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Audible Illusions Modulus 2C enhanced(very detailed)

Posted by Setu (A) on July 11, 2013 at 00:26:23

Background: purchased online auction 3/17/2006 for $438
this 1988 preamp (ser#M7121687) had very low output and seller never
responded to me.(faulty 723 voltage regulator). In Jan 2013
I finally began fixing this stored preamp.
Somehow I located a schematic of the 1980? Mini Mite.
Thanks JonL of AK for the reverse-engineered Mod 2D partial-schematic
with Japanese characters. Otherwise I have totally failed to
find a Modulus 2C schematic! I wonder who designed this
simplified linestage and the phonostage(RIAA via feedback?)
I knew this 2C was far superior to
my PAS3X by the extra detailed sound. The AI build quality was a
pleasure to work with.

Below are subjective details of this Modulus IIC.
I relate to listed vendors only as an amateur hobbyist...

Power supply:
-8x HEXFRED Vishay VS-HFA08TB60P 8A 600V these "ultrafast" diodes
with a 18ns recovery time replaced the B+ and heater
diodes that had dark-brown burn-marks underneath--and
they are now 1/2in above board. Yes,overkill.
-3x Nichicon 10,000uF caps---perhaps I should have spent more here?
however there was only .01 VAC on the DC heaters.
-new 16/3 power cord
-NSC LM723J voltage-regulator(U1) I had been through 2 bad 14-pin ICs before finding a NSC LM723J with military?-125C rating. I heard the
85C more commonly fail..
-new C26 F&T of Germany 100uF @350V this appears to be the smoothing
cap for transistor Q3---the old Mallory had highish ESR(.68).
-new C31 cap Elna Silmic II 470uF @50V The old W German cap strangely
measured 622uF with an ESR=.12 I like to see an ESR of .01
This was a BIG improvement in sonics. Silk fibers?!
-new C28A .47uF@400V K40Y-9 the old Wonder Cap "Ultima Series
with Danor" for filament smoothing probably worked ok but I like
K40Y-9 in this high 60hz EMI area.
-new C24 & C25 100uF@400V Jantzen MKP to replace the 2x 25yr old
electrolytic Mallory Plate supply caps. Bypassed with a K40Y-9 PIO
Russian .47uF these 2 polypropylene caps are BIG and need strapping
thru 1/8in pcb holes. I try to avoid electrolytics.

Linestage:
-6H23n-EB tubes from Upscale Audio AI Tech Note #3 strongly suggests
tube-matching/low-noise so a trusted vendor seemed wise
I also fantasize about E288CC tubes...
-C19 & C20 these 2 old Blackgate electrolytic bipolar caps are
cathode bypass caps that are not in the signal path although
they might be considered so. I used Nichicon Muse BP 470uF at 35V.
I probably should have increased >600uF here but for space.
A very audible improvement! AND I also bypassed these 2 470uF
caps ith .1uF@200V K40Y-9 The Modulus 2D may use 100pF here?
-C32 plate supply smoothing cap---used a K40Y-9 .01uF @630V
-C17 & C18 output coupling caps The old 3.0uF @310V Wondercaps
were replaced by Russian K75-10 3.3 uF @250V. These are polyester/paper
in oil HUGE green steel-encased caps that should be EMI resistant.

Phonostage:
-separated R from L internal phono input wires and replaced them
with Belden 1505F
-2x new .1uF @630V K40Y-9 these are totsig so better quality is needed
than 25 yr old Wondercaps
-6H23n-EB tubes
-2x Nichicon Muse BP to cathode bypass(as above linestage)
-C11 & C12 polystyrene 750pF @630V from Triode Electronics
the 2 Wondercaps seem to be part of a global phono feedback
so polystyrene here.
-C33 plate supply smoothing cap---now a K40Y-9 .01uF@630V
-C13 & C14 2400pF @600V Relcap-51620 polystyrene to replace old
Wondercaps More global feedback so polystyrene best.
-2x K75-10 3.3uF@250V to replace old Wondercap output coupling caps
Totsig so purity needed here---with NO bypass.
-C1,C2,C3,C4 can I remove these 4 24pF caps?

Miscellaneous:
-move various volume-control wiring for better channel separation
-removed 4x 100k resistors from AUX input
-reflow all solder joints with Cardas Quad Eutectic solder
(copper & silver bearing solder blends nicely with the orig AI 2% Ag)
-4x new 3/8in vent holes drilled into pcb
-non-residual contact cleaner to tube-sockets,switchs,pots
-4x Vibrapod Model 1 feet. Now unit weighs 12.2 lbs so Model 2 is
better?
-Vampire CM1F panel-jacks The old Tiffany jacks were gold-plated
brass----Vampire gold-plated copper jacks fit perfectly ($9each)
-unscrew,sand,clean,solder the main ground screw and attach ground
wire from 3-prong power plug and also 22g sheet-metal fence so that
1 central ground serves all(except phono input shield braid).
My Fluke 83 detected much internal 60hz EMI(from worst to least):
power-trans output wires,diode CR2 the last of plate supply diodes,
Q3 rearmost side,470uF C31 cap,CR12 diode,NF4-6V solenoid-switch,Q1
The new&ugly grounded 22g sheet-metal EMI-fence can only help
shield the phono&line. More 1/2in bottom-cover vent holes were
drilled to help with air flow---the emi fence might have hurt
cooling? The green power cord wire took the hum away!
-add silicone caulk to all top-heavy or vibration prone caps etc
Because AI Tech Notes request low microphonics I used probably too
much silicone.. I added blu-tack like material under top&bottom
covers and also 4 corners of power-trans. I touched cabinet
during play and felt sound vibration so the AI metal felt slightly
'tizzy'. Now the dampening material seems to help.
-add flat neoprene plumbing washers to 4x mounting bolts of power- trans
-the 2 Alps vol controls each measured about 87K

Voltage readings(at 121.6 VAC line-voltage):
6.12 VDC tube-heaters
253.4 VDC B+1 for linestage plate supply
228.8 VDC B+2 for phonostage plate supply
65.3 112.2 VDC for Right phono tube plates pins 1 6
63.4 116.3 VDC for Left phono tube plates pins 1 6
126.9 VDC for Right line tube plates
123.0 VDC for Left line tube plates

I found some crosstalk in the push-button input switches and
Dynaco FM-3 tube tuner crosstalk in other inputs.
I saw the AI efforts to minimize hum with the low elegant layout.
The 2U rackmount and aluminum alloy cabinet is very nice.

The sound is Great!

This post is made possible by the generous support of people like you and our sponsors:

The Cable Cooker

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