Audio capture card options

Posted by: @askolds11

I've found that the Behringer LINE 2 USB is amazing (like, seriously perfect for this project and it will be used in version 2) but it may be above your budget. Might be worth dissecting one and seeing what chipset it uses.

This seems to be unavailable in my country, although there is a GUITAR 2 USB, that looks the same just without the 2 leads coming out of it, but it's 20 euros, and I'd much rather get something for much cheaper (5-10 Euros).

 

I have tested that one too and it seems to be basically the same chip etc so performs the same. Not sure about any cheaper options to be honest... I'm sure some of the cheap ones will work well but I've just not gone through and tested them extensively.

I'm gonna give the CM108 card another try - it comes with the CM108B chip, which, according to the datasheet, can have an external EEPROM attached (the CM108 can too), and you can (hopefully) disable the gain (can't do this on the normal CM108, which might be discontinued anyways). The EEPROM chip is about to arrive, but I'm going to have to learn how to use the memory addresses and how to program it, but I'm sure I'll make it haha.

It also could be that the chip could be quiet enough even with the gain enabled (of course not as good as the audio capture cards or that cable), but my circuit was the problem - I'm using my own circuit with a LM4562 op-amp and a dynamic capsule, and I imagine the noise could be from the somewhat lenghty (physical size) feedback loop for the op-amp, but I've bought a THAT1512 (hopefully not fake), which should solve that problem, although I still have to get the isolated +-15v chip, although I'll probably start with a +-12, since that's available in my country.

Keep us posted on that. I've never heard of the CM108B before now and it certainly sounds like an interesting option!

Finally got done with exams and tests, could try some things.

Unfortunately, I haven't tested the EEPROM option, as I'm still working on it - found an Arduino sketch with half of the necessary EEPROM data, successfully programmed the EEPROM chip, but didn't get it to connect to the CM108B (soldering to the tiny pins itself was terribly hard!).

I did however try it out without the microphone circuit component (in my case - LM4562 op-amp, very likely a lot of noise from the circuit) - I recorded some audio on my computer - I've got some filters on it, so it's pretty noiseless -, and played it back on my phone connected to the CM108B sound card microphone jack.

I tested 2 configurations - unmodified and removed bias voltage (don't need it for the microphone either) by removing R13 from the circuit on page 28 in this datasheet (I am not sure how accurate the rest of it is, but I've confirmed that at least the components from the VBIAS pin are exactly the same). I left R10 and C15 there, as I'm not sure how good it is for the pin to be left floating. I also left C11 (confirmed this is the same as well), as I do not want to accidentally fry the chip.

Here are the results (I've tried to normalize the audio levels, so they're the same):

Original audio:

Sound card unmodified:

Sound card removed bias:

There is some minor noise I can hear from the original audio, in the unmodified card I can definitely hear some added noise though - when talking, I can hear the typical buzz, but by removing the resistor it sounds almost the same as original! There is maybe some added noise, but I can't say for sure, as the original audio has some noise.

I was honestly surprised by these results - it must be my circuit that's causing the noise, as these results are not even in an enclosure - I found that my whole desk is a disaster for interference and the front of my computer, where I got these results from, is not any better. I'm going to try to put a circuit together with the THAT1512 (already tested with a dynamic mic capsule and it works, but I have to rethink it a little) and try to find a metal box to minimize interference and feed it into the CM108B with no bias resistor and maybe EEPROM modification, if I manage to finish it, to test the results out!

Here is the stock card with the resistor you have to remove. I did it with my soldering iron and tweezers, but I've seen people do it on YouTube by just snipping it off with flush cutters.

Note: I seem to have forgotten to turn off AGC, so the differences are more noticeable than they would likely be normally, but the comparison should still be fine, as they're under the same conditions.