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#1
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Disabling M-Audio Transit USB Analog Input DC Bias
Looking for a better audio recording interface for my Gateway (Motion
Computing) Tablet PC, I ran across the M-Audio Transit USB. It immediately worked for playing sound, but I've been through a major hassle with recording. It turns out the Transit is designed with 2.24 VDC of bias on each analog audio input, to power cheap PC microphones. When connected to other audio devices, this bias voltage fights the source you are trying to record. Depending on the source impedance, it might even draw enough current from the Transit's tiny USB-driven power supply to interfere with the USB signal and cause full-volume hissing in place of music. With any audio source, I found I could have only one end of the USB cable connected to grounded equipment. If the audio source was grounded, the computer could not be - not even by the supposedly isolated AC-powered battery charger. I spent over a month eMailing M-Audio about these issues, and received no help, only the typical runaround about drivers, SpeedStep, and the kind of IRQ conflicts that went away with the ISA bus. I finally started calling their phone support with the single DC bias issue, and in only a day got them to confirm that the 2.24 V bias was by design. In another hour I had cut the traces and solved almost all of the problems with recording through the Transit. Suddenly there was no more hissing, no more problem with using the battery charger while recording, and the extreme loading of the source signal went away. I don't guarantee that I've found the best way to accomplish removing the DC bias, but here's how I did it. (For a photo of the area I'm referring to, see http://www.psychoros.org/Transit-Input.jpg.) The output end of R4 is a 4.4V supply. R33, R31, and C36, and R34, R32, and C37, form a pair of voltage dividers, creating two independent filtered sources of the 2.2V bias voltage. R21 and R22 connect the bias voltages to the actual analog audio input lines, and in the process connect C36/37 between the audio inputs and ground. (The equivalent sized C32 and C33 connect the audio inputs to the ADC circuits, so the loading is significant.) You can probably see my ugly cuts in the traces to the right of R21 and R22, separating the DC bias supplies from the audio inputs, in the photo. (Guess I need a new X-Acto blade...) Those two tiny cuts solved the problems with my Transit and the Tablet PC, stopped the excessive loading of the audio source, and dramatically improved the recorded sound. With this issue fixed, I'm very pleased with the sound quality of both recording and playback through the Transit. I can't say the same for M-Audio's product documentation or support. Loren |
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#2
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Loren Amelang writes:
Looking for a better audio recording interface for my Gateway (Motion Computing) Tablet PC, I ran across the M-Audio Transit USB. It immediately worked for playing sound, but I've been through a major hassle with recording. It turns out the Transit is designed with 2.24 VDC of bias on each analog audio input, to power cheap PC microphones. When connected to other audio devices, this bias voltage fights the source you are trying to record. Depending on the source impedance, it might even draw enough current from the Transit's tiny USB-driven power supply to interfere with the USB signal and cause full-volume hissing in place of music. With any audio source, I found I could have only one end of the USB cable connected to grounded equipment. If the audio source was grounded, the computer could not be - not even by the supposedly isolated AC-powered battery charger. I spent over a month eMailing M-Audio about these issues, and received no help, only the typical runaround about drivers, SpeedStep, and the kind of IRQ conflicts that went away with the ISA bus. I finally started calling their phone support with the single DC bias issue, and in only a day got them to confirm that the 2.24 V bias was by design. In another hour I had cut the traces and solved almost all of the problems with recording through the Transit. Suddenly there was no more hissing, no more problem with using the battery charger while recording, and the extreme loading of the source signal went away. I don't guarantee that I've found the best way to accomplish removing the DC bias, but here's how I did it. (For a photo of the area I'm referring to, see http://www.psychoros.org/Transit-Input.jpg.) The output end of R4 is a 4.4V supply. R33, R31, and C36, and R34, R32, and C37, form a pair of voltage dividers, creating two independent filtered sources of the 2.2V bias voltage. R21 and R22 connect the bias voltages to the actual analog audio input lines, and in the process connect C36/37 between the audio inputs and ground. (The equivalent sized C32 and C33 connect the audio inputs to the ADC circuits, so the loading is significant.) You can probably see my ugly cuts in the traces to the right of R21 and R22, separating the DC bias supplies from the audio inputs, in the photo. (Guess I need a new X-Acto blade...) Those two tiny cuts solved the problems with my Transit and the Tablet PC, stopped the excessive loading of the audio source, and dramatically improved the recorded sound. With this issue fixed, I'm very pleased with the sound quality of both recording and playback through the Transit. I can't say the same for M-Audio's product documentation or support. Loren Dear Loren, Hey, while you've got that Transit apart, can you show us the rest of the insides??? I'd like to know what ADC chips and mic preamp (opamp?) chips it uses. I'm looking for a cheap USB input device for Laptop recording. It says it has mic inputs, but I don't know how it senses the difference between mic and line. Also, are you able to control the input level via software? That is, can you control the input level *before* ADC is done. Thanks, Richard |
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#3
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Loren Amelang writes:
