Headphone out to line in - looking for the right amp
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drewmk2

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Hi all,
 
I'm new around here so go easy on me.
 
I have a device with a headphone output that must be connected to a line-level input. I know that headphone amps are supposed to drive low impedance loads and that a line input is high impedance. Right now (without an amp) I have a terrible SNR. Could you all recommend an amplifier to do the job? I'd like to keep it analog if possible. Flat frequency response over the audible range would be nice (although I assume that's an obvious requirement on a hifi audio forum
)
 
Cheers,
Andrew
 
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Armaegis

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A "device with headphone output" is awfully vague. Perhaps if you told us what exactly you're trying to do and what devices you're using, we can be of more help.
 
 
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drewmk2

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Sure,
 
This is actually for a biomedical research project. We're using a Doppler ultrasound unit and need to get real-time blood flow information off of it. The only real-time interface it has to the external world is a headphone output. I need to amplify that output and run it into the line-input of a digital signal processor for processing.
 
Andrew
 
EDIT: I didn't make it clear, but the signal from the headphone jack is actually an audio signal for headphones :) Also, the input impedance of the DSP line input is about 35kOhms
 
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Armaegis

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I find it odd that an ultrasound device has no output other than a headphone jack... do you have access to any of the schematics? The device must have a microphone, which itself must be connected to some sort of amping circuit. I also highly doubt that the device uses only frequencies in the audible range.
 
Honestly, an audio forum is not going to be the best place to find answers. Contacting the manufacturers/reps of your equipment would be the best place to start. Your SNR might be completely unrelated to the "audio" aspect.
 
Now, if I take off my audio helmet and put on my researcher's cap (because that's my day job), these are the things you need to start looking for:
- what is the frequency range of your ultrasound
- what are the signal levels within the device (Vrms, something like that)
- what are the output options
- what are the output levels and frequency range
- how exactly is the headphone output connected? (is it a mono signal?)
- what does the headphone output give anyways (does it compress the entire ultrasound band into the audible range? or does it just play back everything, even the inaudible?)
- what are your sources of noise? (old equipment, electrical interference, degraded microphone, microphonics in cabling)
- can you identify what the noise is and filter it out?
- what kind of signal does your DSP require, and what amplitude (assuming the DSP doesn't have gain settings of some sort)
- do you know what an instrumentation amplifier is? because that will probably be more suitable than an audio amplifier
 
Well that's just some stuff off the top of my head without much else to go on. Hope some of it helps.
 
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drewmk2

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Thanks for the reply. The reason I was vague in my first post is because I didn't want to bog people down with the details, as it doesn't really help solve the problem. I believe I just need a headphone amp that can drive a line input.
 
This is a clinical Doppler ultrasound device typically used for one-shot measurements, imaging, or logging short clips of Doppler data to a network drive or USB drive. The device is made by General Electric. It was not designed for real-time streaming of data. I know for certain that there is no way to, in real time, stream the raw Doppler data off the device. I know that others have used the headphone output to calculate meaningful flow values with units before. 
 
The headphone output is provided so that the user can 'listen' to the flow pattern while making measurements. I know that the output is purposefully in the audible range. The Doppler data I collected so far doesn't reach over 12kHz under the typical flow circumstances--well within the range of hearing. Opening the device would void the warrenty, and GE, for good reason, doesn't provide the details of the innards. The DSP has a standard audio line-in 3.5mm stereo mini jack. The utrasound unit transmits only one audio channel. The DSP uses a TI TLV320AIC23 codec.
 
Really my question is, what is a suitable headphone amp to feed into a line input (~35kOhm input resistance)? 
 
 
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Armaegis

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It shouldn't really matter then. Even just a cmoy or an e5 should do the trick. Feeding into a higher impedance is generally *easier* if all you're after is a voltage transfer. There's no guarantee that this will work though, as I don't know what your source of noise is. And keep in mind that any noise being generated from the ultrasound will be similarly amplified. 
 
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