12AU7
– 12BH7 Remote Controlled Preamp
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This
is my remote control 12AU7 – 12BH7 preamp. I built this because I love the
sound of tubes but, am too lazy to keep getting up to adjust the volume and
change functions. This allows me to do it from my couch.
Analog Section
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- The topology is essentially a Electronic Tonalities
Foreplay but, uses DC on the filaments to decrease the noise floor –
preamp is dead quiet.
- Input switching, mute and tape monitor is done via SPDT
relays with gold/silver alloy contacts. The digital and analog sections
are electrically separate with the exception of chassis ground.
- The original circuit was a copy of a basic 12AU7 Foreplay
cathode follower. The only "fancy" thing I did was to use NOS
carbon composition resistors and Sprague paper in oil coupling capacitors.
Volume control is an Alps motorized unit.
Updates – C4S and 12BH7
- After listening to the stock circuit for almost a year, I
replaced the carbon resistors in the voltage amplifier with C4S current
sources. I tried C4S’ing the
cathode follower positions but, removed them because the sound lost some
of its warmth and the C4S in the voltage amplifier positions made the most
improvement.
- While browsing for tubes I came across a pair of
12BH7s. Because the 12BH7 is
pin compatible with a 12AU7 and my power supply will easily handle the
extra filament current, I decided to give them a try. I was totally amazed by the
result. 12BH7’s have a lot of
the qualities I like about 6SN7s while retaining the dynamics and detail
of the original circuit. Thus
far I have tried several brands (RCA, WE, Sylvania, Viking) and all
sounded better than my best 12AU7s à Amperex Orange Globes (Philips
Harleen 1962). Considering
the Amperex’s were hands down the best sounding 12AU7 I have, high praise. Below is a picture of the preamp with
Viking 12BH7s installed.
Preamp
with 12BH7s
Viking
12BH7s and original boxes
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Digital Section
The digital section of the preamp is an amalgamation of Basic Stamp projects (IR decoder, LCD display driver, relay control driver, etc..) pulled together from different sources on the internet. What started as a "I'll knock this out in a weekend" project turned into a half dozen hardware revisions, a couple hundred lines of code and several hundred hours work. I ended up rewriting a lot of the routines I found on the internet to get the thing properly debounced and fit within allowed number of variables and memory space of a BS2IC.
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Close
up of the main control board
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- The digital section is controlled using a Basic Stamp
BS2IC coupled to a Panasonic Infared detector and backlit LCD display.
There are two circuit boards; the main control board (in the back) and a
display board (in the faceplate). The control board holds the Basic Stamp,
control circuitry voltage regulation and filter network, function relays
and motor driver. The display board holds the LCD display, a
serial-parallel display driver and infrared detector.
- The only manual control on the unit is the volume knob -
remote or manual. There are switches at the back for filament/high-voltage
"master" power and a second level volume knob on top to adjust
the input sensitivity of the primary volume. I need this because I use the
preamp to drive two different amplifiers with different input
sensitivities and it gives a wider control range for the main volume.
- The preamp is controlled using a standard universal
remote control with Sony IR control codes. The infared detector behind the
faceplate (just to the right of the LCD display) receives the signal and
filters the 38khz carrier frequency from the IR bit stream (this is why
you should not waste your time with the RadioShack IR detectors). The
Basic Stamp parses and decodes the serial bit stream, determines if the IR
Word is a valid code and responds per it's programming.
- Functions programmed are - Tuner, CD Player, SACD Player,
Tape Player,Tape Monitor, Volume Up, Volume Down, Power, Mute and Info.
These are displayed on the LCD display so you can see what function is
selected and what the unit is doing. When the unit is powered off, high
voltage/filament power is turned off, the LCD is cleared, the LCD
backlight is turned off and the unit ignores all IR control commands until
it receives a valid "power" control code.
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Close of preamp LCD display |
12AU7 Tubes
Comparison
of different 12AU7 tubes in my system. Please don't email me about my
preferences. None of these tubes are really "bad" and will sound
different in your system - so, your milage may vary.
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Amperex |
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Left to right - RCA cleartop, Mullard IEC, Philips 5814, GE and Sylvania |
- RCA Cleartop - I use a pair of these in my AE-25 to help
brighted it up. In my preamp, they have plenty of detail but, are too
bright for my system. A good tube but, can't compete with the Amperex's or
Mullards.
- Mullard IEC - These have a similar level of detail as the
Amperex but, are a bit darker. Would surely be someone's favorite tube if
their system was bright or they prefer a darker kind sound. A great tube.
- Philips JAN 5814A - These are the tubes that came in my
AE-25. In both my preamp and AE-25 they did nothing wrong but, nothing
special.
- General Electric - A good sounding tube. Will get you by
if you cannot get a pair of Amperex, Mullard or Cleartops.
- Sylvania - Would describe these as "Lush"
sounding but, lacking the focus of the Amperex, Mullards or RCAs. Would
definitely get you by if you did not want to spring for Amperex, Mullards
or Cleartops.
12BH7 Tubes
Here is a brief comparison of 12BH7 tubes I have tried. All of the tubes listed here sound at least as good as the best 12AU7s I have used and the better ones, are simply in a different league.
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Right
to left – Sylvania, Western Electric and RCA. Viking boxes in the background – the tubes are in my
preamp |
- Western Electric – This was the first brand of 12BH7 I
tried. I had difficulty
finding a pair as several I tested were microphonic. Good sounding as well as the tube that
got me interested in 12BH7s
- RCA – These were the second pair of 12BH7s I tested. Totally quiet, warm and detailed.
- Sylvania – Good detail, smooth and dynamic. All the pairs I tested were
totally quiet. If I was to
buy a pair without the ability to test them first, this would be the tube
I would buy
- Viking (E.F. Johnson) – Not sure who made this old pair
of tubes. The pair I have is
somewhat microphonic but, not enough to cause a problem. That aside, these are absolutely
stunning sounding tubes. They
are more forward than the RCA’s and WE’s, have great detail, a wonderful
midrange and absolutely stunning presence. This set currently resides in my preamp