The DiDiT DAC212 XLR is a true differential design from input to output.

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1 DiDiT DAC212 XLR Due to the many requests, we decided to launch the DAC212 with a balanced output. With pride we present the DiDiT DAC212 XLR. The balanced vision of the DAC212 RCA. The DiDiT DAC212 XLR is a true differential design from input to output. The design team didn t take this conversion lightly by just adding an XLR connector. Instead we went back to the drawing table, and came up with a design that maintains the superior performance of the highly appreciated DAC212 RCA, but in balanced version. As you can see, this design journey wasn't an easy one, but we are convinced we succeeded and have produced a top class D/A converter that will continue the legacy build up by the DAC212 RCA. Please see below how we DiDiT (did it)

2 Balanced output-stage When designing a balanced XLR output there is actually only one right way to do it. Balanced XLR connections have the reputation to be superior to unbalanced connections. In reality this statement is more subtle and only if done right, a true differential circuit will benefit the sound quality. Making a design balanced often doesn t come without penalties. The noise and distortion will be doubled in most of the cases. When designing a balanced circuit there are basically two things that really matter. CMMR. CMRR stands for Common Mode Rejection Ratio. To clarify how this principle works please see Figure1 below. A differential amplifier receives a differential/balanced signal. This means two poles in opposite phase. Then a subtraction amplifiers amplifies the difference between the two voltage. This has a great benefit that other signals, will not be seen by the circuit. Those other signals can be internal or internal emissions and noise fields. We call this Common Mode Noise hence the name Common Mode Noise Rejection. Low inductance PCB routing. True differential designs give the opportunity (often this possibility is not used in many PCB designs) to route them as a tightly-coupled pair. This implies that the two magnetic fields partly cancel each other out, which results in a lower inductance design. In turn this contributes to lower distortion and a purer signal transmission. Figure 1, Subtraction Amplifier

3 Think Differential When we designed the original DAC212RCA, we made a very thoughtful consideration to have a very short signal path and a very compact circuit with only the most essential components to guarantee the best possible performance. When looking at the block diagram in Figure 2 we can see that essentially the internal architecture is balanced/differential, and has the CMMR benefits. The noise can be kept very low because it is a single-ended circuit with low ohmic resistor values. The added current buffer is to ensure the DAC212 can drive any load, including the low ohmic resistor values in the feedback loop. (those are left out in the diagram of Figure 2 for sake of simplicity) The ESS 9018 DAC is followed by Current to Voltage circuitry (I-V stage) The I-V stage is followed by a subtraction and low pass filter stage. At this stage Common Mode Noise is rejected. It also implies that the internal PCB traces and circuitry is actually truly differential until this stage. This gives the opportunity to route them as differential pairs on PCB level. Despite being a single ended output, the internal architecture is balanced until the subtraction stage, and performance is excellent. Figure 2, DAC 212 RCA Simplified Output Amplifier

4 The wrong way of a balanced output 1 In Figure 3 an Inverter is added to the subtraction amplifier to create the negative output. This circuit is found in over 90% of the commercially available gear with XLR outputs on the market. The inverter will add extra noise and distortion to the original signal by: Resistor noise in the inverter stage Op-amp noise of the inverter The inverter will amplify the positive op-amp s distortion and noise Symmetrical routing at PCB level is not possible The inverter stage will not have the same timing in absolute sense as the driver stage because it is a follower of the positive output Figure 3, A balanced output found in 90% of commercial audio equipment

5 The wrong way of a balanced output 2 This is another topology sometimes found in higher-end audio equipment. Making a symmetrical positive and negative output stage, and being marketed as true balanced. One positive attribute of Figure 4 is that it is possible to route PCB traces and place components closely coupled and cancel magnetic fields (low inductance). The circuit in Figure 4 would actually perform worse then the single-ended design from Figure 1. The main reason is the absence of any CMRR. CMRR will take place in the receiving equipment (e.g. the connected external power amplifier). The circuit from Figure 4 is not immune for common mode noise and higher distortion can be expected. Double output amplifiers No CMRR No benefits over the better performing single-ended stage of Figure 1 Figure 4, A Symmetrical Positive and Negative Stage

6 The right way of a balanced output Figure 5 is DiDiT s way of performing a true differential/balanced output stage. Basically we have copied the differential to single-ended conversion circuit of the DAC212RCA and mirrored it (inverted) for the negative channel of the new balanced output. To avoid the noise penalty, we have added high current buffers in the I-V stage, to be able to drive the very low impedance circuitry of the double mirrored subtraction amplifier. All previous objections are solved. True Differential Output-stage Double CMMR at each halve of the circuit No inverters Differential in > Differential out PCB routing can be as coupled differential pairs, keep loops small and allows very low inductance design. Figure 5, DAC212 XLR True Differential output stage