NEWS: a page with sound samples - link below.
The Behringer ADA8000 is an 8-channel in,
8-channel out Analog/Digital/Analog converter which costs under US$200.
It has already a rather ok sound from factory, but its simple
architecture allows some modifications to be easily done that can make
it
sound
like a US$2K converter, and with a little harder work, an interface
that sounds like no other. There are many reports in forums and pages.
A
simple example is to bypass its input
preamps. If you feed a line signal to its inputs, it takes a small part
of it and passes through its microphone preamplifiers. No matter how
good a preamplifier is, and this is not the case in a budget unit, this
signal-to-noise relationship gets much worse, as the signal is
amplified back to the same line level, but now with an amplified input
noise
acompaining it. Another big issue commented by everyone is the power
supply, with regulators running so hot you cannot touch them.
This is the second Behringer ADA8000 I
am modifying,
and I created this page to put some documents and information that
may be useful to anyone interested in doing similar things. My
motivation was to beat the sound of my expensive Metric Halo ULN-2.
When I first compared the sound of my synthesizers connected
directly to the power amp to the sound passing through the computer I
found that the AD/DA loop completely kills the beauty of the music. The
ULN-2 is a good unit, have nice Linear Technology opamps inside, but
perhaps too many opamps and a lot of cheap coupling caps. The sound of
the first modified Behringer recovers at least half of the original
sound.
The modifications I did in the
first unit, whose
owner is Marcello Sfoggia, to record classical concerts in my city,
were:
- replacement of ceramic caps at the
power supply
for high isolation voltage ones;
- installation of a fan on the top cover
to cool
down the regulators that run very hot (Sfoggia did that);
- bypass the input mic preamps, going
directly to
the main board with appropriate resistors;
- opamps replacement: I put LT1359 at
the inputs and
OPA4134 at the outputs;
- uncrossing the outputs, because the
positive
passed through two opamps, and the negative, only one.
With the mods the unit sounds much much
better. Then I started working on a second unit for myself. My goals
were
to avoid all the
problems
with the built in power supply again, to compare OPA4227, OPA4134 and
LT1359, and avoid the signal coupling caps. From my
experience, every time you can get rid of a coupling capacitor the
sound gets to a new dimension. But it is of course a big challenge to
make some circuits DC-coupled without getting into trouble.
Below you can find:
- Link to the schematics by Gyraf Audio.
The input
coupling
capacitors are inverted in their schematic. They have the right
polarity in the actual unit, with the positive side at the AL1101 chip,
which has 2.5V DC bias at its inputs.
file:Behringer_ada8000_analouge.PDF
-
My annotated picture of the power supply (click on int for a larger
size). I have not figured everything
out, and you are encouraged to help me find out why there are two pins
with
+50V going to the front panel, for example. As you see the bottom
7815/7915 regulators serve the microphone preamps in the front panel,
which I disconnected, and they are responsible for the largest power
consumption on these regulators. If you just cut the +-15V
supply
wires that go to the front panel these stop running hot.
A
little bit of history
This page started one year ago, but it
took me some
time to resume working on these circuits due to my other activities. I
got involved with the DC-inputs again just a few months ago, out of
necessity, and now I can include new information and some
important conclusions below. But let me first highlight how I am
testing them. After the first measurements I did of frequency
response, phase, distortion, I observed that this data alone could not
tell me how the circuit would sound. Check for yourself in the graphics
below, but as I see now they are kind of
complementary, in the sense that the measurements tell me what I cannot
easily determine by ear, while listening tells me how the circuit
sound, which cannot be understood by looking at the data. And as the
specifications come very nice from the factory, yet sound bad, most of
my attention was
given to listening tests.
I also have a relatively uncommon
condition of
testing the recording (in fact, always the complete AD/DA loop) with my
keyboards. This is important because it allows me to compare it to the
sound going straight from the keyboards to the power amplifier, which I
take as the gold standard, for it is actually quite beautiful,
full of natural colors, good depth of field, good stereo image, with
uncompromised bass, and so on. That is why I stated at the beginning
that the AD/DA loop of stock interfaces killed the beauty of the sound
- it is beautiful before recording.
It is of course possible for anyone in a studio's control room to
compare the
analog original sound to the AD/DA loop with a source like vocals,
orchestra, or someone playing guitar, but it is very unlikely that this
person could be making modifications to a circuit, changing resistors
and voltages with those live sources at his disposal! On the
other
hand, I am in some sense very limited as I do not know how the circuit
sounds with vocals, mainly, as I have not tested recording them yet. In
fact, some artifacts do not compromise instrumental sounds as much as
they do to solo vocals or choirs. But in the end I would still say that
as I am not only appreciating the sound but comparing to the original,
I am mainly in the right direction.
You can see parts of my small studio in
the pictures
below. I am listening with a pair of home made speaker cabinets that
contain a 10" weak bass driver, a 5" bass-mid driver from an Event SP5
speaker and some soft dome and paper cone tweeters each, and a pair of
passive Alesis
Monitor One mkII. Each system is driven by a brazillian Nashville
Power-250 amplifier - this is an old hi-fi model that does not have any
harsh
common to other transistor amps. The big cabinets give me more bass and
body, transforming the precise yet irritating sound from the Monitor
Ones into a full and pleasant sound adequate for playing with
satisfaction.
The new input circuits
My goal was to feed the purest
line-level signal to
the AD chips, but it is not possible to connect anything directly to
the chips as they operate from single 5V supply, thus requiring the
signal to be centered at 2.5V. The first SMD board that I made to
perform the level shifting looked very pretty, but had at least one big
flaw: it could not accept a floating signal connected to it.
