ToTo Man
the band not the dog
I've just discovered my DAC is producing DC at its single-ended outputs, 1.61V on the Lch and 1.01V on the Rch. The BAL outputs are fine, Pins 1 & 2, 1 & 3, and 2 & 3 all measure 0.00V.
I only discovered this by chance as I'm currently demo'ing a headphone amp that has an LCD display that warns when there is DC present at an input and shuts off that input as a precaution. Testing for DC voltage from a source isn't something that's been on my radar; amplifiers yes but not sources....
I'm still investigating the source of the DC, i.e. whether it's only from the DAC's USB input or whether it's present across all the DAC's inputs**. All I know so far is that the DC voltage is only present when the DAC is locked onto a source.
I've now tested the coax and optical inputs and the DC is present on these also.
What's of particular concern is the amp I've been using to perform all my loudspeaker bench-testing has been driven from this DAC's single-ended outputs. I always check the amp's speaker terminals before every test session to ensure there's no DC, and I always get readings below 2mV. However, I do not perform this amp test when the DAC is locked onto a source, hence my unawareness of the issue!
The DC offset at the amp's speaker terminals when the DAC is engaged is 140mV on the Lch and 90mV on the Rch, so fortunately the amp appears to be blocking just over 90% of the DAC's DC. I am concerned however that 90mV and 140mV is still quite high to connect to a loudspeaker driver without a crossover, particularly a low-power handling tweeter, if it's subjected to it for a prolonged period of time. Am I right to be worried?
Coincidentally, for the tweeter tests I've performed recently I've connected a 22uf Mundorf bipolar Ecap in series with the amp's output as a safety measure. However I've just measured the DC voltage with this cap in the circuit and it only reduces it from 140mV to 50mV, so it's not really acting as a "DC blocker" but more of a "DC attenuator", and a pretty mild one at that. Is this the behaviour you'd expect from a 22uf bipolar cap?
I only discovered this by chance as I'm currently demo'ing a headphone amp that has an LCD display that warns when there is DC present at an input and shuts off that input as a precaution. Testing for DC voltage from a source isn't something that's been on my radar; amplifiers yes but not sources....
I'm still investigating the source of the DC, i.e. whether it's only from the DAC's USB input or whether it's present across all the DAC's inputs**. All I know so far is that the DC voltage is only present when the DAC is locked onto a source.
I've now tested the coax and optical inputs and the DC is present on these also.
What's of particular concern is the amp I've been using to perform all my loudspeaker bench-testing has been driven from this DAC's single-ended outputs. I always check the amp's speaker terminals before every test session to ensure there's no DC, and I always get readings below 2mV. However, I do not perform this amp test when the DAC is locked onto a source, hence my unawareness of the issue!
The DC offset at the amp's speaker terminals when the DAC is engaged is 140mV on the Lch and 90mV on the Rch, so fortunately the amp appears to be blocking just over 90% of the DAC's DC. I am concerned however that 90mV and 140mV is still quite high to connect to a loudspeaker driver without a crossover, particularly a low-power handling tweeter, if it's subjected to it for a prolonged period of time. Am I right to be worried?
Coincidentally, for the tweeter tests I've performed recently I've connected a 22uf Mundorf bipolar Ecap in series with the amp's output as a safety measure. However I've just measured the DC voltage with this cap in the circuit and it only reduces it from 140mV to 50mV, so it's not really acting as a "DC blocker" but more of a "DC attenuator", and a pretty mild one at that. Is this the behaviour you'd expect from a 22uf bipolar cap?
Last edited: