amps. To the
left is a redrawn schematic of the op-amp. |
| When bench testing the units, I found the distortion was a
lot higher than expected. Further examination revealed that
the first ( input ) amplifier was suffering from crossover distortion.
The image to the left shows the distortion products. |
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Eventually I found that the pcb layout was incorrect, with
reference to the schematic. The layout resulted in the two
series connected diodes having a short across them, effectively
taking them out of circuit. The result of this is that the
output transistors have no base bias and are effectively operating
in class B mode...hence the crossover distortion.
The image to the right shows, the pcb layout that joins the
anode and cathode's of the two diodes together. Note that
the opposite ends of the diodes are connected together on
the other side of the pcb, this places them in series. |
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Restoring the pcb layout to comply with the schematic
( and common sense ) resulted in the crossover distortion
disappearing, leaving fundamentally a third harmonic residue.
The THD levels dropped from ~ 0.45% to less than 0.03%. Most
significantly the unmodified pcb had distortion % rising with
decreasing signal level.
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To the left are spectral plots of the before and
after distortion products. These are measured at -20dBm and
1kHz signal levels. The fine cluster of lines should be ignored,
these are mains hum and harmonics as should the noise floor
which is raised on the modified plot as the scale was increased
by ~ 20dB to show the presence of the 3rd harmonic. What is
significant is the even spread of odd and even harmonics of
the original circuit against the almost pure third harmonic
of the corrected circuit. |
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The twist to this tale is the history of these channel
modules : The console from where these modules were
obtained was originally installed in Burbank Studios.
It was on this very console that the soundtrack for
the second Star Wars movie was mixed. In 1980 Bill Varney,
Steve Maslow, Gregg Landaker and Peter Sutton won an
Oscar for sound for their work on Star Wars - The Empire
Strikes Back using this Quad Eight console.
The Quandary ...... so, it's 2004 and these modules
are being racked up ready to go out in the world for
their next assignment ..... should this "fault"
be left intact to preserve their heritage or should
the pcb be corrected to create the circuit as intended?
I opted for the latter .... although for a fleeting
moment I thought about placing a control on the front
panel to switch in and out the "fault"! It
could have been called the "star wars" mode. |
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