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Audible Illusions Modulus 2C enhanced(very detailed)

98.234.25.39
Posted on July 11, 2013 at 00:26:23
Setu
Audiophile

Posts: 17
Location: Brentwood,Ca 94513
Joined: February 17, 2009



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!








 

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RE: Audible Illusions Modulus 2C enhanced(very detailed), posted on July 11, 2013 at 10:00:12
6bq5
Audiophile

Posts: 4393
Location: SF Bay
Joined: August 16, 2001
 Looks Great - and glad you are pleased!
Art Ferris would cringe - but you like the sound, and that is what matters-
The Modulus came from a Bruce Moore design, and AI tweaked it till it sounded better, then released a new model-
I have an M3, and found that Ferrite beads on the Phono input helped reject RFI GREATLY!
Happy listening
Happy Listening

 

RE: Audible Illusions Modulus 2C enhanced(very detailed), posted on July 11, 2013 at 21:23:01
wheezer
Audiophile

Posts: 4309
Joined: January 24, 2001
 6,
I suggest replacing the stock phono wire with:
*4 X 33ga... 2.5mm D shielded 4 conductor... $6.95 foot
From Michael Percy. The Cardas wire needs to be tinned first.
in a balanced configuration:
Two wires for the signal, two for the return and the shield gets a separate wire > chassis ground strap.
Change the styrene craps on the phono input (24pf) with 338-2951-ND from Digikey

Cheers,
W

 

RE: Audible Illusions Modulus 2C enhanced(very detailed), posted on July 12, 2013 at 15:48:13
Setu
Audiophile

Posts: 17
Location: Brentwood,Ca 94513
Joined: February 17, 2009
 I much appreciate the phono input wire advice and any other changes
you might suggest.
Belden 1505F is 21g with 7x29 stranding. I chose this because of
it's 17pF/foot rating. I have some 20g silver wire with shield but
it's linear capacitance > 60pF/foot which seems too much to add to
phono wiring... I see that internal wire-shield needs a drain.

In this Modulus 2C for Right phono channel I have 2 "input" silver
mica caps 24pF C3&C4 which seem to drain off RFI into cathode
and plate respectively. And there is 1 polystyrene cap (24pF ?)
which seems to use pin 6 plate-output to feedback into cathode pin 3.
Perhaps this local feedback creates the higher-freq RIAA-correction?

Are you suggesting that I might change the polysty feedback cap to
a silver-mica? I cannot get an in-circuit measurement of this 24pF
local-feedback polysty cap. I do worry about polysty thermal stability
which a silver-mica ensures.

I hesitate to remove those 2x 24pF C3&C4 caps on each input channel.
If they really do just simply shunt RFI out of input then why
is it shunted (equally) into cathode&plate rather than into ground?

ONE remaining problem: my phono gain always has been too low even with high catridge output. Can you please suggest reason/solution/mod.

I am mildly ashamed of my cringe-worthy changes to this cleanly
designed preamp. I hunt high performance on a budget so
I used lots of big Russian parts. It is a work-in-progress?
Although I trashed this preamp somewhat, it sure has
much greater detail now.

Much thanks to you and others who generously share advice
on these old preamps without (available)schematics.






 

RE: Audible Illusions Modulus 2C enhanced(very detailed), posted on June 29, 2015 at 20:26:06
Setu
Audiophile

Posts: 17
Location: Brentwood,Ca 94513
Joined: February 17, 2009
 (I accidently also posted similar on AudioKarma).
Changing psu MOSFET Q3 from IRF712 to IRF710 made a big
improvement. I wonder if old Q3 was degraded by
heat since Q3 does use a 3-sided heatsink.
I wonder how/when/why/brand in choosing replacement psu transistors?
Originals are getting harder to find.


