How to volume match phono preamps?
Feb 7, 2024 at 1:02 PM Thread Starter Post #1 of 5

megabigeye

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Hi all,
I recently bought myself a new preamp for my turntable. I thought it sounded slightly off, so decided to buy another, different preamp. I think the second one sounds better... but then, it's prettier and cost more than twice as much as the first one, and I already thought the first one sounded off... So I'm doubting myself.

I already have a hodgepodge of switches kludged together, so switching between preamps is nearly instantaneous.
Both of the preamps have adjustable gain / trim pot. Playing music, I can get the volume close-ish by ear, but not quite matched.

I think on a DAC or amp I could play a tone and then measure the voltage, but I don't have a way to do that on a turntable. So what do I measure? And how exactly?

My aim isn't to prove anything to anybody or be particularly scientifically rigorous. All I want is to do is be able to decide with a little more certainty whether the extra money is worth it.
 
Feb 7, 2024 at 2:37 PM Post #2 of 5
Do you have a computer with an audio input? Either integrated or from an audio interface is fine. You should connect the preamp output to the input using a cable with the correct connectors (rca-jack most likely) for the job. Then, you could record and analyze the audio with whatever program you are already comfortable with. If you don't have any, you could use audacity for that. To record sound, you might need to configure your inputs first by going to Edit->Preferences->Devices and select your input there if it's not already selected correctly. You could measure the RMS of the recorded signal by going into Analyze->Measure RMS. If this option doesn't show up, you need to download the plugin first and copy it into the folder where audacity looks for plugins. I'm using a ~5years old setup of audacity so I don't know exactly to current version works.

If you already have a program you use to record audio, you could just use that for recording and audacity only for analyzing. If you already have foobar and know how to install plugins for that, you could get the dynamic range meter and use that to read the RMS level of the recording.
 
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Feb 7, 2024 at 3:15 PM Post #3 of 5
Ah, Audacity! Of course! How could I forget!

I've actually got a third preamp (that the two new ones are replacing) that has digital output and two line inputs as well as the phono input. I could try that when I have time.

In this case am I just trying to volume match the two tracks so I can switch between them, or should/could I try to do a null test?
I've used Audacity a little bit in the past to record my LPs, but I'm not that well versed in it.

Thanks for the help!
 
Feb 7, 2024 at 4:05 PM Post #4 of 5
I don't think a null test would reveal anything useful. The recorded audio wouldn't null well at all even if you recorded through the same preamp twice. I think you should record a louder segment of the music with both preamps using the RCA out if that's what you use for listening. You don't have to record exactly the same segment twice but make sure when you actually analyze the audio, you do analyze the same segments exactly. I would do this by focusing on a visually distinct part of the waveform (like a nice kick drum) and line the tracks up based on that.

By the way, I don't think there any particular reason you shouldn't use a low level (1-10 millivolts of noise) test signal from one of your DAC/Amps fed into the phono input of the preamp but if you're more comfortable with the turntable, then use that. With that said, I'm not familiar with phono preamps. If you accidentally send a line level signal to the preamp instead of the couple of millivolts, that might damage the amp because I'm almost sure it's made to accept much lower level voltage than that.
 
Feb 8, 2024 at 6:44 AM Post #5 of 5
With most RTAs(real time analyzer) there are options to graph more than just instant levels, I think that might work to check the level of a song or sample of a song, and adjust based on it for 2 devices. If that's no good, there is always the option to graph for peak amplitude at each frequency, and that should work well.
I haven't used any free RTA in recent years, so I'm not sure if I can suggest one, but I'm guessing just about any would do the job.
 

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