Measuring a phono stage properly? what am im doing wrong?

[gamerpaddy]

Hello
i repair hifi stuff for a while now and came across a phonostage from Trigon, model Vanguard 1 with a broken PSU.
after i got it back working (the power brick "volcano" was dead) i wanted to measure its distortion and noise.

I am using a Focusrite 4i4 3rd gen and REW software.
When a do a loopback with a short cable i get about 105dBFS N+D from the interface unless i increase the gain of the generator past -3dB

Also im using a phono converter that basically applies the RIAA in reverse and lowers the signal level passively to be able to hook line level RCA to a unused phono port.
so ive hooked it up like this:

all using RCA, not symmetrical.
REW Signal generator (1khz -12dB) --> Audio IF 1/2 Out --> "phono converter" -> Phono stage -> Audio IF 3/4 In -> REW RTA Plot

Apparently this is the result
the signal should be at -12dB but as i cant change the Gain of Input 3 and 4, its a little lower at -18dB. the Phono stage is at its lowest Gain (all switches off, default loading and impedance)

102.8dB N+D .. this cant be right, what am i doing wrong?
this would beat all measured phono stages by far if its correct.


How to measure it properly?

 

[AnalogSteph]

N+D is relative to full-scale input level, not relative to signal level (which is -18.1 dBFS in your case, leaving N+D of -84.7 dBr(A)). The next REW build should sport an option to change this.

Note that the FR deviation of your inverse RIAA will be added to that of the preamp itself, so if you want to measure that accurately you need known good software IRIAA.

 

[Sokel]

If you want to see the real numbers click "current" at the top and see the results in dBr at the main window.
Lasted beta built is calculating noise strange though at the main window in dBr,it always presents it as if its exactly the same with SNR.use stable version for better.

 

[Balle Clorin]

Without doing a careful calibration the numbers REW give are rubbish and irrelevant . Use dBr to see relative numbers, also google FFT gain…
My Focusrite solo Gen 3 also mess up everything and is highly inconsistent and giving non repeatable results.

 

[AnalogSteph]

My Focusrite solo Gen 3 also mess up everything and is highly inconsistent and giving non repeatable results.

You sure you don't have a problem with dropouts (caused by e.g. system DPC latency issues or too small a buffer size or just a bad USB cable, or possibly even a dodgy XMOS chip if you're unlucky)?

 

[Balle Clorin]

Not sure why it is so miserable , I use a USB dac/ADC from Dlock instead, 20usd and works every time .with REW calibration it is perfectly linear too.

Delock Products 63926 Delock External USB 2.0 Sound Adapter Virtual 7.1 - 24 bit / 96 kHz with S/PDIF

www.delock.com

Good enough for measurements of tube amps and vinyl systems. toslink out too.
For serious stuff I use my Tascam 144 mk2 interface. The Focusrite cannot even be sold due to the poor state