Balanced line driver (DRV134) and noise - diyAudio
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Old 21st December 2009, 01:17 PM   #1
ettos is offline ettos  Italy
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Default Balanced line driver (DRV134) and noise

Hi to all,
I'm building a microphone preamp based on an INA217 chip.
I'm trying to get a balanced output with the DRV134 line driver. I just followed the drv134 datasheet schematic (see the attachment) but I'm getting a lot of noise (a sort of white noise).
The output of ina217 is pretty quiet, all the noise is generated by the drv134.
I tried to remove C4 and C5, and to use different bypass capacitor but... nothing!

Is there anything I can do to solve this? Do you suggest other solutions to get a balanced output?

Thank you very much,
Matteo.
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Old 21st December 2009, 05:51 PM   #2
gfiandy is offline gfiandy  United Kingdom
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Hi, I'm not sure on this one as the output of your INA217 should be faily low impedance but could your problem be injected noise from the 1M servo resistor.

Also what gain have you set the INA217 for as you want to gain it up at this stage, not pass it to the DRV137 at a low level. The DRV is intended to operate at line level not at microphone levels. Your diagram shows no gain resistors if this is the case then you have a gain of 1 which is not suitable.

Regards,
Andrew
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Old 21st December 2009, 06:27 PM   #3
ettos is offline ettos  Italy
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Thank you for your answer, Andrew.
It's not the 1M resistor. I tried disconnecting the whole servo and the noise remains.
The gain of the INA is regulated by a potentiometer. For recording guitar with a condenser microphone, for example, I set a gain of 65 (150 Ohm resistance).

Regards,
M.
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Old 21st December 2009, 06:47 PM   #4
paulb is offline paulb  Canada
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Make sure you have both sides of the differential output loaded. Don't leave either of them open-circuited. Maybe try something close to 600 ohms on each output.
I've seen this same problem as well but didn't look into it too closely at the time. I'll be interested in your results.

Last edited by paulb; 21st December 2009 at 06:57 PM.
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Old 21st December 2009, 06:54 PM   #5
ettos is offline ettos  Italy
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Quote:
Originally Posted by paulb View Post
Make sure you have both sides of the differential output loaded. Don't leave either of them open-circuited.
Yes, the output is connected to a pc audio interface (balanced input). I can actually hear the noise.

Thank you paulb,
M.
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Old 21st December 2009, 08:04 PM   #6
gfiandy is offline gfiandy  United Kingdom
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Hi, Sounds like you have tried most of the obvious things.

How good is your decoupling could it be unstable?

Regards,
Andrew
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Old 21st December 2009, 08:26 PM   #7
ettos is offline ettos  Italy
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Quote:
Originally Posted by gfiandy View Post
Hi, Sounds like you have tried most of the obvious things.

How good is your decoupling could it be unstable?

Regards,
Andrew
Actually I don't know because I haven't got an oscilloscope to test it at present. I used variuos bypass capacitor with no improvements.
This is the power supply unit I use PSU-2448mk2 Kit

Good night,
Matteo.
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Old 21st December 2009, 10:52 PM   #8
gfiandy is offline gfiandy  United Kingdom
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Its unlikely that the PSU regulation would cause noise of this type. I was referring more to the local decoupling; you show some local 1uF caps on the DRV this decouping very close to the supply pins and be a high speed dielectric (i.e not a electrolytic). Although to be honest instability doesn't normally cause a white noise sound, it usually just heats the device up with ultrasonic oscillation.

I am out of ideas. If it is unstable you can normally stop this by touching the circuit. It may inject a bit of hum but it usually stop the oscillation so you could try just touching areas on the circuit to see if you can find a sensitive node.

Regards,
Andy
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Old 22nd December 2009, 02:46 AM   #9
Bonsai is offline Bonsai  Taiwan
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Often, if it sounds like white noise it is in fact high frequency oscillation (200KHz and above). I am not familiar with the balanced line drive r you are using, but it might be you have a capacitive load on the output (figure on 100pf per metre of cable, sometimes higher) and its causing the buffer to go unstable. Additionally, if you are feeding into a sound card, the sound card may have RF decoupling caps from the input to ground designed to work at above audio, but nevertheless causing loading problems in op-amps and buffers. Try inserting a 100 Ohm resistor in series with each output of the driver IC. Place the resistors close to the IC. Can you post a photograph of your layout/setup?

Also, keep in mind that you may have a lot of RF hash around because of the PC - power supply, CPU clock etc - that could be causing problems.
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Last edited by Bonsai; 22nd December 2009 at 02:48 AM.
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Old 22nd December 2009, 02:52 AM   #10
Pano is online now Pano  United States
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I've use this chip (and the 135) several times and had no problems. Hmmm...
I have seen it used with small resistors in the power supply legs, tho I don't know why.
Maybe noise?
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