M2Tech’s Joplin 384/32 ADC White Paper by Marco Manunta

Introducing a groundbreaking product is always a very delicate task as some people may not understand its features and way of using. For this reason, I think a few words on what the Joplin is and how to use it may be useful.

Joplin 384/32 ADC: a gate from analog to digital

With the advent of digital audio (CD leading the way and then SACD, DVD-A, to come to computers, streaming and music files of today), many people decided to quit with analog and adopt digital for their hi-fi systems. Most of them changed the set-up of their hi-fi systems, replacing the preamp with a DAC provided with volume control and steering the preamp budget to improve the other equipment quality: with no analog sources to deal with, the preamp was indeed not necessary anymore.

As often happens, such a definitive decision proved to be too drastic, and with the renaissance of vinyl and reel-to-reel tapes, some hi-fi lovers devoted to digital now feel that burning bridges behind them was not the best thing to do.

Digital audiophiles wishing to go back to analog have to set their system upside down, introducing a preamplifier and, often, a phono preamp, thus altering the subtle balance they achieved when composing their all-digital system.

A different approach may be going for the Joplin 384/32 ADC. As its complete name suggests, the Joplin is a unit capable of transferring analog signals into digital. It has a pair of analog RCA inputs (for left and right) and a comprehensive set of digital outputs (S/PDIF on RCA, AES/EBU on XLR and Toslink optical), plus a USB Audio 2.0 compliant USB port.

Any analog signal applied to its analog inputs is converted in digital and offered at its digital outputs. Connecting one of Joplin’s digital outputs to one digital inputs of a DAC allows for listening to the analog signal through the Joplin and the DAC in a digital system.

Moreover, the USB port allows for connecting the Joplin to any kind of computer (Windows PC, Mac, Linux). The Joplin is then seen by the computer as an audio card or device with only one input channel to the computer: analog signals applied to the Joplin can thus be steered to the computer for archival or redirection (streaming) purposes.

Using an ADC to listen to analog sources in a digital system: the “straight wire with conversion” approach

Any audio equipment has generally its sonic signature which contributes to change the sound of the music we listen to with it. This is even truer with ADC’s and DAC’s, by which an important task is performed: the conversion of the signal from one domain to another. DAC’s perform digital-to-analog conversion, while ADC’s perform analog-to-digital conversion. Thinking of how critical is the D-to-A conversion, users may be scared by the even more critical A-to-D conversion process.

When converting an analog signal into digital, special care must be paid to matching the dynamic and the bandwidth of the signal to that of the ADC being used. This is overly difficult when the ADC must accommodate different sources with different rated output levels and bandwidths. For example, a good phono cartridge allows for bandwidth as large as 50kHz, while its dynamic can be, depending on the quality of the vinyl being read, over 80dB. On the other hand, a reel tape recorded using Dolby or dbx can give over 30kHz bandwidth with more than 90dB dynamic. A good FM tuner can easily give 85dB over a 16kHz bandwidth.

All the dynamic figures given above relate to the maximum output level allowed by the source: 0.5mVrms for a good MC cartridge, 1.5Vrms for a reel tape recorder, 0.5Vrms for a typical tuner. We’re facing more than 69dB change in absolute scale! Should we set the ADC to receive both the signal from the reel tape recorder and the MC cartridge without wasting the SNR of the weakest source, we would need an ADC with 69+80=149dB SNR, that is 24.5 real bits resolution! This is, presently, impossible to achieve. Users generally are obliged to put a phono stage between the cartridge and the ADC to increase the maximum level of the signal driving the ADC when playing records, and also to perform the phono equalization which ADC’s, generally, cannot do.

Moreover, the lower the input signal relative to the ADC acceptance, the stronger the sonic signature imposed by the ADC to the signal itself after the conversion: noise and residual distortion can be comparable to the signal’s magnitude.

Finally, pushing the performance of an ADC to cope with weak signals means that digital artefacts (quantization noise, spurious) become more apparent.

To avoid the above problems while retaining the ability to cope with signals having largely different levels, bandwidths and dynamic, we followed a different approach. The Joplin is provided with a gain-programmable analog front-end, which features extremely high analog performance: noise and distortion largely exceed those of the incoming signals at every selectable gain. Downstream the analog front-end is a very high performance ADC able to operate up to 384kHz. The dynamic, distortion and the signal-to-noise ratio of the ADC are comparable with those of the front-end.

The final result of this set-up (384kHz bandwidth, 122dB SNR and 0.0004% THD @0dB gain) is a conversion path which performance always exceeds that of the analog source driving it. In other words: noise and distortion intrinsic in the signal are always higher than the noise and distortion floor of the Joplin, while the latter’s bandwidth is always larger than the signal’s one. This means that the artefacts we hear after the conversion generally come from the analog source together with the signal. This is very important, as the underlying concept is that no sonic signature is added by the ADC, which can be considered a “straight wire with conversion”.