When connecting an output isolated by a capacitor to that circuit, the
2.5V level at the opamp's output developed the same voltage at the
inputs because of the feedback, messing up with the voltage addition.
So, I simulated a new circuit using this applet
simulator from Paul Falstad
and implemented the circuit below, that has +1.25
and -1.25V references that bias a possible floting input correctly to
0V. This input does not work with a DC-coupled circuit that has a
DC level other than 0V with low impedance, of course. The second
purpose of using the opamps is to invert the signal to feed the
negative input on the AD chip, and I did this in parallel, so as to
keep both polarities going through the same number of stages. If I only
worked with
24bits I would perhaps try using just one polarity, but as I have a
very good
16-bit DAC that I want to try when playing back, loosing the most
significant bit is a serious concern.
Below are the schematics
for my new DC-coupled inputs as
they are working now (click for the pdf). Both circuits take an
unbalanced input to generate positive and negative polarities. The left
version has unity gain but does not limit the input voltage and I
suspect that one could damage the ADC inputs with very high levels
connected to them. Of course you should identify hard clipping in your
software meters and speakers long before any damage occurs, but that
might still be a concern for someone. The version at the right uses a
little adaptation trying to avoid that. It takes the input signal,
multiplies it by 2, than divides the outcome by 4, having a gain of
0.5. This final division would take the maximum opamp swing down to a
level
of +-3.5V, which would add to the 2.5V bias resulting in voltages from
-1 to +6V at the AD
chip at most, and I don't think these can cause any harm. I still think
the
circuit at left provides a better sound, but this is hard to tell as I
need to test them with different input levels, different volume levels
at the power amp, and even the output LP filter is not the same. For
myself I am
going to stick with the first, and I would recommend the safer one to
anyone working outside a studio and that would connect unknown sources
to record.
You can use the PCB
layout I designed for the circuit at the left, if you like, for
non commercial purposes. Yet for the right one you
have to find where to put the 660ohm resistors. Click on the pictures
below to get the postscript files for the single cooper layer and silk
screen, and a pdf with a print of both sides, easier to look at when
placing the components. You would need to place two small SMD resistors
of 0 ohms or make a short for the unused opamps at the center. There
are two wires needed to run the ground to the center parts, and a
resistor marked as 500ohms also corresponds to a short in the
schematic, making that opamp a voltage follower. Do not place any
capacitance at bottom-left and top right. There is also a spare ground
pin for each output, which I don't think will be ever used, as we have
positive and negative outs. There are a few places to put small bypass
capacitors, which I am currently not using. The ones at center-leftmost
and center-rightmost would be used to correct opamps from overshooting
or ringing.
RESULTS
Now let me describe the exciting part. This board sounds
awsome! Have to listen to believe! The DC connection, by eliminating
the coupling capacitors, makes
the sound free, uncompromised, as if it was set free from a can (the
cap can) in which it was confined before. Remember also that I am
listening with a DC coupled output as well. As for opamps, I compared
the circuit with
LT1359 and BB OPA4227 in, and they sound terribly different. I like the
LT1359 most by far. They are cleaner, with deep and precise bass notes,
preserving the color and richness of instrument tones at mid
frequencies. On the other hand, the OPA4227 sound a bit ressonant,
making the bass bigger but more difused, giving the impression that the
sound is duller. That's a similar impression that the OPA4134
gave
me when comparing them on the ADA board (with coupling capacitors). It
is also a bit close to what Jensen transformers give me when connected
directly between the source and ADC chip (more below), yet worse in the
case of the opamp. I must say that the LT1359 are not perfect. They do
sound incredibly pleasant, almost better than the original sound, but I
would say that they are a bit too
flat, and loose just a hint of concreteness and stereo image compared
to the source signal, perhaps highlighting the highs. That is why I am
looking forward to test John Hardy's 990 or other discrete opamps. Not
that the sound now is bad, I insist it is really nice and I could live
forever with it, but I can understand the differences from the original
and other opamps can perhaps surpass the 1359. And
that's also why I will keep a pair of inputs with just the Jensen
JT-11P-1 transformers to isolate and shift levels, and no opamps. They
also make the bass fatter
(in the worse sense) and color the signal, as if I was listening to
something that ressonates (again, not a a good coloring), but they do
preserve better some sound
properties in the mid frequencies and sound coherence, and in this way
some instruments from the
keyboard sound closer to the originals.
Next there are a few pictures of my test environment,
tentative circuits in the
protoboard, PCB fabrication (for it is fun) and experiments.
Finally, I could manage to provide the first files
with sound samples comparing different units and the first mods:
- SOUND SAMPLES PAGE
More to come - this is what I expect to
include here at some time:
- basic measurements showing the
slightly increase
in bass response and low noise/distortion
What I did not like in this circuit and
board:
- the board has room for big capacitors
at reference
voltages, but you should not use them;
- the board does not have a place for
the division
resistors that go from positive to negative outputs;
- the 0 ohm resistors can be omitted once I decided
to stick to the LTs to generate references, as they are good for it.
Things I will probably work on next:
- an SMD board with the same circuit
- testing 990 opamps and
Holco/Rikken resistors
- a circuit tha accepts differential
inputs
- more tests and changes in the
PSU: shunt
regulator, bigger caps (and soft start), ...
- other circuits: outputs (DAC), other
AD/DA chips,
my tracking mixer, I need a simple microphone preamp, ...
More information and results will be
posted in a near future.
Porto
Alegre, Brazil. johann
at inf dot ufrgs dot br
Created on April 24, 2010.
Updated on April 12, 2011.
Graphics on September, 2013.