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Recommend a USB audio interface?


Sauraen
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My desktop has a noise problem. I don't know whether it's the power supply or the motherboard or what, but if I play audio directly from its integrated audio card the signal-to-noise ratio is about 1! (the noise is as loud as the music) I solved this problem by buying a SPDIF optical (TOSlink) to RCA converter and hooking it to the optical audio out on the motherboard, so the grounds of the two are not connected. (I also had to build a linear power supply for the converter, since its $2.00 switching wall-wart added a ground hum.) The overall latency is less than 10ms and the audio is crystal clear, so I'm happy.

 

But now I want to record audio into the computer! If I do it directly (whether mic in or line in), there is again a huge amount of noise. So I need some sort of interface box that takes at least one audio input--that part doesn't have to be fancy, though I guess stereo would be nice--powered by an external linear power supply, and output a USB-audio-device signal of that incoming audio, with the audio input completely electrically decoupled from the USB side (not sharing a ground).

 

Any recommendations?

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I don't have any recommendations, as i do not own a usb audio interface. As for suggestions, i would personally go for one of these three:

 

RME Babyface

Focusrite Safire 2i2

Roland UA-22

 

The RME is pretty expensive. However they have very solid driver support. So you can use it for a long time to come. But then again you could probably pickup a new focusrite or roland in two years and even save some money.

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Hola,

there is just one manufacturer, that I cannot recommend - don´t buy any M-Audio FastTrack USB MIDI/Audio device...

Their drivers under Win7/64 have certainly made me walk away from using a computer and now quite exclusively use the MIDIbox SEQ V4 :-). When seen from that perspective, that company is not all that bad :)

 

Many greets,

Peter

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Hola,

there is just one manufacturer, that I cannot recommend - don´t buy any M-Audio FastTrack USB MIDI/Audio device...

Their drivers under Win7/64 have certainly made me walk away from using a computer and now quite exclusively use the MIDIbox SEQ V4 :-). When seen from that perspective, that company is not all that bad :smile:

 

Many greets,

Peter

M-Audio is indeed a nightmare driver wise. So how do you record your songs then? You cannot do that with the SEQ V4 ;-)

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M-Audio is indeed a nightmare driver wise. So how do you record your songs then? You cannot do that with the SEQ V4 ;-)

 

Hehe, true, it is a Tascam standalone WAV recorder, which is ok, probably not the best analog-digital converter on the market, but it works ok and does not crash :)

 

Many greets,

Peter

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The main problem is the computer's ground is extremely noisy, so it doesn't matter how expensive an interface I get, as long as it has the USB ground connected to audio ground, the audio will be horrible. As a matter of fact, if I connect a single wire from the computer's ground to my mixer's ground, immediately I can hear hard drive data coming out the speakers. As far as the audio side, I only need two channels 48khz 16 bit, nothing fancy.

 

However I did find something that might be useful: a USB ground isolator: http://www.ebay.com/itm/HiFimeDIY-USB-Isolator-ADUM4160-5KV-3KV-signal-voltage-isolation-DC-DC-convert-/141087228001?pt=LH_DefaultDomain_0&hash=item20d9744061 With this plus a cheap audio interface (e.g. http://www.ebay.com/itm/New-Behringer-U-Control-UCA-202-USB-Audio-Interface-Adapter-in-BOX-UCA202-Hot-/290985992006?pt=US_Computer_Recording_Interfaces&hash=item43c01ddf46 ), do you think this will solve the problem?

Edited by Sauraen
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Hola, imho, the ground isolator should work fine.

Another option to try before shelling out 30 bucks would be to build an inexpensive LC-filter for your 5v USB power rail, you would need to cut open an old usb cable for that, but it might be worth a try (am eliminating most noise on the power rails of my flying things with great succcess with those things): http://www.fpvuk.org/how-tos/lc-filters/

 

Many greets!

Peter

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If it just happens you have a DC-DC (5V-5V) converter lying around that might be even better, but, then it's in fact the eBay thingie... And if you don't (and your local electronic parts dealer doesn't have them on-stock) you definitely should try the LC filter first.

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I can power any USB audio interface from the same linear 5v power supply I built for the optical to RCA converter, so getting 5V to it is not the problem. The problem is getting the audio ground at the same average potential as the USB ground from the computer without letting noise on the latter propagate to the former. As far as I know, setting up a filter like the one above will get the +5 rail to be smoothed relative to the ground rail, but it will not smooth out voltage fluctuations in the ground potential itself. Or will it if I put the black wire through an inductor as well?

