I have a DAC that sounds very good to me, it's the Topping D30. The chip is the CS4398, a chip that I have listened to in several implementations and always found it very musical.
I would like to try to improve the output stage. I have several ideas in mind (transformers or tubes). As a first step I should steal the analog signal at the output of the chip. Below in photo I show the PCB. The red arrows indicate the points connected to the positive/negative output pins of the two channels.
What do you think is the best way to get the signal? Try soldering the wires to the pins directly? Or act on the four holes indicated by the arrows? In the latter case I guess it is easier, but I don't know how to remove the blue layer covering the conductor. I would be very happy if someone could point me to the right technique.
I would like to try to improve the output stage. I have several ideas in mind (transformers or tubes). As a first step I should steal the analog signal at the output of the chip. Below in photo I show the PCB. The red arrows indicate the points connected to the positive/negative output pins of the two channels.
What do you think is the best way to get the signal? Try soldering the wires to the pins directly? Or act on the four holes indicated by the arrows? In the latter case I guess it is easier, but I don't know how to remove the blue layer covering the conductor. I would be very happy if someone could point me to the right technique.
I recall that the CS4398 is a voltage output DAC, so keep in mind that you don’t want to overload its signal drive ability by having too low of an input impedance presented by the new output stage. My generic suggestion would be to solder two pairs of thin enough gauge to fit in the via holes, using differently colored signal wires for the polarities. Be sure to use more than sufficient lengths of wire to reach from those PCB holes to where the new output stage is located. Twist each pair of wires (pos and neg) together, for each channel together so to minimize the ‘loop area’ of the pairs. So, you should now have a red and black pair of twisted signal wires for each of the two channel. The twisted pairs will be shorter in length from the untwisted wires, which is why want the untwisted wires longer than at first seems necessary. Simply cut the excess length off the pairs as you connect them to the new output stage PCB’s inputs.
The holes are vias to the underside of the circuit board. Turn it around and you will see where the vias and following traces are going. Next to the CS4398 you will most likely find some in-line resistors you can desolder and whose solder pads you can use to solder some wires to. Get the CS4398's datasheet and have a look at the output stage's schematic to get the idea.
If utilizing those small ‘via’ holes seems problematic in some regard, you could instead carefully solder the signal wires flat on top of the correct chip-lead pins which are already soldered to the PCB. Be careful, this is tricky, and is not advisable unless you experienced at fine solder work. You don’t want to create any solder bridges (shorts) between adjacent chip pins. Test for solder bridges BEFORE powering the DAC circuit using the continuity function of a DMM. Pre-tin the wire ends, and you won’t need to apply any additional solder as you make the wire connections.
It will be easy. And you have to interrupt the signal path to the existing output stage anyway.
Is your plan to sacrifice the existing analogue output of the D30, or do you want to keep it?
If the vias go through the board all the way, you could insert a wire. Scraping the solder resist off the surface of the via with a blade is fairly easy.