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http://tech.groups.yahoo.com/group/micbuilders/files/kuribas/

Kristof

Op 14-02-12 18:14, Jerry Lee Marcel schreef:
> The specified frequency response is +/- 1 dB with a 10k load. I think
> the transformer is loaded with a nearly infinite resistance and the
> input capacitance of the FET stage; that is enough to explain the
> slight HF overshoot.
> Regarding the LF corner frequency, maybe the transformer is not the
> culprit; where is your schemo?
>
> Le 14/02/2012 17:21, kristof Bastiaensen a écrit :
>>
>> Hi,
>>
>> some time ago I posted my schematic of a ribbon microphone buffer
>> with opamp. I have tested my buffer and done some THD measurements.
>> I did the measurements by outputting a sinewave and putting it
>> through my buffer into my focusrite Saffire pro 10 interface,
>> and measuring it with a software audio analyser.
>>
>> I put the output from the focusrite directly (through a 1k resistor)
>> into the buffer (unbalanced) without transformer, and I got 0.003%
>> THD at 100Hz and 0.009% THD at 10kHz.
>> The frequency response was rising to about 0.5 dB at 20kHz.
>> When I measured the distortion through a voltage divider (100k:1k) I
>> was getting much larger distortion levels, but my test setup must
>> have been wrong because with the transformer (see below) there was no
>> large distortion.
>>
>> Then I measured the buffer with the transformer installed (1:110) and
>> a 1k:0.1 ohm voltage divider (balanced), and I got 0.07% THD at 45Hz,
>> unmeasureable (<0.002%) above 150Hz, and 0.005% at 8Khz.
>>
>> The frequency response with the transformer is -3dB at 20Hz, 0dB at
>> 1kHz, -0.4 dB at 4kHz, -0.5 at 10kHz and -0.2dB at 20kHz.
>>
>> I am not sure why I am getting a rising frequency response, maybe it
>> is because of the FET gate capacitance?
>> The -3dB point at 20Hz seems to high, since the transformer datasheet
>> (LL1927) says it should be +- 1dB from 10Hz to 70Hz.
>> Maybe the high -3dB point comes from my buffer instead.
>> Unfortunately the phantom power from my interface has broken down
>> (maybe I shorted something while testing my circuit), and I
>> cannot do any measurements at the moment.
>>
>> The SNR seems very good, the noise is slightly more than my Octava
>> MK-012 condenser, and the output level is about the same.
>>
>> I think my circuit is decent enough for a transparent, professional
>> sound, though I'd like to know if the THD and frequency response
>> could be improved, especially at low frequencies.
>>
>> It's also interesting that the circuit will work at lower voltages
>> (even 12V), by bypassing the zener diode. It would also be possible
>> to add gain by including two resistors in the feedback and a
>> capacitor. For example to use a ribbon microphone with a portable
>> recorder with low gain. I found a similar idea here
>> (http://www.ciphersbyritter.com/RADELECT/PREJFET/JFETPRE.HTM)
>> which uses a BJT instead of an opamp for feedback.
>>
>> Regards,
>> Kristof Bastiaensen
>>
>>



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