Equalizing in the analog domain or in the digital domain: poor Hamlet!

Even when an ADC is matched to work with a single source, it still needs some ancillary component when an equalization is required. This is particularly true when it comes to phono cartridges. The usual solution is interposing a phono stage between the cartridge and the ADC or using the ADC together with a computer running an app which performs phono eq on the digital steam being acquired.

Joplin offers a third solution to get rid of any phono stage and, at the same time, of any specific app or, when it is necessary, of the entire computer: equalization is performed in real time inside the ADC, immediately downstream the conversion. This means that when we connect a cartridge to the Joplin and enable an equalization curve for phono, the signal offered on its outputs is already equalized, ready to be listened to.

We think that this solution offers numerous advantages. For example, another one is the “numeric gain” provided by the equalization algorithms that allows for increasing the input sensitivity over the flat setting. Thanks to it, cartridges with output level down to 0.5mV do not require any additional MC step-up nor transformer. To have a glance of the advantage of the numeric gain, think of an MM cartridge with 5mV output: with an analog phono stage outputting 1.7V (that’s the Joplin’s input sensitivity at 0dB gain), gain should be 70dB: 50dB to compensate for the level difference and 20dB to compensate the equalization loss at 1kHz. With the Joplin, thanks to the fact that the equalization is done after the conversion and also thanks to the numeric gain, 30dB gain is sufficient!

Also, precision is very high, within 0.1dB, whereas analog phono stages hardly allow for 0.5dB precision.

But the most interesting feature, in our opinion, is the possibility to choose amongst more than one equalization curve: the Joplin offers as many as 24 curves for phono, to cope with records from any year, old 78rpm to modern stereo LP’s.

Sampling frequency and resolution: wide flexibility

The Joplin can be set to operate with different sampling frequencies and resolutions. Sampling frequencies range from 44.1kHz (CD standard) to 384kHz, while resolutions can be chosen from 32, 24, 20 and 16 bits, with or without noise-shaping. Users can set the preferred pair of parameter to deal with the chosen destination, media or disk space occupation. Users willing to copy vinyl on CD may set the Joplin to 44.1/16, while audiophiles willing to archive their vinyl on computer at high resolution may choose 192/24 or even 384/32. Who wants to do both may choose 176.4/24 to obtain high resolution files from which to produce CD resolution copies without doing asynchronous sample rate conversion.

An auxiliary digital input: why?

The Joplin is provided with an auxiliary coaxial digital input on RCA. The digital signal applied to it can be routed to the computer via the USB connection. As most computers, even those provided with an audio digital output, lack any audio digital input, no digital source can be connected to them to acquire digital audio. In this case, the Joplin can be used as a bridge to connect a digital source (up to 192/24) to a computer.

Suggested uses for the Joplin

Provided that the only limit in enjoying the Joplin is user’s fantasy, we dare to suggest some interesting uses for the Joplin:

1) analog input for digital systems: any analog source can be interfaced to a digital system via the Joplin, retaining the sound quality of the source;

2) phono stage: audiophiles with good digital equipment may decide to use the Joplin as a phono stage to take advantage of its versatility. Real-time listening, in fact, high quality digital archiving of vinyl on computer is possible. Moreover, digital copies are “portable” and extend the enjoyment of our vinyl treasures outside our listening room;

3) bridge between an analog system and a multi-room digital distribution system: feeding the Joplin with the “tape rec” output of an amplifier allows for making the music from all sources of the system available to all the rooms in a home;

4) bridge between a digital source and a computer lacking digital audio input.

We invite readers to download the application schemes from M2Tech website.

Notes about interfacing

Due to lack of PCB room, we provided the Joplin with no loading options for cartridges: input impedance is 47kOhms, good for MM pick-ups but not for MC ones. Users may handle this in two ways: using an MC step-up or transformer or preparing an impedance adapter by using a “tee” RCA adaptor and RCA plugs with suitable loading resistors.

Another topic to care of is the Joplin’s input acceptance: 1.7Vrms @ 0dB. While older analog sources hardly give more than 0.5Vrms output, some modern sources may have output voltages higher than Joplin’s acceptance, possibly leading the Joplin to clip. In this case, it is necessary to operate on the output volume of the source to reduce it’s output level, when available, or to use an attenuator (a simple stereo potentiometer is sufficient).

When interfacing microphones, users must keep in mind that no phantom power is provided, so an external phantom power supply must be used. Users must check that the phantom power supply output is AC-coupled, as the Joplin is DC-coupled and excessive DC level (such as that provided by some phantom power supplies) can damage it or lead it to misoperate.

Over hermanvandendungen

Almost 40 years of being with and in the world of high-end audio equipment. As distributor of Krell, Sonus Faber, Cello, Conrad-Johnnson and many more, initiator of brands like Kiseki, Cogelco, AH!, PrimaLuna, Mystère and maybe more to follow.
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