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Audible Illusions Tube Preamp Modifications
The Model 2 and 3 tube preamplifiers fit a nice niche in Audio Equipment.
They are well designed, were built to sell at an attractive price point
and offered an excellent value in “High End Audio”. That’s
why they were so popular and why so many of them are available on the
used audio market at affordable prices.
There has been some talk on the web about the modifications Chimera Laboratories
has done to these preamps by people who purchased the “modified”
units. I found this out by e-mails asking me about the modification and
what I would charge to modify theirs. Unfortunately I do not have the
time resources to help them out, so I have made the “modifications”
available to anyone who is interested.
Let me start off by clarifying that I don’t consider the following
information is really a modification of the unit. Modification to me means
optimizing the circuit. I consider the changes to the Audible Illusion
preamps an upgrade or “tweak”. The circuit is not changed, instead
“better” parts are installed to maximize the performance of
the circuit design.
These modifications can be applied to a wide variety to commercial tube
preamplifiers. Manufacturers select the passive components used in products
based on what’s readily available and what they view as cost effective.
The audio engineering community in many cases does not select parts by
subjecting their products to extensive listening evaluations.
The information I am providing does not provide any schematics or diagrams
identifying the specific location of the parts needed to be replaced.
In the past it has been very difficult to get this information from the
manufacturer. If any one has schematics or diagrams, I would like copies.
The mod consists primarily of replacing passive components with a much
better grade of audio components and careful tube selection. The tools
required are a decent soldering iron, solder wick or better yet a de-soldering
iron. Radio Shack sells a nice unit. You will also need some the appropriate
wrenches to replace the potentiometers and RCA phono jacks.
Removing the top and bottom “plates” of the Audible Illusions
preamp provides adequate access for the removal and installation of parts.
Note the location, size and value of the parts discussed below. Make a
list and order the parts from your favorite supplier. I have listed the
suppliers I used t source the parts. I have updated the upgrade to include
parts, now available, that offer better performance. I rank the upgrade’s
impact on sound performance on a scale of five to ten.
This will allow to select the upgrades that meet your budget
Power Supply Upgrade – 7
The silicon rectifiers should be replaced with diodes that offer better
turn-on/turn-offcharacteristics than the stock units. I like the Rectron
HER diodes available through Mouser Electronics. Pick something rated
for at least 2 Amps at 1000 PRV. Nice idea to make sure the lead diameters
will fit in through holes of the circuit board.
Replace the power supply electrolytic capacitors. Choose from a wide array
of modern low ESR, high frequency capacitors. The new modern capacitors
can offer higher capacitance in the same size as the original capacitors.
Go ahead and use them, it just makes your power supply stiffer. Just make
sure the capacitors have the same or higher voltage rating as the original
capacitors.
Elna, Nichicon, and Panasonic offer capacitors with much better performance
than the originals.
Certainly consider the Black Gates, but in my experience, their performance
can be erratic in tube circuits. To realize their dull sound performance
they need to be used in audio equipment that is always turned on. Better
suited to SST equipment like CD players.
Signal Coupling Capacitors – 9
Replace the signal coupling capacitors. Modern “audio” capacitors
offer significant performance improvement over the stock. Suggest your
consider the REL-Cap tin foil and polystyrene and the Infinity Caps for
the larger values. The new Wonder Caps aren’t bad, but I haven’t
had much luck with the MITs (time/phase smear?) or the Hovlands (stranded
silver plated copper wire leads).
The preamp’s large value capacitors (output for the phono and line
stage) with a small value .01 bypass capacitor. If you have a problem
finding the exact value of the large cap, increase the value of the bypass
cap to compensate. For example, if your unit has a 4.5uF capacitor, substitute
a 4.0uF cap and change the .01uF bypass cap to a .47uF. Actually these
values are not that critical. Yes the value determines the low frequency
roll-off but you could always increase the capacitor size to a 5.0uF.
