M2TECH Joplin - Manual (Page 25)

JOPLIN
384
k H z /32bit ANALOG-TO-DIGITAL CONVERTER
REVISION P R B – SEPTEMBER 2012
Copyright © 2012, M2Tech Srl
25
M2Tech
www.m2tech.biz
When the desired parameter is displayed, its value can be changed using the arrow left
and arrow right buttons (Figure 3, 3). To confirm the new value for the current parameter,
press the OK button (Figure 3, 4).
To exit the menu without changing the displayed parameter value, press the Menu/Exit
button once again (Figure 3, 2). Anyway, the system resets to the idle state after some
seconds from the last operation on the knob.
8.3. Parameters meaning and choices
8.3.1. Input gain
The Joplin is provided with an analog front-end which gain can be set to 0dB or to any
value between 10dB and 65dB in 1dB steps. 0dB means that an input voltage of 1.7Vrms
produces and output of 0dBFS (that is the full dynamic of the ADC).
Sources with lower output levels (such as turntable, but also many cassette tape recorders
and also reel-to-reel tape recorders and tuners) may require a higher sensitivity to fully
exploit the dynamic of the Joplin. For example, a tuner with a nominal output voltage of
500mVrms would require a gain of 3, that is 10dB, to achieve the full ADC dynamic. As
well, a phono cartridge with 5mV output level would need 40dB gain. 65dB gain means
that the full dynamic is achieved with an input voltage as low as 0.95mVrms. This means
that even MC cartridges with output voltages as low as 0.5mV can be directly connected to
the Joplin, at the cost of a slight loss of dynamic.
The optimal gain is a matter of the source output level but also of the chosen equalization
curve. Rely on the VU meter and the clip indication to set the gain to the best setting for
the used source. Too high a gain, the unit will saturate and clip and the noise floor will be
higher than desired. Too low a gain and a part of the ADC dynamic will be lost.
8.3.2. Input choice
The Joplin has two inputs: one analog (called “line”) and one coaxial digital (called
“coax”). The use of the former is straightforward, the latter has been included to allow
users to connect a digital source to a computer which is not provided with a digital input,
making a bridge between the S/PDIF and the USB. Please note that the input sampling
frequency on the coax input (set by the digital source) is generally not the same as the
output sampling frequency (set by either the USB driver or the user via the configuration
menu). An asynchronous sample rate converter performs the sampling rate conversion
between input and output. When such an operation is not desired, user needs to set the
output sampling frequency to the same value as the incoming stream sampling frequency.