Occasionally, some of you are tempted to experiment with different TDA1541(A) in the Audial DACs. And, while the TDA1541A pieces recently installed in the S5b are a very good series, it is still generally fine with me if you do that. However, please note that such experiments are normally not recommended for anyone lacking a basic understanding of how these things work, and, in addition, there are still several cautions you would have to keep in mind while doing this.
The first is about the chip's physical replacement procedure. Practically all Audial DACs use a socket for the TDA1541A, so this is not hard to do. However, care is highly needed, so you don't bend the pins, and thus possibly turn the chip, or maybe even the socket, unusable.
The following video, recently made by Matt Philpot, demonstrates how this should be done. As a lever, you can also use a flat-head screwdriver of an appropriate size.
In addition, unless you ground yourself properly, it is recommended to avoid touching the pins by hand, so you prevent possible electrostatic discharge.
https://www.youtube.com/shorts/1bRsQyBZGE0
And the second very important part here is about the output DC offset.
Normally, the output DC offset is factory nullified with the factory-installed chip. However, each TDA1541(A) sample has a somewhat different "zero" (mid-) point that can vary between different samples up to approximately 0.15 mA, which, with a 1.5 kOhm I/V resistor, translates to roughly about 200 mV.
For the I/V stage and output buffers, this is not critical - although their performance may also benefit somewhat from trimming this mid-point as close as possible to zero. Such a relatively high offset, however, may be a real problem for the units using the output transformers. The output transformers used in the S4/S5/S5b DACs have quite a low winding resistance of 13 Ohms, and the DC currents higher than 2.5-3 mA running through their primary will deteriorate their performance. In addition, the output buffers will be forced to supply this as idling current. Overall, DC currents above 10 mA may start producing audible distortion.
So, you can check this output DC offset by measuring it across the primary leads of the output transformers, and these are the black and red leads. The DAC input should be set to USB, and the source should be stopped or paused, or you can completely disconnect the USB cable from the DAC. What is important is to set the input to USB, because this is the simplest way to provide the proper signals to the TDA1541A, which ensure its correct DC operating points. Otherwise, its operation is not "under control", and its output acts erratically in DC terms, so the output DC becomes "undetermined". The output DC value measured under such conditions is useless, and the DC set that way will most probably be wrong.
So, with the input set to USB, the Voltmeter should be set to mV range (or to auto-range). Then, you should be careful to ensure good contacts. The soldering flux residue can form the isolation layer above, so the probes with sharp tips are the best, as they cut through. Also, instead of trying to tap a proper point for both negative and positive probes at the PCB, you can use the ground terminal at the back plate to put the negative one. The error voltage (due to internal traces' resistance) will be negligible, almost always below 1 mV, which is, for this purpose, acceptable. So, you will have one hand for the (positive) probe, and the other to tune the offset, by using the blue multi-turn trimmers beside.
Also, when trimming the offset, it is good to check it right after powering up, but it is most important to set it finally only once the unit reaches its final working temperature, so probably 20-30 minutes after powering up. The offset trimming circuit we use has changed a bit over time, but you can mostly expect thermal drift of about -30 mV. In other words, it is negative, so as the unit temperature increases, the output DC offset voltage will decrease. Ideally, the final voltage seen here should be about +5 mV. This is because, once the top plate is on, the temperature will rise a bit further, and produce additional drift of about -5 mV.
If you need more details, feel free to ask.
Occasionally, some of you are tempted to experiment with different TDA1541(A) in the Audial DACs. And, while the TDA1541A pieces recently installed in the S5b are a very good series, it is still generally fine with me if you do that. However, please note that such experiments are normally not recommended for anyone lacking a basic understanding of how these things work, and, in addition, there are still several cautions you would have to keep in mind while doing this.
The first is about the chip's physical replacement procedure. Practically all Audial DACs use a socket for the TDA1541A, so this is not hard to do. However, care is highly needed, so you don't bend the pins, and thus possibly turn the chip, or maybe even the socket, unusable.
The following video, recently made by Matt Philpot, demonstrates how this should be done. As a lever, you can also use a flat-head screwdriver of an appropriate size.
In addition, unless you ground yourself properly, it is recommended to avoid touching the pins by hand, so you prevent possible electrostatic discharge.
And the second very important part here is about the output DC offset.
Normally, the output DC offset is factory nullified with the factory-installed chip. However, each TDA1541(A) sample has a somewhat different "zero" (mid-) point that can vary between different samples up to approximately 0.15 mA, which, with a 1.5 kOhm I/V resistor, translates to roughly about 200 mV.
For the I/V stage and output buffers, this is not critical - although their performance may also benefit somewhat from trimming this mid-point as close as possible to zero. Such a relatively high offset, however, may be a real problem for the units using the output transformers. The output transformers used in the S4/S5/S5b DACs have quite a low winding resistance of 13 Ohms, and the DC currents higher than 2.5-3 mA running through their primary will deteriorate their performance. In addition, the output buffers will be forced to supply this as idling current. Overall, DC currents above 10 mA may start producing audible distortion.
So, you can check this output DC offset by measuring it across the primary leads of the output transformers, and these are the black and red leads. The DAC input should be set to USB, and the source should be stopped or paused, or you can completely disconnect the USB cable from the DAC. What is important is to set the input to USB, because this is the simplest way to provide the proper signals to the TDA1541A, which ensure its correct DC operating points. Otherwise, its operation is not "under control", and its output acts erratically in DC terms, so the output DC becomes "undetermined". The output DC value measured under such conditions is useless, and the DC set that way will most probably be wrong.
So, with the input set to USB, the Voltmeter should be set to mV range (or to auto-range). Then, you should be careful to ensure good contacts. The soldering flux residue can form the isolation layer above, so the probes with sharp tips are the best, as they cut through. Also, instead of trying to tap a proper point for both negative and positive probes at the PCB, you can use the ground terminal at the back plate to put the negative one. The error voltage (due to internal traces' resistance) will be negligible, almost always below 1 mV, which is, for this purpose, acceptable. So, you will have one hand for the (positive) probe, and the other to tune the offset, by using the blue multi-turn trimmers beside.
Also, when trimming the offset, it is good to check it right after powering up, but it is most important to set it finally only once the unit reaches its final working temperature, so probably 20-30 minutes after powering up. The offset trimming circuit we use has changed a bit over time, but you can mostly expect thermal drift of about -30 mV. In other words, it is negative, so as the unit temperature increases, the output DC offset voltage will decrease. Ideally, the final voltage seen here should be about +5 mV. This is because, once the top plate is on, the temperature will rise a bit further, and produce additional drift of about -5 mV.
