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Setup Phono Preamplifier Measurements // Spot my error?

kerwin

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Apr 27, 2024
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I was hoping to do some measurements on my phono preamplifier to check some parameters (response, thd+n) and looking for a practical way to set this up.

My plan so far:
1. Use REW with an RME interface. Output 1 goes to input 1 on the preamp, output 2 used as loopback reference
2. Set tone generator at 50Hz, calibrate output voltage using multimeter
3. Dial down output to 3mV (1 kHz) to match mm cartridge level
4. Run sweep
5. Apply inverse RIAA on measurement (found a file to apply to 'measurement microphone')

However, i'm running into the problem that when i get to step 4, i check the levels: all good, but when i start the measurement it stops because of clipping.
Anybody have any idea what i'm missing here? Thanks in advance!
 
i'm running into the problem that when i get to step 4, i check the levels: all good, but when i start the measurement it stops because of clipping.
Is it possible that you've enabled the acoustic timing reference, which has its own level control that's not set low enough?

In any case, the RME without an external attenuator is not transparent enough at those signal amplitudes to give meaningful THD results.
 
Could
Is it possible that you've enabled the acoustic timing reference, which has its own level control that's not set low enough?

In any case, the RME without an external attenuator is not transparent enough at those signal amplitudes to give meaningful THD results.
Could you elaborate?
The reference is looped back to the second input and i gained that down; the exact level shouldn't really matter as long as it's not clipping right?

I'll try to make a 40 dB pad to fix on the output, guess that should do the trick for the THD results?
 
Could

Could you elaborate?
The reference is looped back to the second input and i gained that down; the exact level shouldn't really matter as long as it's not clipping right?

I'll try to make a 40 dB pad to fix on the output, guess that should do the trick for the THD results?
DAC will show more distortion and noise at low output levels. (I used 40 dB attenuator.)
 
You need a 40db attenuation as Thomas indicated.
And make the output resistance low as a Cartridge by using a voltage divider with a 10ohm resistor at the bottom, to emulate a MC cartridge.. say a 1000ohm /10ohm divider. Set source to get appropriate level. Or just turn down the level, but the noise floor will be difficult to judge.

some PC do not accept more than 300mV input voltage
 
DAC will show more distortion and noise at low output levels. (I used 40 dB attenuator.)
Thomas, did you make it or buy it? I’ve never seen a 40db for sale but don’t know if I could make one that wouldn’t contribute any noise. I want to do to be able to measure but if I have to make it I’m hoping to use someone else’s recipe that had low noise. Did you use film resistors? Which brand did you have luck with?
 
Thomas, did you make it or buy it? I’ve never seen a 40db for sale but don’t know if I could make one that wouldn’t contribute any noise. I want to do to be able to measure but if I have to make it I’m hoping to use someone else’s recipe that had low noise. Did you use film resistors? Which brand did you have luck with?
I used two 20 dB attenuators, Monacor. They worked fine.
 
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