 

If it just happens you have a DC-DC (5V-5V) converter lying around that might be even better, but, then it's in fact the eBay thingie... And if you don't (and your local electronic parts dealer doesn't have them on-stock) you definitely should try the LC filter first.

If you're sure a DC-DC converter has both +5 and GND rails decoupled from the source, I can order one from my "local" electronics supplier (Mouser, Digikey, etc.) :smile:.As a matter of fact there's one that might work from Mouser for $3.30. After a little research it looks like the "isolated" keyword on a DC-DC converter means -Vin is not connected to -Vout, which is what I'm looking for.

 

Still one question though: Because the USB data wires have to go across this isolation gap to the ADC chip, I can't have the two grounds floating with respect to each other--they need to be at the same potential. But hooking a wire between the two grounds would destroy the whole point. So what should I connect between them? A very-high-resistance resistor (~1Meg) in parallel with a very-large capacitor (~2200 uF)?

Edited by Sauraen
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As far as I understand (and that may well be wrong :-D, please correct me)...

 

1) the capacitor (C part of the filter) will smooth out quick voltage differences between +5v and GND (so also the ground part is affected). For optimal voltage filtering, add a smaller capacitor parallel to the larger capacitor - a really good working pair is 2200uF in conjunction with 100nF.

 

2) the ferrite ring (L part of the filter) will create a small electromagnetic field (only, when current flows), which builds up a certain "stability" through the ferrite core. The electromagnetic field restricts changes to current flow. It is only necessary to "stabilize" the current flow through one wire (it flows from gnd to +5V, no need to go through the ring twice).

 

So imho, the LC filter might help, even if you only add the +5V wire through the ferrite (you can also include the ground wire, but it won´t help much, it is the same as adding more windings to the +5V wire).

 

Many greets,

Peter

Edited by Hawkeye
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Science!

 

Just connecting a wire from the computer's ground to my mixer's ground, with no other wires between them, makes hard drive data noises come out of the speakers (connected to the mixer).


I found a ferrite ring in a drawer in my electronics stuff, put 16 turns around (through) it, and connected that between the two grounds. Result: same noise. :(

 

Measuring with my portable (and therefore ground-isolated!) scope, the noise between the mixer ground and the computer ground when they are not connected is around 50 mV P-P, and spikes to 100 mV when the computer is loading a large program. Oviously audible!

 

I also tried some antenna coils--with ferrite cores--from the same drawer, and the noise comes right through. However, I then tried a small transformer (from a electronics kit radio), and the noise does not appear when I connect the two grounds through it, even though there is 200 ohms resistance between the two. The scope shows the full noise across it, and I don't hear any; but due to the non-infinite resistance between the two ends of that transformer coil, they must be held at the same potential. So it looks like I found what I'm looking for--and I had it all along! I will buy a cheap USB audio interface and wire the transformer in series with the ground connection and power the interface from an external linear supply on the audio side, and see what happens.

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Also, is the computer supplied by the same wall outlet as the audio system? 

 

FWIW, I got very good results with no ground loops with my RME Fireface 800 as an expensive "soundcard". Although the converters are excellent, I wouldn't recommend going with this though. Firewire is notoriously difficult to configure, especially on PCs and even more so on laptops. There is apparently a magical TI chipset that avoids DMA conflicts but production has since moved onto different hardware. The results are very poor quality audio with many dropouts if you get it wrong. 

 

Does anyone know of a good USB3 or other interface? 

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Is your computer properly grounded?

Probably not, but I don't think that's something I can fix. When I first built it and heard the ridiculous amount of noise, since the motherboard I got was advertised as having a 108 dB signal-to-noise ratio for its integrated audio card, I complained to the manufacturer. In performing the tests they asked me to (and some they didn't), I ended up with the motherboard by itself on my desk just hooked into the PSU, mouse, keyboard, screen, and speakers, with no drives or other peripherals. As soon as I plugged the power cord into the PSU--even before turning on the master switch on the PSU--noise started coming out of the speakers. Turning on said switch caused a burst of noise and then it died down to previous levels; turning on the motherboard produced more noise, and even scrolling through the BIOS screens would produce a "click" of audible noise every time it refreshed the screen. I tried this all with a different PSU but got identical results.

 

Also, is the computer supplied by the same wall outlet as the audio system?

 

The speakers are connected to the same surge protector as the computer, but almost everything else (e.g. mixer) is connected to a different surge protector. However, if I connect a dynamic mic directly to the mic in port on the computer (so there's no question of a separate device that's also plugged into the wall) and hit Record on Audacity, all the noise gets digitally recorded along with the actual sound going into the mic.

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