module from LC Audio
Most modern high quality amplifiers do not
include a RIAA
Phono stage, nessescary to use a record player for input. And in the
amps that does have a built-in RIAA, one would often see it is of a
poor quality. Probably constructed with a cheap operational amplifier,
and some standard disc capacitors.
But why use a turntable with an expensive
to leave the signal amplification to this low-quality RIAA circuitry?
answer to this question is, Don't !
We constructed this state-of-the-art RIAA
the purpose of building it into a separate case with RCA plugs, or into
an existing preamplifier or even the turntable itself, if
This RIAA phono preamplifier is intended for
cartridges of the Moving Magnet type. If You have a Moving Magnet type,
we recommend using a suitable step-up transformer or amplifier, such as
our own top-of-the-line MC96 stage.
The technical construction of this RIAA preamp is
advanced! It uses a passive correction with non feedback circuit to
perfect sound quality. The most noise-free operational amplifiers in
world (AD743 from Analog Devices) are utilized, to make sure You can
hear any noise at music intersections.
The whole amplifier is - except for the very
- DC coupled, so no capacitor is in the signal path, apart from the one
we placed in the input circuit for protecting Your cartidge if a power
supply fault should occur.
And this one is a high grade polypropylene type
We think the issue of noise is very important
RIAA stages. Between the music pieces, there should be nothing but
silence. Therefore we have searched the market for ultra-low noise
amplifiers. The quietest bipolar operational amplifier, has an
input voltage noise of 0,9 nV/SHz, which is close to the practical
limit of today's technology.
A normal metal film resistor of 75 Ohms makes
than this, due to the molecular (Brownian) Movements in the resistance
However this data is the only the voltage noise,
since the input load from the cartridge is a very high impedance, the
noise should also be taken into account. In this device we found the
noise to be some 2 pA ( Pico Ampere or 0,000000000002 Ampere ).
by the 47000 Ohms load impedance, the resulting voltage noise increases
to about 100 nV/SHz, which is way over the acceptable limit.
However using a special FET input amplifier,
Devices can solve the problem.
This device, AD743, has a voltage noise of 2,9
which is higher than the 0,9 nV, but the current noise is extremely
Just 7 fA/SHz ( or 350 times lower than the bipolar device ) gives us a
current inducted noise of 329 pV. This means that the 2,9 nV Voltage
is dominant, and a total very low level of noise.
The AD743 is coupled as a 10 times linear
no frequency correction. After this stage the AD844 Current feedback
is used to make another 40 times amplification, a passive frequncy
with no feedback, and last a signal buffering.
The special thing about this IC is that it is
and it has a compensation pin (pin 5) which can be used to reduce the
loop gain right down to zero. Coupling pin 5 to a RIAA compensation
running the high frequncies to ground, results in a perfect RIAA
compensation, with a maximum of musical sound performance, since no
loops are used at all.
Using a Battery Power Supply
Here it is shown how the relay is shorted with
two 200 Ohms
Many DIY people have asked us, if it was possible
use a battery power supply. Therefore we have specified the power
at + / - 13 Volts, to enable use of two simple lead accumulators. This
does not mean that we endorce the use of battery power supply, because
we have taken every measure to ensure that a normal power supply will
in very high grade sound quality. We just felt that since a number of
were interested in the battery issue, we should make it possible to use
it. However there is a problem with this, if the positive battery (
the heaviest load ) runs out of power before the negative, and no
In this case the result could be several volts of
DC voltage coming out of the RIAA stage, that could destroy the next
stages or loudspeakers. To avoid this we have included a small safety
that breaks the output signal with a relay, if the positive voltage is
below 10 Volts.
Normally the relay circuit is not used, since
mains-based power supply systems do not create DC problems, it is
to short the relay with a couple of low-ohmic resistors, just to remove
the relay contacts from the signal path.
Only top-grade Panasonic low-ESR High
capacitors are used for supply-rail bypassing.
The RIAA network consist of Roederstein 1%
capacitors and ROHM low distorsion resistors.
IMPORTANT! When connecting a battery power
should first apply a normal power supply, since the IC's could be
from having only positive or negative supply at the moment of
Signal / noise ratio 88 dB (A) 47k
Distorsion THD <0,1% 20-20.000 Hz
Frequency Response 16-100.000 Hz ( Inverse RIAA
DC offset out (max) 1 mV
Power Supply +/- 12 - 18 V
Supply Current + 40 mA - 30 mA
1) RIAA Phono Amplifier Module - for MM cartriges
2) RIAA Amp. with spec. opamp for MC - for high
(0,3 to 0,7mVrms) MC cartriges
3) RIAA Phono Amplifier Module + MC Stepup Amp.
Supply - for low output MC cartriges
Stepup Amp. w/ Optical Supply
We did not provide load shifting for this MC
because our step up amplifier works in another way than a normal MC
The MC cartridge generates a current signal, as it reads the tracks
the LP. Normally this current signal is made into a stable voltage
by loading with a resistor, of various size, and then amplifying the
voltage signal by maybe 25 times. This is the normal way of doing
Our MC step up, works in another way, it directly
the currents from the MC cartridge, and converts them into a voltage
at the output. This shortens the signal path, and reduces thermal
The input impedance is near to 0, actually around 2,5 Ohms.
The RIAA module is a kit, and You should
it using a soldering iron, and a few normal tools.
The procedure is the normal one: Start with the
and finish with the higher ones. The IC's should be fitted last,
they fit in the PC board in the right direction (with pin 1 in the
position). The electrolytic capacitors should be turned with the minus
pin in the marked hole in the PCB. The assembly is quite simple, and
module is easy to get to function. Remember to use a screened case, to
avoid humming. Connect case to gnd on the RIAA amplifier.
Click here for more LC Audio