Looking for a better audio recording interface for my Gateway (Motion Computing) Tablet PC, I ran across the M-Audio Transit USB. It immediately worked for playing sound, but I've been through a major hassle with recording. It turns out the Transit is designed with 2.24 VDC of bias on each analog audio input, to power cheap PC microphones. When connected to other audio devices, this bias voltage fights the source you are trying to record. Depending on the source impedance, it might even draw enough current from the Transit's tiny USB-driven power supply to interfere with the USB signal and cause full-volume hissing in place of music. With any audio source, I found I could have only one end of the USB cable connected to grounded equipment. If the audio source was grounded, the computer could not be - not even by the supposedly isolated AC-powered battery charger. I spent over a month eMailing M-Audio about these issues, and received no help, only the typical runaround about drivers, SpeedStep, and the kind of IRQ conflicts that went away with the ISA bus. I finally started calling their phone support with the single DC bias issue, and in only a day got them to confirm that the 2.24 V bias was by design. In another hour I had cut the traces and solved almost all of the problems with recording through the Transit. Suddenly there was no more hissing, no more problem with using the battery charger while recording, and the extreme loading of the source signal went away. I don't guarantee that I've found the best way to accomplish removing the DC bias, but here's how I did it. (For a photo of the area I'm referring to, see http://www.psychoros.org/Transit-Input.jpg.) The output end of R4 is a 4.4V supply. R33, R31, and C36, and R34, R32, and C37, form a pair of voltage dividers, creating two independent filtered sources of the 2.2V bias voltage. R21 and R22 connect the bias voltages to the actual analog audio input lines, and in the process connect C36/37 between the audio inputs and ground. (The equivalent sized C32 and C33 connect the audio inputs to the ADC circuits, so the loading is significant.) You can probably see my ugly cuts in the traces to the right of R21 and R22, separating the DC bias supplies from the audio inputs, in the photo. (Guess I need a new X-Acto blade...) Those two tiny cuts solved the problems with my Transit and the Tablet PC, stopped the excessive loading of the audio source, and dramatically improved the recorded sound. With this issue fixed, I'm very pleased with the sound quality of both recording and playback through the Transit. I can't say the same for M-Audio's product documentation or support. Loren Dear Loren, Hey, while you've got that Transit apart, can you show us the rest of the insides??? I'd like to know what ADC chips and mic preamp (opamp?) chips it uses. I'm looking for a cheap USB input device for Laptop recording. It says it has mic inputs, but I don't know how it senses the difference between mic and line. Also, are you able to control the input level via software? That is, can you control the input level *before* ADC is done. Thanks, Richard |
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#4
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Loren Amelang wrote: Looking for a better audio recording interface for my Gateway (Motion Computing) Tablet PC, I ran across the M-Audio Transit USB. It immediately worked for playing sound, .... It's a fine low-cost S/PDIF input device for Mac and PC laptops too. We've tested it an found it's bit-for-bit accurate. [Disclaimer: Core Sound is an M-Audio dealer.] -- Len Moskowitz PDAudio, Binaural Mics, Cables, DPA, M-Audio Core Sound http://www.stealthmicrophones.com Teaneck, New Jersey USA http://www.core-sound.com Tel: 201-801-0812, FAX: 201-801-0912 |
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#5
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Loren Amelang wrote: Looking for a better audio recording interface for my Gateway (Motion Computing) Tablet PC, I ran across the M-Audio Transit USB. It immediately worked for playing sound, .... It's a fine low-cost S/PDIF input device for Mac and PC laptops too. We've tested it an found it's bit-for-bit accurate. [Disclaimer: Core Sound is an M-Audio dealer.] -- Len Moskowitz PDAudio, Binaural Mics, Cables, DPA, M-Audio Core Sound http://www.stealthmicrophones.com Teaneck, New Jersey USA http://www.core-sound.com Tel: 201-801-0812, FAX: 201-801-0912 |
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#6
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#7
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#8
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#9
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#11
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Loren Amelang writes:
On 17 Nov 2004 13:27:39 -0500, (Len Moskowitz) wrote: Loren Amelang wrote: Looking for a better audio recording interface for my Gateway (Motion Computing) Tablet PC, I ran across the M-Audio Transit USB. It immediately worked for playing sound, .... It's a fine low-cost S/PDIF input device for Mac and PC laptops too. We've tested it an found it's bit-for-bit accurate. [Disclaimer: Core Sound is an M-Audio dealer.] I've seen some loopback tests that show it is bit-for-bit during simultaneous play and record on the Toslink side. Just don't think you are going to use it for analog monitoring while recording the optical in - Connecting the Toslink-to-mini adapter to the dual-purpose mini jack input appears to mechanically switch off the analog output. Not that it is such a great loss - there is no direct monitor ability, you must use playthrough in software. The latency is pretty long, more of a distant memory than an echo. Still, for the price, the compromises are tolerable, and the sound is great. Loren More questions. Does this operate in "standalone" ADC mode? That is, if you put analog in, can you get digital out without connecting to a computer? Or could you hack it to do that? This could be a really cheap input device, powered from a 6 or 9V battery. Anyway, I think I'll pick one up for my (Windows) laptop. Richard |
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#12
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Loren Amelang wrote: Just don't think you are going to use it for analog monitoring while recording the optical in - Connecting the Toslink-to-mini adapter to the dual-purpose mini jack input appears to mechanically switch off the analog output. You can use the laptop's internal sound card for monitoring. We've done that here and it works just fine as long as you can stand the latency delays. -- Len Moskowitz PDAudio, Binaural Mics, Cables, DPA, M-Audio Core Sound http://www.stealthmicrophones.com Teaneck, New Jersey USA http://www.core-sound.com Tel: 201-801-0812, FAX: 201-801-0912 |
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#13
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Loren Amelang wrote: Just don't think you are going to use it for analog monitoring while recording the optical in - Connecting the Toslink-to-mini adapter to the dual-purpose mini jack input appears to mechanically switch off the analog output. You can use the laptop's internal sound card for monitoring. We've done that here and it works just fine as long as you can stand the latency delays. -- Len Moskowitz PDAudio, Binaural Mics, Cables, DPA, M-Audio Core Sound http://www.stealthmicrophones.com Teaneck, New Jersey USA http://www.core-sound.com Tel: 201-801-0812, FAX: 201-801-0912 |
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#14
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#16
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#17
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