How accurate is the Audiodinamica BeCube phono?
Summary BeCube phono is a preamplifier for MM cartridges based on a low noise differential design. It has +42dB or +48dB of gain, selectable 47k or 100k ohm input impedance, selectable input capacitance between +0pF and +320pF and it comes with XLR and RCA inputs and outputs, such a rare feature in the market. It works in conjunction with a separate low noise power supply: the BeCube power. We used several dual low noise jFETs, electrically matched on the same substrate and thermally coupled in the same package, in a zero feedback topology to achieve -80dB THD+N, low noise 0.1% tolerance metal film resistors and 1% matched film/foil or metallized film capacitors nail the RIAA equalization with an error typically of 0.05dB. A-weighted noise is 98.8dBu (with input cable connected!) thanks to the internal super regulator. Such an accuracy and performance can be rarely found on discrete RIAA preamps.
You want the phono preamp to be accurate The equalization network must be as accurate as possible to preserve the tonal balance of the records and that’s why we use high quality components with tight tolerance in the BeCube phono. But that would not be enough: the split RIAA is sandwiched between the two amplification stages and it is buffered with jFETs arranged for low output impedances in order to furtherly reduce the deviation from the nominal RIAA curve to negligible values at all frequencies. This buffer has an extraordiarly low distortion and basically no sonic signature, it is key to the accuracy of the equalization and infact is part of the RIAA network itself. In figure 1, the deviation from the RIAA is plotted against a 0.2dB scale: the BeCube phono stays very close to the 0dB (that is no error) and only deviates at 20Hz by a mere 0.2dB. This gentle downward slope at the lowest spectrum is imposed by design by the embedded 12dB/oct anti-rumble filter, it is there to prevent low frequency distrurbances from warped records or form the turntable’s motor to be amplified by the following electronics and reduce the risk to excite long excursions in the woofers of the speakers.
A wide bandwidth
Vinyl records may be able to store information well above the 20kHz audio band
and possibly over 50kHz according to several sources but the mechanical system of
tonearm-suspension-stylus will restrict the actual bandwidth of the whole system.
Moreover, due the pressing process and how the playback happens, the wear of
high frequency information on the record is much faster then the low frequency. Our
BeCube phono can reproduce all frequencies up to 100kHz with the greatest
flatness as you can notice in figure 2. We do not use the ‘infamous’ Neumann 4th
pole (that would cut the high frequencies) as it actually never existed in Neumann
lathes or other cutting lathes in the past.
Do we need all this wide frequency response? Probably not but it comes for free!
Very low noise
MM preamplifiers are demanded to amplify tiny signals of 0.5mVrms at 20Hz so we
need the preamplifier to be really low noise. How much low? The BeCube phono
has a separate power supply to minimize stray magnetic interferences from the
main transformers (we do use a low flux transformer with screens) as it can be kept
at a distance, moreover an additional super regulator capable of more then 100dB
of noise rejections flattens out any power supply noise. Figure 3 shows the residual
noise at 50Hz and 100Hz when an input cable is plugged into the BeCube phono: it
stops at -100dBu that is 7.74uVrms (using a star quad balanced cable). When the
input is shorted, ie no cable connected to the BeCube phono, as per figure 4 those
spikes just vanish, basically the BeCube phono has no power supply related noise.
We strongly encourage fellow audiophiles to use as much as possible balanced
connections to minimize noise pick up, the BeCube phono with its XLR input and
output offers the chances to set up a noise resilient system.
-80dB THD+N
Figure 5 plots the FFT of a 5mVrms signal at 1khz injected at the input showing
second harmonic distortion of -90dB and 3rd harmonic at -80dB. 5th harmonic is
showing below -120dB. This harmonic footprint is typical of differential amplifier
where 3rd and 5th are higher then 2nd and 4th. The absolute level of harmonics is
very low even thou the BeCube phono has no global feedback but only local
degeneration.
The debate about the pros and cons of global feedback would take us very far from
here: basically we have no objections against global feedback, we just felt there is
no need for it in our BeCube phono as performance is much higher then the
minimum requirements for an high end preamplifier.
The sonic footprint of BeCube phono has been described in a number of review
(refer to our press section on our website for details) and you can even listen to high
resolution samples in our download section to make your own idea.
Overload margins
Figure 6 shows how the THD+N depends upon the input frequency. At low
frequency the preamplifier saturates (ie it reaches -40dB THD or 0.1% distortion)
but it is handling a signal 100 times higher then the normal values: the typical output
from an MM cartridge at 20Hz is 0.5mVrms while in the test we were injection
5mVrms corresponding to a whopping 6.3Vrms at the output of the BeCube phono!
At higher frequency it stays comfortably below 60dB but they are much less critical.
The maximum input signal the BeCube phono can amplify before clipping (defined
as 40dB THD+N or 0.1% distortion and noise) is roughly 46mVrms at 1kHz, that is
19dB of overload margin, corresponding to 5.7Vrms at the output. Such a signal
would bring basically any power amp into deep clipping. The BeCube phono
overload margin is very remarkable considering its not so high power supply voltage
(24V bipolar after the local super-regulator).
In figure 7, the straight descending curve on the left of the plot is where the noise is
higher then the THD, while the upward climb on the right is the effect of the
increasing distortion of the BeCube phono: this is a very typical behaviour of linear
amplifiers.
What the BeCube phono can do
The BeCube phono is an MM preamp and would work with any step up in case an
MC cartridge is used but for the best synergy we encourage to use our SUT no3 or
SUT no2 and leverage and the fully balanced connection enabled by Audiodinamica
system. The phono has also RCAs to make it compatible with any other brand but it
will still work as a differential amplifier regardless of the input/output connectors
actually used. Clearly, gain trough RCA’s will be halved. On the back panel you will
find a dip switch to select gain (+42dB or +48dB), input impedance (47k or 100k)
and to add capacitance to the MM cartridges. A ground post is also there for
convenience in case unbalanced cables are used. There is no power switch, the
power consumption is very low and it might be on all the time, the supply is
controlled by the external BeCube power.
The BeCube phono has two separate internal boards for left and right channel to
reduce to the minimum any potential crosstalk.
We made the BeCube phono as accurate as possible in the simplest possible way.
Download the complete PDF file (with all the pictures, graphics, tables) under the section "SUPPORT/DOWNLOAD".
If you have any questions, please don't hesitate to send us an email to the following email address: info@audiodinamica.com
Author: Gianluca Sperti
Audiodinamica co-founder and product development
info@audiodinamica.com