It seems that the issue with the smaller PCB failing to work has to do with the lack of -12V. I don't understand why the larger card doesn't have this problem.

Re: Why isn't this cut down ESS Solo-1 working as expected?

Larger card doesn't have op-amps soldered (empty places). Smaller one has 8 pin chip soldered - which is probably op-amp needed symmetric (+ and -) power supply.

Can I bypass the op-amp for line level output? I don't need the output amplified to my amplified speakers.

I've attached a picture with a close-up of the chip.

It seems like this will need some capacitors as well. I can confirm a lot of noise on the PCI bus.

Re: Low volume out of Cobra AW744L II card

Kahenraz wrote on 2024-08-28, 06:29:

Can I bypass the op-amp for line level output? I don't need the output amplified to my amplified speakers.

I've attached a picture with a close-up of the chip.

The attachment 20240828_023031.jpg is no longer available

Well I'm not an expert, but I think that op-amp is here to drive headphones. It's probably to weak to drive passive speakers.

However, after quick look on datasheets of both Solo-1 (ES1938) and Solo-1e (ES1946) it seems that main difference is IC supply voltage (+5V vs +3.3V), and their pinout looks identical.
So you could trace the paths on the board (of your larger card without an amplifier) from pins 50 and 51 of the ESS chip to the audio output jack.
Quote from datasheet:

1AOUT_L, AOUT_R  |   50, 51  |  O  |  Line-level stereo outputs, left and right. These pins can drive a 5k Ω AC load.

And based on that, recreate the same situation on the smaller card...

A 4558 won't drive headphones either (not enough output current). It's just configured as an inverter (all resistors are the same value, so it doesn't amplify). I don't know what they were thinking with that circuit (maybe to protect the Solo 1 chip from a shorted output).

This is such a simple circuit that it will be easy to bypass, but it will require either removing the chip or cutting traces. The line output connects to C22 and C24, which are probably both severely undersized (meaning that there will be no bass unless you replace them). The output pins on the chip will have a DC offset of about 2.5V, so you will need to keep a series cap of around 10µF. The other option is to use an isolated DC-DC converter to make -12V from +12V or +5V, and then you could use any card you want.
Re: Low volume out of Cobra AW744L II card
You don't have to use a dual-output model like that. Since they're isolated, you can use a single-output version and connect +Vout to ground to get a negative voltage.

I remember seeing flexible PCI risers using ribbon cables many years ago. Those might be your best option if you can still find them (and if the excess cable length can fit inside your thin client).

I ordered some little DC-DC boards from AliExpress to try out, since that seemed like the easiest option.

Which pins should I tap into on the PCI slot to add the -12V? And is there someplace I can probe between the slot and the card to validate that it's connected properly before plugging it into the motherboard?

I have soldering skills but no actual electrical engineering knowledge.