I think small value film bypass caps really belong across power supply
electrolytic capacitors to filter out high frequency noise. Fitting them
to the electrolytic capacitors may be more important . Using them across
a signal coupling capacitor might create problems with phase and time
constants that add additional distortion to the audio signal. On the other
hand, when I was modifying the preamps I did use them and the upgraded
preamps sounded very nice.
If I were going to do one today. I would do one channel with the bypass
caps and one without. This would allow me to do listening evaluations
to determine what sounded best to me.
Hook-Up Wire – 10
There isn’t a whole lot of signal wire in the preamps. Short lengths
from the RCA Chassis jacks to the circuit board and the wire running form
the circuit board to the volume pots. Use your favorite solid core hook
up wire. I always use the 25.5 AWG Continuous Cast Copper wire. I have
never heard better. True, I sell it and you need to use copper content
solder to realize its full potential. But, I ripped all the expensive
silver wire out of my amps when I heard it. It’s the best I have
found so far. You can afford to splurge on whatever you want…The
preamp doesn’t have much wire (in length) to replace.
Potentiometers – 10
Replacing the stock pots with Nobles or better yet (if you can find them)
ALPS high quality potentiometers, makes a huge difference in the sound
performance of the preamp. You need to use good hook up wire to realize
this sound performance. Better hook up wire won’t help the sound
performance of the stock pot, it’s the weal link. Can you use series,
or better yet, ladder type stepped attenuators? Sure you can, it’s
just tough to justify the high costs of these units versus the affordable
price of the Nobles and Alps. Just make sure you purchase a pot with the
same impedance as the stock ones. Sometimes these are hard to source the
mono versions. A stereo pot will fit…You just connect to the wires
to one channel of the pot.
OFC RCA Chassis Jacks - 5
No doubt about it, copper makes a better conductor than brass. Vampire
offers OFC copper RCA chassis jacks at a very affordable price. Replacing
the stock hook up wire to the RCA jacks with
Decent wire and solder really helps. If you install the OFC jacks you
gain even more sound performance. If you replace the RCA plugs on your
interconnects with the OFC types the sound performance improvements increases
to a 9.
Signal Path Resistors – 5
I prefer the Roedersteins or the Welwyns metal films. If you use good
solder to install the new resistors the sound performance improvement
increases to a 7.
Tube Selection – 10+
Many of the 6DJ8 type tubes have an upper midrange glare. The stock tubes
are murky sounding as well. The best tubes to use are the Amperex Mil
Spec 7308, 6922 or ECC-88s. The Amperex ECC88 “A Frame”, Bulge
Boy, and Globe Orange Label are very good tubes. In the 2A phono stage
the Amperex Bugle Boy 6ES8s offer a dynamic smooth presentation with a
rich harmonic structure. This doesn’t work in the 3A. If you are
using Amperex 6922s, 7308, ECC88s, I have found you get better performance
by using different types in the phono and line stage. For example use
7308 in the phono stage and 6922s in the line stage….Or A Frame ECC88s
in the phono stage and Globe Orange Label ECC88s in the line stage.
Odds and Ends - ?
One thing I suggest you do while you have the top and bottom plate off
your preamp is to develop a voltage chart. This is a great thing to have
for trouble-shooting in case anything goes wrong with the preamp. Actually
it’s a good thing to have with any tube gear you use.
Use a ohm meter to determine sequence of the capacitors in the power supply.
Start with the rectifiers and measure the resistance to determine this
sequence. I draw a rough layout of the components I am going to test and
mark them in numerical sequence….C1, C2, V1(tubes).
After you have the power supply mapped. Plug in the unit and measure the
DC voltage from test point to ground on all the power supply capacitors
and the tube pins 1-9 on each tube. There will be pins that do not have
DC voltage on them.
If you measure one tube that has different voltage readings, you better
check your upgrades. If you took your time and your solder joints are
good, you won’t have a problem. If everything checks out, plug it
in and enjoy the Music.
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