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Preamplifier Phono Input Frequency Response

Hello,

I am doing an experiment. I wanted to test CD and Gramophone sound, but first i wanted to see what frequency response do i get with my equipment. As the attached images show, the CD player frequency response is fine (first it's the test wav file frequency response that i burned to the CD-ROM, and next is a frequency response of a CD player with the burned CD-ROM directly attached to the line-in of my sound card. I have no problem with that, my sound card is old and was cheap, so a little low-frequencies cut-off doesn't worry me.


test wav file:


cd player's response:


I have a problem with not knowing how to properly interpret the results from the preamplifier's phono stage. When I have no gramophone connected to the preamplifier and turn up the gain and press record, this is what i get:


I know that normally (in the digital world) I should have gotten a somewhat flat line. I get a curve that falls with the frequency and that quite resembles the RIAA curve. I am surely recording some noise from the enviroment, because there is nothing connected to my phono stage. Is it possible that the reason why this noise looks like this is because the noise "goes" through the RIAA curve filter on the preamplifier's phono stage? I mean, is this what I get "normal" or "abnormal"?

and this is the frequency response of a test record with 20-20000 Hz test track:

Preamplifier Phono Input Frequency Response

Reply #1
Quote
Is it possible that the reason why this noise looks like this is because the noise "goes" through the RIAA curve filter on the preamplifier's phono stage? I mean, is this what I get "normal" or "abnormal"?
Normal!  The phono-preamp noise is normally shaped by the RIAA curve.    With any preamp, you get more noise with the gain turned-up...  So, if you increase the gain for a particular frequency range, the noise for that particular frequency range, etc.

Quote
...because there is nothing connected to my phono stage.
You'll probably get less overall noise with a phono cartridge connected (due to lowered input impedance).  Except, there is a chance that the cartridge will pick up some additional AC hum.

The sweep on your test record seems to be recorded without equalization.  That's OK, and the "documentation" should say so.  I think the sweep on the most test records is RIAA equalized, and you're looking for a flat line from an "ideal" system.

Preamplifier Phono Input Frequency Response

Reply #2
DVDdoug,

thank you for your reply. I'm afraid that there is no label on my test record that would indicate that the record is or isn't RIAA equalized.
I understand that if it was, I should've gotten a straight line. This is my test record:


Here is the link to the recorded file.
http://www.box.net/shared/pzksj2shjp

It can be seen on the frequency analyzer that the volume of lower frequencies is constant when it starts to drop. What do you think?

I think that I get a similar response with and older test record which is RIAA equalized, but am not sure, I have to check as it's on another PC.

here is the frequency response of my unconnected preamp on the phono stage compared to the RIAA curve:

I think that this is ok, because there is a 4dB max difference on high frequencies and a bit more on lower (because of AC hum).

 
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