Limpio

USB Conditioner and Power Injector

2 Channel Limpio


1 Channel Limpio Light Edition


Back Panel View


Internal View


Internal View, Main Board (w/o the Main Module)

Internal Main Module


Range Extender

Introduction

This device is designed to clean USB High Speed signal from a source (PC, media player, etc.), provide clean 5V DC power from a linear power supply to an audio end equipment (DAC).

There are 3 main problems when using the USB interface for an audio DAC:

  1. Signal distortion in interconnecting cables.
    Aa signal of several hundred megahertz passing through the differential pair (D+/D-) of the cable is always distorted in its shape.
    There are many reasons for this - the quality of the cable, the accuracy of its characteristic impedance, board-connector-cable transitions, etc.
    The only question is how large these distortions are at the end of the cable, as long as they cannot affect the decoding of the signal.

  2. Dirty Dirty 5VDC Power (VBUS) from the source
    Usually USB sources are powered by High Power Switching Power Supplies (SMPS).
    These SMPS can generate radiated and conducted noise.
    The USB Audio DAC always uses VBUS as its physical layer control (PLC) signal, and many DACs are partially or entirely powered by VBUS. It is obvious that feeding with a dirty, noisy voltage will not add a sound quality. This is especially important if there is no galvanic isolation between the input USB and analog circuits.

  3. Limited Source-DAC distance.
    The longer the USB cable, the more signal distortion. Also, a higher delay of the signal on the full path (i.e. from the source to the receiver and back to the source).
    If the DAC is also powered by USB and draws a lot of current, then the voltage drop on the cable becomes very significant (if it falls below a certain value, the DAC will stop working).
    The maximum current for USB 2.0 is 500 mA. This is a little, but not a little, since the normal USB PHY operating threshold is 4.6-4.7 volts, which allows only 0.3-0.4 volts to drop in the cable.
    Most USB cables use 26 or 24AWG for the power pair (sometimes AWG22-20, but that's exotic).
    A 0.3V drop at 500mA indicates a maximum cable resistance of 0.6Ω.
    The AWG24 wire has a resistance of 0.084 ohm/m, so for 2 wires (VBUS+GND) 0.17 ohm/m.
    AWG26 wire has a resistance of 0.27 ohm/m for 2 wires.
    The maximum cable length is therefore 3.5 m for AWG24 and only 2 m for the commonly used AWG26.
    Reminder - these are the limiting, boundary conditions! In reality, the operating distance may be less!


Limpio, The Cleaner

To solve the 1st problem, we use a high-speed USB 2.0 signal converter (re-driver) to compensate for both AC losses (due to capacitive loading) and DC losses (due to resistive losses) in transmission channel.

The purpose is to re-store the signal integrity of a USB High-speed channel up to the USB connector. The loss in signal quality stems from reduced channel bandwidth due to high wire losses and other components that contribute a capacitive load. This can cause the channel to fail the USB near end eye mask.

With edge amplifier, it increases static levels with DC boost function. In addition, it includes a pre-alignment feature to improve receiver sensitivity and compensate for intersymbol interference (ISI) jitter in applications with longer cable lengths.

This improves signal quality without changing the packet timing or adding propagation delay or latency and also helps the system to transfer USB 2.0 high speed near end lug to cable matching while 5 meters. Also cable length up to 10 m using an extension cord.



To solve the 2nd problem we use two things:

  • Linear Power Supply with low noise (4uVrms) voltage regulator TPS7A4701.

  • USB Galvanic Isolation (ISOUSB211) - galvanically-isolated USB 2.0 compliant repeater with signal equalization.


The 3rd problem is solved by the same methods that solved the previous two:

  • Using the own power supply, the cable voltage drop is limited by the cable from the Limpio to the DAC, which can be short. The input cable (from USB source to Limpio) can be long.
    For example - 5m PC-Limpio cable and 1m Limpio-DAC cable.

  • Using the signal converter and equalization is also helps to increase the available cable length.

An optional Extender Unit can be used to further increase the length of the input cable. This extender is the same signal converter as used in Limpio, but in the small separate case.

Depending on the quality of the cable, we got 12m total length between the PC and our USB powered ATDAC-8AX.

The extender can be used without Limpio. In this case, it is supposed to solve only the 1st and partially the 3rd problem (see above).

Notes.

In addition to its features, Limpio (full version) has two inputs. That allows you to connect one DAC to two sources (f.e. PC and TV).

Each input has a separate galvanic isolation and a cleaner, which significantly increases the cost of the device. Therefore, in addition to the full one, there is a Lite Edition version of Limpio-LE with only one input.

Since some DACs are designed to be connected with two cables (one for power only, the other for data), the Limpio has an additional USB-A output connector, which has power only. The main output connector contains both data and power.

Depending on the switch on the rear panel, the DAC can be powered either continuously or only when there is input voltage on one of the USB input connectors.

With the "on" button, the power to the DAC can be turned off in any of these options. In the off state, the device consumes meager power, and the switch on the rear panel can be completely disconnected from the outlet.


Limpio Block Diagram