 

RE: Audible Illusions Modulus 2C enhanced(very detailed), posted on June 30, 2015 at 09:32:38
6bq5
Audiophile

Posts: 4393
Location: SF Bay
Joined: August 16, 2001
 The AI equipment was not 'truly' designed in house-
As I said before - most of the designs were based upon Bruce Moore designs-

Parts selection was tightly controled/managed by Art.
what or why he purchased a part - well I can not speak to that-

One other point to bear in mind - the degree to which the mains power was 'always on' for some parts of the pre-amp - while shortening "warm-up time" adds hours to the use of the devices- and shortens the usable life - [if the unit is on for ~8,000 hours-about 1 year- but you only listen to it - say 3 hours/day, 4 days per week - then the listening hours are 624 - leaving the balance of time "un-usable"...

So I am not surprised that a regulator failed - at this point-

One other point from your OP that I did not address originally-
THE "Active RIAA' circuit-
As I understand it, there were always two topologies promulgated by the RIAA/IEEE - one was passive - where the gain stage - though integral to the circuit did not affect the curve, and the other - an Active circuit - where the gain (of the device employed) was integral to maintaining the correct EQ curve. there are a number of old circuits that use the Active topology, and a number - especially after the mid '80s that use the passive.

It is the next item on my list to modify in my M3...


Happy Listening

 

RE: Audible Illusions Modulus 2C enhanced(very detailed), posted on July 8, 2015 at 12:19:55
2con
Audiophile

Posts: 77
Location: lombardia
Joined: February 12, 2003
I had ,among others ,an ai 2c for years .

in my experience medium gain mc (103)with step up xformers

did the best results .

6922 eh (6h23 ev or eb it's the same )is the only choice .

don't even try to put an expensieve nos :marvelous sound for

few hours .

At the beginning Ai used yugo 6dj8 .

Great pre for the money (i have cat reference,c-j permier 3,17ls ,fisher 400cx) still like my Ai (anyway a tube eater and the double

volume pots are a pain in the @ss)

and others)

 

RE: Audible Illusions Modulus 2C enhanced(very detailed), posted on July 8, 2015 at 13:27:35
2con
Audiophile

Posts: 77
Location: lombardia
Joined: February 12, 2003
Sorry forget to say ,

i played a grado 8m (6,5 mv ouput)trough the ai phonostage ,and was

so poor and lacking everything ,while was fantastic in the mx 110

mc intosh phonostage (same ittok arm and sondek table).

When i put the 103 denon (modified by Torlai)with the denon au 320

xformer in the Ai phono ,i was stunned by the quality of sound :

similar at the grado with the aformentioned pre.

Viceversa mc intosh or fisher phono don't like mc coils with

xformers ,but they do like MM or higain mc (es blue point special)

a lot .

 

RE: Audible Illusions Modulus 2C enhanced(very detailed), posted on January 13, 2018 at 11:14:29
optimationman
Audiophile

Posts: 24
Location: west usa
Joined: December 14, 2007
 I too have modded this model 2b(c,d) preamp on a tight budget, mostly using part from the recycle bin of passed projects or dumpster diving. I did rebias/revoltage the tube operation to allow NOS tubes, as suggested and assisted by members here. I also recapped much of the power supplys/coupling circuits. I did find that replacing the .1uf 400v PS bypass cap for the linestage tubes with two(one for each tube) 15uf film cap to vastly improve the soundstage and dynamics. Each cap was attached to the same ground point as the original .1uf cap and the hot side was soldered to the 250v side of the plate resistor for each tube. I believe this maximised the PS separation between channels because of using a single PS. Probably one of many reasons why the model 3s sound better with their multiple PS. Additionally, the data sheet for the 723 regulator indicates that the reference zener bypassed with a 5uf is less noisy than the original .1uf cap made a case for changing the original. I also have extensively changed components. Inline signal resistors have been removed, grid resistors changed to caddock tfts, Mundorf signal caps, input circuit board traces replaced with cables, hexfred diodes, etc. I am surprised how each change affected the sound. I don't need to use double blinded testing becauses if you are likely have removed up to 40% of your changes because it didn't take you where you wanted to go, I figure your on the right track. Of course tube selection makes for changes also. YMNV

 

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