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Topic: Measuring frequency response of a preamp (Read 5704 times) previous topic - next topic
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Measuring frequency response of a preamp

Howdy All,

Looking for a way to take a PCM recorder and check the frequency response on the mic input. Can I do this with software (I have access to RMAA and Amadeus Pro)?

My goal is to determine when/where the low frequency begins to drop off, although I'd like to see the entire plot.

Thanks for any assistance!

Measuring frequency response of a preamp

Reply #1 –
Quote from: dyneq on
Howdy All,

Looking for a way to take a PCM recorder and check the frequency response on the mic input. Can I do this with software (I have access to RMAA and Amadeus Pro)?

My goal is to determine when/where the low frequency begins to drop off, although I'd like to see the entire plot.

Thanks for any assistance!



There are two problems with connecting a mic input to a signal source: to make measurements:

(1) The signal source will have too much voltage if operated at digital full scale in order to get good performance.

(2) The mic input may have phantom power, which may destroy or damage the signal source.

I've built little gizmos with blocking capacitors and attenuators to address this problem. I know of no commercial products that do this.

Measuring frequency response of a preamp

Reply #2 –
Hi Arnie,

Thanks for your insights. I should have provided more info. I have the Olympus LS-11 PCM recorder. Its predecessor, the LS-10 is known to have a low frequency roll-off based on this test.

Presumably, Olympus did this to enhance the performance of the built-in mics, but they did not make it a firmware option when using the mic input. Olympus has claimed that the LS-11's roll off is not as severe. (beginning at 60Hz instead of 70Hz).

I know how to generate a logarithmic sine sweep wav file. Would I be able to somehow record the sweep file into the mic input and then analyze the resulting frequency response of that recording?

Thanks,

John

Measuring frequency response of a preamp

Reply #3 –
Quote from: dyneq on
Hi Arnie,

Thanks for your insights. I should have provided more info. I have the Olympus LS-11 PCM recorder. Its predecessor, the LS-10 is known to have a low frequency roll-off based on this test.

Presumably, Olympus did this to enhance the performance of the built-in mics, but they did not make it a firmware option when using the mic input. Olympus has claimed that the LS-11's roll off is not as severe. (beginning at 60Hz instead of 70Hz).

I know how to generate a logarithmic sine sweep wav file. Would I be able to somehow record the sweep file into the mic input and then analyze the resulting frequency response of that recording?


Once you solve the problems of getting a low noise, undistorted signal (as possible)  into and out of the device under test, you can use RMAA (freeware - search google)) to run a comprehensive test suite which includes frequency response.
 

Measuring frequency response of a preamp

Reply #4 –
Quote from: Arnold B. Krueger on
There are two problems with connecting a mic input to a signal source: to make measurements:

(1) The signal source will have too much voltage if operated at digital full scale in order to get good performance.

(2) The mic input may have phantom power, which may destroy or damage the signal source.


For item 1, I would most likely be feeding the sweep signal from my mobo's line out (is there a better way without specialized equipment?). I'd probably use the internal preamp's low sensitivity setting (set on the recorder's firmware menu). low sensitivity specs: -39 dBv, impedance 2 kOhms. I'm guessing that I'd want to have the gain fairly high without clipping?

For item 2, phantom is not supported, only PIP. I can enable/disable it via a menu setting. I do have a portable battery box to power my microphones, so I'd be most interested in the response while using no PIP.